Magnetic patterning perpendicular anisotropy FePd alloy films by masked ionirradiationD G Merkel L Bottyaacuten F Tanczikoacute Z Zolnai N Nagy G Veacutertesy J Waizinger and L Bommer Citation Journal of Applied Physics 109 124302 (2011) doi 10106313596535 View online httpdxdoiorg10106313596535 View Table of Contents httpscitationaiporgcontentaipjournaljap10912ver=pdfcov Published by the AIP Publishing Articles you may be interested in NiOFe(001) Magnetic anisotropy exchange bias and interface structure J Appl Phys 113 234315 (2013) 10106314811528 Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films Appl Phys Lett 98 232510 (2011) 10106313599492 Study on nanoscale patterning using ferro-antiferromagnetic transition in [001]-oriented L10 FePtRh film J Appl Phys 109 07B705 (2011) 10106313537947 Modification of local order in FePd films by low energy He + irradiation J Appl Phys 104 013901 (2008) 10106312938027 Chemical ordering of epitaxial FePd deposited on ZnSe and the surfactant effect of segregated Se Appl Phys Lett 76 1455 (2000) 1010631126062

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Magnetic patterning perpendicular anisotropy FePd alloy films by maskedion irradiation

D G Merkel1a) L Bottyan1 F Tancziko1 Z Zolnai2 N Nagy2 G Vertesy2 J Waizinger2

and L Bommer31KFKI Research Institute for Particle and Nuclear Physics POB 49 H 1525 Budapest Hungary2Hungarian Academy of Sciences Research Institute for Technical Physics and Materials SciencePOB 49 H 1525 Budapest Hungary3Max-Planck-Institut for Metals Research Heisenbergstr 3 70569 Stuttgart Germany

(Received 15 January 2011 accepted 29 April 2011 published online 16 June 2011)

The nanopatterning of magnetic films by ion implantation is reported Highly L10-ordered Fe47Pd53

epitaxial alloy films on a MgO(001) substrate were covered by a monolayer of silica spheres in a

Langmuir film balance Using this sphere layer as an implantation mask the samples were

irradiated by Nethorn or Fethorn ions with energies of 35 keV and 100 keV respectively After the silica

mask was removed the samples were characterized via conversion electron Mossbauer

spectroscopy longitudinal and polar magneto-optical Kerr effect and atomic force and magnetic

force microscopy We find that the magnetic stripe domains observed in the nonirradiated sample

were converted into a regular 2D magnetic pattern of hcp character upon 1 1015cm2 35 keV

neon or 1 1014cm2 100 keV iron irradiation with the direction of magnetization remaining out

of plane in the nodes of the hcp lattice and relaxed into the film plane in the inter-node region

resulting in an overall in-plane magnetic softening of the film VC 2011 American Institute ofPhysics [doi10106313596535]

I INTRODUCTION

In the past decade studies of materials with perpendicular

magnetic anisotropy (PMA) especially Fe(PdPt) and

Co(PdPt) remained in the forefront of academic and industrial

research alike due to their foreseen role in ultrahigh density

magnetic recording1ndash3 The equilibrium phase of FexPd1x in

the 05lt xlt 06 composition range at room temperature

exhibits an L10 (CuAu(I)-type) structure consisting of alternat-

ing Fe and Pd planes along the [001] direction resulting in a

large anisotropy energy of the order of 107ergcm34ndash6 How-

ever FePd can also form a disordered fcc structure with no

preferred orientation of the local magnetic moment a property

that is disadvantageous for magnetic recording

The thin-film media in magnetic hard-disk drives faces a

physical limit caused by the superparamagnetic effect by

which the individual grains in the medium become so small

that they are no longer stable against thermal fluctuations

There have been several proposed solutions for extending

the superparamagnetic limit to higher bit densities and one

of them is perpendicular patterned media In this approach a

periodic array of magnetic nanoparticles of perpendicular

magnetic anisotropy is defined lithographically on a non-

magnetic substrate7ndash9 Many approaches to the preparation

of metal nanoparticles have been reported10 including chem-

ical reduction11 UV photolysis12 thermal decomposition13

metal-vapor decomposition14 electrochemical synthesis15

and even sonochemical decomposition16 Nanopatterned

structures achieved via ion implantation using silica spheres

were investigated elsewhere in detail17

A magnetic pattern created by low energy Hethorn irradia-

tion in CoPt alloys was reported18 The pattern was formed

by 35 keV Hethorn ions using a platinum mask of 1 lm 1 lm

square dots created via electron beam photoresist lithogra-

phy The resulting magnetic pattern reflected the mask struc-

ture Another group studied the He-irradiation-induced 11

replication of features drilled in a SiC stencil mask to the

magnetic properties19 With these techniques a very uniform

patterning was possible the size of the mask however was

rather limited Moreover the minimum mask cell dimension

was of the order of 1 lm

In a recent study we investigated low energy (130 keV)

Hethorn-irradiated L10 FePd alloy films and found that ion irra-

diation destroys the L10 order20 Using this effect it is possi-

ble to create magnetically patterned media provided a

suitable nanomask is used during irradiation As has been

demonstrated ion irradiation combined with nanosphere li-

thography can be used to achieve local patterning on a sub-

micron scale21 Here we introduce a novel method for

magnetic patterning thin films that can be beneficial for

future applications The resulting atomic and periodic mag-

netic structure was characterized via conversion electron

Mossbauer spectroscopy (CEMS) longitudinal and polar

magneto-optical Kerr effect (MOKE) atomic force micros-

copy (AFM) and magnetic force microscopy (MFM)

II EXPERIMENTAL PROCEDURES

The Cr(3 nm)Pd(15 nm)57Fe47Pd53(30 nm)Pd(1 nm)

sample was grown on a MgO(001) substrate using a MBE

system made by MECA-2000 The base pressure of

33 1010 mbar in the evaporation chamber was increased

to 75 109 mbar during deposition The Cr and Pd layers

a)Author to whom correspondence should be addressed Electronic mail

merkelrmkikfkihu

0021-89792011109(12)1243027$3000 VC 2011 American Institute of Physics109 124302-1

JOURNAL OF APPLIED PHYSICS 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

were deposited using an electron gun the deposition rates

were 01 and 015 As respectively The 57Fe47Pd53 alloy

layer was deposited via co-evaporation of Pd and 57Fe (the

latter from a Knudsen cell with a BeO crucible) with deposi-

tion rates of 008 As and 0055 As respectively Deposi-

tions were controlled by quartz microbalance thickness

monitors The sample was rotated during the deposition for

improved lateral homogeneity The substrate temperature

was held at 350 C during the deposition in order to promote

the formation of an epitaxial FePd alloy of L10 structure

with the c-axis out of the film plane22 With similar growth

conditions we previously reported20 ordered FePd films with

an L10 fraction of 81 and an order parameter of 087

A single monolayer of silica spheres with a nominal di-

ameter of 200 nm was deposited on top of the as prepared

FePd film sample using the Langmuir technique2324 The

silica spheres were prepared with Stoberrsquos method25 via the

controlled hydrolysis of tetraethyl-orthosilicate The silica

sphere film deposition was carried out using a KSV2000 film

balance First the sample was immersed vertically into the

water and then the solution containing silica spheres was

spread onto the water surface in the Langmuir film balance

After evaporation of the spreading liquid the layer was com-

pressed with two barriers while its surface pressure was

monitored At about 80 of the collapse pressure of the sam-

ple the immersed MgO substrate was gradually pulled out

from the water vertically at constant surface pressure By

this careful adjustment an ordered single monolayer of silica

spheres was formed and maintained on top of the FePd film

MgO substrate system Further details of the silica synthesis

and film deposition can be found in Ref 21 and the referen-

ces therein

AFM analysis of the nanosphere-covered sample was

performed using an AIST-NT SmartSPM 1010 system

before and after ion irradiation and the evaluation of the

AFM images was carried out with Gwyddion software26

The AFM image of the sample with the deposited spheres in

Fig 1 (left) shows an almost perfect single layer coverage by

the spheres with a structural coherence length of a few

microns From further analysis of the AFM image the grain

size distribution was deduced (Fig 1 right) with a median

grain size and full width at half maximum of 195 nm and 79

nm respectively In the short range the arrangement of the

spheres shows a hexagonal local symmetry but on a longer

scale the hexagonal cells determined by the first neighbors

follow a twisted pattern This feature is the consequence of

the size distribution of the silica spheres and the domainlike

structure of the deposited monolayer

The covered sample was cut into several pieces with an

average size of 4 4 mm2 The pieces were then irradiated

by 35 keV Nethorn ions of (1-50) 1014cm2 or 100 keV Fethorn

ions with a fluence of (5-100) 1012cm2 The energy was

calculated (using the SRIM code27) so that the impinging ions

would not pass the silica spheres to reach the FePd layer and

the covered surface would remain of an L10 structure with

PMA but between the silica spheres the irradiation might

cause structural disordering and magnetization reversal into

the sample plane

The silica sphere layer was removed from the surface

using Scotch tape The CEMS experiments were performed

after removal of the spheres using a 25 mCi 57Co(Rh) single-

line Mossbauer source with a homemade gas-flow single-

wire proportional counter operating with He mixed with

47 CH4 extinction gas at a bias voltage of 800 6 10 V

Longitudinal and polar MOKE measurements with max-

imum available external magnetic fields of 400 and 430 mT

respectively were carried out on all samples using a diode

laser at a wavelength of 670 nm The size of the laser spot on

the sample surface was about 1 mm2 Enhanced sensitivity

was achieved by modulating the power supply of the diode

laser and using a digital lock-in for registering the signal of

the photodiode detector

Magnetic force microscopy was performed using a

Veeco Multimode microscope (with a Nanoscope V control-

ler) without an applied external field in lift-mode and the

spacing between the tip and the sample was kept constant at

FIG 1 (Color online) (left) AFM image of 200 nm silica spheres deposited on the FePd surface and (right) the size distribution of the spheres The median

grain size and full width at half maximum are 195 nm and 79 nm respectively

124302-2 Merkel et al J Appl Phys 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

80 nm At such a height van der Waals force contributions

vanish and only magnetic forces are detected Standard sili-

con cantilevers with CoCr coating were used

III RESULTS AND DISCUSSION

CEMS spectra (Fig 2 left) were recorded in perpendic-

ular incidence on each sample in order to follow the struc-

tural changes caused by the 35 keV Nethorn or 100 keV Fethorn

irradiation The spectra were fitted by the NORMOS code28

allowing for three sextets with histogram-type hyperfine (hf)

field distributions The fitting was realized so that all of the

distributions remained close to symmetric The direction of

the hyperfine field was calculated from the relative line

intensities of 3x11x3 of the corresponding sextet where

x depends on the angle between the hyperfine field and the

c-ray direction29 as

x frac14 4 1 cos2 H1thorn cos2 H

(1)

According to this expression an in-plane hf field local envi-

ronment gives a subspectrum with intensity ratios of

341143 and a local environment with the hf field point-

ing perpendicular to the surface of the film gives ratios of

301103 A random orientation of the magnetic domains

gives an unweighted average of 32112329

The corresponding magnetic hyperfine field distributions

were calculated from the CEMS spectra and are shown in

Fig 2 (right)

In the as-deposited sample all three of the previously

reported iron environments were observed2030 namely the

low-hf-field ordered L10 species (at 26T stripes) the

large-hf field iron-rich species (at 35T) and the intermedi-

ate-hf-field disordered fcc species (at 31T stripes) In

the best fit the hf field in the low-hf-field and high-hf-field

species point out of plane whereas in the intermediate hf

field species it has a nearly random orientation The fraction

of the low-field L10 spectral component relative to the full

spectrum intensity is 75 Its quadrupole splitting is 042

mms and the isomer shifts are 021 mm (somewhat higher

than that of the bulk L10 FePd) 038 mms and 018 mms

respectively31

The variation of the spectral fractions of the three dis-

tinct environments as a function of the fluence of neon and

iron irradiation is displayed in Fig 3 The spectral fraction

will be considered as the fraction of the given structure in

the sample with no accounting for the possibly somewhat

different Lamb-Mossbauer coefficients

It has been reported32 that the iron irradiation of ordered

FePd might result in the disordering of the film structure As

one can see in Fig 3 with increasing neon and iron irradia-

tion fluence the L10 component of FePd transforms into fcc

FIG 2 (Color online) (left) Conversion electron Mossbauer spectroscopy spectra and (right) the calculated hyperfine field distributions corresponding to the

as-deposited 35 keV Nethorn and 100 keV Fethorn irradiated samples

124302-3 Merkel et al J Appl Phys 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

which results in a decay of the L10 contribution as compared

to the disordered fcc fraction In the case of neon irradiation

almost the entire sample evolved into the disordered fcc

FePd with the highest irradiation fluence used here (5 1015

cm2) whereas in the case of iron irradiation about 30 of

the ordered component remained at a fluence of 1 1014

cm2 The iron-rich environment gradually disappears with

increasing fluence and does not seem to play an important

role in the samplersquos magnetic properties Ion irradiation

destroys the L10 order therefore the presence of ordered

regions corresponds to the regions below the silica spheres

where the impinging ions do not reach the FePd layer The

inter-node regions are converted into fcc thereby allowing a

magnetic and structural pattern to develop in the sample

Because the fraction of the fcc structure increases with

increasing irradiation fluence on average the hf field direc-

tion declines with increasing irradiation fluence (Fig 4 solid

line) In the nonirradiated sample the disordered fcc compo-

nent has a nearly random hf field orientation (46 relative to

the direction of the incident c-beam) and with increasing flu-

ence it rotates toward the sample planemdash70 and 62 relative

to the direction of the incident c-beam for the maximum flu-

ence used in the Ne and Fe cases respectively This tendency

might be due to magnetic coupling of the L10 and disordered

fcc components As expected the direction of the average

magnetization (weighted average for all three iron environ-

ments) of the sample also declines as the spectral ratio of the

ordered component decreases with fluence (from 9 to 68

and 44 for the cases of neon and iron respectively) (Fig 4

dashed line) Because the CEMS spectra provide an average

orientation for the sphere-protected and irradiation converted

regions a quantitative evaluation of the magnetization orien-

tations is difficult based on the CEMS spectra alone There-

fore a magnetic force microscopic analysis was performed

on selected regions of the samples

MFM analysis (Fig 5) shows the well-known stripe do-

main structure in the as-deposited (virgin) sample33 For the

lowest fluence used the magnetic pattern remained similar to

that of the virgin sample irrespective of the irradiating ion

Subsequent to an irradiation fluence of 1 1015cm2 of Ne and

1 1015cm2 of Fe the stripe domain character vanishes and

develops into a magnetic pattern reflecting the silica sphere

distribution Because the ions have no access to the volume

under the spheres (besides lateral straggling) the magnetiza-

tion remained perpendicular to the surface under the silica

spheres but the magnetization relaxed toward the sample

plane in the inter-sphere regions According to the MFM

image analysis the area of the circular dots was about 30 of

the full substrate area This is in good agreement with CEMS

results which show a decrease of the fraction of the perpendic-

ular component from 75 to 25 in the investigated Nethorn flu-

ence range ie to about one-third of the initial amount This

means that 33 of the L10 structural component did not trans-

form to fcc with the used irradiation energies and fluences We

obtained an average diameter of 140 nm for the circular

PMA magnetic dots with an average spacing of 230 nm

between the nearest neighbors A similar geometry was found

for Fe irradiation at a fluence of 1 1014cm2 When the sam-

ple was irradiated with a neon fluence of 5 1015cm2 the pat-

tern almost completely faded out and according to the CEMS

results only a fraction of the 3 L10 remained in the sample

This latter result underlines the role of the lateral straggling

FIG 3 (Color online) Variation of the

spectral fractions of the distinct iron

environments following (left) Nethorn and

(right) Fethorn irradiation

FIG 4 (Color online) The fluence de-

pendence of the angle between the c-ray

and the hyperfine field for (left) neon

and (right) iron irradiation Solid line hf

field direction for the disordered Fe

environment Dashed line weighted av-

erage for all three iron environments

124302-4 Merkel et al J Appl Phys 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

In order to study the average in-plane magnetization of

the layers longitudinal MOKE loops were recorded on each

sample (Fig 6) Apart from the as-deposited and the lowest

fluence Fethorn-irradiated sample the available 400 mT in-plane

external field was sufficient for full saturation With increasing

fluence of the neon irradiation the squareness (defined by the

ratio of the remanence (Ms) to the saturation magnetization

(Mr)) of the longitudinal hysteresis loops increased while the

coercivity decreased (Fig 6 left) At the highest neon fluence

where the layer mainly contains disordered fcc FePd the sam-

ple fully saturates the coercivity decreases to 10 mT and

the squareness of the hysteresis loop increases from below

003 to 097 (see Fig 6 left inset) In the partially saturated

cases the indicated values provide an upper limit for the

squareness A similar tendency can be seen in the case of iron

irradiation (Fig 6 right) however the applied Fethorn irradiation

fluence was not enough to completely convert the L10 struc-

tural regions in the layer into fcc Thus the coercivity dropped

to only 30 mT at the highest Fethorn fluence of 1014 ionscm2

and the squareness increased to only 068

The behavior of the out-of-plane magnetization was fol-

lowed by recording polar MOKE hysteresis loops (Fig 7)

Although the available 430 mT external magnetic field was

insufficient to completely saturate the irradiated samples in

the polar geometry it can be seen that the out-of-plane satu-

ration field (loop closing field) and the out-of-plane domain

contribution to the magnetization decrease with increasing

fluence but no significant change in the coercivity is observ-

able As a result the loops gradually flatten out and the

higher fluence magnetization curves are dominated by the

in-plane fcc grains and the shape anisotropy contribution

The results from the virgin sample indicate that at about 400

mT all magnetic moments point out of the film plane The

range of coercivity and the irregular ldquonarrow waistrdquo shape of

the hysteresis loop are characteristic of a continuous L10

maze domain structure34 The out-of-plane domains are rela-

tively easily saturated by domain wall movement but they

are difficult to demagnetize due to a high nucleation field35

FIG 5 (Color online) MFM images for (left) neon and (right) iron irradia-

tion at the fluences indicated in the figure

FIG 6 (Color online) Longitudinal MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

In the insets the corresponding squareness (MrMs) values are plotted

124302-5 Merkel et al J Appl Phys 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

Even the smallest applied irradiation fluence produces a suf-

ficient amount of domain nucleation centers to change the

hysteresis loop back to a regular shape

Note that according to SRIM simulations the projected

range (Rp) and its straggle (DRp) in silica are about 95 6 30

nm for 100 keV Fethorn and 85 6 35 nm for 35 keV Nethorn ions

respectively while the lateral straggling in FePd is about 20

nm for both cases Therefore the geometrical conditions for

masking and substrate patterning are quite similar However

considering that the amount of irradiation-induced disorder in

FePd is proportional to the number of displacements per atom

(dpa) it follows that the normalized fluence should be about

four times higher for Fethorn than for Nethorn irradiation Indeed

SRIM gives about 15 displacements per Nethorn ion and about 60

displacements per Fethorn ion for a unit depth of 1 nm at the dam-

age peak respectively As expected the fluence dependences

in Figs 4 5 6 7 and 8 are similar for neon and iron if they

are transformed to the corresponding dpa scales

IV CONCLUSION

Silica spheres of submicron size combined with the

Langmuir technique were efficiently applied as a periodic

ion-irradiation mask in order to produce magnetically nano-

patterned FePd films in the lateral direction Highly ordered

Fe47Pd53 alloy films were irradiated with Nethorn and Fethorn ions

through the sphere mask The stripe domains in the virgin

state transform into a lateral magnetic pattern at a fluence of

1 1015cm2 and 1 1014cm2 in the cases of neon and iron

respectively The origin of the lateral magnetic pattern is the

result of the alternation of the ordered L10 and disordered

regions appearing as the 2D projection of the silica sphere

mask pattern onto the FePd film surface

The ratios of the ordered and disordered structures were

extracted from CEMS spectral fractions and the magnetic

pattern geometry (dot spacing and size) was determined

from AFM and MFM the two values show agreement within

the experimental error Longitudinal and polar MOKE mag-

netometry fully supports the above-mentioned microscopic

picture These methods are suitable for describing the modi-

fication of local magnetic properties on a submicron scale

The density of magnetic bits in recording media can be

increased by lowering the diameter of the masking silica

spheres As an earlier publication shows36 silica spheres with

a diameter of 50 nm could be produced but with decreasing

mask size lower irradiation energies must be used in order to

hinder the irradiating ions from crossing the masking spheres

Therefore the presented concept calls for application with

spheres consisting of heavier elements (such as Ausilica core-

shell37 TiO238 etc) in order to increase the ion stopping in

the nanomask Further efforts are underway to improve the

long range ordering of the sphere mask technique

ACKNOWLEDGMENTS

This work was supported by the Hungarian National

Science Fund (OTKA) and by the National Office for

Research and Technology of Hungary under contract K

62272 and NAP-VENEUSrsquo05 Support from Janos Bolyai

Scholarships of the HAS for Z Zolnai and N Nagy are

appreciated The authors would like to thank Theresa Dragon

(MPI Metallforschung Stuttgart) for the technical support in

MFM as well as Dr Janos Major (MPI Metallforschung

Stuttgart) and Laszlo F Kiss (Research Institute for Solid

State Physics and Optics Budapest) for fruitful discussions

1T Devolder H Bernas D Ravelosona C Chappert S Pizzini J Vogel

J Ferre J P Jamet J Chen and V Mathet Nucl Instrum Methods Phys

Res B 175ndash177 375 (2001)2K Piao D Lee and D Wei J Magn Magn Mater 303 e39 (2006)3T Suzuki K Harada N Honda and K Ouchi J Magn Magn Mater

193 85 (1999)4M R Visokay and R Sinclair Appl Phys Lett 66 1692 (1995)5H Shima K Oikawa A Fujita K Fukamichi K Ishida and A Sakuma

Phys Rev B 70 224408 (2004)6O Ersen V Parasote V Pierron-Bohnes M C Cadeville and C Ulhaq-

Bouillet J Appl Phys 93 5 (2003)7C T Rettner S Anders J E E Baglin T Thomson and B D Terris

Appl Phys Lett 80 279 (2002)8B D Terris D Weller L Folks J E E Baglin A J Kellock H Rothui-

zen and P Vettiger J Appl Phys 87 7004 (2000)

FIG 7 (Color online) Polar MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

124302-6 Merkel et al J Appl Phys 109 124302 (2011)

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1601032236 On Fri 10 Jul 2015 145924

80 nm At such a height van der Waals force contributions

vanish and only magnetic forces are detected Standard sili-

con cantilevers with CoCr coating were used

III RESULTS AND DISCUSSION

CEMS spectra (Fig 2 left) were recorded in perpendic-

ular incidence on each sample in order to follow the struc-

tural changes caused by the 35 keV Nethorn or 100 keV Fethorn

irradiation The spectra were fitted by the NORMOS code28

allowing for three sextets with histogram-type hyperfine (hf)

field distributions The fitting was realized so that all of the

distributions remained close to symmetric The direction of

the hyperfine field was calculated from the relative line

intensities of 3x11x3 of the corresponding sextet where

x depends on the angle between the hyperfine field and the

c-ray direction29 as

x frac14 4 1 cos2 H1thorn cos2 H

(1)

According to this expression an in-plane hf field local envi-

ronment gives a subspectrum with intensity ratios of

341143 and a local environment with the hf field point-

ing perpendicular to the surface of the film gives ratios of

301103 A random orientation of the magnetic domains

gives an unweighted average of 32112329

The corresponding magnetic hyperfine field distributions

were calculated from the CEMS spectra and are shown in

Fig 2 (right)

In the as-deposited sample all three of the previously

reported iron environments were observed2030 namely the

low-hf-field ordered L10 species (at 26T stripes) the

large-hf field iron-rich species (at 35T) and the intermedi-

ate-hf-field disordered fcc species (at 31T stripes) In

the best fit the hf field in the low-hf-field and high-hf-field

species point out of plane whereas in the intermediate hf

field species it has a nearly random orientation The fraction

of the low-field L10 spectral component relative to the full

spectrum intensity is 75 Its quadrupole splitting is 042

mms and the isomer shifts are 021 mm (somewhat higher

than that of the bulk L10 FePd) 038 mms and 018 mms

respectively31

The variation of the spectral fractions of the three dis-

tinct environments as a function of the fluence of neon and

iron irradiation is displayed in Fig 3 The spectral fraction

will be considered as the fraction of the given structure in

the sample with no accounting for the possibly somewhat

different Lamb-Mossbauer coefficients

It has been reported32 that the iron irradiation of ordered

FePd might result in the disordering of the film structure As

one can see in Fig 3 with increasing neon and iron irradia-

tion fluence the L10 component of FePd transforms into fcc

FIG 2 (Color online) (left) Conversion electron Mossbauer spectroscopy spectra and (right) the calculated hyperfine field distributions corresponding to the

as-deposited 35 keV Nethorn and 100 keV Fethorn irradiated samples

124302-3 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

which results in a decay of the L10 contribution as compared

to the disordered fcc fraction In the case of neon irradiation

almost the entire sample evolved into the disordered fcc

FePd with the highest irradiation fluence used here (5 1015

cm2) whereas in the case of iron irradiation about 30 of

the ordered component remained at a fluence of 1 1014

cm2 The iron-rich environment gradually disappears with

increasing fluence and does not seem to play an important

role in the samplersquos magnetic properties Ion irradiation

destroys the L10 order therefore the presence of ordered

regions corresponds to the regions below the silica spheres

where the impinging ions do not reach the FePd layer The

inter-node regions are converted into fcc thereby allowing a

magnetic and structural pattern to develop in the sample

Because the fraction of the fcc structure increases with

increasing irradiation fluence on average the hf field direc-

tion declines with increasing irradiation fluence (Fig 4 solid

line) In the nonirradiated sample the disordered fcc compo-

nent has a nearly random hf field orientation (46 relative to

the direction of the incident c-beam) and with increasing flu-

ence it rotates toward the sample planemdash70 and 62 relative

to the direction of the incident c-beam for the maximum flu-

ence used in the Ne and Fe cases respectively This tendency

might be due to magnetic coupling of the L10 and disordered

fcc components As expected the direction of the average

magnetization (weighted average for all three iron environ-

ments) of the sample also declines as the spectral ratio of the

ordered component decreases with fluence (from 9 to 68

and 44 for the cases of neon and iron respectively) (Fig 4

dashed line) Because the CEMS spectra provide an average

orientation for the sphere-protected and irradiation converted

regions a quantitative evaluation of the magnetization orien-

tations is difficult based on the CEMS spectra alone There-

fore a magnetic force microscopic analysis was performed

on selected regions of the samples

MFM analysis (Fig 5) shows the well-known stripe do-

main structure in the as-deposited (virgin) sample33 For the

lowest fluence used the magnetic pattern remained similar to

that of the virgin sample irrespective of the irradiating ion

Subsequent to an irradiation fluence of 1 1015cm2 of Ne and

1 1015cm2 of Fe the stripe domain character vanishes and

develops into a magnetic pattern reflecting the silica sphere

distribution Because the ions have no access to the volume

under the spheres (besides lateral straggling) the magnetiza-

tion remained perpendicular to the surface under the silica

spheres but the magnetization relaxed toward the sample

plane in the inter-sphere regions According to the MFM

image analysis the area of the circular dots was about 30 of

the full substrate area This is in good agreement with CEMS

results which show a decrease of the fraction of the perpendic-

ular component from 75 to 25 in the investigated Nethorn flu-

ence range ie to about one-third of the initial amount This

means that 33 of the L10 structural component did not trans-

form to fcc with the used irradiation energies and fluences We

obtained an average diameter of 140 nm for the circular

PMA magnetic dots with an average spacing of 230 nm

between the nearest neighbors A similar geometry was found

for Fe irradiation at a fluence of 1 1014cm2 When the sam-

ple was irradiated with a neon fluence of 5 1015cm2 the pat-

tern almost completely faded out and according to the CEMS

results only a fraction of the 3 L10 remained in the sample

This latter result underlines the role of the lateral straggling

FIG 3 (Color online) Variation of the

spectral fractions of the distinct iron

environments following (left) Nethorn and

(right) Fethorn irradiation

FIG 4 (Color online) The fluence de-

pendence of the angle between the c-ray

and the hyperfine field for (left) neon

and (right) iron irradiation Solid line hf

field direction for the disordered Fe

environment Dashed line weighted av-

erage for all three iron environments

124302-4 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

In order to study the average in-plane magnetization of

the layers longitudinal MOKE loops were recorded on each

sample (Fig 6) Apart from the as-deposited and the lowest

fluence Fethorn-irradiated sample the available 400 mT in-plane

external field was sufficient for full saturation With increasing

fluence of the neon irradiation the squareness (defined by the

ratio of the remanence (Ms) to the saturation magnetization

(Mr)) of the longitudinal hysteresis loops increased while the

coercivity decreased (Fig 6 left) At the highest neon fluence

where the layer mainly contains disordered fcc FePd the sam-

ple fully saturates the coercivity decreases to 10 mT and

the squareness of the hysteresis loop increases from below

003 to 097 (see Fig 6 left inset) In the partially saturated

cases the indicated values provide an upper limit for the

squareness A similar tendency can be seen in the case of iron

irradiation (Fig 6 right) however the applied Fethorn irradiation

fluence was not enough to completely convert the L10 struc-

tural regions in the layer into fcc Thus the coercivity dropped

to only 30 mT at the highest Fethorn fluence of 1014 ionscm2

and the squareness increased to only 068

The behavior of the out-of-plane magnetization was fol-

lowed by recording polar MOKE hysteresis loops (Fig 7)

Although the available 430 mT external magnetic field was

insufficient to completely saturate the irradiated samples in

the polar geometry it can be seen that the out-of-plane satu-

ration field (loop closing field) and the out-of-plane domain

contribution to the magnetization decrease with increasing

fluence but no significant change in the coercivity is observ-

able As a result the loops gradually flatten out and the

higher fluence magnetization curves are dominated by the

in-plane fcc grains and the shape anisotropy contribution

The results from the virgin sample indicate that at about 400

mT all magnetic moments point out of the film plane The

range of coercivity and the irregular ldquonarrow waistrdquo shape of

the hysteresis loop are characteristic of a continuous L10

maze domain structure34 The out-of-plane domains are rela-

tively easily saturated by domain wall movement but they

are difficult to demagnetize due to a high nucleation field35

FIG 5 (Color online) MFM images for (left) neon and (right) iron irradia-

tion at the fluences indicated in the figure

FIG 6 (Color online) Longitudinal MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

In the insets the corresponding squareness (MrMs) values are plotted

124302-5 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

Even the smallest applied irradiation fluence produces a suf-

ficient amount of domain nucleation centers to change the

hysteresis loop back to a regular shape

Note that according to SRIM simulations the projected

range (Rp) and its straggle (DRp) in silica are about 95 6 30

nm for 100 keV Fethorn and 85 6 35 nm for 35 keV Nethorn ions

respectively while the lateral straggling in FePd is about 20

nm for both cases Therefore the geometrical conditions for

masking and substrate patterning are quite similar However

considering that the amount of irradiation-induced disorder in

FePd is proportional to the number of displacements per atom

(dpa) it follows that the normalized fluence should be about

four times higher for Fethorn than for Nethorn irradiation Indeed

SRIM gives about 15 displacements per Nethorn ion and about 60

displacements per Fethorn ion for a unit depth of 1 nm at the dam-

age peak respectively As expected the fluence dependences

in Figs 4 5 6 7 and 8 are similar for neon and iron if they

are transformed to the corresponding dpa scales

IV CONCLUSION

Silica spheres of submicron size combined with the

Langmuir technique were efficiently applied as a periodic

ion-irradiation mask in order to produce magnetically nano-

patterned FePd films in the lateral direction Highly ordered

Fe47Pd53 alloy films were irradiated with Nethorn and Fethorn ions

through the sphere mask The stripe domains in the virgin

state transform into a lateral magnetic pattern at a fluence of

1 1015cm2 and 1 1014cm2 in the cases of neon and iron

respectively The origin of the lateral magnetic pattern is the

result of the alternation of the ordered L10 and disordered

regions appearing as the 2D projection of the silica sphere

mask pattern onto the FePd film surface

The ratios of the ordered and disordered structures were

extracted from CEMS spectral fractions and the magnetic

pattern geometry (dot spacing and size) was determined

from AFM and MFM the two values show agreement within

the experimental error Longitudinal and polar MOKE mag-

netometry fully supports the above-mentioned microscopic

picture These methods are suitable for describing the modi-

fication of local magnetic properties on a submicron scale

The density of magnetic bits in recording media can be

increased by lowering the diameter of the masking silica

spheres As an earlier publication shows36 silica spheres with

a diameter of 50 nm could be produced but with decreasing

mask size lower irradiation energies must be used in order to

hinder the irradiating ions from crossing the masking spheres

Therefore the presented concept calls for application with

spheres consisting of heavier elements (such as Ausilica core-

shell37 TiO238 etc) in order to increase the ion stopping in

the nanomask Further efforts are underway to improve the

long range ordering of the sphere mask technique

ACKNOWLEDGMENTS

This work was supported by the Hungarian National

Science Fund (OTKA) and by the National Office for

Research and Technology of Hungary under contract K

62272 and NAP-VENEUSrsquo05 Support from Janos Bolyai

Scholarships of the HAS for Z Zolnai and N Nagy are

appreciated The authors would like to thank Theresa Dragon

(MPI Metallforschung Stuttgart) for the technical support in

MFM as well as Dr Janos Major (MPI Metallforschung

Stuttgart) and Laszlo F Kiss (Research Institute for Solid

State Physics and Optics Budapest) for fruitful discussions

1T Devolder H Bernas D Ravelosona C Chappert S Pizzini J Vogel

J Ferre J P Jamet J Chen and V Mathet Nucl Instrum Methods Phys

Res B 175ndash177 375 (2001)2K Piao D Lee and D Wei J Magn Magn Mater 303 e39 (2006)3T Suzuki K Harada N Honda and K Ouchi J Magn Magn Mater

193 85 (1999)4M R Visokay and R Sinclair Appl Phys Lett 66 1692 (1995)5H Shima K Oikawa A Fujita K Fukamichi K Ishida and A Sakuma

Phys Rev B 70 224408 (2004)6O Ersen V Parasote V Pierron-Bohnes M C Cadeville and C Ulhaq-

Bouillet J Appl Phys 93 5 (2003)7C T Rettner S Anders J E E Baglin T Thomson and B D Terris

Appl Phys Lett 80 279 (2002)8B D Terris D Weller L Folks J E E Baglin A J Kellock H Rothui-

zen and P Vettiger J Appl Phys 87 7004 (2000)

FIG 7 (Color online) Polar MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

124302-6 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

which results in a decay of the L10 contribution as compared

to the disordered fcc fraction In the case of neon irradiation

almost the entire sample evolved into the disordered fcc

FePd with the highest irradiation fluence used here (5 1015

cm2) whereas in the case of iron irradiation about 30 of

the ordered component remained at a fluence of 1 1014

cm2 The iron-rich environment gradually disappears with

increasing fluence and does not seem to play an important

role in the samplersquos magnetic properties Ion irradiation

destroys the L10 order therefore the presence of ordered

regions corresponds to the regions below the silica spheres

where the impinging ions do not reach the FePd layer The

inter-node regions are converted into fcc thereby allowing a

magnetic and structural pattern to develop in the sample

Because the fraction of the fcc structure increases with

increasing irradiation fluence on average the hf field direc-

tion declines with increasing irradiation fluence (Fig 4 solid

line) In the nonirradiated sample the disordered fcc compo-

nent has a nearly random hf field orientation (46 relative to

the direction of the incident c-beam) and with increasing flu-

ence it rotates toward the sample planemdash70 and 62 relative

to the direction of the incident c-beam for the maximum flu-

ence used in the Ne and Fe cases respectively This tendency

might be due to magnetic coupling of the L10 and disordered

fcc components As expected the direction of the average

magnetization (weighted average for all three iron environ-

ments) of the sample also declines as the spectral ratio of the

ordered component decreases with fluence (from 9 to 68

and 44 for the cases of neon and iron respectively) (Fig 4

dashed line) Because the CEMS spectra provide an average

orientation for the sphere-protected and irradiation converted

regions a quantitative evaluation of the magnetization orien-

tations is difficult based on the CEMS spectra alone There-

fore a magnetic force microscopic analysis was performed

on selected regions of the samples

MFM analysis (Fig 5) shows the well-known stripe do-

main structure in the as-deposited (virgin) sample33 For the

lowest fluence used the magnetic pattern remained similar to

that of the virgin sample irrespective of the irradiating ion

Subsequent to an irradiation fluence of 1 1015cm2 of Ne and

1 1015cm2 of Fe the stripe domain character vanishes and

develops into a magnetic pattern reflecting the silica sphere

distribution Because the ions have no access to the volume

under the spheres (besides lateral straggling) the magnetiza-

tion remained perpendicular to the surface under the silica

spheres but the magnetization relaxed toward the sample

plane in the inter-sphere regions According to the MFM

image analysis the area of the circular dots was about 30 of

the full substrate area This is in good agreement with CEMS

results which show a decrease of the fraction of the perpendic-

ular component from 75 to 25 in the investigated Nethorn flu-

ence range ie to about one-third of the initial amount This

means that 33 of the L10 structural component did not trans-

form to fcc with the used irradiation energies and fluences We

obtained an average diameter of 140 nm for the circular

PMA magnetic dots with an average spacing of 230 nm

between the nearest neighbors A similar geometry was found

for Fe irradiation at a fluence of 1 1014cm2 When the sam-

ple was irradiated with a neon fluence of 5 1015cm2 the pat-

tern almost completely faded out and according to the CEMS

results only a fraction of the 3 L10 remained in the sample

This latter result underlines the role of the lateral straggling

FIG 3 (Color online) Variation of the

spectral fractions of the distinct iron

environments following (left) Nethorn and

(right) Fethorn irradiation

FIG 4 (Color online) The fluence de-

pendence of the angle between the c-ray

and the hyperfine field for (left) neon

and (right) iron irradiation Solid line hf

field direction for the disordered Fe

environment Dashed line weighted av-

erage for all three iron environments

124302-4 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

In order to study the average in-plane magnetization of

the layers longitudinal MOKE loops were recorded on each

sample (Fig 6) Apart from the as-deposited and the lowest

fluence Fethorn-irradiated sample the available 400 mT in-plane

external field was sufficient for full saturation With increasing

fluence of the neon irradiation the squareness (defined by the

ratio of the remanence (Ms) to the saturation magnetization

(Mr)) of the longitudinal hysteresis loops increased while the

coercivity decreased (Fig 6 left) At the highest neon fluence

where the layer mainly contains disordered fcc FePd the sam-

ple fully saturates the coercivity decreases to 10 mT and

the squareness of the hysteresis loop increases from below

003 to 097 (see Fig 6 left inset) In the partially saturated

cases the indicated values provide an upper limit for the

squareness A similar tendency can be seen in the case of iron

irradiation (Fig 6 right) however the applied Fethorn irradiation

fluence was not enough to completely convert the L10 struc-

tural regions in the layer into fcc Thus the coercivity dropped

to only 30 mT at the highest Fethorn fluence of 1014 ionscm2

and the squareness increased to only 068

The behavior of the out-of-plane magnetization was fol-

lowed by recording polar MOKE hysteresis loops (Fig 7)

Although the available 430 mT external magnetic field was

insufficient to completely saturate the irradiated samples in

the polar geometry it can be seen that the out-of-plane satu-

ration field (loop closing field) and the out-of-plane domain

contribution to the magnetization decrease with increasing

fluence but no significant change in the coercivity is observ-

able As a result the loops gradually flatten out and the

higher fluence magnetization curves are dominated by the

in-plane fcc grains and the shape anisotropy contribution

The results from the virgin sample indicate that at about 400

mT all magnetic moments point out of the film plane The

range of coercivity and the irregular ldquonarrow waistrdquo shape of

the hysteresis loop are characteristic of a continuous L10

maze domain structure34 The out-of-plane domains are rela-

tively easily saturated by domain wall movement but they

are difficult to demagnetize due to a high nucleation field35

FIG 5 (Color online) MFM images for (left) neon and (right) iron irradia-

tion at the fluences indicated in the figure

FIG 6 (Color online) Longitudinal MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

In the insets the corresponding squareness (MrMs) values are plotted

124302-5 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

Even the smallest applied irradiation fluence produces a suf-

ficient amount of domain nucleation centers to change the

hysteresis loop back to a regular shape

Note that according to SRIM simulations the projected

range (Rp) and its straggle (DRp) in silica are about 95 6 30

nm for 100 keV Fethorn and 85 6 35 nm for 35 keV Nethorn ions

respectively while the lateral straggling in FePd is about 20

nm for both cases Therefore the geometrical conditions for

masking and substrate patterning are quite similar However

considering that the amount of irradiation-induced disorder in

FePd is proportional to the number of displacements per atom

(dpa) it follows that the normalized fluence should be about

four times higher for Fethorn than for Nethorn irradiation Indeed

SRIM gives about 15 displacements per Nethorn ion and about 60

displacements per Fethorn ion for a unit depth of 1 nm at the dam-

age peak respectively As expected the fluence dependences

in Figs 4 5 6 7 and 8 are similar for neon and iron if they

are transformed to the corresponding dpa scales

IV CONCLUSION

Silica spheres of submicron size combined with the

Langmuir technique were efficiently applied as a periodic

ion-irradiation mask in order to produce magnetically nano-

patterned FePd films in the lateral direction Highly ordered

Fe47Pd53 alloy films were irradiated with Nethorn and Fethorn ions

through the sphere mask The stripe domains in the virgin

state transform into a lateral magnetic pattern at a fluence of

1 1015cm2 and 1 1014cm2 in the cases of neon and iron

respectively The origin of the lateral magnetic pattern is the

result of the alternation of the ordered L10 and disordered

regions appearing as the 2D projection of the silica sphere

mask pattern onto the FePd film surface

The ratios of the ordered and disordered structures were

extracted from CEMS spectral fractions and the magnetic

pattern geometry (dot spacing and size) was determined

from AFM and MFM the two values show agreement within

the experimental error Longitudinal and polar MOKE mag-

netometry fully supports the above-mentioned microscopic

picture These methods are suitable for describing the modi-

fication of local magnetic properties on a submicron scale

The density of magnetic bits in recording media can be

increased by lowering the diameter of the masking silica

spheres As an earlier publication shows36 silica spheres with

a diameter of 50 nm could be produced but with decreasing

mask size lower irradiation energies must be used in order to

hinder the irradiating ions from crossing the masking spheres

Therefore the presented concept calls for application with

spheres consisting of heavier elements (such as Ausilica core-

shell37 TiO238 etc) in order to increase the ion stopping in

the nanomask Further efforts are underway to improve the

long range ordering of the sphere mask technique

ACKNOWLEDGMENTS

This work was supported by the Hungarian National

Science Fund (OTKA) and by the National Office for

Research and Technology of Hungary under contract K

62272 and NAP-VENEUSrsquo05 Support from Janos Bolyai

Scholarships of the HAS for Z Zolnai and N Nagy are

appreciated The authors would like to thank Theresa Dragon

(MPI Metallforschung Stuttgart) for the technical support in

MFM as well as Dr Janos Major (MPI Metallforschung

Stuttgart) and Laszlo F Kiss (Research Institute for Solid

State Physics and Optics Budapest) for fruitful discussions

1T Devolder H Bernas D Ravelosona C Chappert S Pizzini J Vogel

J Ferre J P Jamet J Chen and V Mathet Nucl Instrum Methods Phys

Res B 175ndash177 375 (2001)2K Piao D Lee and D Wei J Magn Magn Mater 303 e39 (2006)3T Suzuki K Harada N Honda and K Ouchi J Magn Magn Mater

193 85 (1999)4M R Visokay and R Sinclair Appl Phys Lett 66 1692 (1995)5H Shima K Oikawa A Fujita K Fukamichi K Ishida and A Sakuma

Phys Rev B 70 224408 (2004)6O Ersen V Parasote V Pierron-Bohnes M C Cadeville and C Ulhaq-

Bouillet J Appl Phys 93 5 (2003)7C T Rettner S Anders J E E Baglin T Thomson and B D Terris

Appl Phys Lett 80 279 (2002)8B D Terris D Weller L Folks J E E Baglin A J Kellock H Rothui-

zen and P Vettiger J Appl Phys 87 7004 (2000)

FIG 7 (Color online) Polar MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

124302-6 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

In order to study the average in-plane magnetization of

the layers longitudinal MOKE loops were recorded on each

sample (Fig 6) Apart from the as-deposited and the lowest

fluence Fethorn-irradiated sample the available 400 mT in-plane

external field was sufficient for full saturation With increasing

fluence of the neon irradiation the squareness (defined by the

ratio of the remanence (Ms) to the saturation magnetization

(Mr)) of the longitudinal hysteresis loops increased while the

coercivity decreased (Fig 6 left) At the highest neon fluence

where the layer mainly contains disordered fcc FePd the sam-

ple fully saturates the coercivity decreases to 10 mT and

the squareness of the hysteresis loop increases from below

003 to 097 (see Fig 6 left inset) In the partially saturated

cases the indicated values provide an upper limit for the

squareness A similar tendency can be seen in the case of iron

irradiation (Fig 6 right) however the applied Fethorn irradiation

fluence was not enough to completely convert the L10 struc-

tural regions in the layer into fcc Thus the coercivity dropped

to only 30 mT at the highest Fethorn fluence of 1014 ionscm2

and the squareness increased to only 068

The behavior of the out-of-plane magnetization was fol-

lowed by recording polar MOKE hysteresis loops (Fig 7)

Although the available 430 mT external magnetic field was

insufficient to completely saturate the irradiated samples in

the polar geometry it can be seen that the out-of-plane satu-

ration field (loop closing field) and the out-of-plane domain

contribution to the magnetization decrease with increasing

fluence but no significant change in the coercivity is observ-

able As a result the loops gradually flatten out and the

higher fluence magnetization curves are dominated by the

in-plane fcc grains and the shape anisotropy contribution

The results from the virgin sample indicate that at about 400

mT all magnetic moments point out of the film plane The

range of coercivity and the irregular ldquonarrow waistrdquo shape of

the hysteresis loop are characteristic of a continuous L10

maze domain structure34 The out-of-plane domains are rela-

tively easily saturated by domain wall movement but they

are difficult to demagnetize due to a high nucleation field35

FIG 5 (Color online) MFM images for (left) neon and (right) iron irradia-

tion at the fluences indicated in the figure

FIG 6 (Color online) Longitudinal MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

In the insets the corresponding squareness (MrMs) values are plotted

124302-5 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

Even the smallest applied irradiation fluence produces a suf-

ficient amount of domain nucleation centers to change the

hysteresis loop back to a regular shape

Note that according to SRIM simulations the projected

range (Rp) and its straggle (DRp) in silica are about 95 6 30

nm for 100 keV Fethorn and 85 6 35 nm for 35 keV Nethorn ions

respectively while the lateral straggling in FePd is about 20

nm for both cases Therefore the geometrical conditions for

masking and substrate patterning are quite similar However

considering that the amount of irradiation-induced disorder in

FePd is proportional to the number of displacements per atom

(dpa) it follows that the normalized fluence should be about

four times higher for Fethorn than for Nethorn irradiation Indeed

SRIM gives about 15 displacements per Nethorn ion and about 60

displacements per Fethorn ion for a unit depth of 1 nm at the dam-

age peak respectively As expected the fluence dependences

in Figs 4 5 6 7 and 8 are similar for neon and iron if they

are transformed to the corresponding dpa scales

IV CONCLUSION

Silica spheres of submicron size combined with the

Langmuir technique were efficiently applied as a periodic

ion-irradiation mask in order to produce magnetically nano-

patterned FePd films in the lateral direction Highly ordered

Fe47Pd53 alloy films were irradiated with Nethorn and Fethorn ions

through the sphere mask The stripe domains in the virgin

state transform into a lateral magnetic pattern at a fluence of

1 1015cm2 and 1 1014cm2 in the cases of neon and iron

respectively The origin of the lateral magnetic pattern is the

result of the alternation of the ordered L10 and disordered

regions appearing as the 2D projection of the silica sphere

mask pattern onto the FePd film surface

The ratios of the ordered and disordered structures were

extracted from CEMS spectral fractions and the magnetic

pattern geometry (dot spacing and size) was determined

from AFM and MFM the two values show agreement within

the experimental error Longitudinal and polar MOKE mag-

netometry fully supports the above-mentioned microscopic

picture These methods are suitable for describing the modi-

fication of local magnetic properties on a submicron scale

The density of magnetic bits in recording media can be

increased by lowering the diameter of the masking silica

spheres As an earlier publication shows36 silica spheres with

a diameter of 50 nm could be produced but with decreasing

mask size lower irradiation energies must be used in order to

hinder the irradiating ions from crossing the masking spheres

Therefore the presented concept calls for application with

spheres consisting of heavier elements (such as Ausilica core-

shell37 TiO238 etc) in order to increase the ion stopping in

the nanomask Further efforts are underway to improve the

long range ordering of the sphere mask technique

ACKNOWLEDGMENTS

This work was supported by the Hungarian National

Science Fund (OTKA) and by the National Office for

Research and Technology of Hungary under contract K

62272 and NAP-VENEUSrsquo05 Support from Janos Bolyai

Scholarships of the HAS for Z Zolnai and N Nagy are

appreciated The authors would like to thank Theresa Dragon

(MPI Metallforschung Stuttgart) for the technical support in

MFM as well as Dr Janos Major (MPI Metallforschung

Stuttgart) and Laszlo F Kiss (Research Institute for Solid

State Physics and Optics Budapest) for fruitful discussions

1T Devolder H Bernas D Ravelosona C Chappert S Pizzini J Vogel

J Ferre J P Jamet J Chen and V Mathet Nucl Instrum Methods Phys

Res B 175ndash177 375 (2001)2K Piao D Lee and D Wei J Magn Magn Mater 303 e39 (2006)3T Suzuki K Harada N Honda and K Ouchi J Magn Magn Mater

193 85 (1999)4M R Visokay and R Sinclair Appl Phys Lett 66 1692 (1995)5H Shima K Oikawa A Fujita K Fukamichi K Ishida and A Sakuma

Phys Rev B 70 224408 (2004)6O Ersen V Parasote V Pierron-Bohnes M C Cadeville and C Ulhaq-

Bouillet J Appl Phys 93 5 (2003)7C T Rettner S Anders J E E Baglin T Thomson and B D Terris

Appl Phys Lett 80 279 (2002)8B D Terris D Weller L Folks J E E Baglin A J Kellock H Rothui-

zen and P Vettiger J Appl Phys 87 7004 (2000)

FIG 7 (Color online) Polar MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

124302-6 Merkel et al J Appl Phys 109 124302 (2011)

[This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationaiporgtermsconditions Downloaded to ] IP

1601032236 On Fri 10 Jul 2015 145924

Even the smallest applied irradiation fluence produces a suf-

ficient amount of domain nucleation centers to change the

hysteresis loop back to a regular shape

Note that according to SRIM simulations the projected

range (Rp) and its straggle (DRp) in silica are about 95 6 30

nm for 100 keV Fethorn and 85 6 35 nm for 35 keV Nethorn ions

respectively while the lateral straggling in FePd is about 20

nm for both cases Therefore the geometrical conditions for

masking and substrate patterning are quite similar However

considering that the amount of irradiation-induced disorder in

FePd is proportional to the number of displacements per atom

(dpa) it follows that the normalized fluence should be about

four times higher for Fethorn than for Nethorn irradiation Indeed

SRIM gives about 15 displacements per Nethorn ion and about 60

displacements per Fethorn ion for a unit depth of 1 nm at the dam-

age peak respectively As expected the fluence dependences

in Figs 4 5 6 7 and 8 are similar for neon and iron if they

are transformed to the corresponding dpa scales

IV CONCLUSION

Silica spheres of submicron size combined with the

Langmuir technique were efficiently applied as a periodic

ion-irradiation mask in order to produce magnetically nano-

patterned FePd films in the lateral direction Highly ordered

Fe47Pd53 alloy films were irradiated with Nethorn and Fethorn ions

through the sphere mask The stripe domains in the virgin

state transform into a lateral magnetic pattern at a fluence of

1 1015cm2 and 1 1014cm2 in the cases of neon and iron

respectively The origin of the lateral magnetic pattern is the

result of the alternation of the ordered L10 and disordered

regions appearing as the 2D projection of the silica sphere

mask pattern onto the FePd film surface

The ratios of the ordered and disordered structures were

extracted from CEMS spectral fractions and the magnetic

pattern geometry (dot spacing and size) was determined

from AFM and MFM the two values show agreement within

the experimental error Longitudinal and polar MOKE mag-

netometry fully supports the above-mentioned microscopic

picture These methods are suitable for describing the modi-

fication of local magnetic properties on a submicron scale

The density of magnetic bits in recording media can be

increased by lowering the diameter of the masking silica

spheres As an earlier publication shows36 silica spheres with

a diameter of 50 nm could be produced but with decreasing

mask size lower irradiation energies must be used in order to

hinder the irradiating ions from crossing the masking spheres

Therefore the presented concept calls for application with

spheres consisting of heavier elements (such as Ausilica core-

shell37 TiO238 etc) in order to increase the ion stopping in

the nanomask Further efforts are underway to improve the

long range ordering of the sphere mask technique

ACKNOWLEDGMENTS

This work was supported by the Hungarian National

Science Fund (OTKA) and by the National Office for

Research and Technology of Hungary under contract K

62272 and NAP-VENEUSrsquo05 Support from Janos Bolyai

Scholarships of the HAS for Z Zolnai and N Nagy are

appreciated The authors would like to thank Theresa Dragon

(MPI Metallforschung Stuttgart) for the technical support in

MFM as well as Dr Janos Major (MPI Metallforschung

Stuttgart) and Laszlo F Kiss (Research Institute for Solid

State Physics and Optics Budapest) for fruitful discussions

1T Devolder H Bernas D Ravelosona C Chappert S Pizzini J Vogel

J Ferre J P Jamet J Chen and V Mathet Nucl Instrum Methods Phys

Res B 175ndash177 375 (2001)2K Piao D Lee and D Wei J Magn Magn Mater 303 e39 (2006)3T Suzuki K Harada N Honda and K Ouchi J Magn Magn Mater

193 85 (1999)4M R Visokay and R Sinclair Appl Phys Lett 66 1692 (1995)5H Shima K Oikawa A Fujita K Fukamichi K Ishida and A Sakuma

Phys Rev B 70 224408 (2004)6O Ersen V Parasote V Pierron-Bohnes M C Cadeville and C Ulhaq-

Bouillet J Appl Phys 93 5 (2003)7C T Rettner S Anders J E E Baglin T Thomson and B D Terris

Appl Phys Lett 80 279 (2002)8B D Terris D Weller L Folks J E E Baglin A J Kellock H Rothui-

zen and P Vettiger J Appl Phys 87 7004 (2000)

FIG 7 (Color online) Polar MOKE loops taken for FePd film samples irradiated with (left) Nethorn and (right) Fethorn at the fluences indicated in the figure

124302-6 Merkel et al J Appl Phys 109 124302 (2011)

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9C Chappert H Bernas J Ferre V Kottler J-P Jamet Y Chen E Cam-

bril T Devolder F Rousseaux V Mathet and H Launois Science 280

1919 (1998)10C Burda X Chen R Narayanan and M A El-Sayed Chem Rev 105

1025 (2005)11S Sun E E Fullerton D Weller and C B Murray IEEE Trans Magn

37 1239 (2001)12K Kakizaki Y Yamada Y Kuboki H Suda K Shibata and N Hirat-

suka J Magn Magn Mater 272 2200 (2004)13M T Reetz and W Helbig J Am Chem Soc 116 7401 (1994)14K S Suslick M Fang and T Hyeon J Am Chem Soc 118 11960 (1996)15T Hyeon Chem Commun 8 927 (2003)16T Thomson B D Terris M F Toney S Raoux J E E Baglin S L

Lee and S Sun J Appl Phys 95 6738 (2004)17Z Zolnai A Deak N Nagy A L Toth E Kotai and G Battistig Nucl

Instrum Methods Phys Res B 268 79 (2010)18M Abes J Venuat D Muller A Carvalho G Schmerber E Beaurep-

aire A Dinia and V Pierron-Bohnes J Appl Phys 96 7420 (2004)19T Devolder C Chappert and H Bernas J Magn Magn Mater 249 452

(2002)20D G Merkel F Tancziko Sz Sajti M Major A Nemeth L Bottyan Z

E Horvath J Waizinger S Stankov and A Kovacs J Appl Phys 104

013901 (2008)21N Nagy A E Pap E Horvath J Volk I Barsony A Deak and Z Hor-

volgyi Appl Phys Lett 89 063104 (2006)22V Gehanno P Auric A Marty and B Gilles J Magn Magn Mater

188 310 (1998)23K B Blodgett and I Langmuir Phys Rev 51 964 (1937)24M E Diaz and R L Cerro Physicochemistry and Hydrodynamics of Lang-

muir-Blodgett Depositions (VDM Verlag Saarbrucken Germany 2008)

25W Stober A Fink and E J Bohn J Colloid Interface Sci 26 62

(1968)26See httpgwyddionnet for more details about the program27J F Ziegler J P Biersack and U Littmark The Stopping and Range of

Ions in Solids (Pergamon New York) details about the srim code can be

found at wwwsrimorg First Edition edition (August 1985)28R Brand J Lauerm and D M Herlach J Phys F Met Phys 13 675

(1983)29G Schatz and A Weidinger Nuclear Condensed Matter PhysicsmdashNuclear

Methods and Applications (Wiley amp Sons Chichester England 1996) p

5530C Issro M Abes W Puschl B Sepiol W Pfeiler P F Rogl G

Schmerber W A Soffa R Kozubski and V Pierron-Bohnes Metall

Trans A 37 3415 (2006)31V A Tsurin A E Ermakov Yu G Lebedev and B N Filipov Phys

Status Solidi 33 325 (1976)32J Fassbender D Ravelosona and Y Samson J Phys D Appl Phys 37

R179 (2004)33V Gehanno A Marty B Gilles and Y Samson Phys Rev B 55 18

(1997)34G Q Li H Takahoshi H Ito H Saito S Ishio T Shima and K Taka-

nashi J Appl Phys 94 5672 (2003)35G Q Li H Saito S Ishio T Shima K Takanashi and Z Xiong J

Magn Magn Mater 315 126 (2007)36A Deak E Hild A L Kovacs and Z Horvolgyi Phys Chem Chem

Phys 9 6359 (2007)37P Mulvaney L M Liz-Marzan M Giersig and T Ung J Mater Chem

10 1259 (2000)38J Jensen M Skupinski K Hjort and R Sanz Nucl Instrum Methods

Phys Res B 266 3113 (2008)

124302-7 Merkel et al J Appl Phys 109 124302 (2011)

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