Ferromagnetic quantum critical endpoint in UCoAl

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at H_m ∼ 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T0 ∼ 11K. With increasing pressure, H_m linearly increases, while T₀ decreases and is suppressed at the quantum critical endpoint (QCEP, Pqcep ∼ 1.5 GPa, H_m ∼ 7T). At higher pressure, the value of H_m identified as a crossover continuously increases, while a new anomaly appears above P_qcep at higher field H^* in resistivity measurements. The field dependence of the effective mass (m^*) obtained by resistivity and specific heat measurements exhibits a step-like drop at Hm at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m^* is observed near the QCEP. Above Pqcep, the enhancement of m^* is reduced, and a broad plateau is found between Hm and H^*. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr₃ Ru₂ O₇.