Giant MR in Granular Systems Prepared by Melt-Spinning and Sputtering

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GIANT MR IN GRANULAR SYSTEMS PREPARED BY MELT-SPINNING AND SPUTTERING B.DIENY, S.R.TEIXEIRA, B.RODMACQ, A.CHAMBEROD, J.B.GENIN, S.AUFFRET. CEA/D~partement de Recherche Fondamentale sur la Mati~re Condens~e, SP2M-MP, 85X, 38041 Grenoble Cedex, France. P.GERARD CENG, LETI/MEM, 85X, 38041 Grenoble Cedex, France O.REDON, J.PIERRE, R.FERRER, B.BARBARA Laboratoire Louis N~el, CNRS, BP166X, 38042 Grenoble Cedex.G

ABSTRACT We report the observation of giant magnetoresistance in granular systems prepared by either melt-spinning or sputtering. For melt-spun CoCulux alloys, with x varying between 5 and 30%, magnetoresitance amplitudes of 20% in 50 kOe at 5 K were obtained, similar to those reported for the same alloys prepared by sputtering. For sputtered (Nig0Fe 20),Agj_, alloys, three different contributions to the magnetoresistance have been clearly identified: The spin-valve (or giant) magnetoresistance, scattering on magnetic fluctuations, and anisotropic magnetoresistanee. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between NiFe and Ag, and on the temperature. In the third series of samples, sputtered (Co 70Fe 30),Agl x, very large magnetoresistance amplitude has been observed (AR/R as high as 20% at room temperature in 10 kOe and 60% at 10 K). INTRODUCTION Recently, giant magnetoresistance (GMR) has been reported in heterogeneous alloys (CoCu1 l 11,21, CoxAg lx[31, NiFexAgI_xl 4l) prepared by sputtering and also by melt-spinningl 5 l. These alloys comprise two immiscible metallic components, one magnetic, the other non-magnetic. For sputtered samples, depending on the substrate temperature during deposition, the samples are homogeneous metastable alloys (if deposited at liquid nitrogen temperature) or granular alloys consisting of single domain ferromagnetic particles embedded in a metallic matrix. The size of the magnetic particles can be increased by subsequent annealings. By melt-spinning, the size of the particles is obviously much larger than in sputtered samples (especially for samples prepared at LN2 temperature). However, this size is smaller than the optimal size of grains required for largest MR. The physical origin of GMR in granular alloys is the same as in multilayers, namely interplay between neighboring magnetic grains of spin dependent scattering of conduction electrons occurring at interfaces between ferromagnetic and nonmagnetic regions or in the bulk of the single domain magnetic regions. In most cases, electrons having spins antiparallel to the magnetization of the ferromagnetic regions are more strongly scattered than those having spins parallel to the magnetization. So, at low magnetic fields, since the ferromagnetic regions are randomly oriented both spin orientations should experience strong scattering. However, at high magnetic fields, the ferromagnetic regions are aligned parallel to the direction of the applied field. Electrons with spins parallel to the magnetization do not experience the strong scattering at the i