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 similar to 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T-0 similar to 11 K. With increasing pressure, H-m linearly increases, while T-0 decreases and is suppressed at the quantum critical endpoint (QCEP, P-QCEP similar to 1.5 GPa, H-m similar to 7 T). 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 H-m 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 P-QCEP, the enhancement of m* is reduced, and a broad plateau is found between H-m and H*. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr3Ru2O7.