Weak antiferromagnetism and superconductivity in pseudo-binaryspinel compounds (Cu,Co)Co2S4 investigated by 59Co and63Cu magnetic resonance

Abstract

We have carried out a nuclear magnetic resonance study onpseudo-binary spinel compounds (CuxCo1-x)Co2S4 withx = 0-1.0 to deduce the varied impact of variations in theantiferromagnetic (AF) spin correlation and density of states (DOS) that can clearly define the combination of magnetism andsuperconductivity in the system. The Curie-Weiss-type behaviour ofthe Knight shift K for both 59Co and 63Cu on thetetrahedral A site and the temperature-independent K for 59Coon the octahedral B site indicate that the magnetism of the systemoriginates from 3d bands associated with the transition-metalelements on the A site. With the Cu substitution x for Co on the Asite, the negative Weiss temperature θ deduced from the Kdata initially decreases, takes a deep minimum around x = 0.7followed by a rapid increase for x→1. For the compoundswith x⩽0.3 and x>0.8, the nuclear spin-lattice relaxationrate T1-1 for the nuclei on both A and B sites has a T1/2dependence at high temperatures, which is characteristic ofthree-dimensional itinerant weak antiferromagnets. In theCu-rich region (x>0.7) the compounds transform into asuperconducting state below TS with no long-range magneticordering. From a T1-1 data analysis, we find that the increaseof TS correlates not only with the development of the AFspin correlation but also with the significant increase in the DOS atthe Fermi level of the 3d bands Nd(EF) associated withCo on the B site. T1-1 in the superconducting state ofCuCo2S4 (TS = 4.4 K), measured for 59Co on the Bsite utilizing the pure quadrupole resonance spectrum, has acoherence peak followed by an exponential decrease, indicating thatCuCo2S4 is an s-wave superconductor with an energy gap of2Δ = 4.14kBTS. We conclude that the appearance ofsuperconductivity for the compounds in the Cu-rich region originatesfrom the large increase in Nd(EF) associated with Coon the B site, and the development of the AF spin correlationoriginates from the increase in Nd(EF) associated withCu on the A site.