Magnetic Domains in Granular GMR Materials

Recently, it has been shown that certain granular ferromagnets exhibit a Giant Magnetoresistance (GMR) comparable to that observed in multilayers. A basic premise in explaining the GMR in these materials is that the material consists of isolated ferromagnetic regions with characteristic diameters of about 10 nm, surrounded by non-magnetic regions. We have found [EPG Pub# 640] that in Co_xAg_1-x alloys the magnetic domains are surprisingly large, about 300-600 nm, which suggest that a significant fraction of the Co does not contribute to the GMR.

This SEMPA image shows low and high magnification views from an as deposited, unannealed Co_0.35Ag_0.65 granular sample provided to us by Prof. C. L. Chien of Johns Hopkins University. The magnetic domains are much larger than the individual Co particles which were independently measured to be 2-3 nm in diameter.

The striped domains have typical dimensions of 300-600 nm which are much larger than the electron mean free paths or spin diffusion lengths involved in GMR. The GMR in these materials therefore results from Co particles that do not participate in the observed domain structure. This result implies that some or perhaps most of the Co does not contribute to the GMR.

Domain Structures in Magnetoresistive Granular Metals

John Unguris
Robert J. Celotta
Daniel T. Pierce

Andrew Gavrin - Indiana University-Purdue University at Indiana
Michael Kelley - NIST
David Tulchinsky - Naval Research Laboratory

John Q. Xiao - Johns Hopkins University
C.L. Chien - Johns Hopkins University

Supported in part by the Office of Naval Research

Online: May 1996
Last Updated: February 2008

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