Simulations of ferromagnetic nano structures

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Description/Abstract

The magnetic properties of nanometre-scale structures are of fundamental scientific
interest and have the potential to play a major role in future data storage technologies.
In particular, arrays of small magnetic elements, also called bit-patterned media, are one
of the most promising candidates for the future generation of data storage devices.
In this thesis we study potential bit patterned element geometries which are below 1
micrometre in size. Their magnetic behaviour is hard to predict using analytical methods
and computer simulations are the principal tool for in-depth analysis. The relevant micromagnetic
equations are solved using the combined Finite Element/Boundary Element
method, and finite differences.
Patterned media are (quasi) periodic arrangements of identical objects, with each
object typically representing one bit. While one or some of these objects can be simulated
with today’s simulation capabilities, the investigation of arrays with hundreds of objects
requires novel simulation methods.
To deal with such large arrays we introduce and evaluate the new “macro geometry”
approach. In most real samples this is superior to using conventional periodic boundary
conditions as it takes account of the macroscopic shape of the sample.
The micromagnetic simulation package Nmag developed at Southampton has been
extended to provide the macro geometry capabilities, and subsequently used to study
demagnetising effects between the elements of triangular ring arrays. We find that in a
square array of 50-nm size triangular elements these effects are governed by the first and
second nearest neighbours and can be considered negligible when the spacing between the
rings is larger than 30 nm.
We also study the transport properties via the Anisotropic Magneto Resistance (AMR)
signal of connected rings arrays using the multi-physics features of Nmag. The simulations
use a self-consistent approach to determine the AMR values, a technique able to explain
experimental AMR measurements of real structures. We also show how the spatially
varying current distribution affects the computation of the AMR values and found that the
uniform current model, sometimes used in the study of AMR effects, is a very inaccurate
approximation and can easily lead to qualitatively wrong results.