Multiphysics simulations of magnetic nanostructures

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 finding 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 1011 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