Interdependence of reversal asymmetry and training effect inIr22Mn78/Ni81Fe19 bilayers probed with magnetoresistanceHimanshu Fulara, Sujeet Chaudhary, and Subhash C. Kashyap Citation: Appl. Phys. Lett. 101, 142408 (2012); doi: 10.1063/1.4757603 View online: http://dx.doi.org/10.1063/1.4757603 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i14 Published by the American Institute of Physics. Related ArticlesMagnetization reversal and magnetoresistance behavior of perpendicularly magnetized [Co/Pd]4/Au/[Co/Pd]2nanowires J. Appl. Phys. 112, 073902 (2012) Magnetization reversal in multisegmented nanowires: Parallel and serial reversal modes Appl. Phys. Lett. 101, 122412 (2012) Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction Appl. Phys. Lett. 101, 122403 (2012) The magnetic Y-branch nanojunction: Domain-wall structure and magneto-resistance Appl. Phys. Lett. 101, 102403 (2012) Kinetics of magnetization processes in a quasi-one-dimensional Ising superantiferromagnet [{CH3}3NH]CoCl3·2H2O Low Temp. Phys. 38, 843 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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Interdependence of reversal asymmetry and training effect inIr22Mn78/Ni81Fe19 bilayers probed with magnetoresistance

Himanshu Fulara, Sujeet Chaudhary,a) and Subhash C. KashyapThin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

(Received 31 July 2012; accepted 21 September 2012; published online 4 October 2012)

Using magnetoresistance as a probe we demonstrate the correlation between reversal asymmetry

and training effect in ion-beam sputtered IrMn/NiFe bilayers. During the training procedure, both

exchange bias field and the degree of asymmetry decrease monotonically following a very similar

trend. The analysis of the magnetoresistance behaviour establishes that the two distinct training

mechanisms are operative. The first one is exhibited by an abrupt single cycle training effect and

an accompanying pronounced reversal asymmetry, attributed to the presence of biaxial anisotropy

in the IrMn layer. The second one displays a gradual cycling dependence due to thermal depinning

of uncompensated antiferromagnetic spins. VC 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4757603]

The interfacial exchange coupling across an antiferro-

magnet (AF)/ferromagnet (FM) interface breaks the symme-

try of the hysteresis loop of the FM layer by causing a shift

of the centre of hysteresis loop (HEB) along the field axis,

known as exchange bias (EB), and often accompanied by

broadening of the hysteresis loop.1–3 The exchange anisot-

ropy associated with the AF/FM interfaces has been a subject

of significant interest to the magnetism community, primarily

due to the elusiveness of its fundamental understanding.3–5

Recently, EB has also gained significant technological im-

portance due to its application in spintronics devices, where

it is employed to pin the magnetization (M) of one of the FM

layers thereby defining a reference magnetization direction,

e.g., in spin valves and magnetic tunnel junctions.1,6,7

Although, there have been considerable efforts devoted to

exchange coupled AF/FM systems to understand the intrigu-

ing manifestations associated with this EB effect,1,2,4,5,8 the

underlying mechanism is still under intensive debate.1–4 In

particular, the training effect1,9–15 (i.e., decrease in HEB upon

consecutive field cycling) and the magnetization reversal

asymmetry10,13,15–22 (i.e., different mechanisms for magnet-

ization reversal on field-decreasing and field-increasing

branches of the hysteresis loop) have received renewed

attention in recent years. It is believed that the training effect

in the EB system takes place due to the rearrangement of

interfacial AF spin structure upon repeated field cycling, and

the variation in HEB follows an empirical dependence HEB

/ 1/�n, where n is the number of field cycles.1,13–15 Binek9

has proposed a phenomenological approach to describe the

training effect within the thermodynamic framework of spin

configurational relaxation of the interfacial AF magnetization

from a non-equilibrium state towards a quasi-equilibrium

state. In a different approach, Hoffmann11 pointed out the cru-

cial role of both inherent frustration of the interface and sym-

metry of anisotropy of the AF layer on the training effect and

reversal asymmetry. The identification of the exact physical

mechanism responsible for the microscopic origin of the EB

training effect is still subject to debate.

It may be pointed out that the asymmetry in the magnet-

ization reversal process has been commonly observed in

exchange biased AF/FM bilayers, although the mechanisms

appear to differ across the systems.10,11,13,16–22 The origin of

this reversal asymmetry has either been linked with the pres-

ence of higher order FM anisotropies17 and local misalign-

ment of the easy magnetization axes of the FM and AF

layers18 or with the irreversibility due to change in spin con-

figuration at the AF/FM interface.10,11,22 More recently

Camarero et al.19 have interpreted the asymmetric magnet-

ization reversal in terms of competing anisotropies. The cor-

relation between the reversal asymmetry and the training

effect in EB systems continues to be elusive due to the lack

of detailed understanding of the initial AF spin configuration

at the buried AF/FM interface.

In this paper, we present a study of thermal evolution of

exchange bias, reversal asymmetry, and the training effect in

IrMn/NiFe bilayers using magnetoresistance (MR) as a probe

of the magnetic state and have made an attempt to under-

stand the origin of reversal asymmetry and training effect. A

significant increase in HEB and coercivity (HC), pronounced

reversal asymmetry, and most importantly a rapid enhance-

ment in the training effect have been realized at low temper-

ature (T < 50 K) in ion-beam sputtered IrMn/NiFe bilayers.

The bottom-pinned polycrystalline Ta(5 nm)/IrMn

(20 nm)/NiFe(10 nm)/Ta(5 nm) heterostructure was grown

by ion-beam sputtering at 300 K on an oxidized n-type

Si(100) substrate. After evacuating the chamber to a base

pressure of 2� 10�6 Torr, each individual layer was sequen-

tially deposited at a working pressure of 1.5� 10�4 Torr.

During deposition, an in situ static dc magnetic field of about

200 Oe was applied parallel to the film plane to induce uni-

axial magnetic anisotropy in the NiFe layer. The as-grownsample was subsequently magnetic annealed under vacuum

conditions (4� 10�6 Torr) at 300 �C for 1 h in a 3 kOe mag-

netic field (applied in-plane) and followed by field cooling to

300 K. Magnetic hysteresis loops were recorded at 10 K on

the magnetic annealed sample by using SQUID magneto-

metry. The MR measurements were performed on the mag-

netically annealed sample with a standard dc four-probe

technique with the current (I) applied in plane (CIP mode) in

a)Author to whom correspondence should be addressed. Electronic mail:

sujeetc@physics.iitd.ac.in.

0003-6951/2012/101(14)/142408/5/$30.00 VC 2012 American Institute of Physics101, 142408-1

APPLIED PHYSICS LETTERS 101, 142408 (2012)

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a commercial He closed cycle cryostat at different tempera-

tures (from 300 K to 15 K) employing zero-field cooled

(ZFC) and field-cooled (FC) procedures. It is important here

to point out that the electromagnet was demagnetized for iso-

thermal ZFC measurement at 15 K in the absence of sample

prior to cooling from 300 K.

The M-H hysteresis loop recorded at 300 K on the mag-

netic annealed sample did not show any shift (HEB¼ 0, and

HC¼ 6 Oe, cf., the inset to Fig. 1). This is indicative of a

very weak exchange coupling at 300 K. The sample was sub-

sequently field cooled (HCF¼ 4 kOe) from 300 K to 10 K,

and hysteresis loops were successively recorded twice at

10 K (see the main panel of Fig. 1). It can be noted that both

HEB and HC increase significantly at 10 K compared to that

observed at 300 K. Another interesting feature is the asym-

metric magnetization reversal, i.e., sharp magnetization re-

versal on the descending branch followed by a rounded

reversal in the ascending branch of the first hysteresis loop

(n¼ 1). Subsequent field cycling (n¼ 2) at 10 K significantly

reduces both HEB and HC, a characteristic of training effect.

Furthermore, the reversal asymmetry apparently vanishes in

the second hysteresis loop indicating the stabilization of the

reversal mechanism with repeated field cycling.

To gain a deeper understanding of the origin of the afore-

mentioned reversal asymmetry and training effect, we have

further examined the temperature dependence of EB and suc-

cessive field cycling by performing MR measurements on the

same magnetic annealed sample. The MR effect originates

due to spin-orbit scattering10,12,23 and is defined as

DR/R¼ [R(H)-R(Hmax)]/R(Hmax) � 100%, where R(H) is the

resistance at different magnetic fields and Hmax is the maxi-

mum applied magnetic field.24 Generally in MR, the resist-

ance is sensitive to the angle between M and I, and usually

results in a symmetric MR response with two maxima (min-

ima) of identical height at 6HC.23,24 Figures 2(a) and 2(b)

show the MR loops recorded at 15 K after following ZFC(i.e.,

HCF¼ 0) and FC (HCF¼ 1.8 kOe) procedures from 300 K,

respectively. The FC case resulted in a large HEB of 235 Oe

compared to 187 Oe observed in ZFC case. In addition to the

marked increase in HEB and HC, the striking difference in the

height of the MR peaks was clearly seen in the field decreas-

ing and field increasing branches of the loop for FC(HCF¼ 1.8

kOe) case, which was absent in the ZFC case. It is noteworthy

that the magnetoresistance data show a large asymmetry in

the reversal process relative to that in SQUID hysteresis loop

of the same sample. Comparison between M-H and MR

responses for the same sample therefore reveals the detailed

kinetics of the magnetization reversal process. As mentioned

above, Fig. 1 shows that the first magnetization reversal after

FC is more abrupt leading to a sudden change in the magnet-

ization, while the second and subsequent reversals are more

rounded revealing a gradual change in magnetization. It can

be seen from the field decreasing branch (as the field is

reversed from high positive field saturation); the MR response

(see Fig. 2(b)) is relatively constant and then increases slightly

at the first magnetization reversal resulting in a peak with

smaller MR response (0.3%). This low MR ratio is specific to

the first magnetization reversal and indicates that the reversal

process is dominated by domain wall motion along the field

decreasing branch.10,13,23,24 On the other hand, the MR

response during the field increasing branch exhibits a gradual

rise leading to a comparatively larger MR response (1.1%) at

the second magnetization reversal. This signifies that a consid-

erable amount of rotation of magnetization takes place in the

ascending branch of the MR loop.12,13,23,24 Recently, Sahoo

et al.25 have reported the vertical asymmetry in the MR

behaviour and its correlation with EB in an exchange biased

Co/CoO bilayer. Furthermore, in contrast to the negative shift

of MR loop in FC, the occurrence of positive shift in the ZFC

MR loop is quite similar to what has been reported earlier by

Miltenyi et al.8 in FeF2/Fe and CoO/Co systems. The positive

shift in our case is associated with the negative remanent mag-

netization state of the sample at 300 K.8,15 This result illus-

trates the critical role of the remanent magnetization state of

the FM layer prior to cooling.

Now we demonstrate the importance of MR measure-

ments in establishing a correlation between the occurrence

FIG. 1. The first and second SQUID hysteresis loops of the magnetically

annealed (300 �C/3 kOe) IrMn/NiFe bilayer after field cooling (HCF

¼ 4 kOe) from 300 K to 10 K. Inset displays the hysteresis loop recorded at

300 K.

FIG. 2. Magnetoresistance responses at 15 K of the magnetically annealed

IrMn(20 nm)/NiFe(10 nm) bilayer after (a) ZFC(HCF¼ 0) and (b)

FC(HCF¼ 1.8 kOe) from 300 K (left and right arrows indicate the peak posi-

tions in the first field cycling corresponding to first and second coercive

fields, respectively).

142408-2 Fulara, Chaudhary, and Kashyap Appl. Phys. Lett. 101, 142408 (2012)

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of reversal asymmetry and the training effect in our

bottom-pinned IrMn/NiFe system. We have recorded MR

responses at different temperatures (in 15 K–100 K range)

which are reached after field cooling from the identical his-

tory of field and temperature, i.e., HCF¼ 1.8 kOe and

T¼ 300 K. This ensured zero contribution from any of the

previous training effect due to repeated field cycling at a par-

ticular T. In Figures 3(a)–3(c), we show three such MR loops

recorded under the FC protocol. The remarkable feature of

the temperature dependence of FC MR response is the signif-

icant increase of HEB, HC, and the pronounced reversal

asymmetry only for T< 50 K. In Figs. 4(a) and 4(b), we

present the training effect investigated by MR measurements

at two representative temperatures, 15 K and 50 K. For mea-

surement at each temperature, the sample was first field

cooled from 300 K to the desired temperature and consecu-

tive MR loops were recorded isothermally. It can be noted

that training effect and reversal asymmetry were found to be

predominant at 15 K as seen by a significant decrease in HEB

between n¼ 1 and n¼ 2 (Fig. 4(a)). It is remarkable to point

out that the HEB decreased by about 76%, of the total train-

ing observed in HEB, after second field cycling (n¼ 2), i.e.,

substantial training effect took place in just one field cycle.

Contrary to this, the observed training effect and reversal

asymmetry were insignificant for T � 50 K, as evident from

the nearly symmetric MR responses and gradual training

observed at 50 K (Fig. 4(b)). In order to explore, if there

exists a relationship between training effect and reversal

asymmetry, the field cycles (n) dependence of both HEB and

degree of asymmetry (f) at 15 K is plotted in Fig. 5(a). The fis estimated as the difference in MR values at two coercive

fields HC1 and HC2 divided by the MR value corresponding

to the second coercive field (HC2). It is noted that both HEB

and f decrease monotonically with field cycling in a very

similar fashion throughout the entire training procedure. The

observed linear dependence of HEB on degree of asymmetry

(f), as shown in the inset of Fig. 5(a), clearly suggests that

the reversal asymmetry and training effect are closely linked

with each other and have similar origin.

In the phenomenological model of the training effect,

due to Binek,9 the evolution of entire EB training effect is

described by the following general recursive sequence

HEBðnþ 1Þ � HEBðnÞ ¼ �c½HEBðnÞ � HEBð1Þ�3 (1)

Here, c describes the characteristic decay rate of training

behaviour and HEBð1Þ is the EB field in the limit of an infi-

nite number of cycles (n¼1). In our case, as shown in

Fig. 5(a), the solid squares display the best fits to HEB using

Eq. (1). The resulting parameters obtained from the fit are

HEBð1Þ ¼ �24.0 Oe and c¼ 1.11356� 10�5 Oe�2. The fits

clearly demonstrate a perfect agreement with the experimen-

tal data for n � 1. It may be pointed out that this phenomeno-

logical theory of training effect which is independent of the

specific details of a particular EB system is, in fact, applica-

ble to a variety of distinct EB systems.26 However, the

detailed understanding of the EB training effect on a micro-

scopic scenario still remains elusive. More recently, the

training effect has been considered as thermal and/or

athermal contribution in order to separate the distinct feature

of the first hysteresis cycle.20,21,27 This distinction was first

brought about by Fernandez-Outon et al.27 based on the

unique behaviour of the first hysteresis cycle. Quite often,

the phenomenon of athermal training effect is reported par-

ticularly in EB systems containing high symmetry antiferro-

magnets, e.g., NiMn, FeMn, and IrMn.11,27 Therefore, in the

following, we analyze the EB training effect in terms of

athermal training induced by spin-flop like coupling due to

AF biaxial anisotropy11 and thermal training due to ther-

mally activated gradual depinning of uncompensated

spins.14,20

FIG. 3. Magnetoresistive responses of magnetic annealed IrMn(20 nm)/

NiFe(10 nm) bilayer at (a) 15 K, (b) 50 K, and (c) 100 K (for each measure-

ment, temperature is reached after field cooling from the same history of

field and temperature, i.e., HCF¼ 1.8 kOe, T¼ 300 K).

FIG. 4. (a) Six consecutive MR loops recorded at 15 K after field cooling

(HCF¼ 1.8 kOe) from 300 K, (b) three consecutive MR loops recorded at

50 K after field cooling (HCF¼ 1.8 kOe) from 300 K.

142408-3 Fulara, Chaudhary, and Kashyap Appl. Phys. Lett. 101, 142408 (2012)

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In Fig. 5(a), the rapid decrease in HEB between n¼ 1

and n¼ 2 followed by a more gradual decrease for subse-

quent loops and the existence of a strong reversal asymmetry

only for n¼ 1 clearly indicate that the training effect is com-

posed of two distinct mechanisms. We believe that the ab-

rupt single cycle training effect accompanying a large

reversal asymmetry, which is possibly due to the onset of

biaxial anisotropy in IrMn layer, is facilitated by Hoffmann’s

model.11 However, in contrast to our data (Fig. 5(a)), the

Hoffmann model suggested the disappearance of training

effect for n � 2 due to relaxation of AF spins into a collinear

arrangement after first magnetization reversal.11 The subse-

quent gradual training effect (n � 2) in the present case is

due to thermally activated depinning of the uncompensated

AF spins during magnetization reversal.1,21 The thermal

training generally follows an empirical dependence as HEB

/ 1/�n.1,14,22 We have plotted HEB as a function of 1/�n (see

Fig. 5(b)) which shows a good agreement with the experi-

mental data for n > 1 (the conventional thermal training re-

gime) but fails completely if HEB corresponding to n¼ 1 is

included (i.e., the Hoffmann athermal training regime). It

should also be noted that the magnitude of HEB for n¼ 1 is

significantly higher than the value obtained by extrapolating

the straight line fit to n¼ 1 point. The straight line fitting to

n > 1 provides a graphical illustration to separate the two

training mechanisms, by which HEB relaxes, i.e., “Hoffmann”

athermal contribution (HHoffEB Þ and “conventional” thermal

contribution (HconvEB Þ. It can be noted here that the coexistence

of significant increase of HEB, pronounced reversal asym-

metry, and an abrupt single cycle training effect all occurring

below 50 K suggest the presence of biaxial exchange induced

anisotropy in our IrMn/NiFe system.

Similar to our results, existence of a crossover from uni-

axial to biaxial anisotropy at low temperature was previously

reported in interdiffused NiMn/Ni20 and polycrystalline Co/

FeMn21 bilayers. Based on polarized neutron reflectome-

try,28 it was found that interdiffused AF/FM interface

resulted in the dominance of AF interactions over FM inter-

actions on field cooling below 50 K. As a result, a transition

from uniaxial anisotropy (at high T) to biaxial anisotropy (at

T < 50 K) due to establishment of a full contact between the

Ni(FM) layer, and the biaxial NiMn(AF) layer was proposed

and demonstrated.20,28 In our case, the interdiffusion at the

IrMn/NiFe interface is facilitated by the energy enhanced

growth mechanism of ion-beam sputtering process in addi-

tion to the magnetic thermal annealing carried out at high

temperature (300 �C/3 kOe). This is corroborated by the fact

that ion-beam sputtered species are known to possess rela-

tively higher energy (�1–20 eV), which could lead to finite

interdiffusion across the AF/FM interface.

In case of top-pinned NiFe/IrMn system, Mishra et al.22

have found a less significant decrease of HEB between n¼ 1

and n¼ 2 cycles and an accompanying well preserved mag-

netization reversal asymmetry even after training and have

interpreted their experimental findings in terms of the

robustness of bulk AF spin structure. Using Eq. (1) given in

Ref. 22, we have also made an attempt to simulate the

relaxation of EB in our IrMn/NiFe bilayer. However, our

experimental data did not fit satisfactorily with this equa-

tion, adding considerable support to our proposal of the

existence of two training mechanisms in the present IrMn/

NiFe system. We believe that Hoffmann’s model offers a

good description of our experimental data, being accounta-

ble for the existence of a strong single cycle training effect

and an accompanying large reversal asymmetry below 50 K

in our interdiffused IrMn/NiFe system. It may be noted that

Binek’s phenomenological thermodynamic description and

Hoffmann’s model are not conflicting but complement each

other in the sense that the free energy landscape of the ther-

modynamic description merges into the T¼ 0 energy land-

scape of Hoffmann’s model. The former has the advantage

to accurately describe the entire training behaviour for all

loops. The latter has the advantage to provide a microscopic

mechanism for the training effect between the first two

loops.

In summary, the present magnetoresistance investiga-

tions on the ion-beam sputtered IrMn/NiFe bilayers below

50 K have shown a significant increase in HEB, distinct rever-

sal asymmetry, and a strong training effect. The underlying

origin has been proposed to be associated with the existence

of temperature driven biaxial exchange-induced anisotropy

at the interdiffused IrMn/NiFe interface. Using magnetore-

sistance as a probe of the magnetization reversal, we have

demonstrated a direct link between reversal asymmetry and

training effect. Although the phenomenological model pro-

posed by Binek accurately describes the evolution of entire

training effect, the significant decrease in HEB taking place

between n¼ 1 and n¼ 2 and the pronounced reversal asym-

metry only for n¼ 1 have been microscopically explained

within the framework of Hoffmann’s model.

FIG. 5. (a) Variation of HEB (black open circles and blue solid squares) and

degree of asymmetry (f) (red open triangles) as a function of number of

cycles (n) obtained from individual MR loops. Open circles represent the ex-

perimental data; blue solid squares are the data points generated from Eq.

(1). Dotted lines are a guide to eyes only. The inset shows the linear depen-

dence of HEB on f and, (b) illustration of separation of the entire training

effect into “Hoffmann” athermal and “conventional” thermal regimes.

142408-4 Fulara, Chaudhary, and Kashyap Appl. Phys. Lett. 101, 142408 (2012)

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H.F. is thankful to UGC-India for financial support. We

gratefully acknowledge Dr. Maria Eleni Belesi and Professor

Jean-Philippe Ansermet for SQUID measurements and use-

ful discussions.

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142408-5 Fulara, Chaudhary, and Kashyap Appl. Phys. Lett. 101, 142408 (2012)

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