Hidden Antiferromagnetic Order in EuB 6 Revealed by X-Band ESR

The temperature dependences of the resonance field and the linewidth were measured on the single crystals EuB6 − x C x with x = 0 and 0.07 by X-band electron spin resonance (ESR). It was observed the magnetic phase separation in spin system of Eu2+ ions. We assume the reason of the separation is due to the self-localization of free charge carriers (electrons and holes), i.e., the formation of magnetic polarons (ferrons) of two types. The self-localization of free charge carriers is associated with local crystal symmetry breaking. Antiferromagnetic interaction between ferrons of different types leads to attenuation of ferromagnetic interactions. It is manifested in the temperature behavior of the resonance field and the linewidth of ESR.