Quantum Criticality in Ferromagnetic Metals: the case of U4Ru7Ge6
Mucio A. Continentino
Centro Brasileiro de Pesquisas Físicas, 22290-180 Rio de Janeiro, Brazil
The study of quantum critical points (QCP) in magnetic metals is an exciting area of research. In particular, the case of a ferromagnetic QCP has brought surprising results and even raised doubts on the existence of this type of criticality in metals. Indeed, in many of these systems as the critical (Curie) temperature TC is reduced, by pressure or doping, the ferromagnetic transition becomes first order and the critical behavior is aborted. Pressure is a fundamental tool in the study of this phenomenon, as it drives the system to the QCP, reducing TC without introducing disorder that necessarily accompanies the doping of the material. We will present and discuss recent results [1] in a heavy fermion ferromagnetic metal as it is driven to a QCP by external pressure. The U4Ru7Ge6 system that we study is unique among Uranium based ferromagnets in that it has a negligible magnetic anisotropy. The low energy magnetic excitations in this material are spin-waves with no gap in their spectrum. These modes scatter the conduction electrons and an investigation of the low temperature electrical resistivity as a function of pressure allows monitoring the behavior of the stiffness of the magnons as TC is reduced.
We will show that this system can be driven continuously to a ferromagnetic QCP [1] and discuss the possible reasons for the peculiar behavior of this material. We finally discuss the value obtained for the critical exponent governing the vanishing of the spin-wave stiffness, close to the QCP, on the light of existing models.
[1] M. P. Nascimento, M. A. Continentino, A. López, Ana de Leo, D. C. Freitas, J. Larrea, Carsten Enderlein,
J. F. Oliveira, E. Baggio-Saitovitch, Jirí Pospíšil, and M. B. Fontes, Phys. Rev. B 98, 174431 (2018).