Finite-size scaling of complex magnetic materials near the phase transition

Abstract

Nano-technologies such as heat-assisted magnetic recording, magnetic particle hyperthermia and skyrmion racetrack memory have been developing rapidly in recent years. However, there have been a number of issues relating to the finite-size of materials relevant to prospective nano-technologies. FePt is the leading candidate material in the development of heat-assisted magnetic recording. In recent years, there have been conflicting reports of the phase transition behaviour and critical exponents of FePt which have been attributed to its complex, long-ranged internal interactions. This thesis identifies anisotropy as the cause of the inconsistent measurements of the critical exponents, and introduces a two-dimensional finite-size scaling framework which is capable of overcoming the anisotropic interactions of FePt. Using this new technique, the critical exponents of FePt are shown to be those of the Heisenberg model. Granular technologies, such as heat-assisted magnetic recording and magnetic particle hyperthermia, have been shown to have a distribution of Curie temperatures. Identifying the Curie temperature distribution is an important step in accelerating the development of granular technologies which operate across the magnetic phase transition. This thesis presents a new characterisation technique which can be used to identify the parameters of the Curie temperature distribution. This technique only requires measurements of the temperature dependent magnetisation, realisable from standard magnetometry. Other materials, such as MnSi and FeGe, are being targeted towards applications such as skyrmion racetrack memory. The internal magnetisation of these helimagnetic materials has a tendency to twist, forming complex topological structures known as skyrmions. The finite-size effects of such materials is little understood and work is needed to quantify them. This thesis presents a qualitative exploration of the finite-size effects of helimagnetic materials. We demonstrate that both finite-size and choice of experimental protocol can significantly impact on the extent of the skyrmion phase. This thesis also identifies the critical exponents of the Dzyaloshinskii-Moriya interaction by using finitesize scaling. This is a key step towards proper finite-size scaling of general helimagnetic materials in the future.

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Identifiers

ORCID for Ondrej Hovorka: orcid.org/0000-0002-6707-4325

ORCID for Tim Sluckin: orcid.org/0000-0002-9163-0061

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