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Cu2FeBO5 ludwigite single crystals have been grown from a solution–melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mössbauer spectroscopy data, Fe3+ ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K ≤ T ≤ 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at T1 = 35 K and T2 = 20 K and spin-glass effects. Specific heat measurements in the interval 4–300 K have not discovered magnetic-transition-related anomalies. [ABSTRACT FROM AUTHOR]

The high-resolution optical absorption spectra of NdFe3(BO3)4 single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd3+ ion. Light linearly polarized at different angles to the C2 axes in the basal plane has been passed along the trigonal C3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (TN ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C2 axes up to about 17 K, the number of domains with variously directed C2 axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C2 axis within the ±10° interval. [ABSTRACT FROM AUTHOR]

The Raman spectra of four crystals of TbFe3–xGax(BO3)4 solid solutions (x from 0 to 0.54) were studied in the temperature range from 8 to 350 K. The temperatures of structural phase transitions were determined. The observed spectral behavior is characteristic to condensation and restoration of soft modes. Soft modes are associated with a structural phase transition from the R32 phase to the P3121 phase. The Compositions-Temperature phase diagram was constructed. [ABSTRACT FROM AUTHOR]

Experimental and theoretical study of submillimeter (terahertz) spectroscopic and magnetic properties of the rare-earth aluminum borate HoAl3(BO3)4 were performed at temperatures 3–300K. In the transmittance spectra a number of resonance lines were detected at frequencies 2–35 cm–1 for different radiation polarizations. These modes were identified as transitions between the lower levels of the ground multiplet of the Ho3+ ion split by the crystal field, including both transitions from the ground state to the excited ones and transitions between the excited states. The established excitation conditions of the observed modes and the simulation of the spectra made it possible to separate the magnetic and electric dipole transitions and to determine the energies of the corresponding states, their symmetry, and the matrix elements of the transitions. Low-frequency lines that do not fit into the established picture of the electron states of Ho3+ were also found; these lines, apparently, correspond to the ions with the distorted by defects local symmetry of the crystal field. [ABSTRACT FROM AUTHOR]

The inverse magnetoelectric MEE-effect in the SmFe3(BO3)4 single-crystal sample in an applied electric field has been studied. The electric field E consists of two components: a constant one, e0, and a variable one, e × cos(ωt). The investigated compound evokes great interest due to the existence of both linear and quadratic contributions to the magnetoelectric effect. According to theoretical analysis, the effective susceptibility of the first harmonic of magnetic moment oscillations depends on the dc electric field component e0. Indeed, according to the measurement data, the dc field e0 significantly affects the MEE-effect first-harmonic amplitude: depending on the e0 sign, these oscillations can be either amplified or suppressed. This offers an opportunity for controlling the magnetic moment oscillation amplitude at the applied electric field frequency, including the signal modulation, which is promising for application. In the applied dc field e0, a slight change in the susceptibility of the MEE-effect term quadratic to the electric field was detected. The mathematical apparatus used correctly describes the qualitative dependence of the MEE-effect amplitude on the dc field e0, while the quantity appears to be approximately twofold overestimated. [ABSTRACT FROM AUTHOR]

The temperature dependences of the structural parameters in a HoFe3(BO3)4 single crystal, studied by X-ray diffraction below and above the structural phase transition at TS = 365 K, correlate well with behavior of the Mössbauer parameter quadrupole interaction. However, two structural positions Fe1 and Fe2 of iron ions formed in the phase with space group P3121, which appears at temperatures below TS, cannot be distinguished by Mössbauer spectroscopy at 57Fe nuclei. This becomes possible only below the Néel temperature TN. It has been established that below TN, iron ions form a 3D magnetic order of the Ising type with critical parameters β = 0.283(1) and the dimension of the order parameter n = 1. The refined value of the Néel temperature is TN = 37.42(1) K. The dynamics of changes in the Mössbauer parameters of the quadrupole shift and the magnetic hyperfine field Bhf, observed near the temperature T = 4.4 K for iron ions in the Fe1 and Fe2 positions, indicates a reorientation of the magnetic moments of iron. This correlates with the spin-reorientation transition of Fe and Ho ions, previously observed by neutron diffraction in HoFe3(BO3)4. [ABSTRACT FROM AUTHOR]