Ferromagnetic Quantum Criticality Studied by Hall Effect Measurements in UCoAl

TL;DR

This study investigates ferromagnetic quantum criticality in UCoAl through Hall effect measurements under pressure and magnetic field, revealing changes in magnetic transitions and potential Fermi surface modifications near the quantum critical end point.

Contribution

It provides the first detailed Hall effect analysis of UCoAl under pressure, identifying the evolution of metamagnetic transitions and quantum critical behavior.

Findings

Hall effect anomaly at transition diminishes under pressure

Metamagnetic transition disappears above the quantum critical end point

Significant change in Hall coefficient near the quantum critical end point

Abstract

Hall effect measurements were performed under pressure and magnetic field up to 2.2 GPa and 16 T on a single crystal of UCoAl. At ambient pressure, the system undergoes a first order metamagnetic transition at the critical field B_m = 0.7 T from a paramagnetic ground state to a field-induced ferromagnetic state. The Hall signal is linear at low field and shows a step-like anomaly at the transition, with only little change of the Hall coefficient. The anomaly is sharpest at the temperature of the critical end point T_0 = 12 K above which the first order metamagnetic transition becomes a crossover. Under pressure B_m increases and T_0 decreases. The step-like anomaly in the Hall effect disappears at P_M ~= 1.3 GPa and the metamagnetic transition is not detected above the quantum critical end point (QCEP) at P_Delta ~= 1.7 GPa, B_m ~= 7 T. Using magnetization data, we analyse our Hall resistivity data at ambient pressure in order to quantitatively account for both ordinary and anomalous contributions to the Hall effect. Under pressure, a drastic change in the field dependence of the Hall coefficient is found on crossing the QCEP. A possible Fermi surface change at B_m remains an open question.