Spin correlations and reentrant spin-glass behavior in amorphous Fe-Mn alloys: Statics

Neutron scattering studies have been performed on amorphous (Fe_1-xMnₓ)₇₅P₁₆B₆Al₃ 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 ^-1, and changes considerably for x near the multicritical concentration. For the most Mn-rich sample ($x=0.35$), the small-angle scattering is well described by a single Lorentzian. The corresponding inverse ferromagnetic correlation length κ remains nonzero (less than 0.04 ^-1) at all temperatures. For $x=0.32$, the Lorentzian scattering profile persists. As T is reduced, κ 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^-α with 2<α<3. 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.