Supervisors: Michael Carpenter (Earth Sciences) and Richard Harrison (Earth Sciences)
Importance of the area of research:
Elasticity and magnetism are the two physical properties of minerals which have provided the most information relating to the structure and geological history of the solid earth. This is reflected in long term but independent research efforts to measure elastic constants, which determine the velocity of seismic waves, and magnetic properties, which determine the natural magnetic remanence of rocks. In the separate (but highly topical) field of the materials physics of multiferroics, it has been rediscovered that the two properties are fundamentally linked via their shared elastic strain. This “magnetoelastic” coupling has implications for the stability of magnetically ordered structures, the preferred orientation of magnetic moments and the mobility of magnetic domain walls.
If a crystal undergoes any type of phase transition, the mechanism almost invariably involves some degree of structural relaxation that gives rise to local or macroscopic strain. This must inevitably give rise also to changes in elastic properties. If any part of the microstructure associated with the magnetic transition can move under the application of an external shear stress, in particular if the magnetic domain walls have a ferroelastic component, distinct and characteristic anelastic losses will be observed in measurements made by dynamical methods. It follows that elastic/anelastic properties are indicative both of the influence of strain relaxation and of the dynamics of pinning processes for domain walls in magnetic materials. The main objective of the project will be to make use of these relationships to investigate the physics of strain coupling in relation to the magnetic properties of some of the key magnetic minerals used in geomagnetism. The overall approach is made possible by recent advances in the application of Resonant Ultrasound Spectroscopy (RUS) for dynamical measurements of elastic properties as functions of temperature and applied magnetic field.
What the student will do:
Magnetic minerals, including magnetite, ilmenite, ulvospinel and pyrrhotite will be investigated by RUS in the temperature interval ~4–1000 K and with applied magnetic field up to 14 Teslas, using equipment funded in recent years by NERC and EPSRC. Spectra will be analysed using the software package IGOR with a view to characterising variations in elastic constants and anelastic losses. Landau theory will be used to provide formal descriptions of the coupling between different order parameters (magnetic, Jahn-Teller, cation ordering, vacancy ordering, etc.) in these phases and strain. Phenomenological treatments based on the Debye equation will be used to develop a quantitative understanding of the dynamics of domain wall motion and pinning.
Carpenter, M.A., Zhang, Z. & Howard, C.J. 2012. A linear-quadratic order parameter coupling model for magnetoelastic phase transitions in Fe1-xO and MnO. Journal of Physics: Condensed Matter vol. 24, 156002.
Zhang, Z, Church, N. , Lappe, S.-C., Reinecker, M., Fuith, A., Jackson, I., Saines, P.J., Harrison, R.J., Schranz, W. & Carpenter, M.A. 2012. Elastic and anelastic anomalies associated with the antiferromagnetic ordering transition in wustite, FexO. Journal of Physics: Condensed Matter vol. 24, 215404.
Oravova, L., Zhang, Z., Church, N., Harrison, R.J., Howard, C.J. & Carpenter, M.A. 2013. Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe2O3. Journal of Physics: Condensed Matter vol. 25, 116006.
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