Structural features associated with multiferroic behavior in the RX3(BO3)4 system.

The magnetoelectric effect in the RX3(BO3)4 system (R = Ho, Eu, Sm, Nd, Gd; X = Fe, Al) varies significantly with the cation R despite very similar structural arrangements. Our structural studies reveal a symmetry reducing tilting of the BO3 planes and of the FeO6 polyhedra in the systems exhibiting low magnetic field induced electric polarization. Neutron scattering measurements reveal a lack of magnetic ordering indicating the primary importance of the atomic structure in the multiferroic behavior of this system.

Switching of Magnons by Electric and Magnetic Fields in Multiferroic Borates.

Electric manipulation of magnetic properties is a key problem of materials research. To fulfill the requirements of modern electronics, these processes must be shifted to high frequencies. In multiferroic materials, this may be achieved by electric and magnetic control of their fundamental excitations. Here we identify magnetic vibrations in multiferroic iron borates that are simultaneously sensitive to external electric and magnetic fields. Nearly 100% modulation of the terahertz radiation in an external field is demonstrated for SmFe_{3}(BO_{3})_{4}. High sensitivity can be explained by a modification of the spin orientation that controls the excitation conditions in multiferroic borates. These experiments demonstrate the possibility to alter terahertz magnetic properties of materials independently by external electric and magnetic fields.

Bifurcations of Coupled Electron-Phonon Modes in an Antiferromagnet Subjected to a Magnetic Field.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

Electric-field-induced linear birefringence in TmAl3(BO3)4.

The linear birefringence induced by the electric field was first detected in a TmAl3(BO3)4 single crystal. The electric field dependence of the birefringence was investigated. The estimation of the electro-optical coefficient of the material gives ≈1.5×10-10 cm/V for a wavelength 632.8 nm.

Antiferromagnetic Dichroism in a Complex Multisublattice Magnetoelectric CuB(2)O(4).

Magnetic control of the crystal chirality was announced by Saito et al. [Phys. Rev. Lett. 101, 117402 (2008)] on the ground of experiments in CuB(2)O(4). This claim has raised a sharp dispute in the literature because it seemed to contradict the fundamental symmetry principles. We settle this dispute on the basis of a high-resolution optical spectroscopy study of excitonic transitions in CuB(2)O(4). We find that a large sublattice-sensitive antiferromagnetic linear dichroism (LD) emerges at the Néel temperature T(N)=21 K and show how it could simulate a "magnetic-field control of the crystal chirality." We prove that the discovered LD is related microscopically to the magnetic Davydov splitting. This LD is highly sensitive to subtle changes in the spin subsystems, which allowed us to observe a splitting of the phase transition into an incommensurate magnetic phase into two transitions (T(1)(*)=8.5 and T(2)(*)=7.9 K) and to suggest elliptical spiral structures below T(1)(*), instead of a simple circular helix proposed earlier.

Magneto-optical properties of terbium iron borate.

The Faraday effect induced by an external magnetic field in TbFe3(BO3)4 and TbAl3(BO3)4 borates at a wavelength 633 nm has been investigated. It was found that the terbium subsystem brings the dominant magnetic contribution to the Faraday rotation at low temperatures in borate TbFe3(BO3)4. For both TbFe3(BO3)4 and TbAl3(BO3)4 the magneto-optical coefficients of the terbium subsystem were determined.

Phase transitions and crystal-field and exchange interactions in TbFe3(BO3)4 as seen via optical spectroscopy.

High-resolution polarized broadband (1800-23 000 cm(-1)) optical absorption spectra of Tb(3+) in TbFe(3)(BO(3))(4) single crystals are studied between room temperature and 4.2 K. The spectral signatures of the structural (R32-P3(1)21, T(S ) = 192 K) and magnetic (T(N ) = 41 K) 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(1)21 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 Γ(1) + Γ(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 Tb-Fe exchange integral and of the effective magnetic field created by the ordered Fe subsystem were estimated as J(fd) = 0.26 K and B(eff) = 3.92 T, using the observed splitting Δ = 32 cm(-1) of the Tb(3+) ground quasi-doublet at the temperature 5 K. 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+).

Magnetic and specific heat properties of YFe3(BO3)4 and ErFe3(BO3)4.

The present paper reports on the specific heat and magnetization of the YFe(3)(BO(3))(4) and ErFe(3)(BO(3))(4) single crystals. In both compounds, antiferromagnetic order of the iron spins evolves at T(N) = 38 K. The experimental data suggest that the magnetic moments are in the basal plane of the trigonal crystal for both compounds. In the magnetically ordered state the crystal is subdivided into three types of domains, the magnetic moments of the Fe(3+) ions being aligned along the a axis within each domain. For ErFe(3)(BO(3))(4), two non-equivalent magnetic positions of the Er(3+) ions in each domain are observed.

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.

Evidence for differentiation in the iron-helicoidal chain in GdFe3(BO3)4.

A single-crystal X-ray structure study of gadolinium triiron tetraborate, GdFe3(BO3)4, at room temperature and at 90 K is reported. At room temperature GdFe3(BO3)4 crystallizes in a trigonal space group, R32 (No. 155), the same as found for other members of the iron borate family RFe3(BO3)4. At 90 K the structure of GdFe3(BO3)4 transforms to the space group P3(1)21 (No. 152). The low-temperature structure determination gives new insight into the weakly first-order structural phase transition at 156 K and into the related Raman phonon anomalies. The presence of two inequivalent iron chains in the low-temperature structure provides a new perspective on the interpretation of the low-temperature magnetic properties.