Coexistence of antiferromagnetism and spin-glass ordering in the Ising system Fe0.55Mg0.45Cl2

Abstract
Fe1−xMgxCl2 is a diluted Ising antiferromagnet with competing first‐ and second‐neighbor exchange interactions. For x≊45, the system undergoes a Néel transition at TN ≊7.4 K and a reentrant spin‐glass transition at Tsg ≊3.0 K. From neutron scattering experiments we find that (i) the antiferromagnetic Bragg peak persists down to 1.2 K, well below Tsg; and (ii) the diffuse scattering becomes temperature independent below about 6.0 K, with the correlation length frozen at about 2.8 lattice spacings. These results suggest that long‐range antiferromagnetic order and spin‐glass‐like short‐range order coexist. Such a behavior is predicted by the infinite‐range model for spin glasses with strong uniaxial anisotropy.