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Probing magnetism in magneto-electric multiferroics using circularly polarized x-rays

Fabrizi, F.; (2011) Probing magnetism in magneto-electric multiferroics using circularly polarized x-rays. Doctoral thesis, UCL (University College London). Green open access

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Multiferroic materials (in which electric and magnetic order coexist) that also display magneto-electric coupling have recently raised considerable interest as candidate compounds for advanced applications in electronics and data storage. In particular, the magneto-electric coupling is relevant in those materials such as TbMnO3 and Ni3V2O8 in which the onset of a cycloidal magnetic order drives the formation of a ferroelectric state. A key feature in these compounds is the possibility to control the population of magnetic domains (defined by the handedness of the cycloids) by an in situ electric field. The combination of magnetic non-resonant diffraction by circularly polarised X-rays with the full linear polarimetry of the scattered beam opens the way to a new class of experiments, in which the magnetic order of complex magnetic materials under applied electric and magnetic fields is probed. This technique brings a strong experimental sensitivity to the imbalance in the domain populations, since the handedness of the circular polarisation naturally couples to the sense of rotation of the magnetic moments, leading to an accurate description of the domain state and to the refinement of the magnetic structure. The results shed more light on the complex magnetic structure of TbMnO3, a challenging test case due to its two magnetic sublattices on the Mn and Tb sites, by identifying components of the ordering on the Tb sublattice and phase shifts that earlier neutron diffraction experiments could not resolve. In the case of Ni3V2O8, the method not only facilitated the refinement of the magnetic structure, but also allowed real space images of the magnetic cycloidal domains to be obtained. Their evolution is followed as they are controlled via magneto-electric coupling by the applied electric field and cycled through a hysteresis loop, thus collecting valuable information on domain formation, inhomogeneities and domain wall movement.

Type:Thesis (Doctoral)
Title:Probing magnetism in magneto-electric multiferroics using circularly polarized x-rays
Open access status:An open access version is available from UCL Discovery
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Physics and Astronomy

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