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

Neutron scattering studies have been performed on amorphous (Fe/sub 1-x/Mn/sub x/)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 A-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 kappa remains nonzero (less than 0.04 A-1) at all temperatures. For x = 0.32, 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/sup -alpha/ 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