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

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.