Ferromagnetic Quantum Critical Endpoint in UCoAl

概要

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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_{\text{m}}\sim 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T_{0}\sim 11 K. With increasing pressure, H_{\text{m}} linearly increases, while T_{0} decreases and is suppressed at the quantum critical endpoint (QCEP, P_{\text{QCEP}}\sim 1.5 GPa, H_{\text{m}}\sim 7 T). At higher pressure, the value of H_{\text{m}} identified as a crossover continuously increases, while a new anomaly appears above P_{\text{QCEP}} at higher field H^{\ast} in resistivity measurements. The field dependence of the effective mass (m^{\ast}) obtained by resistivity and specific heat measurements exhibits a step-like drop at H_{\text{m}} at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m^{\ast} is observed near the QCEP. Above P_{\text{QCEP}}, the enhancement of m^{\ast} is reduced, and a broad plateau is found between H_{\text{m}} and H^{\ast}. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr3Ru2O7.
2011-09-15