Professor Stephen Blundell

Abstract:

We study by means of bulk and local probes the d-metal alloy Ni_{1-x}V_{x} close to the quantum critical concentration, x_{c}≈11.6%, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly x dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.

Abstract:

The magnetic properties attributed to the hydroxide layer of Li1-xFex(OH)Fe1-ySe have been elucidated by the study of superconducting and nonsuperconducting members of this family of compounds. Both ac magnetometry and muon spin relaxation measurements of nonsuperconductors find a magnetic state existing below ≈10 K which exhibits slow relaxation of magnetization. This magnetic state is accompanied by a low-temperature heat capacity anomaly present in both superconducting and nonsuperconducting variants suggesting that the magnetism persists into the superconducting state. The estimated value of magnetic moment present within the hydroxide layer supports a picture of a glassy magnetic state, probably comprising clusters of iron ions of varying cluster sizes distributed within the lithium hydroxide layer.