Magnetic Excitations of Frustrated Metals

Frustrated magnetism has been a source of inspiration for decades. Much research has focused on insulating magnets where spins are due to localized electrons. Recently, however, there has been renewed interest in metallic magnets – in which itinerant electrons can carry spin – because of their ability to host responses such as the topological Hall effect. These responses are associated with spin textures that are often driven by frustrated interactions.

It is therefore important to understand the interplay of frustration and itineracy in magnetic metals. Magnetic excitation spectra, as measured in neutron spectroscopy experiments, provide vital information about a material's magnetic interactions. However, modeling such spectra is challenging even in itinerant ferromagnets.

In this talk, I explore recent developments in modeling magnetic excitations of metals, and compare modeling results with neutron-scattering data for the frustrated metal β-Mn. I begin by introducing the Heisenberg-Landau Hamiltonian, an extension of the classical Heisenberg model that accounts for itineracy by allowing both longitudinal and transverse spin fluctuations. I show how magnetic excitation spectra of this Hamiltonian can be calculated using atomistic Langevin dynamics, and explore the effect of varying itineracy on the magnetic excitation spectrum of the Heisenberg pyrochlore antiferromagnet.

Finally, I present neutron-scattering data on the canonical frustrated metal β-Mn, in which magnetic Mn atoms occupy a 3D network of corner-sharing triangles. Neutron-diffraction data reveal the absence of long-range magnetic order to T ~ 0.05 K. Inelastic neutron-scattering data reveal magnetic excitations with a bandwidth exceeding hundreds of meV and an increase in intensity with increasing temperature, revealing large antiferromagnetic interactions and a temperature-dependent local moment. I discuss the extent to which these unusual observations can be explained by the interplay of frustrated interactions and longitudinal spin fluctuations. I conclude by discussing the outlook for parametrizing magnetic interactions in metals from first-principles theory and fits to neutron-scattering data.