Temperature dependence of the magnetization of nickel usingNi61NMR

The temperature dependence of the hyperfine field of ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ in pure nickel has been measured in the range 4.2-292 K using continuous-wave NMR. The hyperfine field is a measure of the magnetization in this temperature range since the hyperfine coupling constant at constant volume is shown to be essentially independent of the temperature. The present measurements confirm the difference, noted by Aldred, between the spin-wave parameters calculated from neutron scattering and magnetization measurements. This discrepancy is discussed in terms of the Stoner theory of single-particle excitations and (because of the improved accuracy attainable using NMR) it is possible to show that the simple expression for the magnetization for a strong ferromagnet, $M\sim \exp (-\frac{\Delta }{k_{B}T})$, is unsatisfactory, and a large term proportional to $T^2$ definitely exists. It is concluded that although nickel is a strong ferromagnet, the value of the gap parameter $\Delta$ is an order of magnitude smaller than has been estimated from magnetization measurements.