TCM
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Jonathan Nilsson Hallén

 Jonathan Nilsson Hallén

Jonathan Nilsson Hallén

Member of Downing College
PhD student in Prof Castelnovo's group

Office: 540 Mott Bld
Phone: +44(0)1223 3 37354
Email: ejn41 @ cam.ac.uk

TCM Group, Cavendish Laboratory
19 JJ Thomson Avenue,
Cambridge, CB3 0HE UK.

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Research

My research generally concerns the study of relaxation and response processes in spin liquids and other systems with geometric frustration. This is done using a combination of effective theoretical modelling and Monte Carlo simulations. The aim is to make practical predictions that can be compared to experimental results, or used to guide future experiments.

My focus is primarily on classical spin ice materials, such as the rare earth pyrochlores Dysprosium Titanate (Dy2Ti2O7) and Holmium Titanate (Ho2Ti2O7). In these materials a strong local field anisotropy gives the magnetic moments an Ising symmetry. The combination of ferromagnetic interactions between these Ising spins and the topology of the lattice make these systems highly frustrated, with an extensive number of degenerate ground states that satisfy the so-called ice rules. The excitations in spin ice are emergent magnetic monopoles, and I study the dynamics of these monopoles, making predictions about the experimentally observable magnetic noise that they generate. Recent experiments on Dysprosium Titanate crystals have found that the magnetic noise in the spin ice phase is anomalous, something that earlier models failed to predict. A key aim of my current research is to find the cause of this anomalous behaviour.

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In Plain English

Permanent magnets, like the ones that go on a refrigerator, get their magnetic properties from the alignment of the magnetic moments of the particles that make up the material. In some materials the positioning of the magnetic particles and the nature of their interactions with each other are incompatible, in the sense that not all interactions between magnetic moments can be satisfied simultaneously. This phenomenon is called frustration, and generally leads to lack of magnetic order even at low temperatures.

My research focuses on the magnetic dynamics of different frustrated materials. In any material (that is not at absolute zero temperature) thermal fluctuations cause the magnetic state to change over time. By measuring these changes, we can improve our understanding of the microscopic interactions in the material. Furthermore, it is important to understand how a material's state evolves under different conditions if we want to make use of that material in any technological applications. In my work, I use a combination of theoretical calculations and computer simulations to model the magnetic dynamics of different frustrated systems, and then compare the findings to experimental results obtained by others.