Electron spin resonance study of the two-dimensional randomly mixed system K₂Cu_1-xCo_xF₄

The authors report the experimental results of electron spin resonance (ESR) studies made on K₂Cu_1-xCo_xF₄, a randomly mixed system of the two-dimensional Heisenberg ferromagnet K₂CuF₄ and the two-dimensional Ising antiferromagnet K₂CoF₄ which have orthogonal anisotropies to each other. From both the temperature and the angular dependence of the resonance field at low temperatures, three different kinds of ferromagnetic phase are confirmed, i.e. XY, oblique and Ising types, which coincides well with the results obtained by neutron scattering and susceptibility measurements. Critical behaviors of both linewidth and resonance field are also reported and discussed. In addition, it is reported that the temperature dependence of the linewidth and the resonance field of the sample on the Co concentration of the spin-glass region show a common behaviour generally observed in many other spin glasses.