arXiv subjects

S. -W. Cheong

Publications and source records attributed to S. -W. Cheong.

At least 91 records · Page 5Linked to original sources

arXiv2010

Helical spin-waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14

We have investigated the spin fluctuations in the langasite compound Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The low temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously reported by Marty et al. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second order phase transition for a two dimensional triangular antiferromagnet. We will show that the physical properties and energy scales including the ordering wavevector, Curie-Weiss temperature, and the spin-waves can be explained through the use of only symmetric exchange constants without the need for the Dzyaloshinskii-Moriya interaction. This is accomplished through a set of ``helical" exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering which displays a strong vector chirality up to high temperatures well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound.

cond-mat.str-el↗

arXiv2010

Analysis of magnetization and a spin state crossover in the multiferroic Ca$_3$Co$_{2-x}$Mn$_x$O$_6$

Ca_3Co_{2-x}Mn_xO_6 (x ~ 0.96) is a multiferroic with spin-chains of alternating Co(2+) and Mn(4+) ions. The spin state of Co(2+) remains unresolved, due to a discrepancy between high temperature X-ray absorption (S=3/2) and low temperature neutron (S=1/2) measurements. Using a combination of magnetic modeling and crystal-field analysis, we show that the existing low temperature data cannot be reconciled within a high spin scenario by invoking spin-orbit or Jahn-Teller distortions. To unify the experimental results, we propose a spin-state crossover with specific experimental predictions.

cond-mat.str-el↗

arXiv2009

Spin-Coupled Local Distortions in Multiferroic Hexagonal HoMnO3

Local structural measurements have been performed on hexagonal HoMnO3 in order to ascertain the specific changes in bond distances which accompany magnetic ordering transitions. The transition from paramagnetic to the antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn (nearest and next nearest) and Ho-Ho correlations. These changes in bond distances reveal strong spin-lattice coupling. The similarity in magnitude of the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The structural changes are interpreted in terms of a model of competing spin order and local structural distortions. Density functional calculations are used to estimate the energies associated with ionic displacements. The calculations also reveal asymmetric polarization of the charge density of Ho, O3 and O4 sites along the z-axis in the ferroelectric phase. This polarization facilitates coupling between Ho atoms on neighboring planes normal to the z-axis.

cond-mat.str-el↗

arXiv2009

Spin freezing and dynamics in Ca_{3}Co_{2-x}Mn_{x}O_{6} (x ~ 0.95) investigated with implanted muons: disorder in the anisotropic next-nearest neighbor Ising model

We present a muon-spin relaxation investigation of the Ising chain magnet Ca_{3}Co_{2-x}Mn_{x}O_{6} (x~0.95). We find dynamic spin fluctuations persisting down to the lowest measured temperature of 1.6 K. The previously observed transition at around T ~18 K is interpreted as a subtle change in dynamics for a minority of the spins coupling to the muon that we interpret as spins locking into clusters. The dynamics of this fraction of spins freeze below a temperature T_{SF}~8 K, while a majority of spins continue to fluctuate. An explanation of the low temperature behavior is suggested in terms of the predictions of the anisotropic next-nearest-neighbor Ising model.

cond-mat.str-el↗

arXiv2009

Magnetic excitations and optical transitions in the multiferroic spin-1/2 system LiCu$_2$O$_2$

Magnetic excitations in cycloidal magnet LiCu$_2$O$_2$ are explored using THz absorption spectroscopy in magnetic fields up to 12 T. Below the spin ordering temperature we observe eight optically active transitions in the spin system of LiCu$_2$O$_2$ in the energy range from 4 to 30 cm$^{-1}$. In magnetic field the number of modes increases and the electric polarization flop is seen as a change in magnetic field dependence of mode energies. The polarization dependence of two of the modes in zero magnetic field fits the selection rules for the cycloid tilted by $θ=41\pm1^{\circ}$ from the bc plane. For the remaining six modes electric and magnetic dipole approximations cannot explain the observed polarization dependence. We do not see the electromagnon in the explored energy range although there is evidence that it could exist below 4 cm$^{-1}$.

cond-mat.str-el↗

arXiv2009

Temperature Dependent Local Structure of LaFeAsO_{1-x}F_x: Probing the Bond Correlations

The local structure of the parent and doped LaFeAsO1-xFx (pnictide) compounds were studied by x-ray absorption spectroscopy. In the doped system, the Fe-As and Fe-Fe correlations are well modeled by an Einstein model with no low temperature anomalies. For the Fe-As bonds, the Einstein temperatures are identical for the doped (11%) and undoped samples, but the doped sample is found to have a lower level of static disorder. For the Fe-Fe correlation, doping enhances the effective Einstein temperature of Fe-Fe atom correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic correlations. Density functional calculations of the charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers.

cond-mat.supr-con↗

arXiv2008

X-ray absorption spectroscopy measurement on the LaO1-xFxFeAs system

Results of Fe K-, As K-, and La L3-edge x-ray absorption near edge structure (XANES) measurements on LaO1-xFxFeAs compounds are presented. The Fe K- edge exhibits a chemical shift to lower energy, near edge feature modifications, and pre-edge feature suppression as a result of F substitution for O. The former two changes provide evidence of electron charge transfer to the Fe sites and the latter directly supports the delivery of this charge into the Fe-3d orbitals. The As K- edge measurements show spectral structures typical of compounds with planes of transition-metal tetrahedrally coordinated to p-block elements as is illustrated by comparison to other such materials. The insensitivity of the As-K edge to doping, along with the strong Fe-K doping response, is consistent with band structure calculations showing essentially pure Fe-d character near the Fermi energy in these materials. The energy of the continuum resonance feature above the La-L3 edge is shown to be quantitatively consistent with the reported La-O inter-atomic separation and with other oxide compounds containing rare earth elements.

cond-mat.supr-con↗

arXiv2008

Terahertz wave generation via optical rectification from multiferroic BiFeO3

We detected broadband coherent terahertz (THz) emission from multiferroic BiFeO3 after illuminating a high-quality bulk single ferroelectric domain crystal with a ~100 fs optical pulse. The dependence of the emitted THz waveform on the energy and polarization of the optical pulse is consistent with the optical rectification mechanism of THz emission. The THz emission provides a sensitive probe of the electric polarization state of BiFeO3, enabling applications in ferroelectric memories and ferroelectric domain imaging. We also report room-temperature THz optical constants of BiFeO3.

cond-mat.mtrl-sci↗

arXiv2008

Observation of a multiferroic critical end point

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a "metaelectric" critical end point has not yet been realized. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (\emph{H}) as a tuning parameter. Herein, we report the discovery of a magnetic-field-induced metaelectric transition in multiferroic BiMn$_{2}$O$_{5}$ in which the electric polarization (\emph{P}) switches polarity along with a concomitant Mn spin-flop transition at a critical magnetic field \emph{H}$_{\rm c}$. The simultaneous metaelectric and spin-flop transitions become sharper upon cooling, but remain a continuous crossover even down to 0.5 K. Near the \emph{P}=0 line realized at $μ_{0}$\emph{H}$_{\rm c}$$\approx$18 T below 20 K, the dielectric constant ($\varepsilon$) increases significantly over wide field- and temperature (\emph{T})-ranges. Furthermore, a characteristic power-law behavior is found in the \emph{P}(\emph{H}) and $\varepsilon$(\emph{H}) curves at \emph{T}=0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn$_2$O$_5$ near zero temperature.

cond-mat.mtrl-sci↗

arXiv2008

Electromagnons in multiferroic RMn2O5 compounds and their microscopic origin

We summarize the existing experimental data on electromagnons in multiferroic RMn2O5 compounds, where R denotes a rare earth ion, Y or Bi, and discuss a realistic microscopic model of these materials based on assumption that the microscopic mechanism of magnetically-induced ferroelectricity and electromagnon absorption relies entirely on the isotropic Heisenberg exchange and magnetostrictive coupling of spins to a polar lattice mode and does not involve relativistic effects. This model explains many magnetic and optical properties of RMn2O5 manganites, such as the spin re-orientation transition, magnetically-induced polarisation, appearance of the electromagnon peak in the non-collinear spin state and the polarisation of light for which this peak is observed. We compare experimental and theoretical results on electromagnons in RMn2O5 and RMnO3 compounds.

cond-mat.mtrl-sci↗

arXiv2008

Giant Magnetoelectric Effect in a Multiferroic Material with a High Ferroelectric Transition Temperature

We present a unique example of giant magnetoelectric effect in a conventional multiferroic HoMnO3, where polarization is very large (~56 mC/m2) and the ferroelectric transition temperature is higher than the magnetic ordering temperature by an order. We attribute the uniqueness of the giant magnetoelectric effect to the ferroelectricity induced entirely by the off-center displacement of rare earth ions with large magnetic moments. This finding suggests a new avenue to design multiferroics with large polarization and higher ferroelectric transition temperature as well as large magnetoelectric effects.

cond-mat.str-el↗

arXiv2007

Colossal magnon-phonon coupling in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$

We report the spectra of magnetically induced electric dipole absorption in Eu$_{0.75}$Y$_{0.25}$MnO$_3$ from temperature dependent far infrared spectroscopy (10-250 cm$^{-1}$). These spectra, which occur only in the $e||a$ polarization, consist of two relatively narrow electromagnon features that onset at $T_{FE}=30$ K and a broad absorption band that persists to temperatures well above $T_N=47$ K. The observed excitations account for the step up of the static dielectric constant in the ferroelectric phase. The electromagnon at 80 cm$^{-1}$ is observed to be strongly coupled to the nearby lowest optical phonon which transfers more than 1/2 of its spectral weight to the magnon. We attribute the origin of the broad background absorption to the two magnon emission decay process of the phonon.

cond-mat.mtrl-sci↗

arXiv2007

From Cooperative Paramagnet to Néel Order in Y_2Ru_2O_7

Spin correlations in the pyrochlore antiferromagnet Y_2Ru_2O_7 with Curie-Weiss temperature $Θ_{CW}=-1100$ K and critical temperature T_N=77 K were examined through neutron scattering. For $T_N<T<Θ_{CW}/3$ the data show spin relaxation with a rate $\hbarΓ= 1.17(9)k_BT$. For T<T_N spectral weight moves to higher energies with substantial changes up to $4\times k_BT_N$. For T<<T_N there is a $Δ=11(1)$ meV energy gap and a pronounced spectral maximum at 19.7 meV. Throughout the temperature range examined the wave vector dependence of inelastic scattering exhibits a broad peak for $Qd\approx 3.8$ (d is Ru-Ru spacing) consistent with dipolar spin correlations.

cond-mat.str-el↗

arXiv2007

Non-resonant and Resonant X-ray Scattering Studies on Multiferroic TbMn2O5

Comprehensive x-ray scattering studies, including resonant scattering at Mn L-edge, Tb L- and M-edges, were performed on single crystals of TbMn2O5. X-ray intensities were observed at a forbidden Bragg position in the ferroelectric phases, in addition to the lattice and the magnetic modulation peaks. Temperature dependences of their intensities and the relation between the modulation wave vectors provide direct evidences of exchange striction induced ferroelectricity. Resonant x-ray scattering results demonstrate the presence of multiple magnetic orders by exhibiting their different temperature dependences. The commensurate-to-incommensurate phase transition around 24 K is attributed to discommensuration through phase slipping of the magnetic orders in spin frustrated geometries. We proposed that the low temperature incommensurate phase consists of the commensurate magnetic domains separated by anti-phase domain walls which reduce spontaneous polarizations abruptly at the transition.

cond-mat.str-el↗

arXiv2007

Local Structure of Multiferroic TbMn2O5: Evidence for an Anomalous Terbium Oxygen Distribution

The temperature dependent local structure of TbMn2O5 was determined by x-ray absorption spectroscopy. An anomalous Tb-O distribution is found. At high temperature it is broad but resolves into two distinct peaks below approximately 180 K. The distributions sharpen below the Tb magnetic ordering temperature (approximately 10 K). The distortions in the Tb-O distribution, away from the Pbam structure, are consistent with rotations of the MnOx polyhedra about the c-axis and suggest that Tb-O bond polarization may play a significant role in the observed ferroelectric properties of this system.

cond-mat.str-el↗

arXiv2006

Magnetoelastic Effects and Magnetic Phase Diagram of Multiferroic $DyMn_2O_5$

The magnetoelastic coupling in multiferroic $DyMn_2O_5$ is investigated by magnetostriction measurements along the three crystallographic orientations. Strong lattice anomalies as a function of the magnetic field are detected at the low temperature magnetic and ferroelectric phase transitions. The sign and magnitude of the magnetostrictive coefficient as well as the lattice anomalies at the transitions are correlated with the Dy moment order and with the sharp changes of the dielectric constant and the ferroelectric polarization. With the magnetic field applied along the c-axis a new high-field phase has been detected the magnetic structure of which has yet to be explored.

cond-mat.str-el↗

arXiv2006

Spin thermal conductivity of the Haldane chain compound Y2BaNiO5

We have measured the thermal conductivity of the spin S=1 chain compound Y2BaNiO5. Analyzing the anisotropy of the thermal transport allows to identify a definite spin-mediated thermal conductivity κ_s along the chain direction. The calculated spin-related energy diffusion constant D_E(T) shows a broad peak around 120 K. Close to room temperature, D_E(T) approaches the theoretically predicted high-temperature value, while scattering of spin excitations by magnetic impurities seems to be the major limiting factor of κ_s at low temperature.

cond-mat.str-el↗

arXiv2005

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.

cond-mat.str-el↗