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Register a new userMagnetic resonances of multiferroic TbFe$_3$(BO$_3$)$_4$
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Low-energy magnetic excitations of the easy-axis antiferromagnet TbFe$_3$(BO$_3$)$_4$ are investigated by far-infrared absorption and reflection spectroscopy in high magnetic fields up to 30 T. The observed field dependence of the resonance frequencies and the magnetization are reproduced by a mean-field spin model for magnetic fields applied both along and perpendicular to the easy axis. Based on this model we determined the full set of magnetic interactions, including Fe-Fe and Fe-Tb exchange interactions, single-ion anisotropy for Tb ions and $g$-factors, which describe the ground-state spin texture and the low-energy spin excitations of TbFe$_3$(BO$_3$)$_4$. Compared to earlier studies we allow a small canting of the nearly Ising-like Tb moments to achieve a quantitative agreement with the magnetic susceptibility measurements. The additional high energy magnetic resonance lines observed, besides the two resonances expected for a two-sublattice antiferromagnet, suggest a more complex six-sublattice magnetic ground state for TbFe$_3$(BO$_3$)$_4$.
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Hard x-ray scattering (HXS) experiments with a photon energy of 100keV were performed as a function of temperature and applied magnetic field on selected compounds of the RFe$_3$(BO$_3$)$_4$ family. The results show the presence of several unexpected diffraction features, in particular non-resonant magnetic reflections in the magnetically ordered phase, and structural reflections that violate the diffraction conditions for the low temperature phase $P3_121$ of the rare-earth iron borates. The temperature and field dependence of the magnetic superlattice reflections corroborate the magnetic structures of the borate compounds obtained by neutron diffraction. The detailed analysis of the intensity and scattering cross section of the magnetic reflection reveals details of the magnetic structure of these materials such as the spin domain structure of NdFe$_3$(BO$_3$)$_4$ and GdFe$_3$(BO$_3$)$_4$. Furthermore we find that the correlation length of the magnetic domains is around 100 AA{} for all the compounds and that the Fe moments are rotated $53^circpm3^circ$ off from the hexagonal basal plane in GdFe$_3$(BO$_3$)$_4$
We have investigated electronic structures of La$_3$S$_4$ and Ce$_3$S$_4$ using the LSDA and LSDA+$U$ methods. Calculated density of states (DOS) are compared with the experimental DOS obtained by the valence band photoemission spectroscopy. The DOS at $E_{rm{F}}$ indicates the 5$d$ character in La$_3$S$_4$ and 4$f$ character in Ce$_3$S$_4$. It is found to be nearly half metallic in the ferromagnetic ground state of Ce$_3$S$_4$. %Ce$_3$S$_4$ has ferromagnetic ground states with spin and orbital magnetic %moments of 1.27 $mu_{rm{B}}$ and $-$2.81 $mu_{rm{B}}$ per Ce, respectively, %and shows nearly half metallic ground state. We discuss the superconductivity and structural transition in La$_3$S$_4$, and the absence of structural transition in Ce$_3$S$_4$.
Multiferroic BiFeO3 undergoes a transition from a distorted spiral phase to a G-type antiferromagnet above a critical field H_c that depends on the orientation m of the field. We show that H_c(m) has a maximum when oriented along a cubic diagonal parallel to the electric polarization P and a minimum in the equatorial plane normal to P when two magnetic domains with the highest critical fields are degenerate. The measured critical field along a cubic axis is about 19 T but H_c is predicted to vary by as much as 2.5 T above and below this value. The orientational dependence of H_c(m) is more complex than indicated by earlier work, which did not consider the competition between magnetic domains.
The bulk magnetic properties of the lanthanide metaborates, $Ln$(BO$_2$)$_3$, $Ln$ = Pr, Nd, Gd, Tb are studied using magnetic susceptibility, heat capacity and isothermal magnetisation measurements. They crystallise in a monoclinic structure containing chains of magnetic $Ln^{3+}$ and could therefore exhibit features of low-dimensional magnetism and frustration. Pr(BO$_2$)$_3$ is found to have a non-magnetic singlet ground state. No magnetic ordering is observed down to 0.4 K for Nd(BO$_2$)$_3$. Gd(BO$_2$)$_3$ exhibits a sharp magnetic transition at 1.1 K, corresponding to three-dimensional magnetic ordering. Tb(BO$_2$)$_3$ shows two magnetic ordering features at 1.05 K and 1.95 K. A magnetisation plateau at a third of the saturation magnetisation is seen at 2 K for both Nd(BO$_2$)$_3$ and Tb(BO$_2$)$_3$ which persists in an applied field of 14 T. This is proposed to be a signature of quasi one-dimensional behaviour in Nd(BO$_2$)$_3$ and Tb(BO$_2$)$_3$.
The structural phase transition in hexagonal BaMnO$_3$ occurring at $T_c$=130 K was studied in ceramic samples using electron and X-ray diffraction, second harmonic generation as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P6_{3}cm$) is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary [$q = (frac{1}{3},frac{1}{3},0)$]; this mode activates in infrared and Raman spectra below $T_c$ and it hardens according to the Cochran law. Upon cooling below $T_c$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing-down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P6_{3}/mmc$ or $P6_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $approx220,rm K$ which develops fully near 140 K and, below $T_{n} approx 59,rm K$, it transforms into a three-dimensional antiferromagnetic order.
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