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 x 10(-3) in Fe2VAl, in good agreement with the value deduced from previous field-dependent specific heat measurements.