Optical spectroscopy of PrFe3 (BO3) 4: Crystal-field and anisotropic Pr-Fe exchange interactions

High-resolution polarized optical absorption spectra of PrFe3 (BO3) 4 in the paramagnetic and antiferromagnetic phases are reported. The measured energies of the crystal-field (CF) levels within the 4 f2 configuration of Pr3+ in the paramagnetic PrFe3 (BO3) 4 are described by the CF model that involves the 4 f2 /4f5d and 4 f2 /4f6p configuration interactions. Ordering of Fe spins along the crystalline c axis below TN =32K is confirmed by the analysis of the spectra of Er3+ introduced as a probe into PrFe3 (BO3) 4. To account for the observed changes in the optical spectra of Pr3 + at temperatures below TN, in particular, for the shift of the CF levels, splitting of the CF doublets, and the appearance of forbidden lines, the Pr-Fe exchange Hamiltonian defined by seven parameters is considered. The theoretical approach has been tested by calculating the temperature dependence of the magnetic susceptibility. A good agreement between theory and optical and magnetic experimental data is found demonstrating the validity of the model used. The obtained results confirm that the model of the iron dimers inside the spiral chains of Fe3+ (O2-) 6 octahedrons introduced by us earlier for NdFe3 (BO3) 4 and modified in the present work may serve as a basis for analyzing the low-temperature properties of other rare-earth iron borates. © 2009 The American Physical Society

Investigating the magnetic structure and anisotropic Pr-Fe exchange interaction in a PrFe3(BO3)4 single crystal by optical spectroscopy

Optical spectroscopy was used to obtain information on the energy and symmetry of the crystal-field (CF) levels within the 4f 2 configuration of Pr3+ in PrFe3(BO3) 4, along with changes in the frequencies and intensities of the f-f transition lines upon magnetic ordering (T N = 32K). Analysis of the experimental data yielded the values of the parameters for the CF and the anisotropic Pr3+-Fe3+ exchange-interaction Hamiltonians. © 2010 Allerton Press, Inc

Phase transitions and crystal-field and exchange interactions in TbFe 3(BO 3) 4 as seen via optical spectroscopy

High-resolution polarized broadband (180023000cm 1) optical absorption spectra of Tb 3+ in TbFe 3(BO 3) 4 single crystals are studied between room temperature and 4.2K. The spectral signatures of the structural (R32P3 121, T S=192K) and magnetic (T N=41K) phase transitions are found and analyzed. Energies and symmetries of the Tb 3+ crystal-field (CF) levels were determined for both the high-temperature R32 and the low-temperature P3 121 structures of TbFe 3(BO 3) 4 and compared with the calculated ones. It follows unambiguously from the spectral data that the ground state is the 1+ 2 quasi-doublet of the local D 3 point symmetry group for Tb 3+ in the R32 high-temperature structure. The CF calculations revealed the CF parameters and wavefunctions for Tb 3+ in TbFe 3(BO 3) 4. The value of the TbFe exchange integral and of the effective magnetic field created by the ordered Fe subsystem were estimated as J fd=0.26K and B eff=3.92T, using the observed splitting =32cm 1 of the Tb 3+ ground quasi-doublet at the temperature 5K. The reliability of the obtained parameters was proven by modeling the literature data on the magnetic susceptibility of TbFe 3(BO 3) 4. Lattice distortions below T S were evidenced by the observed changes of probabilities of the forced electric dipole transitions of Tb 3+. © 2012 IOP Publishing Ltd

Breaking of the selection rules for optical transitions in the dielectric PrFe3(BO3)4 crystal by a praseodymium-iron exchange interaction

We report on the emergence of new lines in the optical spectrum of the PrFe3(BO3)4 single crystal at the magnetic ordering temperature. The transitions between singlet crystal-field sublevels of Pr3+ ion with the same transformational properties, strictly forbidden for the trigonal D3 point symmetry of this ion in PrFe3(BO3)4, appear below the Néel temperature and grow in intensity as a square of the order parameter. We show that the phenomenon originates from the mixing of wave functions of different Pr3+ sublevels by the Pr-Fe exchange interaction. © 2009 The American Physical Society

Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4

We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering TN = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We perform crystal-field calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g(perpendicular) = 2.385, g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange integral, respectively, using the experimentally measured Nd3+ ground-state splitting of 8.8 cm-1. To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.Comment: 13 pages, 8 figures, 2 table

Bifurcations observed in the spectra of coupled electron-phonon modes in multiferroic PrFe3(BO3)4PrFe_3(BO_3)_4 subjected to a magnetic field

We report on bifurcations effect mediated by the electron-phonon coupling in a concentrated rare-earth-containing antiferromagnet, observed in the spectra of coupled $4f$-electron-phonon modes under the influence of an external magnetic field. The effect was observed in the low-temperature far-infrared (terahertz) reflection spectra of a multiferroic easy-axis antiferromagnet $PrFe_3(BO_3)_4$ in magnetic fields $\textbf{B}_{ext}||c$. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T. We show that the field behavior of the coupled modes can be successfully explained and modeled on the base of the equation derived in the frame of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant $|W| = 14.8 cm^{-1}$.Comment: 5 pages, 4 figure

High-resolution spectroscopy, crystal-field calculations, and quadrupole helix chirality of DyFe<inf>3</inf>(BO<inf>3</inf>)<inf>4</inf>

© The Authors, published by EDP Sciences.High-resolution polarized transmission spectra of Dy Fe3(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