Magnetic order in the fcc kagome antiferromagnet

Abstract

The research in this thesis was inspired by a particular magnetic compound, IrMn3, which has been used extensively as the antiferromagnetic layer for exchange pinning in spin valves for computer hard disk drives and other magnetic storage technologies. Its magnetic structure is the fcc kagome lattice, where 2D kagome layers are ABC stacked along [111] to give a full 3D lattice. To gain a better understanding of its basic spin structure and dynamics, several theoretical techniques are considered and an experiment was performed with a focus on examining the effect of adding magnetic anisotropy to the Heisenberg fcc kagome lattice model. Monte Carlo simulations using the standard Metropolis algorithm are performed for the fcc kagome lattice with the addition of cubic magnetocrystalline anisotropy K. Comparisons are made between previous K = 0 results and K > 0 through spin order parameters and the specific heat, to study anisotropy effects on the spin degeneracies associated with the 3D kagome spin lattice. A look at energy histograms and Binder energy cumulants reveals a change from a first order to a continuous phase transition with the addition of anisotropy, associated with the removal of ground state degeneracies. Magnetic neutron scattering experiments were carried out on a IrMn3 powder sample at the Oak Ridge National Laboratory to study the magnetic structure and determine whether anisotropy could be detected. Spin waves along with elastic and inelastic magnetic scattering theories are developed for the fcc lattice in an attempt to compare to the experiment. While noise in the data makes this quite difficult, the expected results may help shed some light on this material for future experiments on single crystals.