Исследование магнитных свойств ферроборатов неодима и самария методом спектроскопического эрбиевого зонда

Iron borates NdFe3(BO3)4 and SmFe3(BO3)4 activated with 1% erbium, with a huntite structure (space symmetry group R32) were investigated by the method of erbium spectroscopic probe. From an analysis of the temperature dependence of the transmission spectra in the region of the 4I15/2→4I13/2 transition in the Er3+ ion, it was found that both studied compounds order antiferromagnetically at TN ≈ 33 K into an easy-plane magnetic structure. No other phase transitions were found.

Crystal structure, phase transition and structural deformations in iron borate (Y0.95Bi0.05)Fe3(BO3)4in the temperature range 90–500 K

An accurate X-ray diffraction study of (Y0.95Bi0.05)Fe3(BO3)4single crystals in the temperature range 90–500 K was performed on a laboratory diffractometer and used synchrotron radiation. It was established that the crystal undergoes a diffuse structural phase transition in the temperature range 350–380 K. The complexity of localization of such a transition over temperature was overcome by means of special analysis of systematic extinction reflections by symmetry. The transition temperature can be considered to beTstr≃ 370 K. The crystal has a trigonal structure in the space groupP3121 at temperatures of 90–370 K, and it has a trigonal structure in the space groupR32 at 375–500 K. There is one type of chain formed by the FeO6octahedra along thecaxis in theR32 phase. When going into theP3121 phase, two types of nonequivalent chains arise, in which Fe atoms are separated from the Y atoms by a different distance. Upon lowering the temperature from 500 to 90 K, a distortion of the Y(Bi)O6, FeO6, B(2,3)O3coordination polyhedra is observed. The distances between atoms in helical Fe chains and Fe—O—Fe angles change non-uniformly. A sharp jump in the equivalent isotropic displacement parameters of O1 and O2 atoms within the Fe—Fe chains and fluctuations of the equivalent isotropic displacement parameters of B2 and B3 atoms were observed in the region of structural transition as well as noticeable elongation of O1, O2, B2, B3, Fe1, Fe2 atomic displacement ellipsoids. It was established that the helices of electron density formed by Fe, O1 and O2 atoms may be structural elements determining chirality, optical activity and multiferroicity of rare-earth iron borates. Compression and stretching of these helices account for the symmetry change and for the manifestation of a number of properties, whose geometry is controlled by an indirect exchange interaction between iron cations that compete with the thermal motion of atoms in the structure. Structural analysis detected these changes as variations of a number of structural characteristics in thecunit-cell direction, that is, the direction of the helices. Structural results for the local surrounding of the atoms in (Y0.95Bi0.05)Fe3(BO3)4were confirmed by EXAFS and Mössbauer spectroscopies.

Coupled R and Fe Magnetic Excitations in RFe3(Bo3)4 Multiferroics

Various resonance modes were observed in the transmission spectra of rare-earth iron borates RFe3(BO3)4 (R = Nd3+, Sm3+, Gd3+) at the frequency range 100-600 GHz, which were attributed to collective magnetic excitations in the exchange coupled Fe-and R-subsystems, i.e. antiferromagnetic (Fe) resonance and electron transitions in the R-ions. Strong interaction of the Fe and R oscillations was revealed and theoretically analyzed taking into account feature of the R-ion ground state. Intensities of the coupled modes (contributions to magnetic permeability) strongly depend on a difference of Fe and R ions g-factors that allows defining the sign of the latter. In particular, an appreciable intensity of exchange (Nd) modes in NdFe3(BO3)4 is caused by gNd,|| < 0 whereas in GdFe3(BO3)4 with gGd gFe 2 the exchange (Gd) modes were hided due to compensation of Fe and Gd contributions. In SmFe3(BO3)4, despite a negligible Sm g-factor, the Sm modes were clear observed due to their excitation via coupling with the Fe-subsystem.