Franchin , Matteo
Multiphysics simulations of magnetic nanostructures.
University of Southampton, School of Physics and Astronomy,
Multiphysics simulations of magnetic nanostructures
by Matteo Franchin
In recent years the research on magnetism has seen a new trend emerging, characterised
by considerable effort in developing new nanostructures and nding new ways to control
and manipulate their magnetisation, such as using spin polarised currents or light
pulses. The field of magnetism is thus moving towards the multiphysics direction, since
it is increasingly studied in conjunction with other types of physics, such as electric and
spin transport, electromagnetic waves generation and absorption, heat generation and
diffusion. Understanding these new phenomena is intriguing and may lead to major
technological advances. Computer simulations are often invaluable to such research,
since they offer a way to predict and understand the physics of magnetic nanostructures
and help in the design and optimisation of new devices.
For the preparation of this thesis the Nmag multiphysics micromagnetic simulation
package has been further developed and improved by the author. The software has also
been extended in order to model exchange spring systems. Using Nmag, we carried
out micromagnetic simulations in order to characterise the magnetisation dynamics in
exchange spring systems and derived analytical models to validate and gain further
insight into the numerical results. We found that the average magnetisation moves
in spiral trajectories near equilibrium and becomes particularly soft (low oscillation
frequency and damping, high amplitude) when the applied field is close to a particular
value, called the bending field.
We studied spin transport in exchange spring systems and investigated new geometries
and setups in order to maximise the interaction between spin polarised current
and magnetisation. We found that by engineering a trilayer exchange spring system in
the form of a cylindrical nanopillar, it is possible to obtain microwave emission with
frequencies of 5-35 GHz for applied current densities between 0.5-2.0 x 10 (superscript 11) A/m2 and
without the need for an externally applied magnetic field. We proposed a one dimensional
analytical model and found a formula which relates the emission frequency to
the geometrical parameters and the current density.
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