Anion substitution effects on structure and magnetism of the chromium chalcogenide Cr 5Te 8—Part II: Cluster-glass and spin-glass behavior in trigonal Cr (1+x)Q2 with basic cells and trigonal Cr (5+x)Q8 with superstructures ( Q=Te, Se; Te:Se=6:2)

Abstract

The effect of substitution of up to two Te atoms by Se atoms on the crystal structure, the magnetic and electronic properties has been studied in the system Cr (5+ x )Te 8− y Se y . Trigonal basic cells with space group P - 3 m 1 for Cr (1+ x ) Q 2 and trigonal supercells (doubling of the unit cell in all directions) with space group P - 3 m 1 for Cr (5+ x ) Q 8 ((1+ x)=1.27, 1.32, 1.36; (5+ x)=5.08, 5.28, 5.44; Q=Te, Se; Te:Se=6:2) have been identified in X-ray powder diffraction patterns and Rietveld refinements as high-temperature and low-temperature phases, respectively. The crystal structures are related to the hexagonal NiAs-type structure with metal vacancies in every second metal layer. The magnetic properties are closely related to the Cr content and the structure type. Cluster-glass and spin-glass behavior at low temperatures are observed for high and low Cr contents, respectively. For the same Cr content, the phases with trigonal basic cells have higher values for the Curie temperature T c and the freezing temperature T f, and larger magnetization than those for the phases with trigonal supercells. For the same structure type, the values for T c/ T f do not show a linear relationship with the change of Cr content but exhibit a V-shape fashion. Our experimental investigations were accompanied by spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) electronic structure calculations. Cr deficiencies as well as atomic disorder on the chalcogen sites was accounted for using the coherent potential approximation (CPA) alloy theory. Calculation of the exchange coupling parameters J ij provided the basis for subsequent Monte Carlo simulations of the magnetic properties at finite temperatures.