Non-collinear magnetism: microscopic origin, all-electrical probe, unconventional states and topological excitations
This thesis studies the microscopic mechanisms for noncollinear spin ordering in magnetic materials. I discuss interactions between spins that can give rise to noncollinear and multiply-periodic states, and the interplay between different interactions provides an additional degree of complexity.
Our results on the Double Exchange model show that a non-collinear spiral ordering of spins can be energetically more favorable than the collinear ferromagnetic state, even in the limit of strong Hund's Rule coupling. We obtained a rich phase diagram for the triangular lattice with many competing magnetic phases.
Our study done in collaboration with A. Aqeel and co-workers establishes all-electrical Spin Hall Magnetroresistance measurements as a probe for detection of complex non-collinear orders, such as the skyrmion lattice. In addition, we introduce a new term in the transverse Spin Hall Magnetroresistance that is nonzero only for noncollinear magnets.
Our study establishes the frustrated chiral magnet Pb2MnO4 as a new model system for the realisation of complex non-collinear magnetic ordering and topological defects.
Our study of the collinear antiferromagnet CaFe2O4 shows that magnetic anisotropy structure is crucial for understanding of the time-resolved THz spectroscopy measurements of T. Mai and co-workers.
The main goal of this research is to gain a better fundamental understanding of the properties of non-collinear magnets. Magnetic orders of particular interest to this thesis are topological magnetic structures, such as the compact magnetic vortices known as skyrmions. The results can be used to study and design magnetic materials of special relevance to technological applications, such as smaller and more efficient memory devices.
Maria Azhar has won the Shell Graduation Prize for Physics for her research into microstructures