YbNi\(_4\)Mg: Superheavy fermion with enhanced Wilson ratio and magnetocaloric effect

A comprehensive study of the low-temperature properties of YbNi\(_4\)Mg has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient \(\gamma_0\) \(\approx\) 5.65 J mol\(^{-1}\) K\(^{-2}\) and an elevated Wilson ratio \(R_W\) = 32.1. No magnetic ordering was observed down to 70 mK; however, a broad maximum appears in the specific heat at \(T^*\) = 0.3 K, along with a shoulder in the derivative of susceptibility d\(\chi\)/d\(T\) and resistivity d\(\rho\)/d\(T\). These features indicate a cooperative yet short-ranged magnetism entwined with the superheavy Fermi liquid. The large Wilson ratio, which is also detected in other superheavy-fermion compounds lacking long-range order, might be a signature of residual spin fluctuations. Applying a weak magnetic field of \(\sim\)0.1 T induces a metamagnetic-like crossover, as demonstrated by the quasi-adiabatic demagnetization measurements showing a broad minimum in the temperature-field trace. Here, an enhanced magnetocaloric cooling effect stemming from the field-sensitive superheavy-fermion state is observed, rivaling that of the well-established insulating magnetic coolants like the rare-earth garnet Gd\(_3\)Ga\(_5\)O\(_{12}\).