The formation of three-dimensional magnetic ordering has been studied on a quasi-one-dimensional magnet CoNb 2 O 6 by mean-field calculations as well as neutron scattering measurements down to Tϭ1.5 K under magnetic fields up to H ʈ c ϳ600 Oe. Measurements of a deviation of the magnetic Bragg scattering function from the delta function in the ordered state reveal a surprisingly rich variety of the magnetic formation arising from an isosceles triangular arrangement of the magnetic chain with competing interchain interactions in the a-b plane. The competing interactions result in quasidegenerate ground states with different propagation wave numbers along the b* direction in the sinusoidally amplitude-modulated incommensurate magnetic ͑IC͒ phase. Our mean-field calculations qualitatively reproduce the complicated H ʈ c -T magnetic phase diagram and give evidence for a high degeneracy of ground states by calculating the H ʈ c -T dependence of the free energy curve in the propagation wave number space. In addition, a partial cancellation of the exchange field at the apex site from the base sites on the isosceles triangular lattice leads to a quasi-long-range ordering along the a axis in both IC and antiferromagnetic states where the correlation length along the a axis depends on the propagation wave number along the b* direction.

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We have carried out a nuclear magnetic resonance study on pseudo-binary spinel compounds (Cu x Co 1−x )Co 2 S 4 with x = 0-1.0 to deduce the varied impact of variations in the antiferromagnetic (AF) spin correlation and density of states (DOS) that can clearly define the combination of magnetism and superconductivity in the system. The Curie-Weiss-type behaviour of the Knight shift K for both 59 Co and 63 Cu on the tetrahedral A site and the temperatureindependent K for 59 Co on the octahedral B site indicate that the magnetism of the system originates from 3d bands associated with the transition-metal elements on the A site. With the Cu substitution x for Co on the A site, the negative Weiss temperature θ deduced from the K data initially decreases, takes a deep minimum around x = 0.7 followed by a rapid increase for x → 1. For the compounds with x 0.3 and x > 0.8, the nuclear spin-lattice relaxation rate T −1 1 for the nuclei on both A and B sites has a T 1/2 dependence at high temperatures, which is characteristic of three-dimensional itinerant weak antiferromagnets. In the Cu-rich region (x > 0.7) the compounds transform into a superconducting state below T S with no long-range magnetic ordering. From a T −1 1 data analysis, we find that the increase of T S correlates not only with the development of the AF spin correlation but also with the significant increase in the DOS at the Fermi level of the 3d bands N d (E F ) associated with Co on the B site. T −1 1 in the superconducting state of CuCo 2 S 4 (T S = 4.4 K), measured for 59 Co on the B site utilizing the pure quadrupole resonance spectrum, has a coherence peak followed by an exponential decrease, indicating that CuCo 2 S 4 is an s-wave superconductor with an energy gap of 2 = 4.14k B T S . We conclude that the appearance of superconductivity for the compounds in the Cu-rich region originates from the large increase in N d (E F ) associated with Co

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.