Freya Johnson

Dr Johnson is an Eric and Wendy Schmidt Research Fellow in the Department of Physics, Imperial College London. She received her PhD from Imperial College London, followed by postdoctoral research at Imperial and at University College London, before moving to the Cavendish Laboratory, Cambridge for an 1851 Research Fellowship. She is Deputy Chair of the AFMNET advisory committee and a member of the Magnetism Committee at the Institute of Physics.

Her research involves atomic-precision growth of antiferromagnetic thin films, advanced electronic and magnetic characterisation, and ultrafast spectroscopy, to develop materials for high speed, energy-efficient computing. These materials naturally operate at terahertz frequencies, one thousand times faster than current technologies, and may be switched using energy-efficient spin current that doesn't produce waste heat. They also host an electron wavefunction phase effect known as Berry curvature, which gives rise to anomalous physical properties not normally expected in antiferromagnets. These properties provide new opportunities to read and write the underlying spin state quickly and easily using a large variety of stimuli such as electric current, heat, light and strain. Her current interests include the use of deep learning to predict thin-film properties from in situ growth diagnostics.

Magnetometry Techniques and Measurements: volumetric vs local probes

Probing the bulk and microscopic magnetic properties of a material is essential to understand complex magnetic phenomena. But under the umbrella term "magnetic characterisation" hides a colossal number of different techniques, each of which requires a certain experimental set-up, and many of which function only for specific types of magnetic ordering. And once the data is collected – how may it be interpreted?

This talk makes a start on cataloguing these techniques, organised around one distinction: volumetric probes measure the total moment of a sample, while local probes map the magnetic field above it. We begin with vibrating sample magnetometry (VSM), used to measure the volumetric magnetisation of a sample, and the typical measurements performed with it – magnetisation as a function of field, M(H), and of temperature, M(T), including zero-field-cooled and field-cooled protocols. We will also cover how this method has been adapted to create the superconducting quantum interference device VSM (SQUID-VSM).

We then turn to magnetic force microscopy (MFM), a powerful tool for mapping magnetic structures and features on the nanoscale, which has been used to image magnetic domains, nanomagnetic arrays and topological features – and its recent evolution, the integration of a diamond tip to create the nitrogen-vacancy scanning magnetometer.

Returning to the question of interpretation, a substantial part of the talk is given to the ways these measurements can mislead: substrate backgrounds, trapped flux mistaken for exchange bias, mounting and contamination artefacts, tip-induced switching in MFM, and the dominance of sample-volume uncertainty in thin-film magnetisation. Examples are drawn from my current work on antiperovskite nitride antiferromagnets, where small net moments and tiny domains make separating signal from artefact challenging.