Frustrated magnets: non-collinear spin textures, excitations and dynamics

PhD ceremony: Mr. S. Artyukhin, 12.45 uur, Aula Academiegebouw, Broerstraat 5, Groningen

Dissertation: Frustrated magnets: non-collinear spin textures, excitations and dynamics

Promotor(s): prof. M. Mostovoy, prof. J. Knoester

Faculty: Mathematics & Natural Sciences

The information in the modern data centers is stored on magnetic hard drives. Although the technology used for reading of information was revolutionized by the discovery of giant magnetoresistance (2007 Nobel prize), recording of information still relies on the miniature electromagnets, resulting in energy dissipation, which increases the energy consumption and limits the information density.

Multiferroic materials could provide a solution to this problem. It was recently demonstrated by Tokunaga et al (Nature Materials, 2009) that the magnetization in multiferroic GdFeO3 can be changed by the applied electric field, without the need for direct currents. However, the changes of magnetization were small. Our study suggests this smallness to be due to the clamping of ferroelectric and ferromagnetic domain walls, resulting from the interactions between the Fe and Gd spins.

We also show that interactions between rare earth and Fe spins can stabilize a very unusual incommensurate state recently discovered in TbFeO3. In multiferroic hexagonal YMnO3 experiments demonstrated clamping of ferroelectric and antiferromagnetic domain walls as well as peculiar hexagonal vortices, around which electric polarization changes sign 6 times.

We formulated the Landau-type theory describing interacting non-uniform trimerization, ferroelectric and antiferromagnetic orders in this material, which explains the experimental observations.

In other chapters we study an unusual ferromagnetic insulator state in Fe-doped FeTiO3 and resolve the long-standing contradiction between the results of excitonic magnetoabsorption and cyclotron resonance experiments in Cu2O.