Low-temperature magnetization and spin-wave excitations in amorphous Ni-rich transition-metal—metalloid alloys

Magnetization as a function of temperature has been measured for a number of amorphous Ni-rich transition-metal—metalloid alloys in the temperature range 4.2-300 K at various constant applied magnetic field values. These data when analyzed with caution not only yield reliable values for the coefficients $B$, $C$ of the $T^{\frac{3}{2}}$, $T^{\frac{5}{2}}$ terms appropriate for zero spin-wave energy gap and the mean-square range $〈r^2〉$ of the exchange interaction but also give $g$ values in agreement with those determined directly from the ferromagnetic resonance experiments. As a function of the Curie temperature $T_C$, the above parameters ($B$, $C$, $〈r^2〉$) are found to exhibit a systematic trend which is consistent with the predictions of a spinwave theory based on the nearest-neighbor (NN) Heisenberg model. An empirical relation $D=D_0+m{T}_C$ is found to exist between the spin-wave stiffness coefficient, $D$, and $T_C$. While the collective-electron and NN Heisenberg models both fail to explain the finite positive value observed for $D_0$, the latter model gives the observed slope, $m$, value for $S=1\mathrm{}$. Besides providing a theoretical justification for the observed relation between $D$ and $T_C$, it has been shown that the Ruderman-Kittel-Kasuya-Yosida interaction plays a negligible role so far as the exchange mechanism leading to the present magnetization behavior is concerned. An estimate of the NN and next-nearest-neighbor (NNN) exchange coupling constants $J_1$ and $J_2$ reveals that $J_2$ is at least 1 order of magnitude smaller than $J_1$. Arguments are presented to show that the superexchange interactions brought into play by the presence of metalloid atoms and leading to an antiferromagnetic coupling between the spins localized on the NNN transition-metal atoms cannot be present in the amorphous alloys in question.