Layered magnetic structures: magnetoresistance due to antiparallel alignment

Vacuum/volume 41/numbers 4-6/pages 1241 to 1243/1990

0042-207X/90S3.00 + .00 ~) 1990 Pergamon Press plc

Printed in Great Britain

Layered magnetic structures: magnetoresistance due to antiparallel alignment J B a r n a ~ * , A Fuss, R E C a m l e y t ,

U Walz, P Griinberg

W Z i n n , Kernforschungsanlage, IFF,

and

Postfach 1913, 51 70-JOllch, FRG

Magnetoresistance in epitaxial and polycrystalline ( F e - C r ) , - F e multilayers with n = 1, 2 and 4, and in C o - A u - C o sandwiches has been investigated experimentally and theoretically. A significant increase of the resistance has been observed when the magnetizations of the ferromagnetic films rotate from parallel to antiparallel alignment.

1. Introduction

Investigations of electron transport properties of magnetic layered systems ~-6 have revealed very interesting and attractive features resulting from the interplay between electronic and magnetic properties. It has been found recently that the resistance of F e - C r layered structures with an effective antiferromagnetic interlayer coupling decreases when the film magnetizations are forced to rotate from the antiparallel to parallel alignment due to an external field ]-4. The relative decrease of the resistivity was about 1.5% in sandwiches at room temperature 2 and about 50% in superlattices at liquid helium temperature ~. A simple theoretical explanation of this effect has been proposed 5, which is based on the phenomenological Fuchs-Sondheimer theory with spin-dependent interface electron scattering. Here, we present new experimental data on the magnetoresistance in epitaxial and polycrystalline ( F e - C r ) , - F e layered structures with n = 1, 2 and 4. Apart from this we present also results on the magnetoresistivity in C o - A u - C o double layers with the Au interlayer thick enough to exclude any exchange coupling between the Co films, and where the antiparallel alignment was achieved by different coercive fields of both Co films. Experimental data on the temperature dependence of the effect are compared with appropriate theoretical calculations.

along the strip. An external magnetic field was applied in the film plane and parallel to the easy direction (and consequently parallel to the flowing current). In Figure l we present temperature dependence of the relative change of the resistivity at the transition from antiparallel (resistivity p T~) to parallel (resistivity p TT) alignment of the Fe film magnetizations. As one can see, the effect increases with decreasing temperature and with increasing number of the Cr interlayers. The effect has also been investigated in polycrystalline F e - C r layered structures grown on SiO2 substrates. The magnetoresistivity had the same qualitative features as in the epitaxial F e - C r structures. The relative effect, however, was a little smaller. 3. Magnetoresistivity in the C o - A u - C o system

In the F e - C r layered structures described above the antiparallel alignment was induced by an antiferromagnetic exchange coupling between the neighbouring Fe layers across the Cr interlayer. One can also expect similar effects in double

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The ( 1 2 F e - 1 C r ) , - 1 2 F e samples (layer thicknesses in nm) with n = 1, 2 and 4 have been evaporated on [ll0]-oriented GaAs. The Fe layers then grow parallel to the (110) atomic planes. The magnetic easy axis is in the film plane and is parallel to the [100]-direction, whereas the in-plane hard axis is normal to the easy direction. To measure resistivity each sample was prepared in the shape of a long strip with the easy axis

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*Permanent address: Institute of Physics, Technical University, Piotrowo 3, 60-965 Poznafi, Poland. tPermanent address: Institute of Physics, University of Colorado, Colorado Spings, CO, 80933-7150 USA.

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d Barnag e t al: Layered magnetic structures

layers with no antiferromagnetic coupling between the ferromagnetic layers but with the antiparallel alignment obtained by different coercive fields of both magnetic films. To investigate this problem we prepared C o - A u - C o double layers with the Au interlayer thick enough to exclude any exchange coupling between the Co films. The samples were grown on (111)Si. X-ray analysis showed that both Co films were polycrystalline. The outer Co film was oxidized and the samples were cooled in a magnetic field. The CoO overlayer then produces a unidirectional exchange anisotropy field in the outer Co film. In Figure 2(a) the MOKE hysteresis loop obtained at T = 130 K is shown for the sample I0 C o - 6 A u - 1 0 Co. Due to the unidirectional anisotropy field created by the CoO overlayer, there is a range of magnetic fields where the magnetizations of the two Co layers are aligned antiparallel as indicated by arrows in Figure 2(a). In Figure 2(b) the resistance trace by scanning through the hysteresis loop is shown for T = 80 K and for a magnetic field applied along the strip. The resistivity increases considerably when the antiparallel alignment is achieved. The relative change of the resistivity is now about 6% at low temperature. 4. Theoretical description Consider a layered structure consisting of ferromagnetic layers of thickness d (magnetized in the film plane) and separated I

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from each other by a non-magnetic spacer of thickness do. Suppose that an electric field E is applied in the film plane along the x axis of the coordinate system, and the z axis of this system is normal to the film plane. Let us assume that the angle between the magnetizations of neighbouring ferromagnetic layers is arbitrary and the magnetizations are spatially uniform inside each layer. To describe electron transport in such systems we extend the Fuchs-Sondheimer theory by including spin-dependent electron scattering at each interface between ferromagnetic and nonmagnetic material (we neglect the magnetic properties of Cr). We also assume that this scattering has the same features as the electron scattering in the corresponding alloy, where the electron transport is described by a two-current model 7 which takes into account a difference in scattering rates for spin-up and spindown electrons. Electric current in each layer is determined by the corresponding distribution function for spin-up and spin-down electrons, frt*)(z, b) = fo(b) + gt<*)(z, t~) where fo(t~) is the equilibrium distribution function and gr(~)(z, t~) is a small contribution induced by external fields. Inside each layer the Boltzmann equation for gt(*)(z, v0 can be written in the form:

5. Comparison of theorectical and experimental results The basic features of the effect, which have been found experimentally, are: (i) the effect increases with decreasing tempera-

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Figure 3. Theoretical (line) and experimental (points) results for the relative magnetoresistivity effect in the (12 Fe-1 Cr)4-12 Fe structure vs the electron mean free path in bulk Fe. the parameters assumed in numerical calculations are P = 0.2 and D T= 0.515.

the conductivities of Fe and Cr results from a difference in the corresponding mean free paths. Since the conductivity of Cr is about twice as high as of Fe, we assumed 20/2 = 2, where 20 and 2 are the intrinsic electron mean free paths in Cr and Fe, respectively. Besides, we assumed DT/D ~ = 6 according to the data obtained for F e - C r alloys 7. The appropriate results for the multilayer with four Cr interlayers are shown in Figure 3, where the solid line is a theoretical curve and the points represent the corresponding experimental data taken from Figure 1 and adapted to the 2 scale on taking into account the temperature dependence of the electron mean free path in Fe. This temperature dependence of 2 has been obtained from the best fit of the theoretically calculated resistivity at parallel alignment and magnetoresistivity to the appropriate experimental data, as described elsewhere 4.

Acknowledgement The work of R E C was partially supported by the U S A r m y Research Office under Grant N o DAALO3-88-K-0061.

References ture. (ii) It increases with decreasing film thickness. (iii) It increases also with increase in the number of interlayers. All these features can be used to verify theoretical predictions. However, the most reliable one seems to be the temperature dependence of the effect. This results from the fact that the appropriate experiments are performed on a single sample. Using the other features one has to compare data obtained from different samples. The basic parameters, however, are not strictly reproducible and have some statistical distribution. In the numerical calculations we assumed p). = constant and for Fe we used p = 9.7/~f~ cm and 2 = 20 nm at room temperature 6. For simplicity we assumed that the whole difference in

t M N Baibich, J M Broto, A Fert, F Nguyen Van Dau, F Petroff, P Etienne, G Creuzet, A Friederich and J Chazelas, Phys Rev Lett, 61, 2472 (1988). 2 G Binasch, P Griinberg, F Saurenbach and W Zinn, Phys Rev, B39, 4828 (1989). 3 F Saurenbach, J Barna~, G Binasch, M Vohl, P Griinberg and W Zinn, Thin Solid Films, 175, 317 (1989). 4j BarnaL A Fuss, R E Camley, P Grunberg and W Zinn, To be published. 5 R E Camley and J Barnag, Phys Rec Lett, 63, 664 (1989). 6 M Rubinstein, F J Rachford, W W Fuller and G A Prinz, Phys Rev, B37, 8689 (1988). 7 I A Campbell and A Fert, In Ferromagnetic Materials (Edited by E P Wohlfarth), Vol 3, p 747. North-Holland, Amsterdam (1982).

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