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
Personal web site

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

Research

My research is primarily on relaxation and response processes in spin liquids and other systems with geometric frustration. I am generally interested in the dynamical behaviour of topological defects in all types of systems both in and out-of equilibrium. My work 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 has primarily been on classical spin ice materials, such as the rare earth pyrochlores Dysprosium Titanate (Dy₂Ti₂O₇) and Holmium Titanate (Ho₂Ti₂O₇). These are topological magnetic systems that host fractionalised excitations in the form of emergent magnetic monopoles. I study the dynamics of these monopoles. A particular aim has been to explain the anomalous magnetic noise observed in Dysprosium Titanate, something we were recently able to do through the discovery of emergent dynamical fractals. We are currently searching for other systems where dynamical fractals may appear. I am also actively investigating the behaviour of magnetic monopoles in spin ice out-of-equilbrium.

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.

Featured Publications

Dynamical fractal and anomalous noise in a clean magnetic crystal. JN Hallén et al., Science 378 1218 (2022)
Structural magnetic glassiness in spin ice Dy₂Ti₂O₇. AM Samarakoon et al., Phys. Rev. Research 4 033159 (2022)
Anomalous magnetic noise in an imperfectly flat landscape in the topological magnet Dy₂Ti₂O₇. AM Samarakoon et al., Proc. Natl. Acad. Sci. 119 e2117453119 (2022)