Magnetic Relaxation in Nickel above the Curie Temperature

Abstract

Magnetic relaxation in nickel above the Curie temperature (354\ifmmode^\circ\else\textdegree\fi{}C) has been studied up to a temperature of 450\ifmmode^\circ\else\textdegree\fi{}C. The spin-relaxation rate $\frac{1}{{\ensuremath{\tau}}_{r}}$, which is equal to the magnetic resonance half-width $\ensuremath{\Delta}\ensuremath{\omega}=\ensuremath{\gamma}\ensuremath{\Delta}H$, is determined from the field dependence of the microwave Kerr rotation. We fit the relaxation to an expression: $\frac{1}{{\ensuremath{\tau}}_{r}}=3(\frac{C}{T})(\frac{H}{M})\ensuremath{\omega}_{d}^{}{}_{}{}^{2}{\ensuremath{\tau}}_{c}$, where $C$ is the Curie constant, $T$ the absolute temperature, $\frac{M}{H}$ the transverse static susceptibility, ${\ensuremath{\omega}}_{d}$ the pseudodipolar coupling frequency, and ${\ensuremath{\tau}}_{c}$ the spin correlation time. At high temperatures, ${\ensuremath{\tau}}_{c}$ is constant and is set equal to $\frac{1}{{\ensuremath{\omega}}_{e}}$, the reciprocal exchange frequency, by a suitable choice of ${\ensuremath{\omega}}_{d}$. As the temperature is dropped below 420\ifmmode^\circ\else\textdegree\fi{}C, ${\ensuremath{\tau}}_{c}$ begins to increase as ${(T\ensuremath{-}{T}_{c})}^{\frac{\ensuremath{-}2}{3}}$. This increase in ${\ensuremath{\tau}}_{c}$ is compatible with critical-neutron-scattering studies of nickel and with measurements of the static susceptibility. Below 385\ifmmode^\circ\else\textdegree\fi{}K, ${\ensuremath{\tau}}_{c}$ 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 $g=2.22$ below ${T}_{c}$ to $g=2.29$ 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.