Low-temperature absorption spectra and electron structure of HoFe₃(BO₃)₄ single crystal

Polarized absorption spectra of HoFe₃(BO₃)₄ single crystal in the range of 8500–24 500 cm−¹ were studied as a function of temperature beginning from 2 K. The ground and excited electron states of Ho³⁺ were identified. The abrupt changes of the spectra at the reorientation phase transition at 4.7 K were observed. The exchange splitting of some excited states were revealed and measured. They changed at the reorientation phase transition. Several vibronic transitions were observed. The splitting of absorption lines corresponding to the C₂ local symmetry of the Ho ion was not observed. Moreover, spectra of some absorption bands correspond to splitting in the cubic crystal field. There are some absorption lines, whose polarization cannot be explained both in D₃ and C₂ local symmetries. Some lines appear or disappear as a result of the transition from the easy axis to the easy plane state of the crystal. All these observations testify to the substantial changes of the local magnetic and structural properties in the excited states of the Ho³⁺ ion and to the strong influence of the magnetic moments orientation on the polarization of the electron transitions

High-resolution optical spectroscopy and modeling of spectral and magnetic properties of multiferroic ErFe<inf>3</inf>(BO<inf>3</inf>)4

© 2020 American Physical Society. We carried out the high-resolution broadband temperature-dependent polarized optical spectroscopy and theoretical studies of ErFe3(BO3)4 single crystals in the paramagnetic and antiferromagnetic (T<TN=39K) phases. On the basis of the experimentally determined 45 crystal-field (CF) levels of Er3+ ions at sites with the C2 point symmetry, CF calculations were performed, a set of physically grounded CF parameters was obtained and used to model the temperature dependences of the Er magnetic moments measured in neutron-scattering experiments, as well as the magnetic susceptibility and magnetization of the compound; the contributions of the quasi-one-dimensional iron magnetic subsystem were calculated in the frame of the previously developed self-consistent four-particle cluster model. The modeling strongly supports an easy-plane collinear structure of iron magnetic moments and excludes earlier proposed additional magnetic phase

Holmium iron borate: high-resolution spectroscopy and crystal-field parameters

High-resolution transmission spectra of HoFe3(BO3)4 single crystals were measured in broad spectral (5000-23000 cm−1) and temperature (1.7-300 K) ranges. Crystal-field energies of the Ho3+ ions were determined for a paramagnetic and easy-axis antiferromagnetic phases of the compound. On the basis of these data and of preliminary crystal-field calculations in the frame of the exchange-charge model, crystal-field parameters were found. A parameter of the isotropic Ho-Fe exchange interaction was estimated

Magnetic phase transitions in the NdFe₃(BO₃)₄ multiferroic

Low-temperature studies of the behavior of the sound velocity and attenuation of acoustic modes have been performed on a single crystal NdFe₃(BO₃)₄ Transitions of the magnetic subsystem to the antiferromagnetically ordered state at TN ≈ 30.6 K have been revealed in the temperature behavior of the elastic characteristics. The features in the temperature behavior of elastic characteristics of the neodymium ferroborate and its behavior in the external magnetic field, applied in the basic plane of the crystal, permit us to suppose that the transition to an incommensurate spiral phase is realized in the system. This phase transition behaves as the first order one. H–T phase diagrams for the cases H || a and H || b have been constructed. The phenomenological theory, which explains observed features, has been developed

High-resolution spectroscopy, crystal-field calculations, and quadrupole helix chirality of DyFe

High-resolution polarized transmission spectra of DyFe3(BO3)4 single crystals were investigated in broad spectral (10-23000 cm−1) and temperature (3.5-300 K) ranges. Energies of the dysprosium levels in the paramagnetic and antiferromagnetic phases were determined. On the basis of these data and preliminary calculations in the frameworks of the exchange-charge model, we determined the crystal-field and Dy-Fe exchange interaction parameters of the Dy3+ ions at sites with the point C2 symmetry corresponding to the enantiomorphic P3121 and P3221 space groups. The values of electronic quadrupole moments of the Dy3+ ions were calculated, which enabled us to interpret results of the work [Usui et al., Nature Mater. 13, 611 (2014)] on the observation of domains of different quadrupole chirality in DyFe3(BO3)4

Nature of optical properties of GdFe

Influence of the partial substitution of paramagnetic Fe3+ ions by diamagnetic Ga3+ ions in the trigonal crystal GdFe3 (BO3)4 on its optical and magnetic properties is studied and discussed in connection with problems common for all antiferromagnets containing 3d5 ions. Polarized optical absorption spectra and linear birefringence of GdFe3 (BO3)4 and GdFe2.1Ga0.9 (BO3)4 single crystals have been measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K) and structure of GdFe2.1Ga0.9 (BO3)4 crystal have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature diminishes from 38 till 16 K, the strong absorption band edge shifts on 860 cm-1 (0.11 eV) to higher energy and the d-d transitions intensity decreases substantially larger than the Fe concentration does. Strong absorption band edge is shown to be due to Mott-Hubbard transitions. Correlation between position of the strong absorption band edge and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly forbidden d-d transitions in the studied crystals and in other antiferromagnets are explained within the framework of the model of the exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model permitted us to determine the connection between parameters of d-d absorption bands (intensity, width and their temperature dependences), on the one hand, and the exchange, spin-orbit and electron-lattice interactions, on the other hand

Magnetoelastic studies of Nd₀,₇₅Dy₀,₂₅Fe₃(BO₃)₄ in the external magnetic field: Magnetic phase transitions

We report on results of sound-velocity and sound-attenuation measurements in the Nd₀,₇₅Dy₀,₂₅Fe₃(BO₃)₄ in external magnetic fields up to 5 T, applied along several directions with respect to crystallographic axes, and at temperatures down to 1.7 K. The experimental data are analyzed with a microscopic theory based on exchange-striction coupling and phenomenological theory resulting in a qualitative agreement between theoretical results and experimental data

High-resolution optical spectroscopy, magnetic properties, and single-crystal neutron diffraction of multiferroic HoFe3(BO3)4: Magnetic structure

The magnetic structure is usually determined by the neutron diffraction measurements. However, in the case of complex multisublattice magnetics, this method fails to give an unambiguous result. Here, on the example of multiferroic HoFe3(BO3)4, we show that in the case of rare-earth (RE) compounds the right magnetic structure can be determined by additionally using optical spectroscopy and a theoretical analysis based on spectroscopic data. HoFe3(BO3)4 demonstrates a series of phase transitions and interesting magnetic and magnetoelectric properties. The available information on the magnetic structure of the compound, necessary for understanding and utilizing these properties, is contradictory. To resolve the existing ambiguities, we apply a combined approach. The high-resolution spectroscopy data deliver a set of the Ho3+ crystal-field (CF) levels in the paramagnetic and both easy-plane and easy-axis magnetic phases. These data are used to determine CF and Ho3+-Fe3+ exchange parameters and, then, to calculate the temperature dependencies of the magnetic susceptibility tensor of HoFe3(BO3)4. Based on these calculations, we suggest an easy-plane antiferromagnetic structure with a collinear arrangement of the Fe spins along the a axis and induced noncolinear moments of magnetically nonequivalent Ho ions. The suggested structure is further confirmed by single-crystal elastic neutron scattering experiments. We argue that specific features of the magnetic properties of RE iron borates isostructural to HoFe3(BO3)4 are governed by the energy patterns and the symmetry properties of the wave functions of the lower CF levels of the RE ground multiplet in the crystal field of the C2 symmetry