Universality and critical behavior at the critical endpoint in the itinerant-electron metamagnet UCoAl

We performed nuclear-magnetic-resonance measurements on itinerant-electron metamagnet UCoAl to investigate the critical behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We derived c-axis magnetization M _c and its fluctuation S _c from the measurements of Knight shift and nuclear spin-lattice relaxation rate 1/T ₁ as a function of the c-axis external field (H _c) and temperature (T). We developed contour plots of M _c and S _c on the H _c-T phase diagram, and observed the strong divergence of S _c at the CEP. The critical exponents of M _c and S _c near the CEP are estimated and found to be close to the universal properties of a three-dimensional Ising model. We indicate that the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common feature as a gas-liquid transition.