COMBINATORIAL INVESTIGATION OF RARE-EARTH FREE PERMANENT MAGNETS

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.