Characterization of the Weak Itinerant Ferromagnetic Order in Single-Crystalline UIr

Abstract

The weak itinerant ferromagnetic order of single-crystalline UIr has been characterised by specific heat, thermal expansion and neutron diffraction measurements for temperatures from 5 to 250 K and applied magnetic fields up to 14 T. At the Curie temperature T C =45 K, pronounced anomalies are observed in the specific heat and in the thermal expansion. The coefficients of linear thermal expansion along the [10\bar1], [101], and [010] axes of the monoclinic crystal structure all exhibit negative steps. This implies that the uniaxial pressure dependencies of the Curie temperature, determined by the Ehrenfest relation, are all negative. The calculated hydrostatic pressure dependence of the Curie temperature is found to amount to d T C /d p =-1.6 K/kbar. The effective Gruneisen parameter is determined in order to analyse the volume dependence of the characteristic energy scales. An enhanced volume dependence of the ferromagnetic spin fluctuations is observed at low temperatures.