Conceptual phase diagram and its application to the spontaneous magnetism of several pyrites

A one-electron energy diagram is constructed for FeS ₂, and its modification for CoS ₂ and NiS ₂ are discussed. A T- n_l- b phase diagram is constructed, where n_l gives the occupancy of the narrow σ∗ band determined by chemistry and b ∼ ∊λ_σ is proportional to the width of the narrow σ∗ band in the itinerant-electron domain. Here ∊ is a one-electron energy and λ_σ is the e_g-( sp³) covalent-mixing parameter for σ-bonding orbitals. These two diagrams are used to interpret the varied magnetic properties of the systems Fe _{1- x}Co _xS ₂, Co _{1- x}Ni _xS ₂, CoS _{2- x}Se _x, and CoS _{2- x}As _x. In the mixed systems, the σ∗ bands must be separated into donor and acceptor bands, and it is shown how this may be done for each system. The ferromagnetic to antiferromagnetic transition in CoS _{2- x}Se _x is attributed to the appearance of a spin-density wave in the narrow transitional range of b between full-moment ferromagnetism ( μ_Co = 1 μ_B) and Pauli paramagnetism.