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chromium chalcogenide; anion substitution; Rietveld refinement; low field magnetization; cluster-glass; spin-glass; band-structure calculations
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)Te8-ySey.
Trigonal basic cells with space group P = 3ml for Cr((1+x))Q(2) and
trigonal supercells (doubling of the unit cell in all directions) with
space group P - 3ml 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.
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. (C) 2006
Elsevier Inc. All rights reserved.
