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 fi
Trang 1Anomalous 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, 1 Dai Co Viet, Hanoi, Viet Nam b
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
Abstract
MnPd/Co bilayers sputtered onto Si (1 0 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
r2002 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-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 double-shifted loops, has been observed in some ex-change-biased systems, such as in Co/NiMn[7–8], Fe/MnPd [9] and NiO/NiFe [10] bilayers Gen-erally, this phenomenon is observed when measur-ing perpendicular to the coolmeasur-ing-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 measure-ment conditions and can be tuned by changing
*Corresponding author International Training Institute for
Materials Science (ITIMS), Dai hoc bach khoa, 1 Dai Co Viet,
Hanoi, Viet Nam Tel.: +84-4-8692518; fax: +84-4-8692963.
E-mail address: thuy@itims.edu.vn (N.P Thuy).
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 1 - 9
Trang 2the thickness of the MnPd layer This result
may contribute to elucidate the above
ques-tion on the spin structure at the FM/AFM
interface
Samples with structure Si(1 0 0)/MnPd/Co were grown at room temperature by the RF sputtering technique with base pressure of 106mbar and Ar pressure of 103mbar During the growth, an in-plane 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 (105mbar) at various temperatures, and then cooled in a magnetic field
of 5 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 vibrating-sample magnetometer in the temperature range from 123 K to room temperature
The XRD patterns for some selected samples are shown inFig 1 For MnPd single-layered film,
(d) (c) (b) (a) MnPd (111)
Fig 1 XRD patterns for some selected samples: (a)
MnPd(30 nm) thin film, (b) as-deposited MnPd(30 nm)/
Co(36 nm) bilayer, (c) annealed (Ta¼ 2401C) bilayer, and
(d) annealed (Ta¼ 3201C) bilayer.
-0.0008 -0.0004 0.0000 0.0004 0.0008
-0.0010 -0.0005 0.0000 0.0005 0.0010
-0.0008 -0.0004 0.0000 0.0004 0.0008
-0.0010 -0.0005 0.0000 0.0005 0.0010
H (Oe)
-0.0010 -0.0005 0.0000 0.0005 0.0010
-0.002 -0.001 0.000 0.001 0.002
H (Oe)
H (Oe)
H (Oe) -2000 -1000H (Oe)0 1000 2000
H (Oe)
Fig 2 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 (t ) is varied as indicated.
Trang 3the result shows a prominent peak corresponding
to the MnPd (1 1 1) reflection Both as-deposited
and annealed MnPd(30 nm)/Co(36 nm) bilayer
samples also show strong MnPd (1 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,
how-ever, the magnetization process is rather
unex-pected as illustrated inFig 2 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 (seeFig 3, right
panel)
The above-mentioned results can be explained
reasonably by postulating that there is a
transfor-mation of easy axis of the AFM layer as its
thickness decreases A schematic illustration of our
qualitative explanation is shown in Fig 4 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
config-uration corresponds to a zero angle between FM
spins and cooling-field direction Therefore, no double-shifted loops are observed since the mag-netization 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
-0.0006 -0.0003 0.0000 0.0003 0.0006
H (Oe)
=90 °
-0.0006 -0.0003 0.0000 0.0003 0.0006
H (Oe)
=0 °
H
Fig 3 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.
FM AFM
FM AFM
FM AFM
FM AFM
FM AFM
FM AFM
H
M
H M
HFC
Cooling field direction
Fig 4 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 (t AFM ) is larger than the critical one (t C
AFM ), while the right side corresponds to the opposite situation (t AFM pt C
AFM ).
Trang 4into 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 inFig 4(C2)
In summary, we have found an anomalous
magnetization process (double-shifted loops) in
exchange-biased MnPd/Co bilayers The
appear-ance of these double-shifted loops can be
ex-plained 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.