Crystal asymmetry and low-angle grain boundary governed persistentphotoinduced magnetization in small bandwidth manganitesS. Majumdar, T. Elovaara, H. Huhtinen, S. Granroth, and P. Paturi Citation: J. Appl. Phys. 113, 063906 (2013); doi: 10.1063/1.4789596 View online: http://dx.doi.org/10.1063/1.4789596 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i6 Published by the American Institute of Physics. Related ArticlesControlled epitaxial integration of polar ZnO(0001) with Si(001) Appl. Phys. Lett. 102, 101602 (2013) Oxygen segregation at coherent grain boundaries of cubic boron nitride Appl. Phys. Lett. 102, 091607 (2013) Magnetotransport properties of quasi-one-dimensionally channeled vertically aligned heteroepitaxial nanomazes Appl. Phys. Lett. 102, 093114 (2013) Dewetting of Au and AuPt alloy films: A dewetting zone model J. Appl. Phys. 113, 094301 (2013) Direct atomic imaging of antiphase boundaries and orthotwins in orientation-patterned GaAs Appl. Phys. Lett. 102, 081905 (2013) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Crystal asymmetry and low-angle grain boundary governed persistentphotoinduced magnetization in small bandwidth manganites

S. Majumdar,1 T. Elovaara,1,2 H. Huhtinen,1 S. Granroth,3 and P. Paturi11Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku,FI-20014 Turku, Finland2The National Graduate School in Material Physics (NGSMP), Helsinki, Finland3Laboratory of Materials Research, Department of Physics and Astronomy, University of Turku,FI-20014 Turku, Finland

(Received 19 November 2012; accepted 11 January 2013; published online 11 February 2013)

Persistent photo-induced magnetization (PPM) in low bandwidth manganite Pr1�xCaxMnO3

(PCMO) thin film is reported in the low hole doped regime (x¼ 0.1). X-ray diffraction, x-ray

photoelectron spectroscopy, and magnetic measurements in dark and under illumination on the

as-grown, vacuum, and oxygen annealed thin films of PCMO showed that the PPM effect is

strongly dependent on the crystal symmetry, low angle grain boundaries, and the Mn valence states

in the material. Compared with the as-grown and vacuum annealed film, the oxygen annealed film

shows much less low-angle grain boundaries, higher crystal symmetry, and long range

ferromagnetic (FM) ordering and in this sample PPM is not significant. These results clearly

indicate that in this large band gap material PCMO, photo-induced FM interaction mainly arises

due to improved domain-wall movement of the short range FM clusters. Significant PPM can be

obtained for films with higher amount of grain boundaries, oxygen vacancies, and strain related

defects. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789596]

I. INTRODUCTION

The optical control of physical properties in electronic

materials forms the basis of many optoelectronic devices like

photo-diodes, photo-sensors, photo-resistors, photo-transistors,

etc.1 Photo-induced change in electronic states in materials

leading to modified magnetization, resistivity, etc., is not only

interesting from the application point of view but also broad-

ens our scientific understanding about material properties.2–5

Perovskite manganites with the formula R1�xAxMnO3 (where

R is a rare-earth trivalent element and A is a divalent alkaline

earth element) are the extremely versatile class of materials

with the coexistence of phase-separated regimes6 and inter-

play between different phases leading to their application in

resistive7 and magnetic data storage8 as well as in other future

technologies.7,9,10 Among the perovskite manganites, low

bandwidth compound Pr1�xCaxMnO3 (PCMO) is especially

interesting due to a stable charge-order (CO) state near the

hole doping range of 0.3–0.5. In addition to that, large photo-

induced change in resistivity has been observed in PCMO in

their CO phase11 and photo-induced increase in the metallic

volume fraction is proposed to be the main reason. Earlier, we

observed a persistent photo-induced magnetization (PPM) in

low hole-doped La0.9Ca0.1MnO3 (LCMO)12,13 and in

PCMO,14 but the effect was clearly lower in PCMO. A

detailed analysis of the PPM in intermediate bandwidth mate-

rial LCMO and low bandwidth PCMO indicated towards the

fact that the incident light energy is sufficient to induce

charge-transfer (CT) transition in LCMO but due to higher

Jahn-Teller (JT) distortion in PCMO, incident light energy is

unable to introduce CT transition in PCMO.15

In a recent investigation,16 micro-Raman analysis of

PCMO showed that ex situ annealing of PCMO thin films

effectively modifies the JT distortion of the PCMO unit cell.

In the present communication, we have investigated whether

this modified crystal symmetry improves PPM response in

the PCMO samples. For this purpose, differently treated

PCMO films were prepared and characterized with x-ray dif-

fraction (XRD), x-ray photoelectron spectroscopy (XPS),

and by magnetic measurements in dark and under optical

excitation.

II. EXPERIMENTAL DETAILS

PCMO films with thickness of 100 nm were deposited

on (100) SrTiO3 (STO) substrates by pulsed laser deposition

(PLD) using an excimer XeCl 308 nm laser with the laser flu-

ence 2 J/cm2 and the repetition rate 5 Hz. The flowing oxy-

gen pressure in the chamber was p¼ 0.3 Torr and the

substrate temperature during the deposition was 700 �C. The

PCMO (x¼ 0.1) target for PLD was prepared by the solid-

state reaction method by drying of high-purity Pr6O11,

Mn2O3, and CaCO3 salts for 5 h at 500 �C in air and prepar-

ing a stoichiometric mixture in the metal ion molar ratio of

1.5:5:1. After grinding, the mixed powder is pressed into a

pellet and in order to produce single phase material, the final

target is treated in a sequence of four grinding-pressing-sin-

tering process at 1200 �C in air. Details of the target prepara-

tion and the film deposition parameters have been reported

elsewhere.17 In order to decrease or increase the oxygen

content of the films and affect the valency of Mn ions, the

post-annealing treatments for as-grown film A were made in

vacuum of 10�3 Torr for 15 min at 700 �C with heating and

cooling rates of 10 �C/min (sample B) and in flowing oxygen

at 800 �C for 24 h with heating and cooling rates of 1 �C/min

(sample C).

0021-8979/2013/113(6)/063906/9/$30.00 VC 2013 American Institute of Physics113, 063906-1

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The XPS spectra were obtained by Perkin-Elmer PHI

5400 spectrometer using Al Ka excitation of a twin anode

(1486 eV). Analyzer pass energy of 89.45 eV and 45� take-

off angle were used to measure the core-level spectra of as-

grown (A), vacuum treated (B), and O2-annealed (C) PCMO

samples. The background pressure in the analyzer chamber

was around 1� 10�9 Torr during the measurements. The

binding energy (BE) scale was calibrated by setting the C 1sphotoemission peak to 285 eV.

The temperature dependence of the zero-field-cooled

(ZFC) and field-cooled (FC) magnetizations was measured

between temperatures 5 and 300 K with a superconducting

quantum interference device (SQUID) magnetometer with

different external magnetic fields B. Virgin magnetization as

a function of B and magnetic hysteresis curves were recorded

in a field of B ¼ 6300 mT at different temperatures between

5 and 100 K. All measurements were made for the sample in

the dark or under illumination through an optical fiber using

AlGaInP laser working at k ¼ 658 nm (1.88 eV). The tem-

perature of the sample is controlled during illumination with

two independent thermometers located in He gas flow before

and after the sample. Depending on the position and align-

ment of the optical fibre, the magnetic background signal

vary considerably, but between the each measurement pair in

dark and under the illumination, the sample, and the optical

fibre alignment is not disturbed and therefore the measured

differences are always reliable. The external field B was ori-

ented along the plane of the films, i.e., along the PCMO

[110] axis.

III. RESULTS

A. Structural analysis

The detailed XRD analysis for as-grown, vacuum-, and

oxygen-annealed PCMO films were made earlier,18 and it

was concluded that all the films were phase pure and only

(0 k0) peaks perpendicular to the surface were observed.

From the peak widths of the traditional 2h scans, we have

observed that the vacuum treatment induces more variation

in the lattice parameters while the oxygen treatment makes

the unit cells more uniform when compared to the results of

as-grown film. According to the (2h,/) scans shown in

Figure 1, relatively broad peaks in the / direction

(D/ � 1:5�) can be seen in samples A and B, indicating

large amount of low-angle grain boundaries (LAGBs). The

oxygen treatment (sample C) decreases D/ significantly, i.e.,

the in-plane variation of the unit cells, while the lattice mis-

match based strain seems to stay at the same level as in as-

grown film. The out-of-plane long range ordering calculated

from the XRD rocking curves is clearly decreased in both

vacuum and oxygen annealed films, which indicates the for-

mation of mosaic spread in both treated films.19 The in-plane

lattice parameters does not change during either of the

annealing treatments while the out-of-plane parameter is

appreciably lengthened with oxygen annealing indicating

much stronger effect of oxygen in comparison with vacuum

treatment. In general, the addition of oxygen contracts the

unit cell in the lattice because the cation-cation bond is lon-

ger than the cation-oxygen-cation bond20 which again takes

place during the oxygen treatment when Mn3þ ions are

replaced by Mn4þ ions. This phenomenon has been widely

discussed earlier and is connected to the release of substrate-

induced strain which again reduces orbital location and

straightens the Mn-O-Mn bond angle.18

To have a better understanding of the Mn valence states

in differently treated samples, the XPS measurements were

performed for samples A, B, and C. The photoelectron

spectra of Mn 3s, Mn 3p, Mn 2p, O 1s, Pr 3d, Ca 2p, C 1score levels and the valence band were acquired. For all the

samples, we measured also the survey spectra (1100-0 eV in

binding energy) to confirm the purity of the samples from

unwanted elements and to give the overall image of the

differences of the samples before observing the core level

spectra more precisely. All the samples contained carbon

and different cleaning procedures by annealing at tempera-

tures between 200 and 250 �C in UHV conditions

(�1� 10�9 Torr) and oxygen atmosphere (base pressure like

above and oxygen pressure of �1� 10�6 Torr) were tested

to get rid of the contamination. Using higher annealing tem-

peratures probably would have not only decreased the con-

tamination level21 but also affected the sample properties in

a way that is hard to determine. Since the effect of the clean-

ing procedures described here was quite small and seen only

in carbon and oxygen spectra, we chose to show the original

spectra taken before any annealing. Sample A contained

slightly less carbon than samples B and C. In the O 1s spec-

tra, the biggest difference is seen when the samples A and C

FIG. 1. The /� 2h scans of the (211) peaks as-grown (a), vacuum-treated

(b) and O2-annealed (c) films scaled to the same maximum intensity. The

vertical dashed lines are guides for the eye to ease the comparison of

/-widths of the samples. The separation of these lines is 1.5�.

063906-2 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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are compared to the sample B (Figure 2) which contains an

additional component at higher BE of 533 eV than other

samples. This difference in O 1s spectra of sample B can be

due to higher amount of Od�-species, Pr-O-C22 or surface

degradation or contamination.23 The O 1s spectra of samples

A and C look very similar only difference being the high BE

shift of the spectra of sample C. The feature around 529–

529.5 eV BE is suggested to originate from Pr-O and Mn-O

bondings based on the XPS study of PrMnO3.24 The O 1s

features at BE of about 532 eV, which is present in all the

spectra, can be related to Pr/Ca-O,25 but also the signal origi-

nating from chemisorbed, physisorbed or bound H2O has

approximately the same BE.26 In some references for LSMO

crystals, the low (high) BE component of O 1s spectrum is

caused by O atoms below the surface, “bulk” (surface) O.27

Based on the changes in the spectra that were taken after

cleaning tests, at least part of this high BE component of our

samples is due to surface contamination since the intensity

of the component decreased during the annealing.

In our previous papers including XPS results, the

changes in Mn valence have been studied by analyzing the

Mn 2p core levels which is known to possess very compli-

cated multiplet splitting26,28 that makes it difficult to resolve

small changes in BE or peak asymmetry. In addition to the

Mn 2p spectra (Figure 2(b)), we have measured the Mn 3s(Figure 3) and 3p core levels and in this work the most

obvious changes caused by sample treatments were observed

in Mn 3s spectra. The splitting of 3s core level spectra, DE3s,

of 3d transition metals is due to the exchange interaction

between the 3s core hole produced in a photoemission

process and 3d electrons.27,29,30 After emission of one 3selectron, the exchange coupling of the remaining 3s core-

electron produces two different final states seen as a split

multiplet structure. In the case of PCMO, this multiplet

splitting depends on the valence state of Mn ions being pro-

portional to the local spin of the 3d electrons in the ground

state. The splitting is known to decrease as the Mn valency

increases.31,32 For example, the exchange splitting of MnO is

about 6 eV, of Mn2O3 is 5.5 eV, and of MnO2 is 4.7 eV, that

is for Mn2þ, Mn3þ, and Mn4þ ions, respectively.29,33

Figure 3 presents the Mn 3s core level spectra of the sam-

ples A, B, and C. The interpretation of the spectra is compli-

cated because of the changes in BE, peak asymmetry, and

DE3s splitting as well as the additional satellite visible at high

BE side of the spectra of post-annealed samples B and C. In

the spectra of sample C, also the low BE component has

shifted towards low BE in a comparison to the A and B spec-

tra. Mannella et al.27 have reported similar behaviour of DE3s

and shifting of the low BE (high BE) peak of Mn 3s (O 1s) as

the temperature passed TC during the heating of UHV frac-

tured La0.7Sr0.3MnO3 crystals. The most obvious difference

between the samples is the increased DE3s from sample A

(5.1 eV) to samples B (5.6 eV) and C (6.4 eV) suggesting,

according to a very simplified interpretation, that post-

annealing would increase the amount of Mn3þ in sample B

and Mn2þ in sample C relative to sample A. However,

improved crystal structure obtained by XRD and improved

ferromagnetic (FM) interaction in magnetic measurements

suggest higher Mn4þ/Mn3þ ratio. The increase of Mn4þ/

Mn3þ ratio also resulted in the increase in the amount of

Mn2þ which is known to give larger DE3s values. The high

BE feature observed in the Mn 3s spectra of samples B and C

cannot be caused by Auger peaks since it does not exist in the

spectra of sample A. Similar structure has been seen in

La0.75Sr0.25MnO3 film34 but was not explained or commented.

In the temperature dependent electronic structure study of

La0.7Sr0.3MnO3, a high BE satellite is visible on the high BE

FIG. 2. The O 1s (a) and Mn 2p3=2 (b) core level spectra of as-grown (A),

vacuum-treated (B), and O2-treated (C) PCMO films at photon energy of

1486 eV measured at room temperature.

FIG. 3. Comparison of Mn 3s core level XPS spectra of as-grown (A),

vacuum-treated (B), and O2-treated (C) PCMO films. The spectra are

adjusted by removing Shirley background and normalized by setting the in-

tensity of the low binding energy component to 1. Binding energy differen-

ces between the component maxima in the doublet, DE3s, are given for

samples A, B, and C.

063906-3 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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side of the Mn 3s spectrum measured above the TC.35 This

satellite, which has slightly larger shift from the Mn 3s com-

ponents than what is observed in our spectra, is suggested to

be a reminiscent of the configuration satellites that have been

reported for higher-spin Mn compounds.29 However, after

Shirley type background subtraction and by fitting with Voigt

line shape doublets, we could not quantitatively describe the

Mn2þ, Mn3þ, and Mn4þ intensity ratios. Therefore, we can

qualitatively conclude that, in comparison with samples A

and B, in oxygen annealed film C increased amount of 3delectrons can be expected due to larger DE3s splitting as well

as presence of some Mn2þ on the surface.36,37

B. Photoinduced persistent magnetization

The virgin magnetization curves M(B) measured at 5 K

and up to 500 mT field before and during illumination for as-

grown (A), vacuum-treated (B), and O2-annealed (C) films

are shown in the main panels of Figure 4. The M(B) curves

of sample A show relatively linear increase in both dark (up

to �150 mT) and illumination (up to �120 mT) measure-

ments while in both M(B) curves of sample B two distinctly

different slopes around 90 mT in dark and around 70 mT in

illumination measurements can be observed. These changes

in slopes indicate a metamagnetic transition and confirm the

coexistence of antiferromagnetic (AFM) and FM phases in

the ground state of PCMO38 which is suppressed under the

illumination because the field dependent transition is

decreased around 20 mT. This can be explained by the opti-

cal excitation induced FM ordering which decreases the

field-induced melting of the strain that occurs at the phase

boundary between AFM and FM clusters and thus making

the AFM-FM transition much sharper.39 The effect of illumi-

nation is approximately similar in samples A and B where

the maximum increase in FM can be observed around 90–

100 mT as can be seen from the difference DM ¼Mill �Mdark plotted as a function of applied magnetic field in

the insets of Figure 4. Slight convex shape of DM curve

below 90 mT in sample A can be related to the existence of

minute metamagnetic behaviour in M(B) measured in dark

which disappears completely during illumination where Mill

increases even more rapidly than in sample B. Above

200 mT, the Mill and Mdark curves in both samples A and B

follow each other and the net magnetization is not improved

under illumination. The virgin curves of sample C differ

clearly from the curves of both samples A and B. This leads

to much steeper field dependence of M(B) where Mill and

Mdark curves merge together already around 100 mT which

on the other hand is the field value of maximum DM for sam-

ples A and B. As can be seen from the inset of Figure 4(c),

the sharp peak in DM around 45 mT indicates clearly

improved FM ordering by oxygen treatment which again

restricts the effect of optical excitation. The metamagnetic

transition for sample C can be observed at around 25 mT

field in both measurements made in dark and under illumina-

tion. This again indicates that better lattice symmetry due to

oxygen treatment as observed by micro-Raman analysis16

assists the destabilization of strain related blocking causing

improved FM ordering with lower magnetic field.

Figure 5 shows the magnetic hysteresis loops M(B) meas-

ured at 5 K between �300 and þ300 mT for the PCMO films

A, B, and C. The vacuum-annealed film B measured at 5 K in

dark has the most open loop in terms of coercivity,

Bc � 83 mT, while in as-grown film A, Bc is clearly smaller

of 58 mT and the loop shrinks and sharpens even more by

oxygen-annealing having Bc in the level of 40 mT. This can

be explained by the vacancy related defect structure which

pins the domain wall movement and hence increases the coer-

civity in samples A and B when compared to sample C.18 At

5 K in films A and B, a clear two-step magnetic transition in

the hysteresis loop can be seen, which is in line with earlier

FIG. 4. Virgin curves of the magnetic hysteresis loops measured at 5 K for as-

grown (a), vacuum-treated (b), and O2-annealed (c) PCMO films before (black

symbols) and during (red symbols) illumination with an AlGaInP laser at

658 nm. The background signal of the optical fibre is subtracted in non-

illuminated and illuminated virgin curves in order to obtain the actual differ-

ence between the curves. The insets show the differences of these curves

DM ¼ Mill �Mdark in low fields measured for the same samples A, B, and C.

063906-4 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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observation in virgin curves and associated with metamag-

netic transition and irreversible FM phase. In all the samples

measured in dark or under the illumination, the loops at 5 K

are almost saturated in about 200 mT field, and the saturation

magnetization ms for samples A and B is approximately same

of 4� 105 A/m while in sample C, ms � 6� 105 A/m being

roughly one and a half time larger. This is a clear indication

that FM ordering has been increased by oxygen annealing to

the optimal level and therefore the photoinduced effect is very

limited. A slight increase in M(B) above 200 mT in all the

samples can be explained by the existence of small amount of

AFM clusters in the films.

The effect of illumination can be observed by moderate

narrowing the hysteresis loops in all the samples A, B, and C.

The shape of the loops before and during illumination stays

similar and this differs clearly from the situation observed ear-

lier for La0.9Ca0.1MnO3 films where illumination closes the

hysteresis loops completely in as-grown and in similarly

oxygen-annealed films.13 When comparing the photoinduced

effect with the PCMO films prepared at lower temperature

500 �C,14 we can conclude that higher deposition temperature

(700 �C) induces low-angle grain boundaries in sample A,

which limit the photoinduced increase of the domain size and

therefore two-step transition is still available during the illu-

mination. On the other hand, the higher deposition tempera-

ture induced Mn4þ ions in sample A or the trapping of

electrons at the oxygen vacancies with formed magnetic

polarons in sample B cannot exclusively explain the narrow-

ing the loops because the ms is not increased during illumina-

tion. Oxygenation at high temperature of 700 �C in partial or

full oxygen pressure is also earlier shown to remove twin

boundary and LAGB structure in PCMO,17 and this fits per-

fectly to our situation where only single transition loops are

seen in sample C. When looking at the hysteresis loops meas-

ured at different temperatures (not shown here), we can con-

clude that the photoinduced effect is limited in the

temperature range below 20 K in all samples A, B, and C.

This is in line with the coercive field shift DBc ¼ Billc � Bdark

c

obtained under illumination where DBc is roughly zero above

20 K for all the samples, as can be seen from Figure 5(d).

These curves also demonstrate that the photoinduced effect is

weakest in sample C where almost maximum FM has already

been reached by the improved crystal symmetry, optimal

Mn4þ/Mn3þ ratio and by eliminating the LAGBs in the film.

To observe the effect of illumination on the magnetic

phases of differently annealing-treated PCMO, the MZFCðTÞand MFCðTÞ curves were measured in different magnetic

fields for all the samples A, B, and C. The temperature

dependences as an example in 50 mT field are shown in

Figure 6. As can be seen from the figures, a minor second

transition around 20 K as well as a dell around 40 K are visi-

ble in both dark and illuminated ZFC and FC curves of sam-

ple A and these features are even smaller in sample B. This

confirms the existence of two magnetic phases in the ground

state of samples A and B and could be related to distribution

in unit cell size and the LAGBs observed by XRD. At tem-

peratures above these anomalies, a relatively broad maxi-

mum in ZFC curves around a spin-glass transition

temperature Tg � 70 K can be observed. Traditionally, the

point at which Mirr ¼ MFC �MZFC becomes non-zero defines

a spin-glass transition temperature at the applied field and

the characteristic measuring time, but in our case, the effect

of annealing treatment in Mirr is easier to observe from a

FIG. 5. Magnetic hysteresis loops measured up to 6300 mT fields at 5 K for

as-grown (a), vacuum-treated (b), and O2-annealed (c) PCMO films. (d) The

temperature dependence of the coercive field shift (and the relative shift in

the inset) obtained under the illumination DBc ¼ Billc � Bdark

c for all the sam-

ples A, B, and C.

063906-5 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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cross-over temperature where the variation in Mirr is nearly

linear and which can be defined by the extrapolation to zero

of the sharp increase of Mirr (Ref. 40) (Figure 6(d)). Above

this temperature, the FM to paramagnetic (PM) transition

occurs, having onset temperatures of �125 K and �130 K

for samples A and B, respectively. From the FC curves, simi-

lar kind of PM-FM transition can be observed for both A and

B films where M(T) increases evenly down to 5 K without

saturation. This indicates short-range spin ordering in PCMO

like ferromagnetic spin-glass La0.7-xNdxPb0.3MnO3 (Ref. 41)

and SrRuO3,42 and existence of AFM or canted AFM order-

ing which is expected according to the phase diagram for the

ground state of 10% Ca-doped PCMO.6 This is also in agree-

ment with theoretical predictions where PM-FM transition in

materials with low transition temperature may become dis-

continuous, depending on doping level and competition with

super-exchange AFM interactions.43 As a result of illumina-

tion, the ZFC magnetization improves considerably below Tg

in samples A and B which again indicates the suppressed

AFM and increased FM ordering at low temperature. In con-

trary to our earlier measurements for films deposited at

500 �C,14 the FM ordering is also increased upon field-

cooling and during the illumination. Similar behaviour is

also observed for intermediate electronic bandwidth

La0.9Ca0.1MnO3 films where the bandgap is suitable for CT

transition and the lattices were relatively strained.12,15

As can be seen from Figure 6(c), in 50 mT field, the

oxygen-annealed sample C shows clearly improved FM

ordering in ZFC curve at low temperature and under illumi-

nation ZFC curve approaches the FC curves indicating that

almost complete FM ordering is reached. Similar behaviour

is also observed from the C curve in the main panel of Figure

6(d) where the irreversibility magnetization is shifted clearly

to lower temperatures when compared to Mirr of samples A

and B. This is actually explained by the much steeper field

dependence of M(B) seen also in virgin curves discussed

above and confirmed with the M(T) curves measured in dif-

ferent magnetic fields (not shown here). Based on these

measurements, the inset of Figure 6(d) shows that spin-glass

phase disappears in much lower field range in sample C

(below 100 mT) when compared to the samples A and B

where the transition is still visible with maximum external

field of 150 mT used in our measurements.

When fitting the FC spontaneous magnetization below

TC to the scaling law

M ¼ CðTC � TÞb; (1)

we can estimate the type of ferromagnet where the critical

exponent b is related to different domain structures, orienta-

tion and strain effects.44 In Figure 7, the fits to the experimen-

tal data of samples A, B, and C are shown in 10 mT field, and

in general, we can conclude that the fitting works for all the

samples below TC, especially when the external field induced

gradual part of the curve next to the onset temperature is

ignored. The critical exponent values b ¼ 0:372ð60:008Þ and

b ¼ 0:375ð60:005Þ for samples A and B, respectively, are

only slightly higher than the theoretical value for 3D isotropic

Heisenberg-type ferromagnet (b ¼ 0:367).45,46 For O2-treated

sample C, b ¼ 0:289ð60:004Þ is clearly smaller, even smaller

than for 3D Ising-type ferromagnet where the theoretical value

b ¼ 0:326 is expected.45,47 The field dependence of critical

exponent can be calculated from the fits of FC curves

FIG. 6. Temperature dependence of MZFC (open symbols) and MFC (solid

symbols) magnetizations measured in magnetic field of 50 mT before (black

squares) and during (red circles) illumination for as-grown (a), vacuum-

treated (b), and O2-annealed (c) PCMO films. The main panel of (d) shows

the temperature dependence of the irreversibility magnetizations Mirr ¼MFC �MZFC in 50 mT field measured before (solid symbols) and during

(open symbols) illumination and the inset of (d) shows the field dependence

of the spin-glass transition temperature Tg, where Mirr becomes non-zero

and starts to increase rapidly.

063906-6 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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measured in higher fields and when compared to values meas-

ured in 10 mT field, the value of b for as-grown sample A is

clearly increased up to 0.463 and 0.482 in fields of 50 and

100 mT, respectively. These values are already close to

b ¼ 0:5, as expected by the mean-field theory. In vacuum

treated sample B, b value is more or less field independent

being 0.365 and 0.376, while for O2-treated sample C, the crit-

ical exponent is slightly decreased with external field being

0.244 and 0.235 in fields of 50 and 100 mT, respectively.

These values match well with those derived from the tricritical

mean-field model (b ¼ 0:25) where the existence of tricritical

point clearly sets a boundary between first-order and second-

order magnetic transitions.48

In order to clarify the physical significance of these pa-

rameters, we take a closer look at the critical properties of

the Heisenberg ferromagnets and tricritical point of the phase

transition. For Heisenberg model, exchange integral is a

strong function of interaction distance49 and the long range

exchange integral decays as50

JðrÞ � 1

rðdþrÞ ; (2)

where d is the spatial dimension, r > 0. For a three dimen-

sional material, this means JðrÞ � r�ð3þrÞ with 3=2 � r � 2.

For Heisenberg ferromagnets where r ¼ 2, J(r) decreases

faster than r�5. For r ¼ 3=2 where mean field exponents

(b ¼ 0:5) are valid, J(r) decreases slower, i.e., as r�4:5. In

samples A and B, b close to but slightly higher than the theo-

retical value of Heisenberg exponent then suggests that J(r)

decays even faster than r�5. Clearly smaller b values in sam-

ple C indicate exchange interaction to be more long range in

this case. It is known that exchange interaction gets affected

by the spin-lattice interaction.49 Hence, experimentally by

micro-Raman spectroscopy and XRD16 observed improved

crystal properties in sample C, lead to modified spin-lattice

interaction causing deviation of the critical exponents. Appli-

cation of magnetic field suppresses the charge, lattice, and

orbital degrees of freedom in the system and changes in criti-

cal exponent b under magnetic field indicate slower decay of

J(r) for sample A, almost unchanged for sample B and a

clear shift towards the boundary of first and second order

phase transition in sample C.48 Therefore, we can conclude

that PPM effect is higher in samples with short-range FM

ordering.

The isothermal growth of magnetization Mexc, i.e., exci-

tation of a PPM state is measured by switching the illumina-

tion on at an applied magnetic field and then collecting the

magnetization data as a function of time. From Figure 8, the

excitation in all the samples A, B, and C measured at 5 K

and in 50 mT field follows a stretched exponential law

MexcðtÞ ¼ Msatexcexp½�ðt=sÞc�; (3)

where sðT;BÞ is a characteristic time and the exponent c is

dispersion parameter 0 <c < 1. Qualitatively, the increase of

magnetization by light seems to be much faster in oxygen-

annealed sample C when compared to the curves of samples

A and B. This is clearly supported by the fitting parameters

where s is less than a second (�0:7 s) for sample C while s �30 s and 132 s for samples A and B, respectively. In any case,

the growth of PPM in sample C reaches the maximum level

much faster than we have observed for any other PCMO or

LCMO films.12–14 This is in good agreement with other struc-

tural and magnetic results where the effect of light in sample

C is also smallest while higher structural symmetry as well as

almost maximum FM ordering are already reached by oxygen

treatment. The stretching excitation exponents c of 0.34, 0.42,

and 0.21 for samples A, B, and C, respectively, indicate that

in vacuum annealed sample B, c agrees well with the value

0.43 predicted for charge transfer processes induced by pho-

nons in the presence of randomly distributed traps.51 In the

vacuum-treated sample B, this can be connected to the

increased amount of oxygen vacancies where photoexcited

electrons can be trapped. From the reached saturation level of

Mexc, both the illumination and magnetic field are switched

off and the relaxation of the remanent magnetic moment of

the PPM state Mrel is recorded at 5 K, as can be seen from the

FIG. 7. FC magnetizations of samples A, B, and C measured in 10 mT field

and their fits (solid lines) to the scaling law given also in the figure. Critical

exponents of the fits are also given in the key on the right top corner.

FIG. 8. The time dependence of the excitation of the photoinduced magnet-

ization Mexc measured at 5 K and in 50 mT field for as-grown (black

squares), vacuum-treated (red circles), and O2-annealed (green triangles)

PCMO films. The solid lines show the fits to the stretched exponential law

and the zero level of Mexc in the main panel indicates the value where the

magnetic field is applied and therefore the plots correspond only the real

effect of light. The inset shows the relaxation of the photoinduced state in

50 mT field where the fast relaxation component dominates and the relaxa-

tion is basically completed due to the relative long start-up time of the mag-

netometer of approximately 150 s.

063906-7 Majumdar et al. J. Appl. Phys. 113, 063906 (2013)

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inset of Figure 8. Unfortunately, the fast relaxation component

cannot be measured due to relatively long start-up time of the

magnetometer being approximately 150 s, and therefore only

constant but from the original level elevated magnetization

remains without showing any further relaxation in all the sam-

ples A, B, and C. As can be seen from the figure, the elevated

magnetization level in sample C is roughly doubled when

compared to the levels in samples A and B. This can be quali-

tatively explained by improved original magnetization level in

sample C due to lesser structural distortion and smaller

amount of LAGBs, as well as by smaller effectiveness of illu-

mination due to less Mn3þ ions as observed by XPS.

IV. CONCLUSIONS

Structural characterization, made by x-ray diffraction,

indicates large number of low-angle grain boundaries in as-

grown and vacuum annealed films while oxygen treatment

clearly decreases the structural distortion indicating higher

crystal symmetry in oxygen annealed PCMO film. The effect

of post-annealing treatments on the Mn configuration meas-

ured by x-ray photoelectron spectroscopy was clearly visible

between the samples A, B, and C, especially in the Mn 3score level spectra which are known to be very sensitive to

the changes in Mn valency. Based on the increase in the

DE3s splitting from sample A to B and C, it is concluded that

the oxygen annealing affected the sample C, induces the

largest DE3s split which is related to the increase of Mn2þ on

the surface. For the PCMO films, higher amount of Mn2þ

might be a consequence of Mn3þ changing to Mn4þ which is

consisted with other results presented in this study.

In as-grown and vacuum treated samples A and B, the

amount of oxygen vacancies as well as slightly increased

Mn3þ content with LAGBs in crystal structure results in the

suppressed FM state and increased FM ordering under illu-

mination at low temperature, as confirmed by magnetic

measurements. The suppressed FM in the ground state is a

consequence of higher Mn3þ content that favor AFM super-

exchange interaction between Mn3þ ions in magnetic ground

state. Also the oxygen vacancies in samples A and B can

trap the mobile electrons that induce FM double-exchange

interaction between Mn3þ and Mn4þ ions, forming magnetic

polaron traps. FM ordering is increased in low-field range

and after zero-field cooling by improved domain wall move-

ment under optical excitement. In sample C, where almost

optimal Mn4þ/Mn3þ ratio is reached by oxygen annealing,

the increased FM ordering by double-exchange interaction

limits the effect of illumination since less oxygen vacancies,

i.e., traps and increased amount of mobile electrons are al-

ready achieved by oxygen treatment. These results indicate

that low-angle grain boundaries in crystal structure and

modified oxygen distribution play an important role in deter-

mining PPM in the thin films of manganites.

ACKNOWLEDGMENTS

The Wihuri Foundation, the Turku Collegium for Sci-

ence and Medicine (TCSM) and the Academy of Finland are

acknowledged for financial support.

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