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 Mc and its fluctuation Sc from the measurements of Knight shift and nuclear spin-lattice relaxation rate 1/T₁ as a function of the c-axis external field (Hc) and temperature (T). We developed contour plots of Mc and Sc on the Hc-T phase diagram, and observed the strong divergence of Sc at the CEP. The critical exponents of Mc and Sc 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.