Neutron scattering studies have been performed on amorphous ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_{75}{\mathrm{P}}_{16}{\mathrm{B}}_6{\mathrm{Al}}_3$ alloys for several concentrations $x$ bracketing the spin-glass-ferromagnetic multicritical point found from magnetization measurements. The amorphous structure factor has been measured to 4.0 ${\mathrm{\AA }}^{-1\mathrm{}}$, and changes considerably for $x$ near the multicritical concentration. For the most Mn-rich sample ($x=0.35\mathrm{}$), the small-angle scattering is well described by a single Lorentzian. The corresponding inverse ferromagnetic correlation length $\kappa$ remains nonzero (less than 0.04 ${\mathrm{\AA }}^{-1\mathrm{}}$) at all temperatures. For $x=0.32\mathrm{}$, the Lorentzian scattering profile persists. As $T$ is reduced, $\kappa$ decreases to a value indistinguishable from zero and subsequently increases, as it should for a ferromagnet which evolves into a reentrant spin-glass. For progressively smaller $x$, the scattering function at low temperatures shows increasing deviations from the Lorentzian form, and instead is consistent with a power law $Q^{-\alpha }$ with $2<\mathrm{}\alpha <3\mathrm{}$. These results are very similar to those found in other alloy series which display both ferromagnetic and spin-glass behavior. We argue that this power-law form of the spin correlations in the reentrant phase provides an important clue to the nature of the ferromagnet—spin-glass transition and the reentrant state itself. This leads us to a detailed heuristic model for the phase diagram and phase-transition behavior, including the reentrant phenomenon. The model is based on a decomposition, via the frustration mechanism, of spin systems with exchange interactions of random sign, into spin-glass-like and ferromagnetic networks. Many of the experimental results are explained in terms of random-field effects which arise when the ferromagnetic and spin-glass order parameters are coupled together.

• Received 14 June 1983

DOI:https://doi.org/10.1103/PhysRevB.28.5160