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Journal of Magnetism and Magnetic Materials 315 (2007) 82–88

www.elsevier.com/locate/jmmm

Giant exchange bias in MnPd/Co bilayers

Nguyen Thanh Nama,c,�, Nguyen Phu Thuya,b, Nguyen Anh Tuana,Nguyen Nguyen Phuoca,c, Takao Suzukic

aInternational Training Institute for Materials Science, Hanoi University of Technology, Hanoi, VietnambCollege of Technology, Vietnam National University, Hanoi, Vietnam

cInformation Storage Materials Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya, 468-8511, Japan

Received 17 January 2007

Available online 16 March 2007

Abstract

A systematic study of exchange bias in MnPd/Co bilayers has been carried out, where the dependences of exchange bias, unidirectional

anisotropy constant and coercivity on the thicknesses of MnPd and Co layers were investigated. A huge unidirectional anisotropy

constant, JK ¼ 2:5 erg=cm2 was observed, which is in reasonable agreement with the theoretical prediction based on the model by

Meiklejohn and Bean. The angular dependences of exchange bias field and coercivity have also been examined showing that both

exchange bias and coercivity follow 1= cos a rule.

r 2007 Elsevier B.V. All rights reserved.

PACS: 75.70.Cn; 75.70.�i; 75.25.+z; 75.30.Gw

Keywords: Giant exchange bias; Magnetic thin films; Unidirectional anisotropy

1. Introduction

Exchange bias (EB) is the phenomenon associated withthe exchange anisotropy created at the interface betweenantiferromagnet (AF) and ferromagnetic (FM) layerswhen these layers are cooled in a magnetic field throughthe Neel temperature of the AF layer [1]. Exchange biasinghas attracted much interest because of its technologicaluse in magnetic sensors and high-density magneticrecording systems. Although it was discovered more thanhalf a century ago and there have been a lot of studieson this intriguing subject, its physical origin is still incontroversy [2].

It is known that the theoretically predicted EB field islarger than the experimental value by two orders ofmagnitude. There are several theoretical works, such as

- see front matter r 2007 Elsevier B.V. All rights reserved.

/j.jmmm.2007.02.203

onding author. Information Storage Materials Laboratory,

hnological Institute, 2-12-1 Hisakata, Tempaku, Nagoya,

apan. Tel.: +8152 809 1872; fax: +8152 809 1874.

ddress: sd06508@toyota-ac.ti.jp (N.T. Nam).

the domain wall models [3,4] or spin flopping model [5]proposed to account for this discrepancy. However, there isstill a lack of experimental confirmation for these models.Ohldag et al. [6] reported on the correlation between EBand pinned interfacial uncompensated AF spins, resultingin a vertical offset through the study of X-ray magneticcircular dichroism (XMCD) on several EB systems. Basedon the experimental fact, they argued that the physicalorigin of EB was unambiguously described as due to afraction of uncompensated interfacial spins (about 4%)that are locked to the AF lattice and do not rotate in anexternal magnetic field while most of the other interfacialspins are affected by the external field. Recently, Tsunodaet al. [7,8] found a great enhancement of the unidirectionalanisotropy constant ðJKÞ in MnIr/CoFe bilayer systemwith chemical ordering, resulting a JK up to 1:3 erg=cm2

[7]. They therefore used XMCD to measure element-specific magnetic hysteresis loops of MnIr/CoFe bilayerswith different orderings of MnIr with the expect that thevertical offset should be enhanced with chemical orderingaccording to the model proposed by Ohldag et al. [6].

ARTICLE IN PRESSN.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 83

However, the XMCD results by Tsunoda et al. [8] shownno vertical offset, which is at variance with that reported byOhldag et al. [6]. The fact that this large EB showscontradicted results to that of the ‘‘normal’’ EB thusrequires a modification of the theoretical works for thequantitative understanding of EB coupling. Also, oneshould note that for the giant EB, the discrepancy betweenthe ideal interfacial coupling energy J and the observed

Fig. 1. (a) XRD pattern of Si(1 1 1)/MnPd (540 nm) single layer; (b) XRD

pattern of Si(1 1 1)/MnPd (36 nm)/Co (80 nm) bilayer.

Fig. 2. Hysteresis loops of MnPd ðtMnPdÞ=Co (10 nm) samples wit

exchange anisotropy energy JK is only one order comparedto two orders in ‘‘normal’’ EB system. Therefore, from thefundamental viewpoint, studies of materials with giantunidirectional anisotropy play a vital role on the way to geta better understanding of the mechanism of EB phenom-enon. From the application point of view, the quest formaterials exhibiting giant unidirectional anisotropy isindispensable for the realization of very thin read headsensors used for ultra-high density magnetic recording[7,8].The present work reports about the observation of a very

large unidirectional anisotropy constant in MnPd/Cobilayers. The dependences of EB, unidirectional anisotropyconstant and coercivity on thicknesses are investigated anddiscussed. Also, the angular dependences of EB andcoercivity in out-of-plane configuration are presented anddiscussed.

2. Experimental

Samples with the structure of Co/MnPd/Si(1 1 1) weregrown at room temperature using the RF sputter-deposi-tion system with the base pressure of 10�6 mbar. MnPdlayers were fabricated from Pd target with bonded Mn

h tMnPd ¼ 2, 3, 6, 12, 18, and 36 nm measured at T ¼ 120K.

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Fig. 3. The dependences of exchange biased field ðHEÞ, coercivity ðHCÞ

and unidirectional anisotropy constant ðJKÞ on the thickness of MnPd

layer ðtMnPdÞ for MnPd ðtMnPdÞ=Co (10 nm) bilayers measured at

T ¼ 120K.

N.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–8884

chips. No external field was applied during deposition. Theargon pressure during deposition process is 10�3 mbar. Thecomposition of MnPd layer identified by the energydispersion X-ray spectroscope (EDS) is Mn30Pd70. Thestructural properties of the samples were investigated byX-ray diffraction (XRD). For field cooling procedure, allthe samples were heated in a vacuum oven ð10�5 mbarÞ atT ¼ 570K for an hour and then cooled in a magnetic fieldof about 5 kOe down to room temperature. The magneticproperties of samples were characterized by the vibratingsample magnetometer (VSM) at various temperatures from120 to 300 sK.

3. Results and discussion

Fig. 1 shows the XRD patterns of MnPd (540 nm) singlelayer and MnPd (36 nm)/Co (80 nm) bilayer. It is observedthat in the sample with very thick MnPd single layer,MnPd is FCC phase with several peaks indicating that thesample is polycrystalline without texture. However, for thebilayer of MnPd (36 nm)/Co (80 nm), only the peak ofMnPd(3 2 1) appears. This may suggest that for the bilayerMnPd has the weak texture in (3 2 1) orientation. Anotherpoint worthwhile noting is that there is no peak for Colayers. It may therefore be concluded that Co is inamorphous state.

Hysteresis loops of MnPd ðtMnPdÞ=Co (10 nm) sampleswith tMnPd ¼ 2, 3, 6, 12, 18, and 36 nm measured at T ¼

120K are shown in Fig. 2. It is observed that for thethickness of MnPd layer is less than 18 nm, the M–H curvesshow double-shifted loops. These double-shifted loops aretentatively attributed to the association of the coexistenceof positive and negative EBs [9]. The blocking temperatureis found to be 220K, which is close to the Neel temperatureof Mn25Pd75 [10]. More detailed discussion about thedouble-shifted loops and the temperature dependence ofEB of the present system can be found elsewhere [9].

The dependences of EB HE, coercivity HC and theunidirectional anisotropy constant JK on MnPd thicknessðtMnPdÞ estimated from the hysteresis loops are summarizedin Fig. 3. Here, JK is the unidirectional anisotropy which isdefined from the equation JK ¼ HE �MS � tFM, where HE

is the EB field, MS the saturation magnetization of the FMlayer and tFM the thickness of FM layer. It should be notedthat for the M–H curve with dual loops, the EB andcoercivity are estimated from the average values of positiveand negative EBs and coercivities, respectively. It can beseen that the trend of the dependence of HE and JK ontMnPd is same as many other exchange biased systems[2,11–13,16]. For the samples with tMnPd larger than 6 nm,HE and JK are independent of tMnPd with HE ¼ 690Oe andJK ¼ 1 erg=cm2. As the tMnPd is reduced, HE and JK

decrease abruptly. This behavior can be rationalized withina simple model following the initial interpretation of EB byMeiklejohn and Bean [1]. In essence, EB can only besupported when the anisotropy energy in the AF layer issufficiently large. Otherwise, the magnetization reversal of

the FM layer simply induces a reorientation of the AFlayer surface spins. Within the simple Meiklejohn–Beanmodel this condition can be written KAFtAF4JK, whereKAF is the anisotropy constant of the AF layer, tAF thethickness of the AF layer. Another possible explanation forthis dependence is that the antiferromagnetic phase is onlypartially formed when the thickness of the AF layer issmall[11–13,16]. However, in the present study, there is noexperimental support for this argument because the XRDdata show no peaks for all the samples in this series as aresult of the less crystallinity of MnPd layers.It is noted that even for such a MnPd film as thin as

2 nm, the EB is still observed, thus, the critical thicknessof MnPd layer being less than 2 nm. Normally, inother systems [2,11–13], the critical thickness of AF layeris more than 2 nm. Hence, this fact may possibly beconsidered as an indication of the large value of KAF of theMnPd layer providing a large thermal stability for the AFdomain structure and consequently resulting in a largecoercivity.

ARTICLE IN PRESSN.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 85

Fig. 4 shows the hysteresis loops of the bilayer sampleswhere the thickness of Co layer is changed from 10 to60 nm while the MnPd thickness is fixed at 30 nm. All thehysteresis loops were measured at T ¼ 120K. The depen-dences of the EB field HE, the coercivity HC and theunidirectional anisotropy constant JK on thickness of theCo layer are shown in Fig. 5. It is observed that both EBfield and coercivity show peak values at tCo ¼ 20 nm.Normally, in EB systems, EB field is inversely proportionalto the thickness of the FM layer as an indication of theinterfacial effect, which is usually confirmed by the factthat the unidirectional anisotropy JK is constant over theFM thickness range. However, in the present study, JK isstrongly dependent on the thickness of Co layer. The trendshowing the decrease of JK and HE with decreasing thethickness of Co when tCoo20 nm is observed in severalsystems [14] which were attributed to the less well crystal-line of the FM layer when its thickness is small. However,in this study, this argument may not be correct since theXRD pattern shows that Co layer is amorphous. Thereason for the decrease of JK with Co thickness whentCo420 nm is not clear at present.

It is of great interest to see that the maximum JK is2:5 erg=cm2 when the thickness of Co layer is in the rangefrom 40 to 50 nm. These JK are several orders of magnitudelarger than those observed on most of the metallic

Fig. 4. Hysteresis loops of MnPd (30 nm)/Co ðtCoÞ samples wi

exchanged bias thin films [2]. For example, in MnPd/Febilayers, the JK value is only 0:032 erg=cm2 [15] and0:017 erg=cm2 [16], in MnIr/Co bilayers, the JK value is0:14 erg=cm2 [17] and MnFe/Co bilayers, the JK value is0:059 erg=cm2 [17]. In the literature, there are few systems,which exhibit such large unidirectional anisotropy con-stant. The largest JK ever reported in the literature is2:11 erg=cm2 in the system of Fe3O4=CoO bilayers mea-sured at T ¼ 5K [18]. It is noted that there was one paperby Strom et al. [19] reporting on the JK up to 3:5 erg=cm2 inCo/CoO bilayers. However, the JK value in that paper wasdetermined from the AC susceptometry. If determinedfrom the conventional method, i.e. from the equationJK ¼ HE �MS � tFM, the obtained maximum JK value wasonly 0:7 erg=cm2 [19]. Therefore, it may be concluded thatthe present obtained JK of 2:5 erg=cm2 is the largest valueever found in the literature.It is interesting to find the physical origin of giant EB

energy. Recently, Tsunoda et al. [7,8] reported a very largevalue of the unidirectional anisotropy constant JK ofMnIr/CoFe bilayers up to 1:3 erg=cm2 and found a strongcorrelation between EB anisotropy and chemical ordering.In the present study, it is difficult to find such a correlationsince the giant EB is strongly dependent on the Cothickness. Based on the XRD patterns, it can only beconcluded that Co is in amorphous state and consequently,

th tCo ¼ 10, 16, 20, 40, 50, 60 nm measured at T ¼ 120K.

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Table 1

The experimental value of the unidirectional anisotropy constant and the

theoretical estimation based on the model of Meiklejohn and Bean

System Experimental value Theoretical value

JK ðerg=cm2) JK (erg=cm2)

CoO/Co particles [1] 2 1.8–14.1

CoO/Co bilayers [20] 0.1 1.8–14.1

MnIr/Co [17] 0.14 4.6–14.1

MnFe/Co [17] 0.059 1.0–14.1

MnPd/Fe [15] 0.032 1.8–17.2

MnIr/CoFe [8] 1.3 4.6–19.9

FeF/Fe [21] 1.1 1.3–17.2

The present system 2.5 1.8–14.1

Fig. 5. The dependences of exchange biased field ðHEÞ, coercivity ðHCÞ

and unidirectional anisotropy constant ðJKÞ on the thickness of Co layer

ðtCoÞ for MnPd (30 nm)/Co ðtCoÞ bilayers measured at T ¼ 120K.

N.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–8886

one cannot find any structural change when changing thethicknesses of Co layers.

In this paper, the simple model put forward byMeiklejohn and Bean [1] is employed to compare thetheoretical value with the obtained experimental result.According to this model, the AF spins at the interface areuncompensated and fixed during the FM magnetizationrotation. Hence, the unidirectional anisotropy constant canbe calculated as follows:

JK ¼JexSiSj

a2. (1)

Here, Jex is the exchange integral at the interface, which isbelieved to be in the range of the exchange integrals of theAF and FM materials, Si and Sj the spins of the interfacialatoms, and a the lattice parameter. If using Jex from theexchange integral of bulk materials, then JK is in the rangefrom 1.8 to 14:1 erg=cm2. Therefore, roughly speaking, onemay conclude that the present result lies in the range oftheoretical prediction by the model of Meiklejohn andBean [1].

Table 1 shows the experimental value of JK and thetheoretical estimation based on the simple model ofMeiklejohn and Bean [1] for several EB systems in the

literature. It is clearly seen that only some systems whichexhibit huge unidirectional anisotropy [1,8,21] are inreasonable agreement with the Meiklejohn and Bean model[1]. Therefore, even though the present result is consistentwith the theoretical prediction, it may not considered as afirm support for the simple model by Meiklejohn and Bean[1]. A more complicated model which is able to cover for allthe cases should therefore be developed.Fig. 6 illustrates the angular dependence measurement in

out-of-plane configuration. In this configuration, sampleswere first undergone a field cooling process with thecooling field applied in the plane of the films. Then thehysteresis loop measurement was carried out with theapplied magnetic field rotating out-of-plane, which makesan angle a with the cooling field direction. Fig. 7 showssome representative hysteresis loops of MnPd ð30 nmÞ=Coð20 nmÞ samples measured with a changing from 01 to 3601.These extracted values of HE and HC are plotted in Fig. 8as a function of the angle a.In the literature, there have been several reports on the

angular dependence of EB and coercivity showing that thecoercivity HC has a two-fold symmetry with maximal valuealong the field cooling axis (a ¼ 0� and 180�) and HE showsa unidirectional symmetry which can be described by aseries of odd cosine [22]. The present results of the angulardependences of HE and HC are quite different from eachother. Although the angular dependence of HC behavesalso a two-fold symmetry very large maxima appear atdirections perpendicular to the FC direction (a ¼ 90� and2701) and the HCðaÞ curve can be fitted to functionHCðaÞ ¼ HCð0Þ=j cosðaÞj. Concerning the behavior of theHE ðaÞ dependence, it cannot be fitted to the cosine series asusual but it follows the 1= cos a rule as shown in Fig. 8. Theabove specific behavior of HEðaÞ and HCðaÞ was alsoobserved by Sun et al. [23] and was explained by theassumption that magnetization is pinned along the FCdirection unless the projection of the applied magnetic fieldinto the film surface exceeds the saturation field. The in-plane field component of the applied field is H cos a andhence the switching fields HSW should be equal toHSWð0Þ= cos a which lead both the coercivity HC ðaÞ and

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Fig. 6. The geometry of the angular dependence measurement where

magnetic field has been rotated in the plane containing the field cooling

direction and the film normal.

Fig. 7. Hysteresis loops of MnPd (30 nm)/Co (20 nm) bilayer measured at diffe

field.

N.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 87

the EB field HEðaÞ obey the 1= cos a rule. It is worth notingthat the present result is at variance with the work byPhuoc and Suzuki [13], which shows that HE and HC doesnot follow the 1= cos a rule although their measurementwas also carried out in out-of-plane configuration. Thereason for this discrepancy is possibly due to the strong in-plane magnetic anisotropy of the present samples arisingfrom demagnetization field while in their work [13], thesamples exhibit perpendicular magnetic anisotropy.

4. Conclusion

In summary, the present study reported on the largestunidirectional anisotropy constant ever found in theliterature up to 2:5 erg=cm2 in MnPd/Co bilayers. Thishuge unidirectional anisotropy constant is found to be in

rent angles a between the field cooling direction and the applied magnetic

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Fig. 8. Angular dependences of HE and HC for MnPd (30 nm)/Co (20 nm)

bilayer. The solid line is the fitting curve following 1= cos a rule.

N.T. Nam et al. / Journal of Magnetism and Magnetic Materials 315 (2007) 82–8888

reasonable agreement with the simple model proposed byMeiklejohn and Bean, which predicts that JK in MnPd/Cosystem is in the range from 1.8 to 14 erg=cm2. The thicknessdependences of exchange bias and unidirectional aniso-tropy constant show a complex behavior, which is not yetfully understood. The angular dependences of exchangebias and coercivity show a 1= cos a dependence, which maybe interpreted as due to the magnetic anisotropy along thefield cooling direction. Although the present system ofMnPd/Co bilayer has low blocking temperature, suggesting

that this system cannot be applied for spin-valve sensors,the finding of giant exchange bias may provide some usefulinformation for better understanding of the mechanism ofexchange bias.

Acknowledgment

This work is partially supported by the VietnameseFundamental Research Grant #4.049.06 (2006–2008).Also, the supports from the Academic Frontier Centerfor Future Storage Materials Research [MEXT HAITE-KU (2004–2008)] and from the Japanese Storage ResearchConsortium are gratefully acknowledged.

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