Friday, July 26, 2013

1307.6817 (Tristan Combier et al.)

Ferromagnetic Quantum Criticality Studied by Hall Effect Measurements in

Tristan Combier, Dai Aoki, Georg Knebel, Jacques Flouquet
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.
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