Andrew Boothroyd

Abstract:

The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies $(E \gtrsim 100\,meV)$ the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum becomes more three-dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.

Abstract:

LiOsO3 is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field μSR, along with a combination of electronic structure and dipole field calculations, to determine the magneticground state of LiOsO3. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below ≈0.7 K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08 K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures.