Magnetic phase diagram of UCoAl

We report precision c-axis magnetic measurements on a high-quality single crystal of the heavy fermion metamagnet UCoAl. The metamagnetic transition at H _M changes from 1st order at low temperature to a crossover at high temperature. H _M is nearly linearly increasing with increasing temperature up to a critical temperature T ₀ . The critical temperature T ₀ is determined from both the field and the temperature dependences of magnetization to be ∼ 11 K. The field dependence of the Sommerfeld coefficient γ is estimated from M(T) by using a Maxwell relation. γ(H) shows a step-like decrease at H _M . This behavior is consistent with the previous reports of specific heat and resistivity measurements at low temperatures.