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We describe here a metallic ferromagnetic system Fe1 Co S2 and Co1 &Ni S2 that appears to have a single doubly degenerate band. The electrons in the band are itinerant as manifested by metallic electrical conductivity, but the magnetic moment is that of the isolated ion.For several decades localized as opposed to itinerant-electron ferromagnetism in metals has been investigated theoretically. ' However, experimental systems on which to base theoretical models of band ferromagnetism are often complicated by the presence of composite bands as, for example, in the elemental transition metals which are comprised of overlapping broad s and narrow d bands. Such quantities as density and itinerancy of d electrons, magnetization of the & electrons, and the magnitude of +-d exchange are difficult to measure directly. The compounds to be discussed appear to avoid the complicating features introduced by the presence of such & electrons.All transition metal disulfides, FeS, through ZnS2, crystallize in a cubic pyrite structure in which the cations are in face-centered positions.The physical properties of the series' are summarized in Table I. Each successive member of the series represents the progressive addition of one d electron. In much of this series, a continuous range of solid solutions of adjacent members, e.g. , Fe1 xCoxS2 and Co1 &Ni~S2 for 0 &x~1 , can be formed. 3 The unit cell dimension increases monotonically with increasing atomic number of the cation. These solid solutions are metallic conductors' (Fig. 1) except for dilute concentrations (&0.1%) of CoS2 in either FeS, or NiS2. Since the beginning and end members of the series are both semiconducting while all other members (except near NiS, ) show metallic conductivity, it is not inconsistent to conclude that Table I. Magnetic and transport properties of pyrite compounds. FeSs Temp. independent paramagnetic -6 X = 0.31 + .04 x 10 emu/gm Semiconductor M = 0.92 + .05 ev M 0.96 + .02 ev 6 2g CoS~Ferromagnetic =0. 9~~T = 120K Metallic Plasma Edge

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