Evidence for a collinear easy-plane magnetic structure of multiferroic EuF e3(B O3)4: Spectroscopic and theoretical studies

We performed high-resolution polarized optical transmission spectroscopy and theoretical studies of multiferroic EuFe3(BO3)4 single crystals in the three phases: paramagnetic R32(T>Ts=84K) and P3121(Ts>T>TN=34K), and antiferromagnetic (T<TN) ones. The analysis of the spectra reveals interesting details of the magnetic structure, namely, a collinear arrangement of the iron magnetic moments along the C2 symmetry axis in the ab crystallographic plane of EuFe3(BO3)4 below TN. Spectral signatures of the phase transitions and the spin-phonon interaction are observed and discussed. Reliable crystal-field and exchange-interaction parameters are obtained and used to model the magnetic susceptibility of the compound. The results of detailed calculations of the electric polarization of EuFe3(BO3)4 in the R32 phase are presented, and mechanisms of the magnetoelectric response are discussed. We detect a strong effect of impurities (that enter the crystal from a flux in the course of the crystal growth) on the structural phase-transition temperature and demonstrate a coexistence of both R32 and P3121 phases down to the lowest temperatures in a EuFe3(BO3)4 crystal grown with the Bi2Mo3O12 based flux, due to inhomogeneous distribution of impurity Bi3+ ions. Our study can be considered as a demonstration of the abilities of optical spectroscopy in delivering new information on a magnetic compound, even in the case when other methods fail.