Superparamagnetism and magnetic defects in Fe₂VAl and Fe₂VGa

We have investigated the magnetic properties of Heusler-type Fe₂VAl and Fe₂VGa by means of susceptibility, magnetization, and nuclear magnetic resonance (NMR) measurements. As evidenced by the static and dynamic magnetic susceptibilities, both compounds exhibit the characteristic features of a superparamagnetic glass. The freezing temperatures of our samples are 23 K for Fe₂VAl and 9 K for Fe₂VGa. A detailed analysis indicates the coexistence of individual magnetic defects with large-moment superparamagnetic clusters. The temperature-dependent magnetization and NMR linewidth are dominated by isolated magnetic defects, identified as Fe antisite defects. The concentration of these defects, determined from NMR linewidths, is 4.5 × 10^-3 in Fe₂VAl, in good agreement with the value deduced from previous field-dependent specific heat measurements.