Amorphous antiferromagnets and spin glasses

Buyers et al (1971) discussed evidence for the existence of Anderson-localized spin states m antiferromagnets in which there is disorder. They investigated the substitutionally disordered materials K(Co,Mn)F3 and (Co,Mn)F2, finding two branches of propagating spin waves corresponding to the two constituents and 

In a series of papers on cobalt and other phosphate glasses, Simpson (1970) and Simpson and Lucas (1971) showed that there is no sign of a Neel temperature down to -IK while the corresponding crystals show a Neel temperature near 20 K. In fact the 1//-T curve shows increased slope at low temperatures. Simpson pointed out that the theorem of Ziman (1952) (see Section 2)—that nearest-neighbour interaction cannot give antiferromagnetism for spherical orbitals— may be applicable here the orbitals are not spherical but are oriented at random. 

There is no adequate theory of the Neel temperature of a random distribution of centres in a dilute alloy, of indeed one exists. For higher concentrations of the magnetic matrix, with the assumption that only nearest neighbours interact, there is considerable theoretical work, giving a percolation limit , the concentration c0 at which long-range order disappears. The behaviour of TN is as (c —c0)12. For details see Brout (1965), Elliott and Heap (1962) and Klein and Brout (1963). 

In an amorphous ferromagnet (a sputtered him with the composition Tb0.33Fe0 67) Rhyne et al (1972) used neutron diffraction to demonstrate a random direction of moments with average ferromagnetic orientation. This material has a fairly sharp Curie temperature in the range 380-390 K, the moment below this temperature reaching saturation at about 50kOe. 

It is worth pointing out here, however, that 1/ (-T curves of this form are also predicted for a highly correlated electron gas (Chapter 4, Section 9). 

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