Collinear and noncollinear magnetism in transition-metal alloys

We discuss collinear and noncollinear magnetism in transition metal alloys such as Fe-Ni on the basis of ab initio calculations. It is shown that osci llating interatomic exchange integrals and chemical ordering tendencies det ermine the behavior of the individual magnetic moments leading to antiferro magnetic and noncollinear admixture of magnetic moments in the ferromagneti c-like ground state. We explain how the chemical environment of the magneti c moments leads to an intrinsic magnetovolume instability which is responsi ble, among others, for unusual lattice softening and the Invar effect.