Fnidikib a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam b GPM-UMR 6634, Universit !e de Rouen, 76821 Mon
Trang 1Low-field magnetoresistance of Fe/Cr multilayers
N.H Duca,*, N.A Tuana, N.T Nama, N.H Sinha, J Teilletb, A Fnidikib
a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam
b GPM-UMR 6634, Universit !e de Rouen, 76821 Mont-Saint-Aignan, France
Abstract
Sputtered {Fe/Cr} multilayers with a fixed Cr individual layer thickness tCr¼ 2 nm and variable Fe individual layer thickness (1 nmptFep6 nm) are investigated by means of X-ray diffraction, magnetoresistance and magnetisation measurements At room temperature, the initial magnetoresistive susceptibility of the as-deposited samples is almost constant However, the saturation field increases with decreasing Fe-layer thickness, therefore, a maximal magnetoresistance ratio DR=R of 0.7% is reached in the sample with tFe=1 nm After annealing at 3501C, a DR=R value as large as 2.3% was obtained Further annealing causes a reduction of magnetoresistance As the temperature is decreased, the DR=R ratio measured in m0H ¼ 0:3 T increases linearly At 77 K, the magnetoresistance ratio is about four times larger than that at 300 K Results are discussed in terms of the scattering located at interfaces and the formation of a ferromagnetic state at high-temperature heat treatments
r2002 Elsevier Science B.V All rights reserved
PACS: 75.70.Nt; 75.70.Cn; 75.70.Pa
Keywords: Multilayers; Giant magnetoresistance; Magnetic coupling
Intensive study of spin-dependent transport in
magnetic multilayers has been stimulated by the
discovery of a giant magnetoresistance (GMR)
effect [1] Nowadays, on the basis of this effect,
various types of device such as sensors, read heads,
high-density magnetic random access memories,
etc have been realised, see for example Ref.[2] It
is well known that the origin of GMR is the
spin-dependent scattering of conduction electrons
However, there is controversy on the exact
location of the scattering centers They can occur
at the interfaces and/or in the bulk of the
ferromagnetic layers In addition, with respect to
applications the question arises to what extent the
GMR survives at elevated temperatures In order
to tackle this point, in this paper, we consider the GMR effect of sputtered Fe/Cr multilayers with a fixed Cr individual layer thickness and variable Fe individual layer thickness
The {Fe/Cr}n mutilayers with a number of periods n ¼ 60 and with a fixed Cr individual layer thickness, tCr¼ 2 nm, and a variable Fe individual layer thickness, tFe¼ 1; 2, 3 and 6 nm, were prepared by RF-magnetron sputtering The typical power during sputtering was 100 W and the
Ar pressure was 102mbar The substrates were glass with a nominal thickness of 0.5 mm Both target and sample holder were water-cooled The samples were annealed at temperatures from
TA¼ 200–5001C for 1 h in a vacuum of
5 105mbar The crystalline structure of the
*Corresponding author.
E-mail address: duc@netnam.org.vn (N.H Duc).
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 8 1 - 7
Trang 2films was investigated by X-ray diffraction using
Cu Ka radiation (Siemens D5000 diffractometer)
The magnetoresistance was measured by the
four-point technique in current-in-plane configuration
and longitudinal geometry The magnetisation is
measured in fields up to 1.3 T using a VSM
The room temperature GMR ratio DR=Rð0Þ
ð¼ ðRðHÞ Rð0ÞÞ=Rð0ÞÞ; where Rð0Þ and RðHÞ
are the resistance in zero field and in applied field
m0H; respectively) of the as-deposited Fe/Cr
multilayers is presented inFig 1 The results show
that the initial magnetoresistive susceptibility of
the as-deposited samples is almost constant wR
(=(DR/R)/m0H)E13% T1 The saturation field,
however, increases with decreasing Fe-layer
thick-ness In this way, a maximal magnetoresistance
ratio DR/R of 0.7 % is reached in the sample with
tFe=1 nm This finding shows that the volume
Fe-fraction increases, i.e the volume/interface
frac-tion ratio increases, while the GMR effect decreases
The result seems to support the assumption that
the scattering centers are located at interfaces
Annealing effects on the GMR are presented in
Fig 2 for the Fe/Cr multilayers with tFe¼ 1 nm
The GMR ratio initially increases with increasing
the annealing temperature and reaches a
max-imum value of 2.3% at TA¼ 3501C With further
increasing TA; the GMR ratio decreases, e.g after
annealing at 5001C, the GMR ratio equals 0.3%
only A similar result was observed for samples
with tFe¼ 2 nm Such a tendency of GMR was
recently reported by Hecker et al.[3] These results
can be explained as follows The annealing at
TAp3501C is usually thought to modify the
multilayer structure due to the interdiffusion and the broadening of the interfaces This leads to an increasing interface/volume fraction and then to the enhancement of the GMR The annealing at 5001C, however, is assumed to cause a further breakup of the layers, leading to the formation of heterogeneous structures of small particles This argument was proposed earlier by Flores et al.[4] XRD results of the Fe (1 nm)/Cr (2 nm) multi-layers (Fig 3) strongly support the above argu-ment At TAp3501C, the stability of individual Fe- and Cr-layers is well evidenced by the (1 1 0) BCC-Fe and (110) BCC-Cr reflections At
TA¼ 5001C, however, a broadened Bragg peak
is observed indicating the formation of fine
-0.8
-0.6
-0.4
-0.2
0
-0.3 -0.2 -0.1 0 0.1 0.2 0.3
µo H (T)
6 nm
3 nm
2 nm
tFe = 1 nm
Fig 1 GMR data for Fe/Cr multilayers at 300 K.
Fig 2 GMR data for annealed Fe (1 nm)/Cr films.
Fig 3 XRD patterns of Fe (1 nm)/Cr multilayers.
Trang 3particles of BCC-CrFe phases In this state, the
antiferromagnetic coupling breaks down and the
ferromagnetic one is established (see Fig 4) The
system, thus, can no longer switch between an
antiparallel (ground state) and a parallel aligned
state (applied field)
The GMR curves measured at different tem-peratures are presented inFig 5for a sample with
tFe¼ 1 nm annealed at TA¼ 3501C Note that the GMR ratio measured in m0H ¼ 0:3 T increases linearly with decreasing temperature and reaches a value as large as 7.7% at 77 K This GMR ratio is about four times larger than that at room temperature It may be related to the enhancement
of the antiferromagnetic coupling at lower tem-peratures
In conclusion, our investigation suggests an important role of the scattering at the interfaces It reveals also that the layer structure of sputtered Fe/Cr multilayers remains stable during annealing
up to 3501C At higher temperatures, the multi-layer structure is modified and the onset of ferromagnetic coupling is found, leading to the reduction of the GMR signal
Acknowledgements This work was granted by the State Program for Fundamental Researches of Vietnam, within the project 420.301
References
[1] A Barth !el!emy, A Fert, F Petroff, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 12, Elsevier Science, Amsterdam, 1999, pp 1–96.
[2] K.Y Kim, J.E Evetts, J Magn Magn Mater 198–199 (1999) 92.
[3] M Hecker, D Tietjen, D Elefant, C.M Schneider, J Appl Phys 89 (2001) 7113.
[4] W.H Flores, S.R Teixeira, J.B.M da Cunha, M.C.M Alves, H Tolentino, A Traverse, J Magn Magn Mater.
233 (2001) 100.
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
µ0 H (T)
TA = 30
350
500°C
°C
°C
Fig 4 Hysteresis loops of Fe (1 nm)/Cr samples.
-8
-6
-4
-2
0
-0.3 -0.2 -0.1 0 0.1 0.2 0.3
µo H (T)
T = 300 K
233 K
184 K
145 K
77 K
Fig 5 Low-temperature GMR data of Fe (1 nm)/Cr samples
annealed at 3501C.