DSpace at VNU: Anomalous training effect in exchange-biased MnPd Co bilayers

solidi status pss physica Phys Status Solidi A 206, No 1, 152– 156 (2009) / DOI 10.1002/pssa.200824385 a www.pss-a.com applications and materials science Anomalous training effect in exchange-biased MnPd/Co bilayers Nguyen Nguyen Phuoc*, 1, 2, Nguyen Phu Thuy1, 3, and Nguyen Anh Tuan1 International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam Department of Physics, Faculty of Science, National University of Singapore, 117542 Singapore Department of MEMS and Micro-systems Technology, Faculty of Electronics and Communications, College of Technology, Vietnam National University, Hanoi, Vietnam Received 27 May 2008, revised September 2008, accepted 25 September 2008 Published online 13 November 2008 PACS 75.25.+ z, 75.30.Gw, 75.70.Cn, 81.15.Cd * Corresponding author: e-mail nnguyenphuoc@yahoo.com, Phone: + 65-6516-2816, Fax: + 65-6777-6126 Exchange bias has been studied for a series of MnPd/Co bilayers sputtered onto Si(100) by an RF sputter-deposition system The double-shifted loops with an anomalous training effect have been observed The manifestation of double- shifted loops is interpreted as the coexistence of positive exchange bias and negative exchange bias, which is in agreement with the temperature dependence and the observed anomalous training effect © 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Introduction Exchange coupling at the interface between an antiferromagnet (AF) and a ferromagnet (FM), discovered in 1956 [1], results in a shift of the hysteresis loop along the magnetic field axis called exchange bias (EB) This phenomenon has been studied extensively due to its widespread application in spin valves and magnetic tunnel junctions as well as its intriguing physical origin [2] Normally, exchange bias is described as an additional unidirectional anisotropy induced by the AF into the FM via exchange coupling at the interface, causing a single magnetic hysteresis loop shifted along the magnetic-field axis after the field-cooling procedure through the Néel point of the AF The magnitude of this shift is termed the exchange bias field (HE) and in almost all cases, the magnetic hysteresis loop is shifted in the negative field if one defines the direction of the cooling field (HFC) as the positive direction This case is referred to as negative EB The phenomenon of positive EB was first observed in 1996 by Nogués et al [3] when studying the systems of Fe/FeF2 and Fe/MnF2 They found that the sign of the EB field changes from negative to positive as the cooling field increases Very recently, it was found that the state of coexistence of positive and negative EB could be achieved in some specific systems [4–7] This state manifests itself as a double hysteresis loop In this paper, we report on the observation of doubleshifted loops in MnPd/Co when the MnPd thickness is less than 18 nm, which can be ascribed to the superposition of positive and negative EB Moreover, we present additional evidences to firmly support this assumption, namely the experimental results of temperature dependence and the observed abnormal training effect Experimental procedures The samples used in the present work with the structure of Si(100)/MnPd (x nm)/ Co (10 nm) (x = 1.2, 3.6, 6, 12, 18, 36 nm) were fabricated at room temperature by an RF sputter-deposition system The MnPd layers were sputtered from a composite target constituting a Pd target with Mn chips placed on it The base pressure was about 10–6 mbar, whereas the working argon pressure during deposition was 10–3 mbar The composition of the MnPd films, identified by energy-dispersive X-ray spectroscopy (EDS), is Mn30Pd70 The samples were then annealed in a high-vacuum oven (10–5 mbar) at the temperature of 570 K for h The purpose of annealing the samples at such a high temperature is to enhance the crystallinity of MnPd layers to obtain the antiferromagnetic phase Subsequently, they were cooled in a magnetic field of kOe to room temperature to induce the exchange bias effect The magnetic properties of the annealed bilayers © 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Original Paper Phys Status Solidi A 206, No (2009) were characterized by a vibrating sample magnetometer (VSM) in the temperature range from 10 K to 300 K It is known that Co may be spontaneously oxidized, forming a thin CoO layer on the top of the samples In order to check this possibility, we made a similar sample with the Mo layer on the top and found that the magnetic behavior of that sample is identical with the sample without a Mo layer Hence, the effect of oxidation of the Co layer, if any, may be neglected in our investigation Results and discussion Figure 1a and b show the magnetic hysteresis loops measured at T = 120 K of MnPd (6 nm)/Co (10 nm) and MnPd (36 nm)/Co (10 nm) bilayers, respectively A double-shifted loop is observed in the MnPd (6 nm)/Co (10 nm) bilayer, while the MnPd (36 nm)/Co (10 nm) bilayer shows a single-shifted loop, i.e normal EB In the series of MnPd(x nm)/ Co (10 nm) samples, double-shifted loops are seen in samples with the thickness of MnPd layers smaller than 18 nm These double-shifted loops have also been observed in FeF2/Ni [4, 5], and CrMn/Co [6] systems and have been attributed to the coexistence of positive and negative EB In these systems, an antiferromagnetic coupling at the interface of AF/FM is favored, causing a competition between the Zeeman and the AF/FM exchange energy, which results in the crossover from negative to positive EB as the cooling field is increased When the sample is cooled in an intermediate applied field, the state of coexistence of positive and negative EB is realized, in which a fraction of AF 153 spins is aligned in the cooling field due to Zeeman energy, while the rest is aligned in the opposite direction due to AF exchange coupling In our case, if the cooling field is small (less than kOe), only negative EB is observed and as the cooling field is beyond kOe, double hysteresis loops are seen However, due to the limit of the field-cooling system (maximum kOe), we cannot observe the state of complete positive EB One should note that the present state of superposition of positive and negative EB differs from other studies [8, 9] where a double loop can be found after demagnetizing the FM, since in our case the cooling field is strong enough to saturate the magnetization of Co layer For the sake of convenience, HE1 and HE2 are denoted as the positive and negative exchange bias fields and MS1 and MS2 as the spontaneous positive-biased and negative-biased magnetizations, respectively as seen in Fig 1a Figure 1c and d show the dependences of the EB field, unidirectional anisotropy, and coercivity on the MnPd thicknesses It is noted that for the samples with double loops, the EB field of the whole sample is defined as the average value of HE1, HE2 and HC is estimated as the average value of the coercivities of each subloop The obtained unidirectional anisotropy constant JK is rather large (up to 1.1 erg/cm2) compared to other exchange bias systems [2] Shown in Fig are the magnetization curves for the MnPd (6 nm)/Co (10 nm) bilayer measured at T = 120 K with the applied magnetic field rotated in the plane of the film It is seen that as the angle θ between the applied field Figure Magnetic hysteresis loops of MnPd (6 nm)/Co (10 nm) and MnPd (36 nm)/Co (10 nm) bilayers measured at 120 K (Fig 1a and b) The dependence of exchange bias (HE), unidirectional anisotropy constant (JK) and coercivity (HC) on MnPd thickness (Fig 1c and d) The definitions of HE1, HE2, MS1 and MS2 are shown in Fig 1a (See the text for more detail.) www.pss-a.com © 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim solidi status physica pss a 154 N N Phuoc et al.: Anomalous training effect in exchange-biased MnPd/Co bilayers Figure M – H loops of MnPd (6 nm)/Co (10 nm) at T = 120 K measured at θ = 0°, 45° and 90° with respect to the field-cooling direction as shown in the inset and the cooling field is 45°, we still observe the doubleshifted loops, although the subloops are slanted, indicating that the applied field is now deviated from the easy axis As θ = 90°, i.e the applied magnetic field is now along the hard axis, both subloops become hard-axis curves, resulting in a total single hard-axis loop as observed in Fig The present result suggests that double-shift loops observed in our system are quite different from that reported earlier in the Refs [10–12], where double hysteresis loops are found when the external magnetic field is applied along the hard axis of the AF, attributed to an additional biquadratic AF–FM interaction For more evidences to support the idea of two oppositely oriented AF domains, we have carried out a study of the temperature dependence of this effect Shown in Fig 3a are some representative magnetic hysteresis loops of the MnPd (12 nm)/Co (10 nm) bilayer measured at various temperatures It is seen that the double-shifted loops disappear as the temperature is beyond 220 K, which is the same as the blocking temperature of the MnPd (36 nm)/Co (10 nm) sample that exhibits a singleshifted loop The blocking temperature of about 220 K is very close to the Néel point of Mn30Pd70 bulk materials [13] The temperature dependence of positive EB field (denoted as HE1) and negative EB field (denoted as HE2) is shown in Fig 3b It is clearly seen that the value of positive and negative EB fields, though different, are varied in a similar manner This fact can be considered as additional evidence that the double-shifted loop originates from the coexistence of positive and negative EB It should be mentioned that this explanation is valid only in the case of an atomically flat interface with uncompensated spins In reality, the natural interface roughness may play a vital role in the mechanism of exchange bias as argued by some authors [16, 17] Hence, the AF layer may be split into multidomain structure, which causes the applied field of kOe to be subcritical and supercritical, corresponding to the crossover from negative EB to positive EB More interestingly, we have observed an abnormal training effect in the samples that exhibit double-shifted © 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Figure (a) M – H loops of MnPd (6 nm)/Co (10 nm) bilayer measured at different temperatures (b) Temperature dependence of positive (HE1) and negative (HE2) exchange bias fields in MnPd (6 nm)/Co (10 nm) bilayer HE1 and HE2 are defined in Fig 1a loops Figure 4a and b present some representative magnetic hysteresis loops of a MnPd (12 nm)/Co (10 nm) film after some cycles of measurement and the corresponding values of positive (HE1) and negative (HE2) EB fields as a function of the number of measurements, respectively Since the magnitudes of MS1 and MS2 correspond to the areas that make positive and negative EB respectively, we can denote the normalized values of mS1 and mS2, where mS1 = MS1/(MS1 + MS2) and mS2 = MS2/(MS1 + MS2), as the fractions of positive- and negative-biased areas The dependence of these positive- and negative-biased area fractions on the cycle measurement is shown in Fig 4c At first, the negative EB decreases very rapidly from 840 Oe to 740 Oe and levels off after cycles, while the positive EB decreases slower After 11 cycles, there is no change in mS1 and mS2 At cycle 12, there is a drastic jump of HE1 and HE2 as well as mS1 and mS2 The drastic change of mS1 and mS2, namely the increase of mS2 and the decrease of mS1, implies that there is an enlargement of the negative-biased area at the cost of reducing the positive-biased area A similar abnormal training effect has also been observed in the system of CrMn/Co bilayers [6] and has been explained as the movement of the AF domain wall toward the positivewww.pss-a.com Original Paper Phys Status Solidi A 206, No (2009) Figure (a) Representative of training-effect hysteresis loops of MnPd (12 nm)/Co (10 nm) film M – H loops for the cycles 1, 6, 12, 24 and 48 are shown (b) Positive (HE1) and negative (HE2) exchange bias fields as a function of cycles of measurement (c) Fractions of positive (mS1) and negative (mS2) exchange bias areas as a function of cycles of measurement biased domain Of great interest is the reverse training effect for negative EB as seen in Fig 4b HE2, which is decreased to 740 Oe after 11 cycles of measurement, regains the previous value of 840 Oe at cycle 12 This surprising effect implies that the movement of the AF domain wall, causing the enlargement of the negative-biased area, reinduces the untrained state Recently, Brems et al [14] reported on the reverse training effect in CoO/Co bilayers, in which the EB field regained its untrained value after carrying out a M–H loop measurement along the hard axis and this effect can be interpreted as a change in the magneticdomain structure in the AF layer It should be noted that recently the physical origin of exchange bias was described as due to a fraction of uncompensated interfacial spins (about 4%) that are locked to the AF lattice and not rowww.pss-a.com 155 tate in an external magnetic field, while most of the other interfacial spins are affected by the external field [15] The training effect can therefore be understood as the loss of the pinned uncompensated spins Taking into account this argument, we can state that after the movement of the AF domain wall causing the enlargement of the negativebiased area, the fraction of uncompensated interfacial spins increases to its original value that results in the reverse training effect observed in our system However, for the decrease of the positive-biased area, the positive exchange bias decreases from 400 Oe to 250 Oe after the movement of the AF domain wall, i.e the reverse training effect has not been observed in the positive subloop It is well known that positive exchange bias is in a high-energy state so it is less stable than the negative exchange bias Therefore, we may expect that after the AF domain-wall movement, the fraction of pinned uncompensated spins will be reduced It is also very interesting to see that as we carried out more measurement, namely cycle 12 upward, the positive EB is not changed while the change of negative EB from cycle 12 to cycle 17 is very similar to the change from cycle to cycle This implies that after the movement of the AF domain wall, the negative-biased area is reinduced to the untreated state It is noticed that previously, Nowak et al [17, 18] explained the training effect in terms of the rearrangement of AF domain by using their domain state model Hauet et al [19] studied the mechanism of the training effect in hard/soft Tb12Fe88/Gd40/Fe60 bilayers with positive exchange bias and found that a partial magnetization reversal of soft layers generates new domain of the hard layers, which has been subjected to a training effect It is evidenced from their experiments that the training effect is due to an irreversible reorientation of the hard-layer magnetization Hence, the present explanation employing the idea of the movement of AF domain seems to be reasonable for the interpretation of the abnormal training effect However, one should not rule out other possibilities, e.g the granular model for the AF with thermal activation [20, 21], as a potential explanation for this behavior Recently, Binek [22] considered the training effect in the framework of nonequilibrium thermodynamics and found that training of the exchange bias effect originates from spin configurational relaxation, which is activated through consecutive-cycled hysteresis loops Based on this idea, one can explain the breakdown of the power-law behavior when n = 1, which is quite similar to the nonmonotonic behavior in this work Further investigation may thus need to be performed to get a better understanding of this intriguing effect Summary and conclusion To summarize, we present an EB system exhibiting a double-shifted loop, which results from the overlap of the two oppositely biased loops The angular and temperature dependences are consistent with this argument An abnormal training effect is qualitatively explained using this assumption We believe that the present results will be useful for understanding the mecha© 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim solidi status physica pss a 156 N N Phuoc et al.: Anomalous training effect in exchange-biased MnPd/Co bilayers nism of some peculiar effects associated with exchange bias Acknowledgements We thank Dr Givord and Dr Dempsey from the Laboratory Louis Néel for their kind experimental supports and stimulating discussions This work is supported by the State Programs on Fundamental Research of Vietnam under the Grants 4.049.06 and 4.105.06 References [1] W H Meiklejohn and C P Bean, Phys Rev 102, 1413 (1956) [2] J Nogués and I K Schuller, J Magn Magn Mater 192, 203 (1999) [3] J Nogués, D Lederman, T J Moran, and I 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0°, 45° and 90° with respect to the field-cooling direction... explanation for this behavior Recently, Binek [22] considered the training effect in the framework of nonequilibrium thermodynamics and found that training of the exchange bias effect originates... enlargement of the negativebiased area, the fraction of uncompensated interfacial spins increases to its original value that results in the reverse training effect observed in our system However,