Ferromagnetic quantum criticality studied by hall effect measurements in UCoAl

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 T₀ = 12K above which the first order metamagnetic transition becomes a crossover. Under pressure B_m increases and T₀ 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 PΔ ≈ 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 Bm remains an open question.