Ferrimagnetism of ${\mathrm{Cu}}_{0.45}$${\mathrm{Co}}_{0.55}$${\mathrm{Cr}}_2$${\mathrm{S}}_{4\mathrm{-}\mathrm{y}}$${\mathrm{Se}}_{\mathit{y}}$

Powdered samples of the new ferrimagnetic spinel series ${\mathrm{Cu}}_{0.45}$ ${\mathrm{Co}}_{0.55}$ ${\mathrm{Cr}}_2$ ${\mathrm{S}}_{4\mathrm{-}\mathrm{y}}$ ${\mathrm{Se}}_{\mathit{y}}$ for 0.0≤y≤1.5 have been obtained. The results of measurements of magnetization isotherms in applied fields up to 14 T at 4.2 and 77 K, the magnetization curves in pulsed fields in the temperature range of 77 K≤T≤300 K, the Curie temperature, the anisotropy field, and the asymptotic Curie-Weiss temperature are summarized. The magnetic moment, the temperature ${\mathrm{FTHETA}}_{\mathrm{CW}}$, and the Curie constant exhibit anomalous behavior in the concentration range of 0.5<y<1.0. It turns out that the magnetic properties of these spinels can be explained if one assumes the existence of monovalent Cu ions and of Cr ions in two valence states: ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Cr}}^{4+}$, the concentration ratio [${\mathrm{Cr}}^{3+}$]:[${\mathrm{Cr}}^{4+}$] being equal to 1.55:0.45. The terms of the molecular-field approximation were used to explain the magnetic properties of the compounds under study. The ground-state energy and the exchange integrals for three types of interactions between the nearest neighbors ${\mathrm{Cr}}^{\mathrm{n}+}$-$X^{2\mathrm{-}}$-${\mathrm{Cr}}^{\mathrm{m}+}$, ${\mathrm{Co}}^{2+}$-$X^{2\mathrm{-}}$-${\mathrm{Co}}^{2+}$, and ${\mathrm{Cr}}^{\mathrm{n}+}$-$X^{2\mathrm{-}}$-${\mathrm{Co}}^{2+}$ have been calculated.