Session H12: Lattice and Magnetic Properties of Multiferroics

Infrared phonon dynamics of multiferroic BiFeO$_3$ single crystal

We discuss the first infrared reflectivity measurement on a BiFeO$_3$ single crystal between 5 K and room temperature. The 9 predicted \textit{ab}-plane $E$ phonon modes are fully and unambiguously determined. The frequencies of the 4 $A_1$ \textit{c}-axis phonons are found. These results settle issues between theory and data on ceramics. Our findings show that the softening of the lowest frequency $E$ mode is responsible for the temperature dependence of the dielectric constant, indicating that the ferroelectric transition in BiFeO$_3$ is soft-mode driven.   

Electromagnons and Multiple Phase Transitions in BiFeO$_{3}$ Multiferroic System

Magnetoelectrics (ME) multiferroics has recently become an exciting research area due to its potential technological applications. Of special relevance is the case of Bismuth ferrite, BiFeO$_{3}$ (BFO) where multiferroicity coexist at room temperature. In this work, the Fe-Fe exchange interaction effects on the optical properties of BiFeO$_{3}$ are analyzed by using both, linear and non linear spectroscopy as a function of temperature. Two and three magnons Raman scattering as well as detectable \textit{electromagnons} in the second harmonic generation (SHG) signal are reported. Temperature studies up to 750 K reveals a cascade of phase transitions associated to different dynamic reorientations in the magnetic subsystem. These transitions were detectable by several optical methods including linear absorption, Raman spectroscopy and SHG due to the strong electric dipole coupling found between electromagnetic radiation and spin waves in BFO.   

Temperature studies of multiferroic TbMnO$_3$ with resonant Raman scattering

Using temperature dependent resonant Raman scattering with different excitation energies from the ultraviolet (UV) to near infrared (NIR), we have investigated the complex interplay between the orbital, structural and magnetic ordering in the multiferroic material TbMnO$_3$. Depending on the scattering geometry, the magnetic transition at the N\'{e}el temperature at 41 K or the ferroelectric transition at T$_f$ = 28 K is observed. In resonance studies with an incident frequency of 1.91 eV, the Jahn-Teller mode shows a strong softening below T$_N$. In the low frequency spectra, a quasielastic response is identified. The results give information about the electron-phonon coupling and the correlations between electronic and structural degrees of freedom that contribute to the multiferroic behavior in TbMnO$_3$.   

Temperature- and field-dependent inelastic light scattering studies of TbMnO$_{3}$

TbMnO$_{3}$ is one of several multiferroic materials that have coexisting magnetic and electric orders that are strongly coupled. Because it is exquisitely sensitive both to structural order and magnetic degrees of freedom, field-dependent inelastic light scattering measurements are ideally suited to studying magnetoelastic coupling and multiferroic phases in materials such as TbMnO$_{3}$.~ By carefully examining the temperature- and field-dependent evolution of new magneto-elastic modes in various phases of TbMnO$_{3}$, our study reveals several new features of the IC-C transition in TbMnO$_{3}$, including the co-existence of distinct structural phases in the intermediate field regime around the IC-C phase transition and evidence for dynamical fluctuations of the IC and C phases outside the established phase boundaries.   

Pressure tuned phonon mode splitting in magnetic frustrated spinel ZnCr$_{2}$O$_{4}$

ZnCr$_{2}$O$_{4}$ has cubic spinel structure. Below 390 K, the geometrically frustrated magnet enters a paramagnetic state. Below 12.5 K, it undergoes a first-order phase transitions, resulting into an antiferromagnetic order and a structural distortion simultaneously. An IR-active phonon related to the Cr$^{3+}$ ion's motion undergoes a splitting at 12.5 K. This transition is explained as a spin-Peierls like transition. However, the exact cause and effect in such a transition is not clear. Is it because the lattice undergoes transition first, spin just follows, or is it spins' interaction that forces the lattice to undergo changes? Pressure can provide a crucial service in clarifying this issue, since pressure can change spin and lattice interactions in different ways, it can differentiate these two scenarios. We have measured the infrared absorption spectra of ZnCr$_{2}$O$_{4}$ under pressure. Our data shows that Tc, at which the spin-Peiers like transition occurs and the phonon at about 370 cm$^{-1}$ starts to show the splitting, increases from its ambient pressure value of 12.5 K to about 15.8 K at 1 GPa. This provides an important clue for the exact nature of this transition.   

Calculation of the Order Parameter and the Damping Constant in the Ferroelectric Phase for NaNO$_{2}$

The temperature dependence of the order parameter and the damping constant is calculated in the ferroelectric phase in the range of 27 to 162 $^{o}$C close to the phase transition (T$_{c}$=436 K) for NaNO$_{2}$. The values of the order parameter calculated from the molecular field theory, are used to evaluate the damping constant as a function of temperature on the basis of the soft phonon-hard phonon coupling model for NaNO$_{2}$ in the ferroelectric phase. By representing the damping constant calculated at various temperatures in terms of an Arrhenius plot, the activation energy is computed for this crystal in the ferroelectric phase. Our calculated order parameter agrees with the measured one and also the damping constant predicts the critical behaviour exhibited by the NaNO$_{2}$ crystal near the transition temperature in the ferroelectric phase. From the values of the activation energies obtained here, the mechanism of an order-disorder transition which involves the orientation of the NO$_{2}^{-}$ ions is investigated for NaNO$_{2}$.   

Pressure Dependence of the Phonon Modes of Hexagonal-RMnO3

We present high pressure IR measurements of the phonon spectra of HoMnO3 and YMnO3. Measurements were conducted over the pressure range ambient to $\sim $20 GPa. No phase changes were observed over this broad range of hydrostatic pressures. A strong non-linear variation of frequency with pressure is observed suggesting saturation at higher pressures. A discussion of the effect of hydrostatic pressure on the ferroelectric properties of these systems will be given based on comparisons with density functional calculations.   

Probing Spin-Lattice Correlations in Hexagonal RMnO3 Multiferroics

The hexagonal multiferroic system RMnO3 is known to exhibit strong spin-lattice correlations based on bulk thermal expansion measurements. Enhanced correlations at the spin ordering temperatures are observed. In this work, we examine the local structure about the R and Mn sites in order to determine the changes in atomic interactions which coincide with the spin alignments. Measurements over a abroad range of temperatures are presented and estimates of the changes in atomic bond distances are given.   

Giant magneto-elastic coupling in multiferroic hexagonal manganites

In order to investigate a possible structural change of RMnO$_3 $ at the magnetic transition temperature, we have carried out high-resolution structural studies using neutron diffraction. Here we show that the hexagonal manganites RMnO$_3$ undergo an isostructural transition at T$_N$ with unusually large atomic displacements: two orders of magnitude larger than those seen in any other ordinary materials, resulting in a uniquely strong magneto-elastic coupling. For the first time, we could follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and found consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling of RMnO$_3$ arises from geometrical frustration, and holds the key to the recently observed magnetoelectric phenomenon in this intriguing class of materials.   

Anomalous low-temperature magnetic ordering and spin-phonon coupling in BiFeO$_{3}$ thin films

Low-temperature magnetic properties and Raman spectra of epitaxal BiFeO$_{3}$ (BFO) thin films grown on (111) SrTiO$_{3}$ substrates have been studied. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves showed a large discrepancy beginning at a characteristic temperature which did depend on the magnetic-field strength, suggesting a spin-glass-like behavior of the epitaxial BFO film with R3c symmetry. For all three major A$_{1}$-symmetry Raman modes (138, 170, and 214 cm$^{-1})$, there was a good linear correlation between the mode-frequency softening and the square of the in-plane magnetization in the temperature range between 80 and 300 K. These observations were ascribed to the spin-phonon coupling below the N\'{e}el temperature (T$_{N }$= 643 K).   

Magneto-dielectric study of multiferroicity in biferroic YCrO$_{3}$

Dielectric measurements are used to characterize multiferroicity in the doped monoclinic ferroelectric oxides $Y_{1-x}$\textit{Ca}$_{x}$\textit{MnO}$_{3}$ ($x$ = 0, 0.15, 0.3). The focus of this study is on the effects of the magnetic field and Ca doping on the temperature- magnetic field dependent dielectric response of polycrystalline samples of rare earth chromates YCrO$_{3}$. YCrO$_{3}$ shows dielectric relaxation around 150 K related to the weak ferromagnetic ordering in system. Dipolar response is activated following Arrhenius formalism in the frequency range of 0.01-100 kHz, but it is independent on magnitude of magnetic field. As frequency increases dielectric peaks become broader and smaller. Dielectric constant relaxation behavior and magnetic phase transitions are not coupled through lattice distortions in this ferroelectric ferromagnetic system. YCrO$_{3}$ is an example of system with dipolar response in magnetic field without coupling with magnetic structure. There is strong dispersion of Debye relaxation peaks but absence of influence of magnetic field on ferroelectric system YCrO$_{3}$.   

Multiferroicity in the spin-1/2 quantum matter of LiCu$_{2}$O$_{2}$

Multiferroicity in LiCu$_{2}$O$_{2}$ single crystals is studied using resonant soft x-ray magnetic scattering, hard x-ray diffraction, heat capacity, magnetic susceptibility, and electrical polarization. Two magnetic transitions are found at 24.6 K (\textit{T}$_{1}$ and 23.2 K (\textit{T}$_{2}$. Our data are consistent with a sinusoidal spin structure at \textit{T}$_{2}$ $<$ \textit{T} $<$ \textit{T}$_{1}$ and with a helicoidal spin structure at \textit{T} $<$ \textit{T}$_{2}$ giving rise to ferroelectricity. Surprisingly, above \textit{T}$_{2}$ the correlation lengths of the spin structures increase as the temperature increases with dramatic changes of $\sim $42{\%} occurring along the \textit{c} -axis. Our results demonstrate the intimate connection between frustration and coupling between electronic and magnetic polarizations in LiCu$_{2}$O$_{2}$.   

Magnetically-induced ferroelectric polarization in a molecule-based quantum magnet

Ferroelectricity coupled to antiferromagnetic (AFM) order has been observed in the organic S=1/2 chain compound CDC (CuCl$_{2}$ 2(CH$_{3})_{2}$SO). For magnetic fields along the orthorhombic c-axis, AFM order occurs below T$_{N}$ = 0.93 K and H $\sim $ 4 T. A spin-flop transition above H$_{sf}$ = 0.35 T leads to a magnetically ordered state that breaks inversion symmetry along the b-axis for 0.35 T $<$ H $<$ 4 T. Measurements of the pyroelectric effect and the dielectric constant along b indicate ferroelectricity occurring in this same region of HT phase space with the spin polarization closely tracking the magnetic order parameter. The ferroelectric polarization is observed without electrically poling the material, and polarization switching can be observed by consecutive field sweeps in the same direction. While the magnetically-induced ferroelectricity in CDC is far from practical temperatures and fields, it nevertheless demonstrates that this phenomena can occur in a whole new class of compounds.   

Spin Dynamics and Spin-flop transition in Magnetoelectric Effect LiMnPO$_{4}$

Neutron scattering techniques were used to study~the magnetic phase transition and spin dynamics in single crystal~LiMnPO$_{4}$ both with and without magnetic field.~~Elastic scattering confirmed that~LiMnPO$_{4}$ has a collinear antiferromagnetic ground state with~moments along $a$-axis in zero-field. The temperature dependent~order parameter, calculated from the integrated intensity of the (010)~magnetic reflection, was fit to a power law equation, yielding a~transition temperature $T_{N}$ = 33.8 K. By applying magnetic field along the a-axis, the moments rotate from a-axis to the c-axis at a critical field of 3.5 Tesla at 5 K.~ The field dependent (100) and (001) intensities indicate a complicated intermediate state between the ground state and the spin-flop state. The critical field increased from 3.5 Tesla at 5 K to 4.5 Tesla near the transition $T_{N}$. Spin-wave dispersion curves along the three principal axes were measured in the antiferromagnetic state at 4.5 K in zero magnetic field and were analyzed using a 3D Heisenberg~model.   

Magnetic Excitations in LiCoPO$_{4}$

LiCoPO$_{4}$ continues to attract much attention due to its exceptionally large magnetoelectric (ME) effect coefficient and the observed weak ferromagnetism and ME ``butterfly loop'' anomaly. To gain insight into the microscopic magnetic interactions in LiCoPO$_{4}$, inelastic neutron scattering experiments were performed in the antiferromagnetic phase at T = 8 K (T$_{N} \approx $ 21.8 K). Weak dispersion was detected in the magnetic excitation spectra along the three crystallographic axes measured around the (0 1 0) magnetic reflection. A gap of $\sim$ 4.7 meV was observed below T$_{N}$ that vanished above T$_{N}$. We analyze the data within a linear spin-wave approximation by explicitly including single-ion anisotropy terms in the Heisenberg spin Hamiltonian. The magnitude of the single-ion anisotropy is found to be comparable to the strongest nearest-neighbor magnetic interaction suggesting that the Co$^{2+}$ single ion anisotropy plays an important role in the spin dynamics of LiCoPO$_{4}$.