Weak Ferromagnetism in Co-Si Alloys

Cobalt-silicon alloys are one of the most intriguing classes of magnetic materials. CoSi and Co₂Si are close to ferromagnetism, so that nanostructuring and minor amounts of excess Co can trigger a quantum-phase transition from exchange-enhanced Pauli paramagnetism to weak ferromagnetism. The ferromagnetic states exhibit interesting properties, such as B20-type DM interactions and skyrmionic spin spirals [1, 2]. Our focus is on off-stoichiometric bulk Co₂Si, which we investigate experimentally and by model calculations. Depending on the wave-function overlap, there are two limiting types of magnetism, namely ordinary very weak itinerant ferromagnetism (as in ZrZn₂) and moments induced by excess Co (similar to 3d impurities in 4d/5d hosts). The two scenarios involve quasiparticles whose Curie constants cannot be analyzed in terms of traditional atomic-scale effective moments, namely ZrZn₂-type paramagnons and moments localized around excess Co atoms. The M(H, T) curves convolute random-anisotropy effects, thermal spin disorder, and Stoner excitations, but in both cases, magnetic measurements allow the simultaneous estimation of the two key quantities involved, namely the quasiparticle size and the quasiparticle moment, which we have done for the present system.