Anomalous Temperature Dependence of the Hyperfine Field at 119Sn in Co

The fall of H_eff at the nucleus of ¹¹⁹Sn in fcc Co from −22 kOe at 4.2°K to −10 kOe at 500°K has been reported. We have now measured H_eff between 900° and 1300°K and find values of 8–9 kOe. The sign was not determined, but following the trend of the earlier results leads us to believe that it is positive. We take up the suggestion that there is a component of H_eff due to 5s | 3d overlap which has a temperature dependence due to the strong distance dependence of $\text{〈5s|3d〉}$ . If ${\text{|〈5s|3d〉|}}^2{\mathrm{\propto r}}^{\mathrm{-n}}{\mathrm{=(d-x)}}^{\mathrm{-n}}$ where d is the distance between atoms, x the thermal displacement of an atom, and n large, then ${\text{|〈5s|3d〉|}}^2{\mathrm{\propto 〈x}}^2〉$ , neglecting thermal expansion and higher terms. Using for simplicity an Einstein model for ${\mathrm{〈x}}^2〉$ , we find that the results can be well fitted by H_eff=σ(−27.8+5.1 coth T_E/2T), where σ is the magnetization, and T_E = 281°K is an Einstein temperature.