DSpace at VNU: Anomalous magnetization process in exchange-biased MnPd Co bilayers

Physica B 327 (2003) 385–388 Anomalous magnetization process in exchange-biased MnPd/Co bilayers N.N Phuoca, N.A Tuana, N.P Thuya,b,*, D Babonneauc, J Rabierc a International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, Dai Co Viet, Hanoi, Viet Nam Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Viet nam National University, Hanoi, Viet Nam c Laboratoire de M!etallurgie Physique, UMR 6630 CNRS-Universit!e de POITIERS, F-86962 Chasseneuil Futuroscope Cedex, France b Abstract MnPd/Co bilayers sputtered onto Si (1 0) substrates with varied MnPd thickness have been investigated Structural characterization of the bilayers was carried out with an X-ray diffractometer and their magnetic properties were measured by a vibrating-sample magnetometer As-deposited bilayers exhibit large but quite usual exchange-biased coupling, i.e the shift of the hysteresis loop towards the field direction opposite to the cooling-field direction This exchange bias vanishes if the antiferromagnetic (AFM) layer thickness is below a certain value However, for annealed bilayers with an MnPd layer thickness below a critical value, large exchange bias and asymmetric double-shifted loops are unexpectedly observed when the measuring field is along the cooling-field direction These anomalies in the magnetization process can be explained if one postulates that the orientation of the AFM easy axis with respect to the spin direction of the ferromagnetic layer is changed when the thickness of the AFM layer decreases r 2002 Elsevier Science B.V All rights reserved Keywords: Exchange bias; Magnetic multilayers; Double-shifted loops; Nanostructures The phenomenon of exchange bias between antiferromagnetic (AFM) and ferromagnetic (FM) materials has attracted enormous attention due to its potential application in spin valves Although it was discovered a long time ago [1] and extensive studies have been carried out recently, the physical origin of exchange bias remains poorly understood [2] One of the difficulties in solving this problem is the poor understanding of the spin structure at the interface of the FM/AFM bilayers Earlier models assumed collinear mag*Corresponding author International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, Dai Co Viet, Hanoi, Viet Nam Tel.: +84-4-8692518; fax: +84-4-8692963 E-mail address: thuy@itims.edu.vn (N.P Thuy) netic structures on the FM and AFM side of the interface [3–5] However, in a recent calculation by Koon [6], the interfacial energy is minimized for perpendicular coupling between the FM and AFM layers, showing that this kind of coupling is also possible Recently, a new phenomenon, the doubleshifted loops, has been observed in some exchange-biased systems, such as in Co/NiMn [7–8], Fe/MnPd [9] and NiO/NiFe [10] bilayers Generally, this phenomenon is observed when measuring perpendicular to the cooling-field direction In this paper, we study the magnetization process in MnPd/Co bilayers and show that double-shifted loops appear at some preparation and measurement conditions and can be tuned by changing 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N.N Phuoc et al / Physica B 327 (2003) 385–388 386 the thickness of the MnPd layer This result may contribute to elucidate the above question on the spin structure at the FM/AFM interface Samples with structure Si(1 0)/MnPd/Co were grown at room temperature by the RF sputtering technique with base pressure of 10À6 mbar and Ar pressure of 10À3 mbar During the growth, an inplane biasing field of 300 Oe was applied The thickness of the Co layer was fixed to 18 nm, whereas that of the MnPd layer was varied The composition of the MnPd film, analyzed by energy dispersive X-ray spectroscopy, is Mn19Pd81 After being deposited, the bilayers were annealed for an hour in a vacuum oven (10À5 mbar) at various temperatures, and then cooled in a magnetic field of kOe down to room temperature X-ray diffraction (XRD) measurements were performed to characterize the crystal structure of the samples The magnetic properties of both as-deposited and annealed samples were measured by a vibratingsample magnetometer in the temperature range from 123 K to room temperature The XRD patterns for some selected samples are shown in Fig For MnPd single-layered film, MnPd (111) Intensity (a.u.) (d) (c) (b) (a) 30 35 40 45 50 55 60 65 70 75 80 2θ (deg.) [Cu Kα] Fig XRD patterns for some selected samples: (a) MnPd(30 nm) thin film, (b) as-deposited MnPd(30 nm)/ Co(36 nm) bilayer, (c) annealed (Ta ¼ 2401C) bilayer, and (d) annealed (Ta ¼ 3201C) bilayer 0.0010 0.0004 0.0000 tMnPd =36 nm M (emu) M (emu) 0.0008 -0.0004 0.0005 0.0000 -0.0005 tMnPd =6 nm -0.0010 -0.0008 -2000 -1000 -2000 -1000 1000 2000 H (Oe) 1000 2000 H (Oe) 0.0010 0.0000 -0.0005 tMnPd =18 nm M (emu) M (emu) 0.002 0.0005 0.001 0.000 -0.001 tMnPd =3 nm -0.002 -2000 -1000 1000 2000 H (Oe) 0.0010 0.0008 0.0005 0.0004 0.0000 tMnPd =12 nm -0.0005 -0.0010 -2000 -1000 1000 2000 H (Oe) M (emu) M (emu) -0.0010 -2000 -1000 1000 2000 H (Oe) 0.0000 -0.0004 -0.0008 -2000 -1000 tMnPd =1.8 nm 1000 2000 H (Oe) Fig Hysteresis loops of exchange bias MnPd/Co bilayers at 123 K, in which the Co thickness is fixed to 18 nm, whereas the thickness of MnPd (tMnPd ) is varied as indicated N.N Phuoc et al / Physica B 327 (2003) 385–388 0.0006 H M (emu) M (emu) 0.0006 0.0003 387 0.0000 -0.0003 -0.0006 -2000 -1000 0.0000 -0.0003 =0° 1000 H (Oe) 0.0003 2000 =90° -0.0006 -2000 -1000 1000 H (Oe) 2000 Fig Hysteresis loops of the MnPd(6 nm)/Co(18 nm) bilayers measured at 123 K when the applied field is parallel (left panel) and perpendicular (right panel) to the cooling field the result shows a prominent peak corresponding to the MnPd (1 1) reflection Both as-deposited and annealed MnPd(30 nm)/Co(36 nm) bilayer samples also show strong MnPd (1 1) peaks, but slightly shifted toward the higher 2y angles This may be due to the stress induced by the Co layer The exchange bias observed in as-deposited MnPd/Co bilayers is very large: (B700 Oe) at low temperature (123 K) The dependence of the exchange bias on the thickness of the AFM MnPd layer conforms to that reported in Ref [4], namely, the exchange bias field vanishes as the thickness of the AFM layer is lower than a critical value For annealed, field-cooled, MnPd/Co bilayers, however, the magnetization process is rather unexpected as illustrated in Fig As the thickness of the MnPd layer is below a critical value (12 nm), double-shifted loops appear, whereas the usual exchange bias (single-shifted loop) is evidenced for samples with MnPd thickness larger than 12 nm It should be noted that the double-shifted loops are only observed when the applied field is parallel to the cooling field (HFC ) In case of magnetic hysteresis loops measured in fields perpendicular to the cooling field HFC ; neither exchange bias is observed nor double-shifted loops (see Fig 3, right panel) The above-mentioned results can be explained reasonably by postulating that there is a transformation of easy axis of the AFM layer as its thickness decreases A schematic illustration of our qualitative explanation is shown in Fig For field-cooled FM/AFM bilayers with thick AFM layer, the spin arrangement is illustrated in Fig 4(A1) In this case, the unique stable configuration corresponds to a zero angle between FM Cooling field direction tAFM ≤ tAFMC HFC FM tAFM ≤ tAFMC FM AFM AFM (A1) (A2) FM FM AFM AFM (B1) (B2) FM FM AFM AFM (C2) (C1) (B1) (C1) M (B2) (A1) H (C2) M (A2) H Fig Schematic diagram of the spin configurations of FM/ AFM bilayers and the corresponding hysteresis loops The left side schema corresponds to the case in which the AFM layer thickness (tAFM ) is larger than the critical one (tC AFM ), while the right side corresponds to the opposite situation (tAFM ptC AFM ) spins and cooling-field direction Therefore, no double-shifted loops are observed since the magnetization reversal occurs by coherent rotation (Fig 4(B1) and (C1)) For field-cooled bilayers in which the AFM layer is thinner than the critical thickness, the AFM spin axis is parallel to the FM spins (Fig 4(A2)) When the measuring field is changed, the FM spins rotate coherently, but they are subjected to an extra pinning, when rotated 388 N.N Phuoc et al / Physica B 327 (2003) 385–388 into a direction perpendicular to the cooling field (Fig 4(B2)) Therefore, an extra field decrease DH is needed to continue rotating to the configuration shown in Fig 4(C2) In summary, we have found an anomalous magnetization process (double-shifted loops) in exchange-biased MnPd/Co bilayers The appearance of these double-shifted loops can be explained qualitatively by assuming that the orientation of the AFM easy axis with respect to the spin direction in the FM layer is changed as the AFM thickness decreases Acknowledgements This work is supported by the State Program on Fundamental Research of Vietnam under Grant No 421001 References [1] W.H Meiklejohn, C.P Bean, Phys Rev 102 (1956) 1413; W.H Meiklejohn, C.P Bean, Phys Rev 105 (1957) 904 [2] J Nogu!es, I.K Schuller, J Magn Magn Mater 192 (1999) 203 [3] W.H Meiklejohn, J Appl Phys 33 (1962) 1328 [4] D Mauri, H.C Siegmann, P.S Bagus, E Kay, J Appl Phys 62 (1987) 3047; D Mauri, H.C Siegmann, P.S Bagus, E Kay, J Appl Phys 62 (1987) 2929 [5] A.P Malozemoff, Phys Rev B 35 (1987) 3679 [6] N.C Koon, Phys Rev Lett 78 (1997) 4865 [7] Y.H Wang, C.H Lai, C.R Chang, J.S Yang, C.K Lo, J Appl Phys 89 (2001) 6603 [8] C.H Lai, Y.H Wang, C.R Chang, J.S Yang, Phys Rev B 64 (2001) 094420 [9] Y.J Tang, X Zhou, X Chen, B.Q Liang, W.S Zhan, J Appl Phys 88 (2000) 2054 [10] R.P Michael, A Chaiken, C.T Wang, L.E Johnson, Phys Rev B 58 (1998) 8566  ... is needed to continue rotating to the configuration shown in Fig 4(C2) In summary, we have found an anomalous magnetization process (double-shifted loops) in exchange-biased MnPd/ Co bilayers The... field-cooled FM/AFM bilayers with thick AFM layer, the spin arrangement is illustrated in Fig 4(A1) In this case, the unique stable configuration corresponds to a zero angle between FM Cooling field... a vibratingsample magnetometer in the temperature range from 123 K to room temperature The XRD patterns for some selected samples are shown in Fig For MnPd single-layered film, MnPd (111) Intensity