Supervisor: Michael Carpenter (Earth Sciences)
Importance of the area of research:
Interest in multiferroics is being driven both by possibilities for device applications, such as spintronics and magnetoelectric memories, and by scientific focus on the physics of multiple instabilities in broad classes of new materials. Individual phases such as BiFeO3 have become the classic materials in this context but alternatives are emerging. For example, two-phase systems in which a ferromagnetic material is in mechanical contact with a ferroelectric material offer possibilities for stronger magnetoelectric coupling. A common theme which pervades the explosion of scientific literature in this field is the role of strain. From the perspective of phase transitions in general, it is well known that coupling of a driving order parameter with strain suppresses fluctuations, promotes mean field behaviour and leads to strong coupling between multiple order parameters in systems with more than one instability. From the perspective of potential device applications it is also well known that thin film properties can be engineered by imposing a strain from the substrate. This theme is not limited to multiferroics, but informs investigation of other properties/materials of topical interest, including quantum paraelectrics, colossal magnetoresistance, superconductivity, electrocalorics and magnetocalorics.
The main objective of this project will be to investigate magnetoelastic coupling, ie the specific role coupling between strain and magnetic order parameters, in ferroic and multiferroic materials. This will be achieved by using Resonant Ultrasound Spectroscopy to measure elastic and anelastic properties as simultaneous functions of temperature (2-300 K) and magnetic field (0-14 Teslas).
What the student will do:
The elastic and anelastic properties of multiferroic oxides, such as the classic perovskite TbMnO3 and multicomponent perovskites based on solid solutions between Pb(Fe0.5Ta0.5)O3, Pb(Fe0.5Nb0.5)O3 and Pb(Zr,Ti)O3, and intermetallic phases, such as NiMnGa and CoMnSi, with a view to characterising the phase transitions that take place within them in response to changes of temperature and magnetic field. Landau theory will be used to provide formal descriptions of the coupling between different order parameters (magnetic, Jahn-Teller, cation ordering, vacancy ordering, etc.) 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. Magnetic properties will be characterised by SQUID magnetometry.
Carpenter, M.A., Howard, C.J., McKnight, R.E.A., Migliori, A., Betts, J.B. & Fanelli, V.R. (2010) Elastic and anelastic relaxations associated with the incommensurate structure of Pr0.48Ca0.52MnO3. Physical Review B, vol. 82, 134123.
Thomson. R.I., Chatterji, T., Howard, C.J., Palstra, T.T. & Carpenter, M.A. 2014. Elastic anomalies associated with structural and magnetic phase transitions in single crystal hexagonal YMnO3. Journal of Physics: Condensed Matter, vol. 26, 045901.
Schiemer, J., Carpenter, M.A., Evans, D.M., Gregg, J.M., Schilling, A., Arredondo, M., Alexe, M., Sanchez, D., Ortega, N., Katiyar, R.S., Echizen, M., Colliver, E., Dutton, S. & Scott, J.F. 2014. Studies of the room-temperature multiferroic Pb(Fe0.5Ta0.5)0.4(Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy, dielectric and magnetic phenomena. Advanced Functional Materials, vol. 2014, pp. 1–10.
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