Magnetic structure of the field-induced multiferroic GdFe3(BO3)4

We report a magnetic x-ray scattering study of the field-induced multiferroic GdFe3(BO3)4. Resonant x-ray magnetic scattering at the Gd LII,III edges indicates that the Gd moments order at TN ~ 37 K. The magnetic structure is incommensurate below TN, with the incommensurability decreasing monotonically with decreasing temperature until a transition to a commensurate magnetic phase is observed at T ~ 10 K. Both the Gd and Fe moments undergo a spin reorientation transition at TSR ~ 9 K such that the moments are oriented along the crystallographic c axis at low temperatures. With magnetic field applied along the a axis, our measurements suggest that the field-induced polarization phase has a commensurate magnetic structure with Gd moments rotated ~45 degrees toward the basal plane, which is similar to the magnetic structure of the Gd subsystem observed in zero field between 9 and 10 K, and the Fe subsystem has a ferromagnetic component in the basal plane.Comment: 27 pages, 7 figures, to appear in Phys. Rev.

Raman scattering from phonons and magnons in RFe3)BO3)4

Inelastic light scattering spectra of several members of the RFe3(BO3)4 family reveal a cascade of phase transitions as a function of temperature, starting with a structural, weakly first order, phase transition followed by two magnetic phase transitions. Those consist of the ordering of the Fe-spin sublattice revealed by all the compound, and a subsequent spin-reorientational transition for GdFe3(BO3)4. The Raman data evidence a strong coupling between the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice ordering leads to a strong suppression of the low energy magnetic scattering, and a multiple peaked two-magnon scattering continuum is observed. Evidence for short-range correlations is found in the `paramagnetic' phase by the observation of a broad magnetic continuum in the Raman data, which persists up to surprisingly high temperatures.Comment: 17 pages, 13 figure

Electronic and magnetic states of Fe ions in Co2FeBO5

The ludwigite Co2FeBO5 has been studied experimentally using 57Fe Mössbauer spectroscopy and theoretically using DFT + GGA calculations. The room-temperature Mössbauer spectra are composed of four quadrupole doublets corresponding to the high-spin Fe3+ ions in octahedral oxygen coordination. All components undergo splitting below 117 K due to the magnetic hyperfine fields. The DFT + GGA calculations performed for three models of Fe ion distributions have revealed that the ground state corresponds to the “Fe4(HS)” model with the high-spin Fe3+ ions located at the M4 site and the high-spin Co2+ ions located at the M1, M2, and M3 sites. A ferrimagnetic ground state, with the Co and Fe magnetic moments being nearly parallel to the b-axis and a total magnetic moment of circa 1.1μB f.u.−1, was found. The other Fe distributions cause an increase in the local octahedral distortions and transformation of the spin state. The calculated quadrupole splitting values are in good agreement with the experimental values obtained by Mössbauer spectroscopy.This research is funded by the Russian Foundation for Basic Research (project no. 20-02-00559 and 21-52-12033), the Russian Foundation for Basic Research, Government of Krasnoyarsk Territory, Krasnoyarsk Regional Fund of Science (project no. 19-42-240016) and the President Council on Grants (project no. MK-2339.2020.2). The authors acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO Grant No. MAT2017-83468-R and from the regional Government of Aragón (E12-20R RASMIA project).Peer reviewe

Co5/3Nb1/3BO4: A new cobalt oxyborate with a complex magnetic structure

Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected.This work has been financed by the Russian Foundation for Basic Research (project no. 20-02-00559). We acknowledge financial support from the Spanish MINECO DWARFS project MAT2017-83468-R and Gobierno de Aragón (Group, E12-20R).Peer reviewe

Excitation of multiple phonon modes in copper metaborate CuB_2O_4 via nonresonant impulsive stimulated Raman scattering

The excitation of four coherent phonon modes of different symmetries is realized in copper metaborate CuB2O4 via impulsive stimulated Raman scattering (ISRS). The phonons are detected by monitoring changes in the linear optical birefringence using the polarimetric detection (PD) technique. We compare the results of the ISRS-PD experiment to the polarized spontaneous Raman scattering spectra. We show that agreement between the two sets of data obtained by these allied techniques in a wide phonon frequency range of 4–14 THz can be achieved by taking into account the symmetry of the phonon modes and corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We analytically demonstrate that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes

Crystal Structure and Magnetism of Co_2−xNi_xB₂O₅ Pyroborate

The high quality single crystals of Co2B2O5:Ni were synthesized. The detail study of crystal structure using single-crystal X-ray diffraction was carried out. The monoclinic symmetry was found (P21/c space group). The magnetization and magnetic susceptibility measurements have shown antiferromagnetic behavior below TN = 47 K and paramagnetic temperature = 43 K. The effective magnetic moment per magnetic ion was 3.49 μB, which points out the divalent and high-spin state of Co and Ni ions.Синтезированы высококачественные монокристаллы Co2B2O5:Ni. Проведено детальное исследование кристаллической структуры с использованием монокристаллического рентгеновского дифрактометра. Обнаружена моноклинная симметрия (пространственная группа P21/c [14]). Измерения намагниченности и магнитной восприимчивости выявили антиферромагнитный переход при TN = 47 K и парамагнитную температуру = 43 K. Эффективный магнитный момент, приходящийся на магнитный ион, найден равным 3.49 μB, что указывает на двухвалентное и высокоспиновое состояния ионов Co и Ni