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FeNi sandwiches
Journal of Magnetism and Magnetic Materials 148 (1995) 319-320
~
ELSEVIER
Journalof magnetism and magnetic materials
Spin-valve effect in non-coupled FeNi/Cu/FeNi sandwiches M. Bauer, M. Matner *, H. Hoffmann Institut f~r Angewandte Physik, Lst. Hoffmann, Universitiit Regensburg, Universitiitsstrafle 31, 93040 Regensburg, Germany Abstract Sandwiches containing FeNi and Cu layers were grown by rf and de sputtering in a magnetic field of 30 On. After deposition of the first FeNi and Cu layers the sample was rotated 90 ° in the magnetic field. Due to the relatively thick Cu interlayer (60 A,) ~ve get an uncoupled system with two different easy axis orientations in the upper and lower FeNi layers. M O K E and MR measurements show expected behaviour, although the spin-valve MR effects are relatively small and of the order of the AMR. A simple theoretical separation of the two MR effects was successful.
1. Introduction Metallic multi.layers comprising ferromagnetic transition metal layers (Fe, Co, Ni or their alloys) separated by nonferromagnetie metallic layers sometimes show large magnetoresistanee effects, often called giant magnetoresistance (GMR). The G M R is associated with a change in the relative orientations of the magnetization of adjacent ferromagnetic layers when an external field is applied. The changes in the relative orientations of the magnetizations can be achieved in different ways: systems with strong antiferromagnetic coupling ( C o / C u [1], F e / C r ) , and systems without coupling through the spacer layer (FeNi/Cu/Co, F e N i / C u / F e N i / F e M a [2]). Systems without coupling exhibit large MR effects (,-, 10%) in relatively small magnetic fields (,,~ 50 Oe). In this paper we describe an approach to obtain an angle between the relative orientations of the magnetization in two different ferromagnetic layers by changing the relative orientation between the easy axis (EA) in the upper and lower layers in a sandwich ( F e N i / C u / F e N i ) by 90 °. Thus we get a system where in a certain magnetic field one of the FeNi layers has changed the magnetization direction (external field parallel to this Elk), whereas the other film (external field perpendicular to that EA) still has a magnetic component in the original direction.
p,ressure 8 × i0 - 3 mbar, with sputtering rates of 0.7-1.5 A / s . A nna~,qnetic field of 30 Oe was applied on the substrate position during deposition to fix the direction of the EA in lhe FeNi films. On the basis of previous investigations the thickness of the Cu spacer was chosen to be 60 A to ensure that the two FeNi layers were completely deeoupled. The thielcnesses of the FeNi films were varied between 50 and 150 ,An After sputtering lhe first FeNi layer and the Cu spacer layer, the samples veere rotated 90 ° in the magnetic field to get crossed EAs in the upper and lower layers (see Fig. 1).
3. Results Magnetic investigations (MOKE and VSM) show the expected hysteresis loops (Fig. 2). The coercitivity H e and
magnets " ' * ~
2. Preparation The films were deposited by ff and dc sputtering onto glass substrates; the base pressure was 1 × 10 -7 tabor,
"Corresponding author. Fax: +49-941-9434544; michaeLmatner@physik'nni'regensburg'de"
c-mail:
Fig. 1. Deposition and resulting orientation of the EA in the sandwich.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 5 3 - 7
M. Bauer et al. /Journal of Magnetism and Magnetic Materials 148 (1995) 319-320
320 i)
it)
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1
20-2o-,o
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magnetic field [Oe] magnetic field [Oel Fig. 2. (i)"Theoretical: (a) EA, (b) HA, (c) combin..ation. (it) Experimental: (a) MOKE and (b) VSM. anisotropy field H k are H c ~- 1 0 e and H k = 6 0 e , respectively. MR measurements (convention four-point geometry) show that two different contributions must be considered, namely the common anisotropi¢ rnagnetoresistance (AMR) and the spin-valve effect (SV effec0. The AMR is sensitive to the relative orientation between the applied field and the direction of the current. With two different measurement configurations (H.I_ I ~ trans, H II ! ,--' long) it is possible to separate the two effects. Applying a simple parallel circuit model, we get
AR/R = (
,n/R)sv + (
where ( A / ~ / R ) s v is the magaetoresistance due to the different orientations of the magnetizations in each Fel'~i layer, and (AR/R)Aa~a is the common anisotropic magnetoresistance:
I(A S/R)A ,R I = ½[( A S / S ) (A s/R)s
=
(An/n)T
- (a S/R;'°N°], "s
+
Table 1 SVMR and A1VIRfor different thicknesses of the FeNi layers tF©Ni(A) 50 70 90 110 150
I(AR/R)AMR I(%) 0.08 0.10 O.16 0.18 0.24
(AR/R)sv (%) 0.13 0.16 0.20 0.20 0.20
Fig. 3. Miero~on cross section of the sample FeNi(50 A)/Cu(60 X)/FeNi(50 A)Cu(60 A)/FeNi(50 A).
The maximum value of the SV effect in our sandwiches is 0.20%, and the m a x i m u m value of the AMR is 0.24% (see Table 1). What are the reasons for the small MR effect? First, the relatively thick Cu spacer layer diminishes the SV effect due to shunting. Second, in our system we never get a total antiparallel orientation of the magnetizations in the two different FeNi layers. Third, this small SV effect (compared with the result in Ref. [3]) may be understood by regarding some structural properties. TIEM investigations show clearly the layoered structure but also very small crystallites ( ~ = 20 A) which are limited to each single layer (Fig. 3). Therefore there is a large contribution o f spin-independent grain boundary scattering which reduces the spin.dependent SV effect. Further investigations with buffer layers (Fe, Cr) and different substrates will be conducted to get better structural properties (larger column-like crystaUites throughout the whole film) and so higher SV effects. Also, multilayered structures with crossed uniaxial anisotropies in the different FeiNt layers will be prepared. References
[1] S.S.P. Parldn, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [2] Dieny et al., Phys. Rev. B 63 (1991) 1297, [3] T. Valet, J.C. Jacquet, P. Galtier, J.M, Coutellier, L.G. Peireira, R. Morel, D. Lottis and A. Fert, Appl. Phys. Lett. 61 (1992) 3187.
~
ELSEVIER
Journalof magnetism and magnetic materials
Spin-valve effect in non-coupled FeNi/Cu/FeNi sandwiches M. Bauer, M. Matner *, H. Hoffmann Institut f~r Angewandte Physik, Lst. Hoffmann, Universitiit Regensburg, Universitiitsstrafle 31, 93040 Regensburg, Germany Abstract Sandwiches containing FeNi and Cu layers were grown by rf and de sputtering in a magnetic field of 30 On. After deposition of the first FeNi and Cu layers the sample was rotated 90 ° in the magnetic field. Due to the relatively thick Cu interlayer (60 A,) ~ve get an uncoupled system with two different easy axis orientations in the upper and lower FeNi layers. M O K E and MR measurements show expected behaviour, although the spin-valve MR effects are relatively small and of the order of the AMR. A simple theoretical separation of the two MR effects was successful.
1. Introduction Metallic multi.layers comprising ferromagnetic transition metal layers (Fe, Co, Ni or their alloys) separated by nonferromagnetie metallic layers sometimes show large magnetoresistanee effects, often called giant magnetoresistance (GMR). The G M R is associated with a change in the relative orientations of the magnetization of adjacent ferromagnetic layers when an external field is applied. The changes in the relative orientations of the magnetizations can be achieved in different ways: systems with strong antiferromagnetic coupling ( C o / C u [1], F e / C r ) , and systems without coupling through the spacer layer (FeNi/Cu/Co, F e N i / C u / F e N i / F e M a [2]). Systems without coupling exhibit large MR effects (,-, 10%) in relatively small magnetic fields (,,~ 50 Oe). In this paper we describe an approach to obtain an angle between the relative orientations of the magnetization in two different ferromagnetic layers by changing the relative orientation between the easy axis (EA) in the upper and lower layers in a sandwich ( F e N i / C u / F e N i ) by 90 °. Thus we get a system where in a certain magnetic field one of the FeNi layers has changed the magnetization direction (external field parallel to this Elk), whereas the other film (external field perpendicular to that EA) still has a magnetic component in the original direction.
p,ressure 8 × i0 - 3 mbar, with sputtering rates of 0.7-1.5 A / s . A nna~,qnetic field of 30 Oe was applied on the substrate position during deposition to fix the direction of the EA in lhe FeNi films. On the basis of previous investigations the thickness of the Cu spacer was chosen to be 60 A to ensure that the two FeNi layers were completely deeoupled. The thielcnesses of the FeNi films were varied between 50 and 150 ,An After sputtering lhe first FeNi layer and the Cu spacer layer, the samples veere rotated 90 ° in the magnetic field to get crossed EAs in the upper and lower layers (see Fig. 1).
3. Results Magnetic investigations (MOKE and VSM) show the expected hysteresis loops (Fig. 2). The coercitivity H e and
magnets " ' * ~
2. Preparation The films were deposited by ff and dc sputtering onto glass substrates; the base pressure was 1 × 10 -7 tabor,
"Corresponding author. Fax: +49-941-9434544; michaeLmatner@physik'nni'regensburg'de"
c-mail:
Fig. 1. Deposition and resulting orientation of the EA in the sandwich.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 5 3 - 7
M. Bauer et al. /Journal of Magnetism and Magnetic Materials 148 (1995) 319-320
320 i)
it)
.
-2o
.
.io
.
.
o
,o
1
20-2o-,o
o
io
20
magnetic field [Oe] magnetic field [Oel Fig. 2. (i)"Theoretical: (a) EA, (b) HA, (c) combin..ation. (it) Experimental: (a) MOKE and (b) VSM. anisotropy field H k are H c ~- 1 0 e and H k = 6 0 e , respectively. MR measurements (convention four-point geometry) show that two different contributions must be considered, namely the common anisotropi¢ rnagnetoresistance (AMR) and the spin-valve effect (SV effec0. The AMR is sensitive to the relative orientation between the applied field and the direction of the current. With two different measurement configurations (H.I_ I ~ trans, H II ! ,--' long) it is possible to separate the two effects. Applying a simple parallel circuit model, we get
AR/R = (
,n/R)sv + (
where ( A / ~ / R ) s v is the magaetoresistance due to the different orientations of the magnetizations in each Fel'~i layer, and (AR/R)Aa~a is the common anisotropic magnetoresistance:
I(A S/R)A ,R I = ½[( A S / S ) (A s/R)s
=
(An/n)T
- (a S/R;'°N°], "s
+
Table 1 SVMR and A1VIRfor different thicknesses of the FeNi layers tF©Ni(A) 50 70 90 110 150
I(AR/R)AMR I(%) 0.08 0.10 O.16 0.18 0.24
(AR/R)sv (%) 0.13 0.16 0.20 0.20 0.20
Fig. 3. Miero~on cross section of the sample FeNi(50 A)/Cu(60 X)/FeNi(50 A)Cu(60 A)/FeNi(50 A).
The maximum value of the SV effect in our sandwiches is 0.20%, and the m a x i m u m value of the AMR is 0.24% (see Table 1). What are the reasons for the small MR effect? First, the relatively thick Cu spacer layer diminishes the SV effect due to shunting. Second, in our system we never get a total antiparallel orientation of the magnetizations in the two different FeNi layers. Third, this small SV effect (compared with the result in Ref. [3]) may be understood by regarding some structural properties. TIEM investigations show clearly the layoered structure but also very small crystallites ( ~ = 20 A) which are limited to each single layer (Fig. 3). Therefore there is a large contribution o f spin-independent grain boundary scattering which reduces the spin.dependent SV effect. Further investigations with buffer layers (Fe, Cr) and different substrates will be conducted to get better structural properties (larger column-like crystaUites throughout the whole film) and so higher SV effects. Also, multilayered structures with crossed uniaxial anisotropies in the different FeiNt layers will be prepared. References
[1] S.S.P. Parldn, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [2] Dieny et al., Phys. Rev. B 63 (1991) 1297, [3] T. Valet, J.C. Jacquet, P. Galtier, J.M, Coutellier, L.G. Peireira, R. Morel, D. Lottis and A. Fert, Appl. Phys. Lett. 61 (1992) 3187.