Composition-tuned magnetic order and excitations in Nb(1-x)Fe(2+x) and pressure-induced changes in structural and optical properties of Cu2OSeO3

James Poulten

Research output: ThesisDoctoral Thesis

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Understanding the magnetic properties of matter is of paramount interest from a principal and technological point of view. For example, transition-metal compounds tuned to a magnetic quantum phase transition often form unconventional superconductivity and exhibit non-Fermi liquid behaviour. The discovery of novel magnetic states such as the skyrmion lattices might lead to new magnetic storage technologies if a full understanding of the magnetic, electronic, and structural material properties is obtained. This thesis presents results on composition and pressure-tuned magnetic compounds: the itinerant ferromagnet Nb(1-x)Fe(2+x) and the insulating helimagnet Cu2OSeO3.

The continuous ferromagnetic transition in Fe-rich Nb(1-x)Fe(2+x) can be suppressed and a ferromagnetic quantum phase transition can be reached by replacing Fe with Nb. Nb(1-x)Fe(2+x) follows a trend that nature tends to avoid ferromagnetic quantum critical points. They are typically either replaced by a first order transition or masked by unconventional superconductivity. Nb(1-x)Fe(2+x) offers a predicted third scenario: masking by a spin density wave phase. With neutron diffraction, the extension of the spin density wave phase in the field-temperature phase diagram has been determined. Additionally, cold neutron spectroscopy revealed low-energy magnetic excitations at zero field including soft quasi-elastic scattering in an extended region in reciprocal space, which reflects the proximity of the tuned system to different types of magnetic order.

The recently discovered skyrmion lattice in Cu2OSeO3 has created particular interest in this compound as its insulating properties allow the skyrmion lattice to be moved by an electrical field. Moreover, its magnetic ordering temperature increases with pressure, in contrast to the metallic helimagnets. In order to obtain information on the microscopic changes that might be responsible for the strengthening of the magnetic interactions, the structural properties of Cu2OSeO3 have been examined by X-ray diffraction. These studies reveal unusual changes in some Cu-O distances in the low pressure phase that undergoes an irreversible structural transformation to a yet unsolved high-pressure phase at 9.7 GPa. Complementary Raman scattering and infrared absorption measurements confirm subtle changes in the low pressure phase and a marked change in optical properties accompanies the structural phase transition.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Niklowitz, Philipp, Supervisor
  • Wilhelm, Heribert, Supervisor, External person
Thesis sponsors
Award date1 Jun 2018
Publication statusPublished - 30 Jun 2018

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