ARTICLE IN PRESS 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,Ã, Nguyen Phu Thuya,b, Nguyen Anh Tuana, Nguyen Nguyen Phuoca,c, Takao Suzukic a International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam b College of Technology, Vietnam National University, Hanoi, Vietnam c Information 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, J K 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.Ài; 75.25.+z; 75.30.Gw Keywords: Giant exchange bias; Magnetic thin films; Unidirectional anisotropy Introduction Exchange bias (EB) is the phenomenon associated with the exchange anisotropy created at the interface between antiferromagnet (AF) and ferromagnetic (FM) layers when these layers are cooled in a magnetic field through the Ne´el temperature of the AF layer [1] Exchange biasing has attracted much interest because of its technological use in magnetic sensors and high-density magnetic recording systems Although it was discovered more than half a century ago and there have been a lot of studies on this intriguing subject, its physical origin is still in controversy [2] It is known that the theoretically predicted EB field is larger than the experimental value by two orders of magnitude There are several theoretical works, such as ÃCorresponding author Information Storage Materials Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya, 468-8511, Japan Tel.: +81 52 809 1872; fax: +81 52 809 1874 E-mail address: sd06508@toyota-ac.ti.jp (N.T Nam) 0304-8853/$ - see front matter r 2007 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2007.02.203 the domain wall models [3,4] or spin flopping model [5] proposed to account for this discrepancy However, there is still a lack of experimental confirmation for these models Ohldag et al [6] reported on the correlation between EB and pinned interfacial uncompensated AF spins, resulting in a vertical offset through the study of X-ray magnetic circular dichroism (XMCD) on several EB systems Based on the experimental fact, they argued that the physical origin of EB was unambiguously described as due to a fraction of uncompensated interfacial spins (about 4%) that are locked to the AF lattice and not rotate in an external magnetic field while most of the other interfacial spins are affected by the external field Recently, Tsunoda et al [7,8] found a great enhancement of the unidirectional anisotropy constant ðJ K Þ in MnIr/CoFe bilayer system with chemical ordering, resulting a J K up to 1:3 erg=cm2 [7] They therefore used XMCD to measure elementspecific magnetic hysteresis loops of MnIr/CoFe bilayers with different orderings of MnIr with the expect that the vertical offset should be enhanced with chemical ordering according to the model proposed by Ohldag et al [6] ARTICLE IN PRESS N.T Nam et al / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 However, the XMCD results by Tsunoda et al [8] shown no vertical offset, which is at variance with that reported by Ohldag et al [6] The fact that this large EB shows contradicted results to that of the ‘‘normal’’ EB thus requires a modification of the theoretical works for the quantitative understanding of EB couplinned from the AC susceptometry If determined from the conventional method, i.e from the equation J K ¼ H E Ã M S Ã tFM , the obtained maximum J K value was only 0:7 erg=cm2 [19] Therefore, it may be concluded that the present obtained J K of 2:5 erg=cm2 is the largest value ever 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 large value of the unidirectional anisotropy constant J K of MnIr/CoFe bilayers up to 1:3 erg=cm2 and found a strong correlation between EB anisotropy and chemical ordering In the present study, it is difficult to find such a correlation since the giant EB is strongly dependent on the Co thickness Based on the XRD patterns, it can only be concluded that Co is in amorphous state and consequently, Fig Hysteresis loops of MnPd (30 nm)/Co ðtCo Þ samples with tCo ¼ 10, 16, 20, 40, 50, 60 nm measured at T ¼ 120 K ARTICLE IN PRESS N.T Nam et al / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 86 Table The experimental value of the unidirectional anisotropy constant and the theoretical estimation based on the model of Meiklejohn and Bean Fig The dependences of exchange biased field ðH E Þ, coercivity ðH C Þ and unidirectional anisotropy constant ðJ K Þ on the thickness of Co layer ðtCo Þ for MnPd (30 nm)/Co tCo ị bilayers measured at T ẳ 120 K one cannot find any structural change when changing the thicknesses of Co layers In this paper, the simple model put forward by Meiklejohn and Bean [1] is employed to compare the theoretical value with the obtained experimental result According to this model, the AF spins at the interface are uncompensated and fixed during the FM magnetization rotation Hence, the unidirectional anisotropy constant can be calculated as follows: JK ¼ J ex Si Sj a2 (1) Here, J ex is the exchange integral at the interface, which is believed to be in the range of the exchange integrals of the AF and FM materials, Si and Sj the spins of the interfacial atoms, and a the lattice parameter If using J ex from the exchange integral of bulk materials, then J K is in the range from 1.8 to 14:1 erg=cm2 Therefore, roughly speaking, one may conclude that the present result lies in the range of theoretical prediction by the model of Meiklejohn and Bean [1] Table shows the experimental value of J K and the theoretical estimation based on the simple model of Meiklejohn and Bean [1] for several EB systems in the System Experimental value J K ðerg=cm2 ) Theoretical value J K (erg=cm2 ) CoO/Co particles [1] CoO/Co bilayers [20] MnIr/Co [17] MnFe/Co [17] MnPd/Fe [15] MnIr/CoFe [8] FeF/Fe [21] The present system 0.1 0.14 0.059 0.032 1.3 1.1 2.5 1.8–14.1 1.8–14.1 4.6–14.1 1.0–14.1 1.8–17.2 4.6–19.9 1.3–17.2 1.8–14.1 literature It is clearly seen that only some systems which exhibit huge unidirectional anisotropy [1,8,21] are in reasonable agreement with the Meiklejohn and Bean model [1] Therefore, even though the present result is consistent with the theoretical prediction, it may not considered as a firm support for the simple model by Meiklejohn and Bean [1] A more complicated model which is able to cover for all the cases should therefore be developed Fig illustrates the angular dependence measurement in out-of-plane configuration In this configuration, samples were first undergone a field cooling process with the cooling field applied in the plane of the films Then the hysteresis loop measurement was carried out with the applied magnetic field rotating out-of-plane, which makes an angle a with the cooling field direction Fig shows some representative hysteresis loops of MnPd ð30 nmÞ=Co ð20 nmÞ samples measured with a changing from 01 to 3601 These extracted values of H E and H C are plotted in Fig as a function of the angle a In the literature, there have been several reports on the angular dependence of EB and coercivity showing that the coercivity H C has a two-fold symmetry with maximal value along the field cooling axis (a ¼ 0 and 180 ) and H E shows a unidirectional symmetry which can be described by a series of odd cosine [22] The present results of the angular dependences of H E and H C are quite different from each other Although the angular dependence of H C behaves also a two-fold symmetry very large maxima appear at directions perpendicular to the FC direction (a ¼ 90 and 2701) and the H C ðaÞ curve can be fitted to function H C aị ẳ H C 0ị=j cosaịj Concerning the behavior of the H E ðaÞ dependence, it cannot be fitted to the cosine series as usual but it follows the 1= cos a rule as shown in Fig The above specific behavior of H E ðaÞ and H C ðaÞ was also observed by Sun et al [23] and was explained by the assumption that magnetization is pinned along the FC direction unless the projection of the applied magnetic field into the film surface exceeds the saturation field The inplane field component of the applied field is H cos a and hence the switching fields H SW should be equal to H SW ð0Þ= cos a which lead both the coercivity H C ðaÞ and ARTICLE IN PRESS N.T Nam et al / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 87 the EB field H E ðaÞ obey the 1= cos a rule It is worth noting that the present result is at variance with the work by Phuoc and Suzuki [13], which shows that H E and H C does not follow the 1= cos a rule although their measurement was also carried out in out-of-plane configuration The reason for this discrepancy is possibly due to the strong inplane magnetic anisotropy of the present samples arising from demagnetization field while in their work [13], the samples exhibit perpendicular magnetic anisotropy Conclusion Fig 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 In summary, the present study reported on the largest unidirectional anisotropy constant ever found in the literature up to 2:5 erg=cm2 in MnPd/Co bilayers This huge unidirectional anisotropy constant is found to be in Fig Hysteresis loops of MnPd (30 nm)/Co (20 nm) bilayer measured at different angles a between the field cooling direction and the applied magnetic field ARTICLE IN PRESS 88 N.T Nam et al / Journal of Magnetism and Magnetic Materials 315 (2007) 82–88 that this system cannot be applied for spin-valve sensors, the finding of giant exchange bias may provide some useful information for better understanding of the mechanism of exchange bias Acknowledgment This work is partially supported by the Vietnamese Fundamental Research Grant #4.049.06 (2006–2008) Also, the supports from the Academic Frontier Center for Future Storage Materials Research [MEXT HAITEKU (2004–2008)] and from the Japanese Storage Research Consortium are gratefully acknowledged References Fig Angular dependences of H E and H C for MnPd (30 nm)/Co (20 nm) bilayer The solid line is the fitting curve following 1= cos a rule reasonable agreement with the simple model proposed by Meiklejohn and Bean, which predicts that J K in MnPd/Co system is in the range from 1.8 to 14 erg=cm2 The thickness dependences of exchange bias and unidirectional anisotropy constant show a complex behavior, which is not yet fully understood The angular dependences of exchange bias and coercivity show a 1= cos a dependence, which may be interpreted as due to the magnetic anisotropy along the field cooling direction Although the present system of MnPd/Co bilayer has low blocking temperature, suggesting [1] W.H Meiklejohn, C.P Bean, Phys Rev 102 (1956) 1413 [2] J Nogue´s, I.K Schuller, J Magn Magn Mater 192 (1999) 203 [3] D Mauri, H.C Siegmann, P.S Bagus, E Kay, J Appl Phys 62 (1987) 3047 [4] A.P Malozemoff, Phys Rev B 37 (1988) 7673 [5] N.C Koon, Phys Rev Lett 78 (1997) 4865 [6] H Ohldag, A Scholl, F Nolting, E Arenholz, S Matt, A.T Young, M Carey, J Stoăhr, Phys Rev Lett 91 (2003) 017203 [7] I Imakita, M Tsunoda, M 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Fig illustrates the angular dependence measurement in out-of-plane configuration In this configuration, samples were first undergone a field cooling process with the cooling field applied in the plane