Correlated magnetic oxides studied using muon-spin spectroscopy

Abstract

This thesis is concerned with the application of muon-spin spectroscopy (muSR) in the study of correlated magnetic oxides. The results of four studies are presented, where muSR is used to provide insight into both the static and dynamic magnetic behaviours of the materials in question.

The results of muSR measurements on the double perovskite compounds Sr2BOsO6 (B = Co, Fe, Y, In) are presented. The Co compound exhibits lattice-site-specific ground states and spin dynamics with two distinct, partially ordered antiferromagnetic states encountered upon cooling from room temperature. Here, and also for the Fe compound, the muSR results reveal the evolution of the internal fields through the phase transitions and as dynamic relaxation channels freeze out on further cooling. The possibility of incommensurate magnetic ordering is considered for the Y and In compounds.

The magnetic properties of the tetrahedral spin-chain oxide CsCoO2 are investigated using muSR, with results that are consistent with a magnetic structure comprising ferromagnetically ordered Co-Co spin dimers, themselves arranged in antiferromagnetic chains. The critical behaviour near TN and the magnetostructural coupling in the vicinity of a structural phase transition are examined. These results help elucidate a complex freezing out of relaxation processes upon cooling, induced by a bifurcation of superexchange bond angles.

A revised magnetic phase diagram is reported for the antiferromagnetic insulating series L(2-x)Sr(x)CoO4, which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. It is found that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behaviour within an intermediate x range suggests that the putative stripe ordered phase extends to lower x than previously thought, whereas further charge doping prevents magnetic ordering for T > 1.5 K.

Finally, the results of the first transverse field muSR measurements to be made on a skyrmion lattice phase are presented. The muon response to that phase and the surrounding ones within Cu2OSeO3 is used to demonstrate how the technique is sensitive to the skyrmion lattice via the frequency domain lineshape.