Mechanism for the Magnetoresistance of Pure Bulk Ferromagnets and Composite Thin Film Structures
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MECHANISM FOR THE MAGNETORESISTANCE OF PURE BULK FERROMAGNETS AND COMPOSITE THIN FILM STRUCTURES
MARY BETH STEARNS Department of Physics and Astronomy, Arizona State University, Tempe, Arizona, 85287
ABSTRACT A unified explanation is given of the long-standing question of the origin of the lowfield magnetoresistance, MR, behavior inpure ferromagnetics and the large magnetoresistance effects seen in magnetic layered and granular structures. It is shown that the main contributions to these effects are due to the scattering that occurs at the magnetic boundaries between non-aligned magnetic regions. This scattering occurs because the predominant conduction electrons in 3d ferromagnetics are the highly polarized itinerant d electrons. As a result of this polarization the majority-band d electrons are strongly reflected at an antiparallel magnetic boundary due to a lack of available states for occupancy. The traversing electrons are further scattered as they cross the boundary due to a discontinuity in the potential caused by the interchange of their kinetic and exchange energies at the boundary. Expressions for the magnetoresistance due to these scattering mechanisms are derived and shown to describe very well the wide variety of magnetoresistance values and other features found in the literature for both pure Fe and nano-structures of Fe or Co with non-magnetic materials. The MR magnitude is seen to vary inversely with the domain size. Thus the domain size and sample purity are seen to be the main factors that determine the magnitude of the MR effect in pure ferromagnets. The large MR values seen in layered and granular magnetic structures arise from the small effective domain size attainable in these structures. This is achieved by introducing a non-magnetic material into these structures which allows the effective domain size to be decreased from the micron range of the pure ferromagnetic elements into the nanometer range.
INTRODUCTION The low field magnetoresistance (MR) of the 3d ferromagnetic elements was extensively studied in the 1960's and 1970's. It was found that at liquid He temperatures Fe had huge magnetoresistance effects1 -6. For example, at 4.2K, a transverse MR effect of magnitude 15 was reported in Ref. 1 and longitudinal magnitude of 7.8 along the direction was reported in Ref. 2. It was found' that the largest MR values were obtained in the purest samples. More recently several series of experiments in various laboratories have found that sandwiches 7-11or granular'2-15 nano-structures containing magnetic and non-magnetic materials have "large" low-field magnetoresistance values ranging up to -1. Several highly parameterized versions of a theoretical explanation 7" 6 -17 based on the assumption that the spin-up and spin-down electrons of the ferromagnet have appreciably different scattering amplitudes for solute atoms of the non-magnetic element, have been proposed to explain these results. However, the spin-dependent scattering rates used in these calculations for the solutes in Fe have recentl
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