Magnetic Relaxation in Nickel above the Curie Temperature

Abstract

Magnetic relaxation in nickel above the Curie temperature (354°C) has been studied up to a temperature of 450°C. The spin-relaxation rate $\frac{1}{{\tau }_r}$, which is equal to the magnetic resonance half-width $\Delta \omega =\gamma \Delta H$, is determined from the field dependence of the microwave Kerr rotation. We fit the relaxation to an expression: $\frac{1}{{\tau }_r}=3\left(\frac{C}{T}\right)\left(\frac{H}{M}\right)\omega _d^{}{}_{}{}^2{\tau }_c$, where $C$ is the Curie constant, $T$ the absolute temperature, $\frac{M}{H}$ the transverse static susceptibility, ${\omega }_d$ the pseudodipolar coupling frequency, and ${\tau }_c$ the spin correlation time. At high temperatures, ${\tau }_c$ is constant and is set equal to $\frac{1}{{\omega }_e}$, the reciprocal exchange frequency, by a suitable choice of ${\omega }_d$. As the temperature is dropped below 420°C, ${\tau }_c$ begins to increase as ${(T-T_c)}^{\frac{-2\mathrm{}}{3}}$. This increase in ${\tau }_c$ is compatible with critical-neutron-scattering studies of nickel and with measurements of the static susceptibility. Below 385°K, ${\tau }_c$ remains constant at a value 1.8 times the high-temperature value. It is believed that the development of spin-wave excitations in this temperature range prevents a further increase in the spin correlation time. A shift in the spectroscopic splitting factor from $g=2.22\mathrm{}$ below $T_c$ to $g=2.29\mathrm{}$ above $T_c$ is observed. Well above $T_c$, a small amount of Kerr rotation, which is associated with the Hall effect, is observed. The magnitude of this rotation is consistent with the results of dc studies.