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Title: Ab initio modelling of magnetoelectric materials
Author: Tillack, Natalie
ISNI:       0000 0004 6496 5469
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2016
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The search for magnetoelectric materials, i.e. materials that respond with a polarisation (magnetisation) when an external magnetic (electric) field is applied, has become more important in recent years---both for the standalone magnetoelectricity and as part of the cross-coupling in modern multiferroics. To date, applications of magnetoelectrics (MEs) in technology are hampered by either small coupling strengths or low operating temperatures. In the present work, the linear magnetoelectric coupling is examined by means of density functional theory with the aim to understand the physics of the coupling, and in turn open up pathways in the search for improved coupling properties. The spin part of the ME coupling tensor was shown to be the sum of the electronic and the lattice contribution. Recently developed first-principles frameworks allow us to gain insights into the microscopic mechanisms of the coupling and are applied to compute the response in modifications of the prototypical ME Cr2O3 and isostructural compounds. We find that strains at ambient pressures influence the lattice contribution significantly. At higher pressures, a magnetic phase transition is shown to destabilise magneto-active phonon modes and lead to a divergence of the longitudinal lattice response due to magnetic softness, with a coupling strength 650 times larger than its zero-pressure value. Equivalently, a divergence of the electronic response is measured at the transition from insulator to metal---previously suggested in literature, but to date not measured. The largest value of the electronic response is shown to be 31.4ps/m, which is larger than the ambient value by a factor of 70, and also large in terms of the absolute value. We exemplify both the magnetic softness and band-gap alterations by doping the Cr2O3 parent structure with Fe and Ti. We show that doping provides a powerful way to include magnetic frustration resulting in a ten-fold increase of magnetic contributions to the lattice response. When the dopants lower the Cr2O3 band gap, the electronic response is measured to be ten times larger than reported for pure Cr2O3. Finally, the high-symmetry configurations of the spin flop phase of Cr2O3 are modelled. A combined phenomenological and group-theoretical analysis reveals a novel ME coupling mechanism only present in the spin flop phases. Akin to the ferroaxial mechanism in multiferroics, a chiral in-plane transverse coupling term is observed, whose Landau invariant is of 5th order.
Supervisor: Yates, Jonathan Sponsor: Engineering and Physical Sciences Research Council ; Scatcherd European Scholarship
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available