Helical Spin Resonance in the Cubic B-20 Fe_0.8Co_0.2Si Single Crystal with a Long-Period Helical Spin Structure

Abstract

Electron spin resonance has been studied at various frequencies (6–24 GHz) on B-20 Fe_0.8Co_0.2Si single crystal, which has a long-period helical spin structure (HSS) with a cone-ferromagnetic transition at a critical field H_c. Three resonance absorptions have been observed clearly: (a) behaving like a ferromagnetic resonance, (b) occuring around H_c and (c) assigned to be a helical spin resonance. The resonance frequency of (c) decreases with increasing the magnetic field. Such dependence does not agree with the resonance theory for a HSS resulting from a competition between symmetric exchange interactions (SEI’s), but it agrees well with that for a HSS resulting from a competition between the SEI and the antisymmetric exchange interaction. The present results together with our previous ones give the evidence that the HSS in Fe_0.8Co_0.2Si is of the latter type.