COMBINATORIAL INVESTIGATION OF RARE-EARTH FREE PERMANENT MAGNETS
COMBINATORIAL INVESTIGATION OF RARE-EARTH FREE PERMANENT MAGNETS
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Date
2015
Authors
Fackler, Sean Wu
Advisor
Takeuchi, Ichiro
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Abstract
The combinatorial high throughput method allows one to rapidly study a large
number of samples with systematically changing parameters. We apply this method to
study Fe-Co-V alloys as alternatives to rare-earth permanent magnets. Rare-earth
permanent magnets derive their unmatched magnetic properties from the
hybridization of Fe and Co with the f-orbitals of rare-earth elements, which have
strong spin-orbit coupling. It is predicted that Fe and Co may also have strong
hybridization with 4d and 5d refractory transition metals with strong spin-orbit
coupling. Refractory transition metals like V also have the desirable property of high
temperature stability, which is important for permanent magnet applications in
traction motors.
In this work, we focus on the role of crystal structure, composition, and
secondary phases in the origin of competitive permanent magnetic properties of a
particular Fe-Co-V alloy. Fe38Co52V10, compositions are known as Vicalloys. Fe-CoV
composition spreads were sputtered onto three-inch silicon wafers and patterned
into discrete sample pads forming a combinatorial library. We employed highthroughput
screening methods using synchrotron X-rays, wavelength dispersive
spectroscopy, and magneto-optical Kerr effect (MOKE) to rapidly screen crystal
structure, composition, and magnetic properties, respectively. We found that in-plane
magnetic coercive fields of our Vicalloy thin films agree with known bulk values
(300 G), but found a remarkable eight times increase of the out-of-plane coercive
fields (~2,500 G). To explain this, we measured the switching fields between in-plane
and out-of-plane thin film directions which revealed that the Kondorsky model of
180° domain wall reversal was responsible for Vicalloy’s enhanced out-of-plane
coercive field and possibly its permanent magnetic properties. The Kondorsky model
suggests that domain-wall pinning is the origin of Vicalloy’s permanent magnetic
properties, in contrast to strain, shape, or crystalline anisotropy mechanisms
suggested in the literature. We also studied the thickness dependence of an Fe70Co30-
V thin film library to consider the unique effects of our thin film libraries which are
not found in bulk samples. We present results of data mining of synchrotron X-ray
diffraction data using non-negative matrix factorization (NMF). NMF can
automatically identify pure crystal phases that make up an unknown phase mixture.
We found a strong correlation between magnetic properties and crystal phase quantity
using this valuable visualization.
In addition to the combinatorial study, this dissertation includes a study of
strain controlled properties of magnetic thin films for future applications in random
access memories. We investigated the local coupling between dense magnetic stripe
domains in transcritical Permalloy (tPy) thin films and ferroelectric domains of
BaTiO3 single crystals in a tPy/BaTiO3 heterostructure. Two distinct changes in the
magnetic stripe domains of tPy were observed from the magnetic force microscopy
images after cooling the heterostructure from above the ferroelectric Curie
temperature of BaTiO3 (120°C) to room temperature. First, an abrupt break in the
magnetic stripe domain direction was found at the ferroelectric a-c-domain
boundaries due to an induced change in in-plane magnetic anisotropy. Second, the
magnetic stripe domain period increased when coupled to a ferroelectric a-domain
due to a change in out-of-plane magnetic anisotropy. Micromagnetic simulations
reveal that local magnetic anisotropy energy from inverse magnetostriction is
conserved between in-plane and out-of-plane components.