ANOMALOUS TEMPERATURE-DEPENDENCE OF THE ELECTRIC-FIELD GRADIENT AT THE GD NUCLEAR SITE IN INTERMETALLIC COMPOUNDS

We have used Gd-155 Mossbauer spectroscopy to study the behavior of th e electric field gradient (V-zz) at the nuclear site in a variety of G d intermetallic compounds. We found that V-zz is not always temperatur e-independent as is commonly assumed but can exhibit strong temperatur e dependences when the Gd moments become magnetically ordered. Example s of experimental results are presented, where V-zz either decreases ( GdGa2) or increases (Gd2Zn17) with temperature below the corresponding magnetic ordering temperature. In both cases the temperature variatio n of V-zz appears to correlate with that of the hyperfine field. Possi ble explanations are briefly discussed.