Experimental System for Studying Itinerant d-Electron Ferromagnetism

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 n<0.05 the electrical resistivity vs temperature curves exhibit shallow minima at 20°K indicative of localized states. The loss of ferromagnetism below n≈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<n<0.15, but the curve of magnetization vs applied field does not saturate, and μs 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.