Magnetic susceptibility of ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$S and ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$Se alloys

Static magnetic susceptibility has been investigated in two cobalt-based diluted magnetic semiconductors, ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$S and ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$Se. The measurements were performed in the temperature range 4.2 K≤T≤300 K, using a vibrating-sample magnetometer. The Co concentration (determined by x-ray fluorescence) was x≤0.145 for the sulfide samples, and x≤0.048 for the selenides. The susceptibility in both systems displays a high-temperature Curie-Weiss behavior, which is qualitatively similar to that observed in Mn-based diluted magnetic semiconductors. From quantitative analysis of the high-temperature behavior within the framework of the mean-field approximation, we obtain the value of the ${\mathrm{Co}}^{2+}$ spin as 1.43±0.10 for the sulfides, and 1.47±0.10 for the selenides, i.e., in good agreement with the value of (3/2 expected for the isolated ${\mathrm{Co}}^{2+}$ ion. The nearest-neighbor ${\mathrm{Co}}^{2+}$-${\mathrm{Co}}^{2+}$ exchange integral J/$k_B$ for the sulfides and the selenides is found to be -47±6 K and -54±8 K, respectively. This value is at least three times as large as that for the ${\mathrm{Mn}}^{2+}$-${\mathrm{Mn}}^{2+}$ exchange integrals in ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$S and ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Se alloys. The origin of such strong antiferromagnetic coupling in Co alloys is not presently understood.