Giant Magnetoresistance in Hybrid Magnetic Nanostructures Including Both Layers and Clusters
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P.A.SCHROEDER, P. HOLODY, and R. LOLOEE. Department of Physics and Astronomy, and Center for Fundamental Materials Research. Michigan State University, East Lansing, Ml 48824-1116. J. L. DUVAIL, A. BARTHELEMY, L.B.STEREN, R.MOREL AND A.FERT. Laboratoire de Physique des Solides, Universit6 Paris Sud, 91405 Orsay, France.
ABSTRACT
Early experiments to define oscillations in the CIP magnetoresistance (CIP-MR) of Ag/Co analogous to those for tcu < 5nm in Cu/Co were unsuccessful. The MR in this region was very small. Later experiments by Araki using thin (0.6nm ) Co layers produced much larger MRs and lead us to look at the MRs of similar samples more closely. We conclude that the large MR of such samples is associated with the discontinuous nature of the Co layers. The object of the present paper is to combine the high MR associated with the thin Co layers with the field dependence governed by the magnetization reversal in thick, and magnetically soft permalloy (Py) layers. We have measured the CIP-MR of sputtered samples of the [Co(O. 4 nm)/Ag(tAg)/Py(tpy)/Ag(tAg)]xl 5 system with tAg ranging from 1.05 to 4rnm and with tpy = 2 or 4nm. We obtain MRs at 4.2K as large as 35% in less than 10Oe with slopes as high as 5%/Oe. With CPP measurements slopes as high as 10%/Oe have been obtained. Squid magnetometer measurements indicate that, as the temperature increases, there is a crossover to superparamagnetic behaviour and a resulting gross deterioration of the MR slopes at room temperature. Efforts to increase the room temperature sensitivity are described. Detailed measurements of the CPP-MR of the [Co(0.4nm)/Ag(4nm)/Py(tpy)/Ag(4nm)]x20 series of multilayers are consistent with a two spin band model modified to take account of the granular nature of the Co.
INTRODUCTION
To define the terms Ho, Hp and Hsat we show in Fig. 1 a typical plot of magnetoresistance (MR) versus magnetic field for a Ag(l .8nm)/Co(0.4nm) sample. We define MR(H) as
MR(H) = {R(H) -R(Hsat)}/R(Hsat)
(1)
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Mat. Res. Soc. Symp. Proc. Vol. 384 @1995 Materials Research Society
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Magnetic Field (KOe) Figure 1. Magnetoresistance plotted against magnetic field for a [Ag(1 .8nm)/Co(0.4nm)]x60 sample. The + represents the MR(Ho) point. Hp and Hsat are defined by this figure.
MR(Ho) and MR(Hp) will be the MRs for H = Ho and H = Hp. In this paper we are primarily concerned with standard CIP-MR measurements in which the measuring current is in the plane of the layers. However we will look at some CPP-MR measurements with the measuring current perpendicular to the planes. Early attempts to see the oscillations of MR as a function of tAg, the thickness of the Ag layers, analogous to those seen in the Cu/Co systeml, 2 failed. Most of our work on the Ag/Co 3 system was then pursued on samples with tAg > 5nm which showed a substantial MR. More recently, Araki et al. 4 reported a large and oscillating MR in evaporated Ag/Co multilayers with very thin Co layers, tCo =
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