Electrical Transport Properties of the Insulating Ferromagnetic Spinels CdCr2S4 and CdCr2Se4

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Abstract
Electrical transport properties were measured on the recently discovered ferromagnetic spinels CdCr2S4 (Tc=85°K) and CdCr2Se4 (Tc=130°K). The large Cr−Cr separation (≥3.63 Å) excludes metallic conductivity due to Cr−Cr overlap. Electrical conductivity, Seebeck effect, Hall effect, and magnetoresistance measurements were made on high‐density, polycrystalline samples (CdCr2S4: 99.6%; CdCr2Se4: 99.9% of the theoretical density). Both materials show a negative temperature coefficient of the resistivity in the para‐ and ferromagnetic region without any discontinuity at the Curie temperature. The room‐temperature conductivities of CdCr2S4 and of CdCr2Se4 are 5×10−4 (Ω·cm)−1. At 77°K the conductivity of CdCr2S4 is 3.5×10−10 (Ω·cm)−1 and of CdCr2Se4 6.7×10−5 (Ω·cm). The Seebeck coefficient for both materials is rather small: (CdCr2S4: −60 μV/°K; CdCr2Se4: +60 μV/°K), which indicates that the materials are almost compensated. The room‐temperature mobility of CdCr2Se4 is 7 cm2/Vsec. In the ferromagnetic region CdCr2Se4 exhibits a negative magnetoresistance effect. An applied field of 7 kG results in a 4% increase of the conductivity. Band formation in CdCr2Se4 due to overlap of the 4p orbitals of the Se ions is proposed.