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Co1−xFex bilayers
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Journal of Magnetism and Magnetic Materials 304 (2006) 41–45 www.elsevier.com/locate/jmmm
The exchange bias in MnPd/Co1xFex bilayers N.P. Thuya,b,, N.A. Tuanb, N.N. Phuocb,c, N.T. Namb, T.D. Hienb, N.H. Haid a
College of Technology, Vietnam National University, Hanoi, Vietnam International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam c Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan d Laboratoire Louis Ne´el, C.N.R.S., Grenoble, France
b
Available online 3 March 2006
Abstract A systematic study of exchange bias in MnPd/Co and MnPd/Co1xFex bilayers has been carried out. Very large unidirectional anisotropy constant of 2.2 erg/cm2 and the appearance of double-shifted loops, ascribed to the coexistence of positive and negative exchange bias, have been observed. The dependence of exchange bias, unidirectional anisotropy constant and coercivity on thickness, temperature, annealing regime and Fe content has been investigated and discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.70.–i; 75.25.+z; 75.30.Gw Keywords: Exchange bias; Double-shifted loops; MnPd/Co1xFex bilayers; Magnetic thin film
1. Introduction Discovered by Meiklejohn and Bean (MB) on CoO/Co system, the exchange bias (EB) effect has been explained by these authors using a model based on the suggestion of the existence of the unidirectional anisotropy originated from the exchange coupling between uncompensated spins at the interface between antiferromagnetic (AFM) and ferromagnetic (FM) layers [1,2]. This model has been considered so far as a simple and ideal model due to the fact that the EB field HE and the unidirectional anisotropy constant JK predicted theoretically are two to three orders of magnitude larger than those observed on most of the AFM/FM systems. The JK value measured on the CoO/Co system was 3 erg/cm2 at 10 K being rather unique and still far smaller than the theoretical value of 10 erg/cm2 [3]. Studies on EB in AFM/FM bilayers have therefore been developed in two trends. The first is to develop different theories that can adequately explain the actual experi-
mental EB values [4,5]. The second one, on the other hand, is to bring the experimental values of HE and JK closer to those predicted by MB theory. The later trend has been realized by two ways: (i) by improving the preparation techniques of the well-known AFM/FM systems so that the crystallographic morphologies approach the perfect states [6,7] and (ii) by looking for novel materials which have larger unidirectional anisotropy [8,9]. Recently a large value of JK of 1.3 erg/cm2 at room temperature has been discovered by Tsunoda et al. [10,11] on IrMn/Co1xFex system. We have also reported quite large values of EB field at cryogenic temperatures in MnPd/Co [12] and MnCr/Co [13] bilayers. This paper focuses on a more comprehensive study on MnPd/Co as well as recent experimental results on MnPd/Co1xFex systems hoping to shed some light on the understanding of EB effect.
2. Experimental results Corresponding author. International Training Institute for Materials Science (ITIMS), Dai hoc Bach khoa, 1 Dai Co Viet, Hanoi, Vietnam. Tel.: +84 04 8680787; fax: +84 04 8692963. E-mail addresses: [email protected], [email protected] (N.P. Thuy).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.063
MnPd/Co1xFex bilayer samples have been prepared by RF-sputtering onto Si (1 0 0) wafers in the sequence of Si/ AFM/FM. As-sputtered samples have been subsequently annealed at temperatures varied from T ann ¼ 240 to 320 1C
ARTICLE IN PRESS N.P. Thuy et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 41–45
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in vacuum of 105 m bar for 1 h followed by field cooling to room temperature under a magnetic field of 5 kOe. Scanning electron microscope (SEM) patterns of two MnPd films with thickness of 6 and 36 nm prepared separately on the Si (1 0 0) substrates do not show any structure that ensures the homogeneity of the AFM layers in our present bilayers. The energy dispersion X-ray spectroscopy (EDS) carried out on these films showed that their composition is around Mn30Pd70. The bulk material of the same composition has a CuAu-I type structure with the Ne´el temperature TN around 200 K [14]. Magnetic properties have been characterized by vibrating sample magnetometers (VSM) in temperatures ranging from 10 to 300 K. Shown in Fig. 1a are the hysteresis loops measured at 123 K in the MnPd (tMnPd)/Co (10 nm) samples with tMnPd ¼ 2, 3, 6, 12, 18 and 36 nm which were annealed at 300 1C. For the samples with tMnPd larger than 18 nm only the negative exchange bias, corresponding to a singleshifted loop (SSL), is observed. For samples with smaller 1.0
M / MS
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MnPd thickness, however, the double-shifted loop (DSL) behavior has been observed. As discussed later this DSL can be considered to be the superposition of the subloops with negative exchange bias (NEB) and positive exchange bias (PEB). We can then denote HE1 and HE2 as positive and negative EB fields respectively and use their average value. The thickness dependence of this average HE is plotted in Fig. 1b together with that of the unidirectional anisotropy constant JK derived from it by using the expression JK ¼ |HE MS tFM|, where MS and tFM are the ferromagnetic layer magnetization and thickness, respectively. The coercivity HC of this sample series as a function of tMnPd is presented in Fig. 1c, in which the average HC value of those two sub-loops in the case of DSL has been used. The role of annealing regime has been studied on the MnPd (12 nm)/Co (10 nm) sample which were annealed at different temperature Tann ¼ 240, 260, 280, 300, 320 1C. Hysteresis curves together with the variation of the exchange biased field HE and the coercivity HC as a function of annealing temperature are presented in Figs. 2a and b, respectively. Temperature dependence of the EB phenomenon has been investigated in two samples from the above series, one annealed at 240 1C and has the DSL and the other annealed
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(b) Fig. 1. Magnetic measurement results at 123 K on MnPd (tMnPd)/Co (10 nm) samples with tMnPd ¼ 2 to 36 nm: (a) hysteresis loops, (b) HE and JK versus tMnPd and (c) HC versus tMnPd. (a) quoted from our previous paper [12] but with corrected Co thickness values.
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Fig. 2. Magnetic measurement results on the MnPd (6 nm)/Co (10 nm) samples annealed for 1 h at Tann ¼ 240, 260, 280, 300 and 320 1C: (a) hysteresis loops, (b) HE and HC as a function of Tann.
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Fig. 3. Temperature variation of hysteresis loops and the derived EB fields of the MnPd (6 nm)/Co (10 nm) bilayer which is annealed at Tann ¼ 240 1C (a and c) and Tann ¼ 320 1C (b and d).
at 320 1C which has only SSL behavior. Magnetic measurements in the temperature range from 10 K to 300 K (see Figs. 3a and b) allowed us to evaluate temperature dependence of exchange biased fields HE1,(T), HE2 (T) (for sample with DSL) and HE (T) (for sample with SSL) as shown in Figs. 3c and d, respectively. Hysteresis loops measured at 123 K in the MnPd (30 nm)/Co1xFex (20 nm) sample series in which the alloy composition of the Co1xFex layers varied from x ¼ 0 to 1 show only SSL behavior. The derived HE, JK and HC values are plotted in Figs. 4a and b as a function of iron content x in the Co1xFex layers. 3. Discussion The overall behavior of the systems studied in this paper are very large EB fields found in samples with rather thick FM layers, which results in a huge unidirectional anisotropy constant. Indeed, a JK value of 2.2 erg/cm2 is obtained at 123 K for the MnPd (30 nm)/Co (20 nm) samples (see Fig. 4b). Noting that the maximal JK values reported so far are of 3 erg/cm2 at 10 K on the CoO/Co bilayer samples [3] and of 1.3 erg/cm2 at room temperature
on the MnIr/ Co1xFex system [11], the considered MnPd/ Co1xFex bilayer systems thus can be placed to the class of giant EB effect materials. Coercivity values of our samples, being 3 kOe at 10 K and 1 kOe at around 120 K, are also remarkable because the coercive field of unpinned Co layer is 50 Oe at 2 K and 20 Oe at 295 K [15]. The large magnitudes of both HE and HC are a clear evidence of very large unidirectional and uniaxial anisotropies induced during the field cooling process through the Ne´el temperature in the AFM/FM system which possesses a strong exchange coupling of the AFM and FM moments at their interface and a very large volume magnetocrystalline anisotropy of the MnPd layer. Our recent experiments on these samples on the dependence of JK as a function of the angle between the applied magnetic field and the initial field cooling direction showed that a huge value of JKE10 erg/cm2 could be attained at an angle near 901 [16]. The AFM thickness dependence of both HE and HC presented in Figs. 1b and c shows that the onset of biasing appears at tMnPd less than 2 nm and the EB is fully established at around tMnPd ¼ 12 nm. In contrast the coercive force shows a peak at about 6 nm. We note that the critical thickness for MnPd is approximately of the
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Fig. 4. Variation of HE, HC (a) and JK (b) of MnPd (30 nm)/Co1-xFex (20 nm) samples versus the Fe content x in the Co1-xFex layer measured at 123 K. JK of IrMn/Co1-xFex system and the saturation magnetization MS of Co1-xFex alloy are quoted from Ref. [10] for comparison.
same size as that found in the IrMn/Co system and is much lower than those observed in most of other AFM layers, e.g. FeMn [15]. This can be considered as an indication of the quite large value of the volume anisotropy of the MnPd layer providing a large thermal stability for the AFM domain structure and consequently resulting in a large coercivity value mentioned above. Although in the limited range of Tann (from 240 to 320 1C) the influence of annealing regime seems not to be critical to EB values, it has a stronger effect on the coercivity and especially on the behavior of the hysteresis curve (see Fig. 2b). Indeed, in the sample of MnPd (12 nm)/ Co (10 nm), the appearance of SSL can only be observed with Tann ¼ 320 1C, below which the sample has DSL behaviors. It is thus interesting to note that the appearance of the specific kind of loop behavior (either SSL or DSL) can be tuned not only by the variation of the relative AFM and FM thickness but also by annealing conditions. However, the mechanism for effect of the annealing regime is not clear at the present. The origin of the DSL is not identical according to different authors. It may be due to the contribution of uniaxial anisotropy apart from unidirectional anisotropy that brings about exchange bias [17,18] or the pinning of the FM spin in the direction perpendicular to the cooling field by a mechanism like the Koon trapping [19]. In this paper we suggest that this anomaly may result from the overlap of PEB and NEB [13,20]. This suggestion is verified by our results on temperature dependence of the EB in two
kinds of samples with either DSL or SSL behavior as presented in Figs. 3a–d. In case of sample with DSL, the overall loop is considered to consist of two sub-loops each of which corresponds to one domain of exchange bias, i.e. one sub-loop refers to one domain with NEB (characterized by EB field HE1) and the other to PEB (HE2). We now note in Figs. 3c and d that HE1 and HE2 decrease as temperature increases in the same manner as the decrease of HE in the case of sample with SSL. For the sample with DSL the blocking temperature, the temperature above which DSL disappears, is about 200 K, nearly the same as that of the sample with SSL. This result shows a strong correlation between DSL and EB. One may argue that the DSL can be caused by un-biased loops superimposed on negative biased loops, as often observed in spin-valve structure. However, if this were the reason, the kinked points of the ascending and descending branch would be symmetric through the original point of the M–H loops, which is not seen in our results. Therefore, this possibility can be ruled out. As demonstrated by Roshchin et al. [20], the DSL phenomenon in AFM/FM bilayers can be ascribed to the modification of EB by changing the relative AFM and FM domain sizes. When the AFM domain size is larger or comparable to that in the FM layer, sample can be split into independent subsystems with EB of opposite signs resulting in DSL upon cooling sample through the Ne´el temperature. Although Roshchin et al. [10] have studied the systems with FeF2 as the AFM layers their model does not require the interface exchange coupling to be AFM. Therefore, it can be applied as well for the MnPd/Co1xFex case even when the nature of the interface exchange coupling in this system has not known yet. The observed variation of the HE or the JK as a function of the AFM thickness in our samples can be understood in this line because the thinner AFM layer corresponds to the larger AFM domain. So the decrease of the AFM layer thickness while keeping the FM layer fixed will lead to the appearance of the DSL below some critical MnPd thickness. On the other hand, the change from DSL to SSL behavior with Tann increasing shown in Fig. 2a can also be explained as caused by the improvement of the interface crystallographic morphology. Concerning the results of the last sample series where the FM layer is Co1xFex alloy with different Fe content x, it should be noted that HE and HC depend on the Fe content in different ways (see Fig. 4a). The concentration dependence of JK derived from HE curve is presented in Fig. 4b. For a comparison the result of IrMn/Co1xFex system reported by Tsunoda et al. [10] is also plotted. It is clear that the trends of the composition dependence of JK in the two systems are quite different. Furthermore, they both show large discrepancy with the composition variation of the saturation magnetization of bulk Co–Fe alloy (see the dashed line in the figure). As analyzed by Tsunoda et al. [10] if the Heisenberg expression for the interface exchange coupling energy is valid for the AFM/FM bilayer
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systems, the derived JK values should follow the trend of magnetization. The observed results therefore can be attributed to the electron transfer at the hetero-interface giving rise to the change of the magnetic moments. Our results showed that this changing may be strongly depend on the nature of the AFM layer in a system with a given FM layer. This problem certainly needs more effort to be well understood.
Grenoble for their kind experimental help and stimulating discussion. This work is partly supported by the State Program on Fundamental Research of Vietnam under the Grants 811604 and 811404.
4. Concluding remarks
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
In summary, a systematic study of exchange bias in MnPd/Co1xFex bilayers has been carried out with the investigation of the dependence of exchange bias, unidirectional anisotropy constant and coercivity on MnPd thickness, temperature, annealing regime and the content of Fe. A very large unidirectional anisotropy has been found and the DSL has been observed, the appearance of which can be tuned by both changes of layer thickness and the preparation conditions. The study of Fe-composition dependence of EB in MnPd/Co1xFex bilayers shows a behaviour which is quite different from that observed in IrMn/Co1xFex system [10] and thus needs further study. Acknowledgement We express our sincere thanks to Dr. N. Dempsey and Dr. D. Givord from the Laboratoire Louis Ne´el, CNRS,
References W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. W.H. Meiklejohn, J. Appl. Phys. 33 (1962) 1328. J. Nogues, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. D. Mauri, et al., J. Appl. Phys. 62 (1987) 2929. A.P. Malozemoff, J. Appl. Phys. 63 (1988) 3874. A.J. Devasahayam, et al., J. Appl. Phys. 83 (1998) 7216. K. Yagami, et al., J. Appl. Phys. 89 (2001) 6609. M. Saito, et al., J. Magn. Soc. Japan 21 (1997) 505. S. Araki, et al., IEEE Trans. Magn. 34 (1998) 387. M. Tsunoda, et al., J. Magn. Magn. Mater. 239 (2002) 182. I. Imakita, et al., Appl. Phys. Lett. 85 (2004) 3812. N.N. Phuoc, et al., Physica B 327 (2003) 385. N.N. Phuoc, et al., J. Magn. Magn. Mater. 298 (2006) 43. E. Kren, G. Kadar, Phys. Lett. A 29 (1969) 340. M. Ali, et al., Phys. Rev. B 67 (2003) 172405. N.T. Nam, et al., to be published. Y.J. Tang, et al., J. Appl. Phys. 88 (2000) 2054. C.H. Lai, et al., Phys. Rev. B 64 (2001) 094420. N.C. Koon, Phys. Rev. Lett. 78 (1997) 4865. Roshchin, et al., Europhys. Lett. 71 (2005) 297.
Journal of Magnetism and Magnetic Materials 304 (2006) 41–45 www.elsevier.com/locate/jmmm
The exchange bias in MnPd/Co1xFex bilayers N.P. Thuya,b,, N.A. Tuanb, N.N. Phuocb,c, N.T. Namb, T.D. Hienb, N.H. Haid a
College of Technology, Vietnam National University, Hanoi, Vietnam International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam c Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan d Laboratoire Louis Ne´el, C.N.R.S., Grenoble, France
b
Available online 3 March 2006
Abstract A systematic study of exchange bias in MnPd/Co and MnPd/Co1xFex bilayers has been carried out. Very large unidirectional anisotropy constant of 2.2 erg/cm2 and the appearance of double-shifted loops, ascribed to the coexistence of positive and negative exchange bias, have been observed. The dependence of exchange bias, unidirectional anisotropy constant and coercivity on thickness, temperature, annealing regime and Fe content has been investigated and discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.70.–i; 75.25.+z; 75.30.Gw Keywords: Exchange bias; Double-shifted loops; MnPd/Co1xFex bilayers; Magnetic thin film
1. Introduction Discovered by Meiklejohn and Bean (MB) on CoO/Co system, the exchange bias (EB) effect has been explained by these authors using a model based on the suggestion of the existence of the unidirectional anisotropy originated from the exchange coupling between uncompensated spins at the interface between antiferromagnetic (AFM) and ferromagnetic (FM) layers [1,2]. This model has been considered so far as a simple and ideal model due to the fact that the EB field HE and the unidirectional anisotropy constant JK predicted theoretically are two to three orders of magnitude larger than those observed on most of the AFM/FM systems. The JK value measured on the CoO/Co system was 3 erg/cm2 at 10 K being rather unique and still far smaller than the theoretical value of 10 erg/cm2 [3]. Studies on EB in AFM/FM bilayers have therefore been developed in two trends. The first is to develop different theories that can adequately explain the actual experi-
mental EB values [4,5]. The second one, on the other hand, is to bring the experimental values of HE and JK closer to those predicted by MB theory. The later trend has been realized by two ways: (i) by improving the preparation techniques of the well-known AFM/FM systems so that the crystallographic morphologies approach the perfect states [6,7] and (ii) by looking for novel materials which have larger unidirectional anisotropy [8,9]. Recently a large value of JK of 1.3 erg/cm2 at room temperature has been discovered by Tsunoda et al. [10,11] on IrMn/Co1xFex system. We have also reported quite large values of EB field at cryogenic temperatures in MnPd/Co [12] and MnCr/Co [13] bilayers. This paper focuses on a more comprehensive study on MnPd/Co as well as recent experimental results on MnPd/Co1xFex systems hoping to shed some light on the understanding of EB effect.
2. Experimental results Corresponding author. International Training Institute for Materials Science (ITIMS), Dai hoc Bach khoa, 1 Dai Co Viet, Hanoi, Vietnam. Tel.: +84 04 8680787; fax: +84 04 8692963. E-mail addresses: [email protected], [email protected] (N.P. Thuy).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.063
MnPd/Co1xFex bilayer samples have been prepared by RF-sputtering onto Si (1 0 0) wafers in the sequence of Si/ AFM/FM. As-sputtered samples have been subsequently annealed at temperatures varied from T ann ¼ 240 to 320 1C
ARTICLE IN PRESS N.P. Thuy et al. / Journal of Magnetism and Magnetic Materials 304 (2006) 41–45
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in vacuum of 105 m bar for 1 h followed by field cooling to room temperature under a magnetic field of 5 kOe. Scanning electron microscope (SEM) patterns of two MnPd films with thickness of 6 and 36 nm prepared separately on the Si (1 0 0) substrates do not show any structure that ensures the homogeneity of the AFM layers in our present bilayers. The energy dispersion X-ray spectroscopy (EDS) carried out on these films showed that their composition is around Mn30Pd70. The bulk material of the same composition has a CuAu-I type structure with the Ne´el temperature TN around 200 K [14]. Magnetic properties have been characterized by vibrating sample magnetometers (VSM) in temperatures ranging from 10 to 300 K. Shown in Fig. 1a are the hysteresis loops measured at 123 K in the MnPd (tMnPd)/Co (10 nm) samples with tMnPd ¼ 2, 3, 6, 12, 18 and 36 nm which were annealed at 300 1C. For the samples with tMnPd larger than 18 nm only the negative exchange bias, corresponding to a singleshifted loop (SSL), is observed. For samples with smaller 1.0
M / MS
0.5 0.0 -0.5
MnPd thickness, however, the double-shifted loop (DSL) behavior has been observed. As discussed later this DSL can be considered to be the superposition of the subloops with negative exchange bias (NEB) and positive exchange bias (PEB). We can then denote HE1 and HE2 as positive and negative EB fields respectively and use their average value. The thickness dependence of this average HE is plotted in Fig. 1b together with that of the unidirectional anisotropy constant JK derived from it by using the expression JK ¼ |HE MS tFM|, where MS and tFM are the ferromagnetic layer magnetization and thickness, respectively. The coercivity HC of this sample series as a function of tMnPd is presented in Fig. 1c, in which the average HC value of those two sub-loops in the case of DSL has been used. The role of annealing regime has been studied on the MnPd (12 nm)/Co (10 nm) sample which were annealed at different temperature Tann ¼ 240, 260, 280, 300, 320 1C. Hysteresis curves together with the variation of the exchange biased field HE and the coercivity HC as a function of annealing temperature are presented in Figs. 2a and b, respectively. Temperature dependence of the EB phenomenon has been investigated in two samples from the above series, one annealed at 240 1C and has the DSL and the other annealed
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(b) Fig. 1. Magnetic measurement results at 123 K on MnPd (tMnPd)/Co (10 nm) samples with tMnPd ¼ 2 to 36 nm: (a) hysteresis loops, (b) HE and JK versus tMnPd and (c) HC versus tMnPd. (a) quoted from our previous paper [12] but with corrected Co thickness values.
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Fig. 2. Magnetic measurement results on the MnPd (6 nm)/Co (10 nm) samples annealed for 1 h at Tann ¼ 240, 260, 280, 300 and 320 1C: (a) hysteresis loops, (b) HE and HC as a function of Tann.
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Fig. 3. Temperature variation of hysteresis loops and the derived EB fields of the MnPd (6 nm)/Co (10 nm) bilayer which is annealed at Tann ¼ 240 1C (a and c) and Tann ¼ 320 1C (b and d).
at 320 1C which has only SSL behavior. Magnetic measurements in the temperature range from 10 K to 300 K (see Figs. 3a and b) allowed us to evaluate temperature dependence of exchange biased fields HE1,(T), HE2 (T) (for sample with DSL) and HE (T) (for sample with SSL) as shown in Figs. 3c and d, respectively. Hysteresis loops measured at 123 K in the MnPd (30 nm)/Co1xFex (20 nm) sample series in which the alloy composition of the Co1xFex layers varied from x ¼ 0 to 1 show only SSL behavior. The derived HE, JK and HC values are plotted in Figs. 4a and b as a function of iron content x in the Co1xFex layers. 3. Discussion The overall behavior of the systems studied in this paper are very large EB fields found in samples with rather thick FM layers, which results in a huge unidirectional anisotropy constant. Indeed, a JK value of 2.2 erg/cm2 is obtained at 123 K for the MnPd (30 nm)/Co (20 nm) samples (see Fig. 4b). Noting that the maximal JK values reported so far are of 3 erg/cm2 at 10 K on the CoO/Co bilayer samples [3] and of 1.3 erg/cm2 at room temperature
on the MnIr/ Co1xFex system [11], the considered MnPd/ Co1xFex bilayer systems thus can be placed to the class of giant EB effect materials. Coercivity values of our samples, being 3 kOe at 10 K and 1 kOe at around 120 K, are also remarkable because the coercive field of unpinned Co layer is 50 Oe at 2 K and 20 Oe at 295 K [15]. The large magnitudes of both HE and HC are a clear evidence of very large unidirectional and uniaxial anisotropies induced during the field cooling process through the Ne´el temperature in the AFM/FM system which possesses a strong exchange coupling of the AFM and FM moments at their interface and a very large volume magnetocrystalline anisotropy of the MnPd layer. Our recent experiments on these samples on the dependence of JK as a function of the angle between the applied magnetic field and the initial field cooling direction showed that a huge value of JKE10 erg/cm2 could be attained at an angle near 901 [16]. The AFM thickness dependence of both HE and HC presented in Figs. 1b and c shows that the onset of biasing appears at tMnPd less than 2 nm and the EB is fully established at around tMnPd ¼ 12 nm. In contrast the coercive force shows a peak at about 6 nm. We note that the critical thickness for MnPd is approximately of the
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1400 HC -HE
1200
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1000 800 600 400
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1.0
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Fig. 4. Variation of HE, HC (a) and JK (b) of MnPd (30 nm)/Co1-xFex (20 nm) samples versus the Fe content x in the Co1-xFex layer measured at 123 K. JK of IrMn/Co1-xFex system and the saturation magnetization MS of Co1-xFex alloy are quoted from Ref. [10] for comparison.
same size as that found in the IrMn/Co system and is much lower than those observed in most of other AFM layers, e.g. FeMn [15]. This can be considered as an indication of the quite large value of the volume anisotropy of the MnPd layer providing a large thermal stability for the AFM domain structure and consequently resulting in a large coercivity value mentioned above. Although in the limited range of Tann (from 240 to 320 1C) the influence of annealing regime seems not to be critical to EB values, it has a stronger effect on the coercivity and especially on the behavior of the hysteresis curve (see Fig. 2b). Indeed, in the sample of MnPd (12 nm)/ Co (10 nm), the appearance of SSL can only be observed with Tann ¼ 320 1C, below which the sample has DSL behaviors. It is thus interesting to note that the appearance of the specific kind of loop behavior (either SSL or DSL) can be tuned not only by the variation of the relative AFM and FM thickness but also by annealing conditions. However, the mechanism for effect of the annealing regime is not clear at the present. The origin of the DSL is not identical according to different authors. It may be due to the contribution of uniaxial anisotropy apart from unidirectional anisotropy that brings about exchange bias [17,18] or the pinning of the FM spin in the direction perpendicular to the cooling field by a mechanism like the Koon trapping [19]. In this paper we suggest that this anomaly may result from the overlap of PEB and NEB [13,20]. This suggestion is verified by our results on temperature dependence of the EB in two
kinds of samples with either DSL or SSL behavior as presented in Figs. 3a–d. In case of sample with DSL, the overall loop is considered to consist of two sub-loops each of which corresponds to one domain of exchange bias, i.e. one sub-loop refers to one domain with NEB (characterized by EB field HE1) and the other to PEB (HE2). We now note in Figs. 3c and d that HE1 and HE2 decrease as temperature increases in the same manner as the decrease of HE in the case of sample with SSL. For the sample with DSL the blocking temperature, the temperature above which DSL disappears, is about 200 K, nearly the same as that of the sample with SSL. This result shows a strong correlation between DSL and EB. One may argue that the DSL can be caused by un-biased loops superimposed on negative biased loops, as often observed in spin-valve structure. However, if this were the reason, the kinked points of the ascending and descending branch would be symmetric through the original point of the M–H loops, which is not seen in our results. Therefore, this possibility can be ruled out. As demonstrated by Roshchin et al. [20], the DSL phenomenon in AFM/FM bilayers can be ascribed to the modification of EB by changing the relative AFM and FM domain sizes. When the AFM domain size is larger or comparable to that in the FM layer, sample can be split into independent subsystems with EB of opposite signs resulting in DSL upon cooling sample through the Ne´el temperature. Although Roshchin et al. [10] have studied the systems with FeF2 as the AFM layers their model does not require the interface exchange coupling to be AFM. Therefore, it can be applied as well for the MnPd/Co1xFex case even when the nature of the interface exchange coupling in this system has not known yet. The observed variation of the HE or the JK as a function of the AFM thickness in our samples can be understood in this line because the thinner AFM layer corresponds to the larger AFM domain. So the decrease of the AFM layer thickness while keeping the FM layer fixed will lead to the appearance of the DSL below some critical MnPd thickness. On the other hand, the change from DSL to SSL behavior with Tann increasing shown in Fig. 2a can also be explained as caused by the improvement of the interface crystallographic morphology. Concerning the results of the last sample series where the FM layer is Co1xFex alloy with different Fe content x, it should be noted that HE and HC depend on the Fe content in different ways (see Fig. 4a). The concentration dependence of JK derived from HE curve is presented in Fig. 4b. For a comparison the result of IrMn/Co1xFex system reported by Tsunoda et al. [10] is also plotted. It is clear that the trends of the composition dependence of JK in the two systems are quite different. Furthermore, they both show large discrepancy with the composition variation of the saturation magnetization of bulk Co–Fe alloy (see the dashed line in the figure). As analyzed by Tsunoda et al. [10] if the Heisenberg expression for the interface exchange coupling energy is valid for the AFM/FM bilayer
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systems, the derived JK values should follow the trend of magnetization. The observed results therefore can be attributed to the electron transfer at the hetero-interface giving rise to the change of the magnetic moments. Our results showed that this changing may be strongly depend on the nature of the AFM layer in a system with a given FM layer. This problem certainly needs more effort to be well understood.
Grenoble for their kind experimental help and stimulating discussion. This work is partly supported by the State Program on Fundamental Research of Vietnam under the Grants 811604 and 811404.
4. Concluding remarks
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
In summary, a systematic study of exchange bias in MnPd/Co1xFex bilayers has been carried out with the investigation of the dependence of exchange bias, unidirectional anisotropy constant and coercivity on MnPd thickness, temperature, annealing regime and the content of Fe. A very large unidirectional anisotropy has been found and the DSL has been observed, the appearance of which can be tuned by both changes of layer thickness and the preparation conditions. The study of Fe-composition dependence of EB in MnPd/Co1xFex bilayers shows a behaviour which is quite different from that observed in IrMn/Co1xFex system [10] and thus needs further study. Acknowledgement We express our sincere thanks to Dr. N. Dempsey and Dr. D. Givord from the Laboratoire Louis Ne´el, CNRS,
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