Experimental System for Studying Itinerant d-Electron Ferromagnetism

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Abstract
Typical theoretical models of an itinerate electron ferromagnet consist of a single narrow band, but experimental data have been obtained primarily from the ferrous metals whose behavior is more complex because of overlapping broad s bands and narrow d bands. Transition metal sulfides of the cubic pyrite structure appear to have a doubly degenerate conduction band arising from atomic d orbitals of eg symmetry without overlapping s bands.1 The full band would contain four electrons per metal atom. Metallic conductivity is observed, except for the empty FeS2, exactly half‐filled NiS2, and filled ZnS2 bands. The electron concentration, n, can be varied from zero to one electron per metal ion in the Fe1−xCoxS2 systems (n=x) and can be extended to higher values by use of the Ni1−yCoyS2 system (n=2−y).2 In these systems the effective moment obtained from paramagnetic susceptibility measurements is that of the isolated cobalt and nickel ions. It appears, therefore, that a model based on strong electron‐electron correlation at the metal‐ion sites would be applicable to these pyrite compounds. Such a model for a nondegenerate band has been given extensive theoretical treatment by Hubbard.3,4 Solutions for the degenerate case have also been indicated,5 and calculations of the Hubbard model for the two‐fold degenerate band are currently being carried out.6 Significant features of the pyrite system which should be qualitatively explained by a theoretical model are: (1) Ferromagnetism exists down to very low electron concentrations. A plot of Curie temperature vs n goes to zero at n≈0.05. For nn≈0.05 may thus be due to loss of itinerancy and not to a critical electron concentration. (2) The ferromagnetic moment, μs, is equal to one Bohr magneton per electron over a wide range of electron concentration 0.15 ≤n≤0.95. Ferromagnetic behavior is observed for 0.05<ns appears to be less than n. (3) In Ni1−yCoyS2 (n>1) the saturation magnetization, Curie temperature, and Weiss constant decrease rapidly with Ni concentration, indicating strong antiferromagnetic Ni–Co interactions. Ferromagnetism disappears for n>1.1. This would be expected if strong correlations at the Ni sites dominate. (4) In Fe1−xCoxS2, μs decreases from 0.95 to 0.89 as n is increased from 0.95 to 1.0, but the effective moment in the paramagnetic state continues to increase linearly with cobalt concentration. Neutron‐diffraction studies indicate a collinear spin state.7 A localized moment model which seems to explain much of the magnetic behavior cannot account for this decrease in μs with increasing Co concentration.

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