Splitting of the Hopkinson maximum by heat treatment in Gd foils

Measurements of the ac initial magnetic susceptibility [χactriple bondχ(ω=constant, T)] were carried out on polycrystalline Gd foils in UHV. χ was measured before and after annealing the Gd foils for 16 h at 700degC in a 10-8 Torr vacuo. The χ vs. T spectrum of Gd foils shows, after annealing, two Hopkinson maxima at the temperatures TH1=287.5 K and TH2=291.5 K. The Arrhenius plots indicate that the thermal activation contributions to χ may be partially responsible for the occurrence of the Hopkinson effect in agreement with recent studies of Popov and Mikhov. (orig.)