Spin correlations and reentrant spin-glass behavior in amorphous Fe-Mn alloys. II. Dynamics

Abstract

Inelastic-neutron-scattering measurements have been performed on amorphous ${({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})}_{75}{\mathrm{P}}_{16}{\mathrm{B}}_{6}{\mathrm{Al}}_{3}$ alloys for several concentrations $x$ bracketing the spin-glass---ferromagnetic multicritical point found from magnetization measurements. For $x=0.35$, the alloy is nonferromagnetic, and the inelastic spectra are dominated at all temperatures by a resolution-limited quasielastic peak. We find no evidence for propagating modes of any kind. In a 10-kG applied field, the central peak is substantially reduced, and dispersionless sidebands, centered at the Larmor precession frequency, appear. Three samples ($x=0.30,0.25, \mathrm{and} 0.20$) on the ferromagnetic side of the phase diagram have also been studied. Spectra were collected for one of these ($x=0.25$) above its Curie point. These data are consistent with ordinary spin diffusion. For all three samples, resolvable spin-wave peaks exist at temperatures below the Curie points but above the spin-glass transitions established in bulk measurements. As the temperature is reduced, the spin-wave stiffness first increases, as in normal ferromagnets, and then decreases. At the lowest temperatures, resolvable spin-wave peaks are absent, and the spectra are dominated by a resolution-limited (30 \ensuremath{\mu}eV full width at half maximum) quasielastic peak. For the most iron-rich material ($x=0.20$), true inelastic scattering coexists with the central peak, even at 5 K. This inelastic scattering broadens and decreases in intensity with increasing momentum transfer $Q$, in a manner consistent with simple spin-wave theory. The central-peak intensity decays according to a power law ${Q}^{\ensuremath{-}\ensuremath{\alpha}}$, with $\ensuremath{\alpha}\ensuremath{\gtrsim}2$. At temperatures near the spin-glass---to---ferromagnetic crossover, direct inspection of the raw data for $x=0.20$ unambiguously demonstrates the coexistence of the central peak with well-resolved spin-wave peaks. Our experimental results are discussed in terms of existing theories on the dynamics of spin-glasses and ferromagnets with frozen-in disorder. Also included are speculations on time-dependent random fields, and their relevance to the crossover from ferromagnetic to spin-glass---like behavior.