Neutron diffraction studies of HoFeO 3 single crystals were performed under external magnetic fields. The interplay between the external magnetic fields, Dzyaloshinsky-Moria antisymmetric exchange, isotropic exchange interactions between Fe and Ho sublattices and within the Fe sublattice provides a rich magnetic phase diagram. As the result of the balance of exchange interactions inside the crystal and external magnetic fields, we found 8 different magnetic phases, induced or suppressed dependent on the external field.

The multiferroic and the rotating magnetocaloric properties of Nd0.8Tb0.2Mn2O5 are investigated by microscopic optical probes and macroscopic magnetic measurements. Raman-active phonons as a function of temperature, and Nd3+ and Tb3+ infrared active crystal-field (CF) excitations as a function of temperature and under magnetic fields up to 11 T have been studied in Nd0.8Tb0.2Mn2O5. The obtained results are compared to those of NdMn2O5 and TbMn2O5 reference compounds. The observation of one set of Raman-active phonons and CF excitations rule out possible twinning while their energy positions and thermal evolutions indicate noticeable changes of Mn1-O3-Mn1 and TbO8 structural units. This would explain the nature of separated magnetic phases in Nd0.8Tb0.2Mn2O5. The degeneracy of the ground-state Kramers doublet is lifted (???0 ??? 9 cm???1), indicating that the Nd3+???Mn3+ interaction impacts the magnetic and ferroelectric properties of Nd0.8Tb0.2Mn2O5. The Zeeman splitting of excited crystal-field levels of the Nd3+ ions at low temperatures shows that the gz factor is weak compared to that in NdMn2O5. This indicates that the R3+ spins in Nd0.8Tb0.2Mn2O5 are mostly aligned within the ab-plane. The nature of magnetocrystalline anisotropy in Nd0.8Tb0.2Mn2O5 as well as in all RMn2O5 compounds is quantitatively investigated by studying the anisotropy of paramagnetic Curie temperatures along (??||) and perpendicular (?????) to the c axis, (??|| ??? ?????), as a function of the rare-earth atomic number. It is particularly found that the magnetocrystalline anisotropy is mainly determined by the quadrupolar charge distribution of 4 f shells. The rotating magnetocaloric effect in Nd0.8Tb0.2Mn2O5 is also evaluated and compared to that in NdMn2O5 and TbMn2O5. Our findings show that Nd- and Tb- separated magnetic phases independently contribute to the magnetocaloric effect of Nd0.8Tb0.2Mn2O5.

By the single crystal inelastic neutron scattering the orthoferrite HoFeO3 was studied. We show that the spin dynamics of the Fe subsystem does not change through the spin-reorientation transitions. The observed spectrum of magnetic excitations was analyzed in the frames of linear spin-wave theory. Within this approach the antiferromagnetic exchange interactions of nearest neighbors and next nearest neighbors were obtained for Fe subsystem. Parameters of Dzyaloshinskii-Moriya interactions at Fe subsystem were refined. The temperature dependence of the gap in Fe spin-wave spectrum indicates the temperature evolution of the anisotropy parameters. The estimations for the values of Fe-Ho and Ho-Ho exchange interaction were made as well.

Detailed investigation of the incommensurate magnetic ordering in a single crystal of multiferroic NdMn2O5 has been performed using both non-polarized and polarized neutron diffraction techniques. Below TN = 30.5 K magnetic Bragg reflections corresponding to the non-chiral type magnetic structure with propagation vector k1 = (0.5 0 kz1) occurs. Below about 27 K a new distorted magnetic modulation with a similar vector kz2 occurs, which is attributed to the magnetization of the Nd3+ ions by the Mn-sub-lattice. Strong temperature hysteresis in the occurrence of the incommensurate magnetic phases in NdMn2O5 was observed depending on the cooling or heating history of the sample. Below about 20 K the magnetic structure became of a chiral type. From spherical neutron polarimetry measurements, the resulting low-temperature magnetic structure kz3 was approximated by the general elliptic helix. The parameters of the magnetic helix-like ellipticity and helical plane orientation in regard to the crystal structure were determined. A reorientation of the helix occurs at an intermediate temperature between 4 K and 18 K. A difference between the population of right- and left-handed chiral domains of about 0.2 was observed in the as-grown crystal when cooling without an external electric field. The magnetic chiral ratio can be changed by the application of an external electric field of a few kV/cm, revealing strong magnetoelectric coupling. A linear dependence of the magnetic chirality on the applied electric field in NdMn2O5 was found. The results are discussed within the frame of the antisymmetric super-exchange model for Dzyaloshinsky-Moria interaction.

The 63,65Cu NMR and NQR spectra have been obtained in single crystal sample of CuCrO2 in the magnetically ordered phase. The components of the electric field gradient tensor (EFG) characterizing the structure of the nearest environment of copper ions are determined from the analysis of the spectra. Comparative analysis obtained data with similar data of paramagnetic phase showed that the local charge environment of copper nuclei does not change during the magnetic phase transition.

Antiferromagnetic systems with rich spin dynamics and pronounced interplay between magnetic and lattice degrees of freedom are of great interest in the emerging field of terahertz and subterahertz magnonics and spintronics. Here we report on spin and lattice dynamics of single crystals of Ni2NbBO6 studied with the use of polarized Raman spectroscopy in the temperature range of 10-300 K which includes the antiferromagnetic transition at T-N = 23.5 K. Well-defined and highly polarized magnetic excitations were observed below and partly above T-N. Three magnetic modes at 30, 84, and 113 cm(-1) (at T = 10 K) were detected. The two high-energy magnetic excitations were identified as two-magnon modes. In total, 57 out of the expected 60 Raman-active phonons were registered and identified. Several modes revealed a nontrivial spin-phonon coupling manifesting either hardening or softening mostly below T-N. The possible magnetic space groups were determined through symmetry analysis. Spin-wave spectra and density of states were calculated within the linear spin-wave theory for supporting the experimental findings.

The anisotropy of the components of the complex permittivity of vanadate Co 3 V 2 O 8 and Co 3 V 2 O 8 single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c . The optical properties are also investigated along axis b . The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c . The maximum of the main interband absorption band of Co 3 V 2 O 8 is shifted toward low energies by 0.25–0.3 eV as compared to Co 3 V 2 O 8 . The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.

The 63,65Cu nuclear magnetic resonance (NMR) spectra have been obtained in the external magnetic field H0 = 11.7 T at the temperature range from 20 to 350 K and the static magnetic susceptibility, χ(T), has been measured in single-crystalline CuFeO2. The temperature dependences of the 63,65Cu NMR line shifts, K(T), and the magnetic susceptibility, χ(T), can be satisfactorily described by Curie–Weiss law at T > 60 K. It is worth noting that the behavior of K(T) and χ(T) is different below T = 60 K. The deviation of K(T) from the Curie–Weiss law indicates the short-range order at T < 60 K in CuFeO2.

Raman and infrared spectroscopies are used as local probes to study the dynamics of the Nd-O bonds in the weakly multiferroic NdMn2O5 system. The temperature dependence of selected Raman excitations reveals the splitting of the Nd-O bonds in NdMn2O5. The Nd3+ ion crystal field (CF) excitations in NdMn2O5 single crystals are studied by infrared transmission as a function of temperature, in the 1800 - 8000 cm(-1) range, and under an applied magnetic field up to 11 T. The frequencies of all I-4(j) CF levels of Nd3+ are determined. We find that the degeneracy of the ground-state Kramers doublet is lifted (Delta(0) similar to 7.5 cm(-1)) due to the Nd3+ -Mn3+ interaction in the ferroelectric phase, below T-c similar to 28 K. The Nd3+ magnetic moment m(Nd) (T) and its contribution to the magnetic susceptibility and the specific heat are evaluated from Delta(0) (T) indicating that the Nd3+ ions are involved in the magnetic and the ferroelectric ordering observed below similar to 28 K. The Zeeman splitting of the excited CF levels of the Nd3+ ions at low temperature is also analyzed.

In this paper we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$ drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinon-like fluctuations. At $T T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies, creates unusual quantum spin dynamics on the low energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be "hidden" by a dominating interaction.

For the first time, the CuFeO 2 single crystal has been studied by 63,65 Cu nuclear magnetic resonance (NMR). The measurements have been carried out in the temperature range of T = 100−350 K in the magnetic field H = 117 kOe applied along different crystallographic directions. The components of the electric field gradient tensor and the hyperfine coupling constants are determined. It is shown that electrons of copper 4 s and 3 d orbitals are involved in the spin polarization transfer Fe → Cu. The occupancies of these orbitals are estimated.

Results of studying the paramagnetic and ordered phases of a CuCrO 2 single crystal using nuclear magnetic and nuclear quadrupole resonances on 63,65 Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature ( T = 4.2–300 K) and magnetic-field strength ( Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor ( K a,c ) have been determined. The temperature dependences K a ( H || a ) and K c ( H || c ) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χ a,c ). The hyperfine field on a copper nucleus has been determined, which is equal to h hf a,c = 33 kOe/μB. Below the temperature Т N = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for 63,65 Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.

We present macroscopic and neutron diffraction data on multiferroic lightly Co doped Ni3V2O8. The magnetic H-T phase diagrams have been derived from magnetization and electric polarization measurements with field directions parallel to the principal crystallographic axes. While the phase diagram for H parallel to b is very similar to that of the parent compound Ni3V2O8 for the commonly involved phases, the zero-field phases in (Ni0.9Co0.1)(3)V2O8 show a stronger instability for applied magnetic fields along the a or c axis. Neutron single-crystal diffraction revealed the magnetic structure of the field-induced phase for H parallel to c with a collinear spin alignment along the a and b axes for the two magnetically inequivalent sites. A pronounced irreversibility has been observed for the transition between the zero-field spin cycloid and the field-induced phase, which is manifested in a propagation vector change from q = (0.322 0 0) to q = (0.306 0 0), with slight modifications of the magnetic structure after reentering the zero-field phase. The reentrant phase is characterized by a significantly larger b component of the cross-tie site spin, therefore showing remanent features of the high-field phase. For H parallel to a the magnetization data reveal anomalies, one of which was proved to reflect a field-induced transition from the cycloidally to the sinusoidally modulated magnetic structure.

Spin dynamics in the intermediate ordered phases (between 4 and 9 K) in Ni3V2O8 have been studied with inelastic neutron scattering. It is found that the spin waves are very diffuse, indicative of short lived correlations and the coexistence of paramagnetic moments with the long-range ordered state.

The magnetically ordered phase of the CuCrO2 single crystal has been studied by the nuclear magnetic resonance (NMR) method on 53Cr nuclei in the absence of an external magnetic field. The 53Cr NMR spectrum is observed in the frequency range νres = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τπ/2–t del–τπ–t del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.