We report on the electrical and magnetic properties of the layered ternary sulfide system BaCoi_ x Ni x S2-y . Ni doped samples of BaCoS2 that contain sulfur vacancies exhibit a first-order phase transition from a high temperature antiferromagnetic, semiconducting phase to a low temperature metallic phase. Increasing the sulfur vacancy concentration increases the Neel temperature of the two-dimensional antiferromagnetic phase and decreases the resistivity in the metallic phase. As the Ni content is increased, the system undergoes a smooth transition to a paramagnetic metal at all temperatures.PACS numbers: 71.27.-fa, 71.30.+h, 75.50.Ee Ternary sulfides of alkaline earth metals with transition metals provide a class of materials that might be expected [1,2] to have several common features with highly correlated systems such as the intermediate valence compounds and the copper oxide superconductors. We have investigated the BaCoi_ x Ni a; S2-2/ system, and in this Letter we report the discovery of a discontinuous phase transition in BaCoo.9Nio.iS2-2, for 0.05 < y < 0.20 from semiconducting to metallic behavior with decreasing temperature. Above the transition temperature the materials are Mott-Hubbard insulators with localized states near the Fermi energy arising from the substitution of Ni for Co. There is also evidence of two-dimensional antiferromagnetism above the transition. The metallic phase below the transition is similar to that of the highly correlated paramagnetic metal BaNiS2 where there is mixed valency associated with the Ni and S ions. This first-order phase transition shows substantial hysteresis and involves a structural transformation to a commensurate superlattice state for the low temperature phase.We present resistivity and magnetic susceptibility data for single-phase polycrystalline BaCoi_ x Ni x S2-y samples all having the BaNiS2 crystal structure except at temperatures below the phase transition. The BaNiS2 [3] structure is shown in Fig. 1. In this tetragonal structure the transition metal ions are pentacoordinated to sulfur in a square-pyramidal environment. The apical sulfurs of the edge sharing pyramids alternate above and below the plane formed by the bases of the pyramids. These apical sulfurs together with the Ba atoms form slightly distorted rock salt layers between the Ni2S2 sheets. Within the sheets the transition metal ions alternate above and below the plane containing the sulfurs.In the BaNiS2 structure, the distance between neighboring Ni atoms is less than half the distance between the sheets. Hence it is expected that the Ni2S2 sheets determine the electric and magnetic properties much as the Cu02~2 sheets determine these properties in La2CuC>4. Apart from the net charge on the sheets, these two systems primarily differ by the additional Ni per unit cell of the sheet and the quadratic pyramidal symmetry of the crystal field at the Ni sites in BaNiS2 versus the octahedral symmetry at the Cu sites in La2CuC>4. However, the important nearest-neighbor Ni-S coordination in the she...

We have performed both ac susceptibility and dc magnetization measurements on the diluted magnetic semiconductor Zn 1Ϫx Mn x Te. The measurements clearly indicate spin-glass behavior. For samples with concentrations xϭ0.51 and xϭ0.41, the data for the imaginary part of the complex susceptibility (Љ) were analyzed according to conventional power-law dynamics and good scaling was obtained with the critical exponent values zϭ10Ϯ2 and ␤ϭ1.0Ϯ0.2. These values of z and ␤ are consistent with results obtained in other insulating spin-glass systems with short-range interactions. Because of the presence of significant Dzyaloshinsky-Moriya anisotropy in these materials, an attempt was also made to fit the data using an activated dynamics model. However, the scaling of the Љ data was less satisfactory in this case. Magnetization measurements on the xϭ0.51 sample also showed a spin-glass-like transition. Scaling of the nonlinear magnetization just above the transition gave T c ϭ20.8Ϯ0.2 K, and the critical exponent values ␥ϭ4.0Ϯ1.0 and ␤ ϭ0.8Ϯ0.2. The value of T c obtained from the static magnetization measurements is in good agreement with the value T c ϭ20.7Ϯ0.05 K obtained from the dynamic scaling analysis. Further, the value for the critical exponent ␥ obtained in this work is in fair agreement with values reported for other spin-glass materials. These results represent convincing evidence that diluted magnetic semiconductors are a subset of the class of insulating spin-glass materials with short-range interactions.

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