Magnetic and Electrical Properties of the CuCr2Se4−xBrx System

Abstract

The system CuCr₂Se_4−xBr_x has been prepared both in powder and in single‐crystal forms. The lattice parameter increases linearly from 10.325 to 10.405 Å as x increases from 0 to 1. The magnetic moment also increases linearly from 5 μ_B/mole to 6 μ_B/mole, and the ionic configuration Cu_1−x²⁺Cu_x¹⁺[Cr₂³⁺]Se_4−x⁻²Br_x¹⁻ applies well to this system. The Curie temperature suddenly drops over 100° in the small range 0.98≲x≲1.0. This sudden change in T_c is unusual and is one of the most significant facts found in this work. It has also been found that both the electrical resistivity and Hall effect (p‐type) change very sharply near x=1 and that the system becomes semiconductive only for 0.98≲x≲1. A strong correlation is thus found between the Curie temperature and the electrical properties (electrical resistivity ρ and anomalous Hall coefficient R′). Replotting the electrical resistivity and the Hall coefficient as a function of the Curie temperature for arbitrary composition x, we found a simple relationship between them. $$\mathrm{\rho \propto R\prime \propto }\exp {\text{[−(2.5±0.3)×10}}^{\text{−2}}{\mathrm{·T}}_c]$$ . This relationship is satisfied over the whole system, 0≥x≥1 (covering both the metallic and semiconductive ranges). Here, the temperature dependence of ρ is not large and the above equation covers the measured temperature range, 77° to 300°K. The Hall voltage for the semiconductive range is found to be proportional to the magnetic moment. On the other hand, the Hall voltage for the metallic range takes its maximum value at the Curie temperature and saturates in high fields. Therefore, R′ in the above equation was chosen as the maximum value of the curves of R′ versus T with x as a parameter. These values of R′ correspond to points of maximum scattering in the (R′, X, T) surface. This indicates that both ρ and R′ are dominated by the magnetic exchange interactions. These properties are qualitatively understood in terms of the itinerant d‐hole carriers of the Cu²⁺ ions which exchange couple to the localized spins of the Cr³⁺ ions and enhance the magnetic exchange interactions in the system (Carrier Enhanced Magnetic Exchange). Nevertheless, no theory yet exists to explain the remarkable relation described above. A more complete description of this research will be published elsewhere.