Magnetism and Superconductivity in Copper Spinels

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Abstract
Spinel type copper sulfides, Cu1+x Rh2-x S4(x=0–1) and Cu1-x Co2-x S4(x=0–0.75), were synthesized, and the crystallographic properties, ac and dc susceptibilities and electrical resistivity were experimentally studied in relation with their magnetic and superconductive properties. Cu1+x Rh2-x S4 showed Pauli paramagnetism, metallic conductivity and a superconducting transition at 4.75 K with a ∼100% Meissner fraction. The copper doping in the B site did not cause any prominent effect on these properties. In contrast to this, Cu1+x Co2-x S4 showed a Curie-Weiss type antiferromagnetic susceptibility with a constant term χ0=2.35×10-6 emu/Oe·g, Néel temperature T n=18.0 K, effective Bohr magneton µeff=0.89µB/f.u. and paramagnetic Curie temperature θP=-52 K. AC susceptibility showed diamagnetism due to superconductivity as x increases and reached ∼100% Meissner fraction at x=0.5. Metallic type resistivity was observed for the whole range of x. The zero resistivity temperature depends on x and is 2.3 K for x=0.5. It was concluded that the magnetic moment of the itinerant 3d hole of Cu2+(3d9) in the system Cu1+x Co2-x S4 orders antiferomagnetic way below the Néel temperature and transfers to or coexists with a superconductive state below the critical temperature. The copper doping in the B site thus caused a prominent effect on the superconductivity, on which collaborative effect of the coexisting second phases of Cu7S4 and Cu9S4 with the XRD intensity of 0.5–5% still remains to be clarified.

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