Antiferromagnetic Spintronics and Magneto-optics

During my PhD in experimental spintronics and magnetooptics at the University of Nottingham, I have experience with many experimental techniques for magnetic characterisation such as transport measurements and magnetooptical measurements. My research focussed on studying CuMnAs as an example of an antiferromagnet (AFM) with promise in spintronic applications with it's relatively low resistance and ability to be switched electrically. My studies can be categorised into three main types of experiment: optical pump probe measurements, magnetoresistive electrical measurements and transport measurements (it would also be possible to break down my research into three major aims: to understand spin flop; to improve switching and electrical readout signals; and to understand the domain structure and domain formation in CuMnAs). Other smaller experiments were also conducted and I also spent a lot of my time assisting other researchers with their experiments while I was waiting for parts or equipment availability.

Spin flop is a sudden reorientation of antiferromagnetic moments which occurs above some threshold field when an external magentic field is applied along the easy axis of the sample. This reproducible and predictable method of reorientation of magnetic moments is ideal for demonstrating the magnetic origin of phenomena which may be applicable to new AFM characterisation techniques. Before studying spin flop using neutron diffraction, the full volume extents of spin flop were not known. By using magnetic neutron diffraction to detect the change in amplitudes of diffraction peaks, the full thickness extent of spin flop was detected as 98 % at 8 T, increasing confidence in the use of spin flop as a test of characterisation techniques.

The optical pump probe experiments were performed with the aim of demonstrating asyncrhonous optical sampling (ASOPS) as a convenient and rapid way of characterising the anisotropy of an AFM sample by using a heating pump pulse to induce demagnetisation in the material which is detected as a reduction in the polarisation angle change induced resulting from the Voigt effect. This same measurement process was also performed in collaboration with TU Dortmund, Germany, using aa delay line pump probe technique which detected spin flop as a change in the magnitude of this demagnetisation related Voigt effect polarisation angle change to unambiguously demonstrate that the signals are of magnetic origin.

The electrical switching possibilities of CuMnAs have been demonstrated previously (todo cite switching paper) and since then, there has been an upsurge in interest in CuMnAs and its possible applications. To this end, many studies have investigated the origins and possible alternative mechanisms for both switching and readout. Typically anisotropic magnetoresistance (AMR) is used as the readout signal to detect magnetic changes induced by current pulses but these signals are small. I performed various switching experiments and also used short-duration high-power pulses with simultaneous measurement to detect larger signal changes with the aim of detecting the steady state anisotropy of a sample without the need for magnetic fields to induce some changes. These studied led to the discovery of a new mechanism capable of generting much larger resistance change signals than expected.

Below are links to a paper summarising the neutron diffraction studies, a review article summarising AFM spintronics and the first paper demonstrating electrical switching and readout using CuMnAs.