I completed my M.Phys. in Physics at the University of Warwick in 2016. I am currently researching the experimental properties of permanent magnetic materials under the joint supervision of Rachel Edwards and Martin Lees. My project can broadly be split into two parts- an ultrasound part, and a magnetism part.
Ultrasound can track the changes in elastic constant of a material as the temperature is changed or when the material is subjected to a ramping magnetic field. Sudden changes in elastic constant indicate the material has undergone a phase change. This technique is cheaper and more accessible than standard x-ray or neutron scattering experiments. I am currently working on BCC compounds of FeGa, an alloy notable for its dramatically enhanced magnetostrictive properties. Piezoelectric transducers made of quartz are often used to generate and detect ultrasound signals. More exciting are Electromagnetic Acoustic Transducers (EMATs) which induce ultrasonic stress waves in the material via magnetic interactions between the sample and a variable field created by a coil. Experiments were carried out on Fe75Ga25 and Fe81Ga19 using a spiral EMAT in a superconducting cyrostatic magnet that allowed the elastic constants of the materials to be measured as a function of temperature and biasing field. As these materials are themselves magnetic, the propagation of ultrasonic stress waves created by the EMAT is complicated and interesting, revealing detail about the magnetic anisotropy, magnetostrictive and structural properties of these samples. This work was intended to demonstrate the potential of ultrasonic detection methods in materials analysis. Further insite into these samples can be gained by using a different geometry EMAT. A "racetrack" EMAT is being developed to be used next on these samples. This will isolate the ultrasound generation to a particular crystal direction, and will provide more insight into the behaviour of the elastic constants and the anisotropy of these materials.
The "magnetism" part of my PhD is currently looking at Ni-doped compounds of GdCo5 (GdCo5-xNix), as part of the PRETAMAG collaboration study into rare earth transition metal magnets . The Ni substitionally replaces the Co on the sublattice, and changes properties such as the saturation magnetisation and coercivity. I have created polycrystalline samples using an arc furnace for x=0 to x=5 in approximately 0.5 increments. The samples were stuided under powder x-ray diffraction to confirm the formation of a single phase (complicated by this material's tendency to eutectically decompose at 775 degrees C) and Rietveld refinement was carried out to find the lattice parameters as a function of Ni content. The Ni causes the contraction of the unit cell, mostly in the ab plane, but to a smaller extent in the c axis as well. Metallographic slides were produced and studied under optical and SEM microscopy. These methods allowed secondary phases to be observed, even if the powder x-ray diffraction pattern appeared clean (the pXRD method can only detect phases contributing around 5% or above to the signal). EDS analysis allowed this secondary phase to be identified as Gd2(Co, Ni)7, which is consistent with the behaviour of pure GdCo5. Samples with Ni content x>2.5 contained no secondary phase, possibly owing to the stability of GdNi5 compared to GdCo5. Once these structures were proved to be sufficiently phase pure the magnetic properties were studied in a SQUID magnetometer. The magnetisation and coercivty show an interesting dependence on Ni content, with magnetisation (coercivtiy) having a minimum (maximum) at approximately x=1. Theoretical calculations using the disordered local moment formulation of DFT produced results agreeing with this finding.
It is hoped that the collaboration can make or acquire some single crystals, in which case I could carry out ultrasound studies on them, linking the two disparate elements of my PhD so far. The final aim is to use a magnetic material produced and characterised by the PRETAMAG collaboration as the pernament magnet component of an EMAT and to test this EMAT in high temperatures or otherwise hostile environments, where other magnets will stop working.
Quantum Theory of Interacting particles (4th year undergraduate module)
DST module 2:Properties and Charaterisation of Materials
CH934: Focussed academic writing
PX446: Condensed matter physics 2 (4th year undergraduate module)
Demonstrator and marker in the 2nd year Microelectronics Laboratory, PX271.