Ferromagnetism in VAu4 and related compounds
Abstract
VAu4 undergoes an order-disorder transformation at about 750°K from a fcc to a bct (Ni4Mo) structure. In the ordered (bct) structure, VAu4 exhibits a paramagnetic moment with a θ ranging from 41° to 53°K. It undergoes a ferromagnetic transition at Tc∼53°K. Both the paramagnetic moment and saturation moment depend on the heat treatment and thus the degree of crystallographic order. To determine whether VAu4 is an intrinsic ferromagnet, we have made magnetization and susceptibility measurements on alloys containing up to 5% Fe replacing V. For these alloys no enhancement of the paramagnetic or ferromagnetic moment was observed. We have observed the kinetics of the moment formation in VAu4 by high-temperature-susceptibility measurements between 700° and 800°K. Starting with a disordered sample, it only takes approximately 10 min to achieve μeff=1.40 μB. After this initial rapid increase, the rate of growth slows considerably, and if we extrapolate our results, we estimate that it requires approximately 5 h to reach a susceptibility corresponding to 1.73 μB. Above 800°K, a temperature-independent susceptibility χB of 0.71×10-6 emu/gm·Oe is observed. Using this value to correct the susceptibility data below the ordering temperature, one obtains a very good fit to the Curie-Weiss law. If one corrects χB for the core diamagnetism and assumes this value is due solely to Pauli paramagnetism, a value of 23.6 states/eV·(VAu4) is obtained for the density of states in good agreement with Luo, Creveling and Maita's1 determination of the electronic specific-heat density of states, 23.2 states/eV·(VAu 4). However, since one expects the orbital paramagnetism to be nonnegligible, this is certainly an overestimate. To further study this material, we have made magnetic measurements on the alloys VAu 4(1-x)Ag4x where x=0.025, 0.10, and 0.20. Ferromagnetism was observed in the alloy with x=0.025, with a saturation magnetization of 0.12 μB/VAu4 and Tc=28°K, even though XRDF measurements showed no evidence of long-range order. Hc was found to be 16 kOe for VAu3.9Ag0.1, even though we found Hc>25 kOe for VAu4. Ag was found to inhibit the formation of the ordered phase and narrow the range of temperature over which it occurs. For x=0.10 and 0.20, no ferromagnetism and no time-dependent susceptibility were observed. © 1969 The American Institute of Physics.