Investigations of materials hosting exotic magnetism with spin-density functional theory

Abstract

Spin-density functional theory (spin-DFT) has long been used to study materi- als exhibiting collinear spin textures. Recently it has become more common to apply non-collinear spin-DFT to tackle systems with more exotic magnetism.
In this thesis, we use spin-DFT both as a predictive tool and as a means of explaining phenomena uncovered by experiment. We use calculations to com- plement muon-spin spectroscopy (μSR) studies of two skyrmion hosting materi- als, Cu2OSeO3 and GaV4S8−ySey, as well as crystallography and magnetometry measurements of [Cu(pyz)0.5(gly)]ClO4. For Cu2OSeO3 we use structural re- laxation to find likely stopping sites for an implanted muon. Our predicted muon sites lead to good agreement with the measured μSR spectra. For the GaV4S8−ySey series, we are able to see how the spin density changes upon sub- stitution, consistent with observations of a spin-glass at high substitution. In a pressure study of [Cu(pyz)0.5(gly)]ClO4, we find that calculated changes to the structure well match X-ray crystallography measurements, capturing the rele- vant changes to the Cu separation. We see dramatic change in the secondary exchange mechanism consistent with structural distortions at high pressure.
Next, in Cr1/3MS2 (M = Nb or Ta), we predict a gap-like feature in the density of states (DoS) which is then confirmed through magnetometry mea- surements. This gap explains the low temperature transport and magnetism in these materials. Extending the study to N1/3NbS2, for N in the first period transition metals, we show that only Cr intercalation results in this gap. We predict the magnetic properties of the series through a band filling mechanism.
Finally, we make use of a recently developed exchange and correlation (xc) functional and implement it in castep. This functional improves the treatment of non-collinear spin which allows us to realise a spin-ice state in Dy2Ti2O7.