Spin correlations near the ferromagnetic-to-spin-glass crossover (invited)

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Abstract
We have performed neutron scattering studies on two very different alloys which undergo transitions from ferromagnetic (FM) to spin‐glass (SG) states as the temperature is reduced. The alloys are EuxSr1−xS, a crystalline insulator, and (FexMn1−x)75P16B6Al3, an amorphous metal, and their FM‐SG multicritical points are at x≂0.50, T=4 K and x≂0.65, T=42 K respectively. In spite of the substantial differences between these materials, the neutron scattering data show that their spin correlations are remarkably similar. In particular, for the samples near the multicritical points, a single Lorentzian describes the magnetic scattering very well. Its width κ corresponds to a ferromagnetic correlation length ξ which, as T is reduced, first increases to a value indistinguishable from infinity, and then decreases to a finite value, as expected for a ferromagnet which evolves into a reentrant spin glass. As the Fe or Eu content is raised, the scattering function at low temperatures deviates increasingly from the Lorentzian form, and is better described by a power law Q−α with 2<α<3. We find no evidence for coexistence of ferromagnetic order with freezing of the transverse spin components, as proposed by Gabay and Toulouse. We argue on general grounds that the zero‐field Gabay‐Toulouse state cannot exist in real magnets. However, our results can be explained in terms of the random field effects which arise when ferromagnetic and spin‐glass order parameters are coupled together.