P4: Spin dynamics and spin freezing at ferromagnetic phase transitions

C. Pfleiderer, P. Böni

Disordered ferromagnets near quantum phase transitions (QPTs), e.g. Pd1-xNix, display the properties of quantum criticality expected in pure compounds, but the thermal expansion suggests the absence of quantum criticality. In contrast, pure itinerant ferromagnets such as Ni3Al, ZrZn2 and CoS2 display first-order QPTs under hydrostatic pressure that suggest the absence of critical fluctuations, but the resistivity displays an anomalous temperature dependence over a wide pressure range that suggests novel types of soft magnetic modes and a breakdown of Fermi liquid theory. Theoretical scenarios that may account for the inconsistencies in itinerant ferromagnets near a QPT are for instance anomalous magnetic textures, singular higher-order spin correlation functions or special topological features of the Fermi surface.

We propose comprehensive studies of single crystals of the disordered systems Pd1-xNix, Pd1-xFex and the pure system Ni3Al by means of their low-temperature bulk properties (resistivity, magnetisation, specific heat, thermal expansion) and state-of-the-art neutron spin resonance techniques, almost uniquely available at the neutron source Heinz Maier-Leibnitz (FRM II), to study magnetic critical slowing down, higher-order spin correlation functions, the interplay of collective excitations, and the formation and properties of magnetic textures. The proposed study will be of prime importance for the development of a general theory of QPTs in itinerant-electron systems.