Critical behaviour of ZrZn₂

The nature of the metallic state of the low temperature itinerant ferromagnet ZrZn₂ in the vicinity of a magnetic quantum phase transition has been studied in a series of susceptibility and resistivity measurements under high pressures. A pressure-temperature phase diagram has been constructed on the basis of the susceptibility data. The magnetic transition temperature is observed to follow T_c ^{4 3} ∼ (p_c - p) down to temperatures of order 1 K. No evidence for a first order transition was found at any pressure, but a clearly defined and moderately pressure-dependent maximum was observed in the susceptibility in the paramagnetic regime, i.e. for p > p_c. The temperature dependence of the resistivity is found to differ from that of the conventional Fermi liquid model over a wide range of temperature and pressure. The shape of the phase diagram and the observed behaviour of the resistivity have been interpreted in terms of a quantum Ginzburg Landau model for the magnetic fluctuation spectrum.