Exchange coupling and giant magnetoresistance in nickel-iron/copper multilayers
Pettit, Kevin
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https://hdl.handle.net/2142/22249
Description
Title
Exchange coupling and giant magnetoresistance in nickel-iron/copper multilayers
Author(s)
Pettit, Kevin
Issue Date
1996
Doctoral Committee Chair(s)
Salamon, Myron B.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Electricity and Magnetism
Physics, Condensed Matter
Engineering, Materials Science
Language
eng
Abstract
In recent years interlayer exchange coupling and giant magnetoresistance (GMR) in transition metal ferromagnet/nonmagnetic multilayers have been extensively studied. Bilinear interlayer exchange favors the collinear alignment of the magnetization of adjacent ferromagnetic layers, oscillates with the thickness of the non-magnetic spacer layer, and has been observed in a large number of magnetic multilayer and trilayer systems. Biquadratic exchange favors the perpendicular alignment of the magnetization of adjacent ferromagnetic layers and has been observed in several magnetic multilayer and trilayer systems. Unlike bilinear exchange, the origins of biquadratic exchange are uncertain. Generally, biquadratic exchange is significantly weaker than bilinear exchange.
The connection between the magnetic configuration, which is determined by the interlayer coupling, and the magnetoresistance is well established. The GMR effect occurs when the antiferromagnetically aligned moments of alternating magnetic layers are brought into alignment by an external magnetic field. This causes a dramatic drop in the electrical resistance. Much of the interest in magnetic multilayers stems from the many possible applications of the GMR effect.
This work is a study of biquadratic coupling and its effects on the magnetic phase diagram and giant magnetoresistance of sputter grown single crystal NiFe/Cu multilayers. It is often difficult to determine the bilinear and biquadratic coupling strengths separately, particularly in samples with pinhole defects in the spacer layers, because the zero field configuration of the moments is not easily determined. However, I will show that under certain conditions biquadratic coupling can lead to configurations in which the net magnetization of the coupled magnetic moments is not aligned parallel to the applied magnetic field. In samples with magnetocrystalline anisotropy, these asymmetric configurations lead to a novel minimum in the GMR at low fields when the field is applied along the hard axis of magnetization. In such cases, the bilinear and biquadratic coupling strengths can be determined from resistance measurements only.
In this work the magnetic phase diagram is calculated for a two layer system with bilinear coupling, biquadratic coupling and magnetocrystalline anisotropy. New asymmetric configurations, some of which cannot be determined analytically, are found. Numerical simulations confirm the contribution of these configurations to the GMR and the accuracy of the interlayer coupling strengths extracted from the magnetoresistance data.
These measurements indicate that the biquadratic coupling can be considerably stronger than predicted theoretically. The data are found to be inconsistent with the intrinsic, loose spin, dipole field, and fluctuation models of biquadratic coupling. To explain the data I propose a new model in which ferromagnetic coupling through pinholes in the spacer layer, combined with the antiferromagnetic bilinear coupling, frustrates the magnetic layers and leads to orthogonal coupling. Unlike the fluctuation model, the pinhole-frustration model allows for very strong biquadratic coupling. Although the pinhole mechanism can lead to biquadratic coupling, closer analysis reveals that the bilinear-biquadratic formalism may break down for strong frustration.
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