Magnetoelectricity in the ferrimagnetic Cu₂OSeO₃: symmetry analysis and Raman scattering study

We report Raman scattering studies and magneto/structural symmetry analysis of the many sublattices ferrimagnet Cu₂OSeO₃ with a cubic symmetry and a linear magnetoelectric effect. There is no spectroscopic evidence for structural lattice distortions below TC = 60 K, which are expected due to magnetoelectric coupling. Using symmetry arguments we explain this observation by considering a special type of ferrimagnetic ground state which does not generate a spontaneous electric polarization. Interestingly, Raman scattering shows a strong increase of electric polarization of media through a dynamic magnetoelectric effect as a remarkable enhancement of the scattering intensity below TC. New lines of purely magnetic origin have been detected in the magnetically ordered state. A part of them are attributed as scattering on exchange magnons. Using this observation and further symmetry considerations we argue for strong Dzyaloshinskii–Moriya interaction existing in the Cu₂OSeO₃

Phonon Raman scattering in LaMn₁₋xCoxO₃ (x = 0, 0.2, 0.3, 0.4, and 1.0)

The Raman-active phonons in perovskite-like LaMn₁–xCoxO₃ (x = 0, 0.2, 0.3, 0.4, and 1.0) were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly more phonon lines than allowed for the rhombohedral LaCoO₃ structure were observed

Magnetic ordering in Co2+-containing layered double hydroxides via the low-temperature heat capacity and magnetisation study

The low-temperature heat capacity and the magnetisation of Co2+ n Al3+ layered double hydroxides (LDH) with the cobalt-to-aluminium ratio n = 2 and 3 and intercalated with different anions have been studied in a wide range of magnetic fields up to 50 kOe. The heat capacity, C(T), was found to demonstrate a Schottky-like anomaly observed as a broad local maximum in the temperature dependence below 10 K. The effect is caused by a splitting of the ground-state Kramers doublet of Co2+ in the internal exchange field and correlates with magnetic ordering in these LDH. In low applied fields, the temperature-dependent dc magnetic susceptibility demonstrates a pronounced rise, which is associated with an onset of magnetic ordering. Both the heat capacity anomaly and the magnetic susceptibility peak are more pronounced for the LDH with n = 2 than for those with n = 3. This feature is associated with an excess of the honeycomb-like Co–Al coordination (which corresponds to a 2:1 Co–Al ordering) over the statistical cation distribution in Co2Al LDH, while a rather random cobalt-aluminium distribution is typical for Co3Al LDH. The temperature of the Schottky-like anomaly measured in a zero field is independent of the interlayer distance. Application of the magnetic field results in a widening of the anomaly range and a shift to higher temperatures. The observed experimental data are typical for a cluster spin glass ground state.publishe

Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4 detwinned crystals as a function of doping, temperature and magnetic field. Two magnetic modes were observed within the AF region of the phase diagram. The one at lower energies was identified with the spin-wave gap induced by the antisymmetric DM interaction and its anisotropic properties in magnetic field could be well explained using a canonical form of the spin Hamiltonian. A new finding was a magnetic field induced mode whose dynamics allowed us to discover a spin ordered state outside the AF order which was shown to persist in a 9 T field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these single magnon excitations we map out the Raman selection rules in magnetic fields and demonstrate that their temperature dependent spectral weight is peaked at the N\'eel temperature. The second aspect is related to phononic and magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We observed that around 1/8 Sr doping and independent of Nd concentration there exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers. The weak magnitude of existing charge/spin modulations in the Nd doped structure did not allow us to detect specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.Comment: 26 pages, 22 figure

Magnetic and transport properties of Gd0.9A0.1CoO3-δ (A=Ba, Sr)

The X-ray diffraction, magnetization and electrical conductivity measurements for Gd0.9A0.1CoO3-δ (A=Ba, Sr) have been made. The complicated magnetic behavior, including the paramagnetic-ferromagnetic-antiferromagnetic and paramagnetic-ferromagnetic transitions, was found for Ba- and Sr-doped samples. The gradual insulator-metal transitions were observed in a wide temperature range T=600-800 K. The complex magnetic and transport data could be explained on the basis of the structural phase separation. © 2009 Elsevier B.V. All rights reserved

Magnetic fields in Proto Planetary Nebulae

The X-ray diffraction, magnetization and electrical conductivity measurements for Gd0.9A0.1CoO3-? (A=Ba, Sr) have been made. The complicated magnetic behavior, including the paramagnetic-ferromagnetic-antiferromagnetic and paramagnetic-ferromagnetic transitions, was found for Ba- and Sr-doped samples. The gradual insulator-metal transitions were observed in a wide temperature range T=600-800 K. The complex magnetic and transport data could be explained on the basis of the structural phase separation. " 2009 Elsevier B.V. All rights reserved.",,,,,,"10.1016/j.jmmm.2008.11.087",,,"http://hdl.handle.net/20.500.12104/42640","http://www.scopus.com/inward/record.url?eid=2-s2.0-61849143325&partnerID=40&md5=90f25c0a8cff31749916101a9d8284bb",,,,,,"9",,"Journal of Magnetism and Magnetic Materials",,"126