Hexagonal Rare-earth Manganites Research Articles

Pressure-induced charge-transfer and structural transition in hexagonal multiferroic HoMnO3

The improper ferroelectricity and frustrated magnetism observed in hexagonal rare-earth manganites are tied to Mn trimerization in the basal plane. Multiferroic properties interplay via the lattice leading to complex structural behavior with temperature and pressure. Applying hydrostatic pressure to hexagonal HoMnO${}_{3}$ induces a phase transition to a rhombohedrally distorted superstructure, accompanied by disproportionation of Mn(III) and compressibility of the basal plane. These findings reveal new insights into multiferroic manganites under extreme conditions, highlighting the complicated interactions between their properties.

Molten Salt Synthesis of Micro-Sized Hexagonally Shaped REMnO3 (RE = Y, Er, Tm, Yb)

Yttrium manganite (YMnO3) is a compound belonging to the hexagonal rare earth manganites family, which demonstrates multiferroic properties. This material can be prepared by several synthetic approaches, with the most common one being the solid-state synthesis. In this work, we show a possibility of preparing this material via molten salt synthesis using the NaCl-KCl mixture as the reaction medium and yttrium and manganese nitrates as the starting materials. We demonstrate that, by varying the reaction temperature and the nitrates-to-chlorides ratio, it is feasible to synthesize hexagonally shaped particles of microscopic dimensions. A similar synthesis procedure can be successfully applied for the preparation of other hexagonal manganites—ErMnO3, TmMnO3, and YbMnO3.

Influence of the triangular Mn-O breathing mode on magnetic ordering in multiferroic hexagonal manganites

We use a combination of symmetry analysis, phenomenological modeling, and first-principles density functional theory to explore the interplay between the magnetic ground state and the detailed atomic structure in the hexagonal rare-earth manganites. We find that the magnetic ordering is sensitive to a breathing mode distortion of the Mn and O ions in the $ab$ plane, which is described by the ${\mathrm{K}}_{1}$ mode of the high-symmetry structure. Our density functional calculations of the magnetic interactions indicate that this mode particularly affects the single-ion anisotropy and the interplanar symmetric exchanges. By extracting the parameters of a magnetic model Hamiltonian from our first-principles results, we develop a phase diagram to describe the magnetic structure as a function of the anisotropy and exchange interactions. This in turn allows us to explain the dependence of the magnetic ground state on the identity of the rare-earth ion and on the ${\mathrm{K}}_{1}$ mode.

Influence of Doping on the Transport Properties of Y1−xLnxMnO3+δ (Ln: Pr, Nd)

It has been documented that the total electrical conductivity of the hexagonal rare-earth manganites Y0.95Pr0.05MnO3+δ and Y0.95Nd0.05MnO3+δ, as well as the undoped YMnO3+δ, is largely dependent on the oxygen excess δ, which increases considerably at temperatures below ca. 300 °C in air or O2. Improvement for samples maintaining the same P63cm crystal structure can exceed 3 orders of magnitude below 200 °C and is related to the amount of the intercalated oxygen. At the same time, doping with Nd3+ or Pr3+ affects the ability of the materials to incorporate O2, and therefore indirectly influences the conductivity as well. At high temperatures (700–1000 °C) and in different atmospheres of Ar, air, and O2, all materials are nearly oxygen-stoichiometric, showing very similar total conduction with the activation energy values of 0.8–0.9 eV. At low temperatures in Ar (δ ≈ 0), the mean ionic radius of Y1−xLnx appears to influence the electrical conductivity, with the highest values observed for the parent YMnO3. For Y0.95Pr0.05MnO3+δ oxide, showing the largest oxygen content changes, the recorded dependence of the Seebeck coefficient on the temperature in different atmospheres exhibits complex behavior, reflecting oxygen content variations, and change of the dominant charge carriers at elevated temperatures in Ar (from electronic holes to electrons). Supplementary cathodic polarization resistance studies of the Y0.95Pr0.05MnO3+δ electrode document different behavior at higher and lower temperatures in air, corresponding to the total conduction characteristics.

Room-Temperature Antiferroelectricity in Multiferroic Hexagonal Rare-Earth Ferrites.

The antiferroelectric (AFE) phase, in which nonpolar and polar states are switchable by an electric field, is a recent discovery in promising multiferroics of hexagonal rare-earth manganites (ferrites), h-RMn(Fe)O3. However, this phase has so far only been observed at 60-160 K, which restricts key investigations into the microstructures and magnetoelectric behaviors. Herein, we report the successful expansion of the AFE temperature range (10-300 K) by preparing h-DyFeO3 films through epitaxial stabilization. Room-temperature scanning transmission electron microscopy reveals that the AFE phase originates from a nanomosaic structure comprising AFE P3̅c1 and ferroelectric P63cm domains with small domain sizes of 1-10 nm. The nanomosaic structure is stabilized by a low c/a ratio derived from the large ionic radius of Dy3+. Furthermore, weak ferromagnetism and magnetocapacitance behaviors are observed. Below 10 K, the film exhibits an M-shaped magnetocapacitance versus magnetic field curve, indicating unusual magnetoelectric coupling in the AFE phase.

Hexagonal rare-earth manganites and ferrites: a review of improper ferroelectricity, magnetoelectric coupling, and unusual domain walls.

Hexagonal rare-earth manganites and ferrites are well-known improper ferroelectrics with low-temperature antiferromagnetism/weak ferromagnetism. In recent decades, new multi-functional device concepts and applications have provoked the exploration for multiferroics which simultaneously possess ferroelectric and magnetic orders. As a promising platform for multiferroicity, hexagonal manganites and ferrites are attracting great research interest among the fundamental scientific and technological communities. Moreover, the novel type of vortex-like ferroelectric domain walls are locked to the antiphase structural domain walls, providing an extra degree of freedom to tune the magnetoelectric coupling and other properties such as conductance. Here, we summarize the main experimental achievements and up-to-date theoretical understanding of the ferroelectric, magnetic, and magnetoelectric properties, as well as the intriguing domain patterns in hexagonal rare-earth manganites and ferrites. Recent work on non-stoichiometric compounds will also be briefly introduced.

An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites.

Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realized that domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures. Here, a novel improper ferroelectric, CsNbW2 O9 , with the hexagonal tungsten bronze structure, is reported. Powder neutron diffraction and symmetry mode analysis indicate that the improper transition (TC = 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However, in contrast to the manganites, the symmetry breaking in CsNbW2 O9 is electronically driven (i.e., purely displacive) via the second-order Jahn-Teller effect in contrast to the geometrically driven tilt mechanism of the manganites. Nevertheless CsNbW2 O9 displays the same kinds of domain microstructure as those found in the manganites, such as the characteristic six-domain "cloverleaf" vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics.

Hexagonal YMnO3 films as promising ultraviolet photodetectors

Hexagonal rare-earth manganites (h-RMnO3, R = Sc, Y, Ho-Lu) exhibit intricate multiferroic properties and may be utilized for various technological applications ranging from non-volatile memories to solar cells and photo-detectors. Here, we report the epitaxial growth of hexagonal YMnO3 (h-YMO) films with different thicknesses deposited on yttria-stabilized zirconia (YSZ) substrates and their photoresponses to ultraviolet (UV) light. The structural characterizations illustrate that all the films present high quality epitaxy with out-of-plane h-YMO(0001)//YSZ(111) and in-plane h-YMO[1230]//YSZ[110] relationship. Meanwhile, UV light driven photoresponses of these as-prepared films based on the planar photodetector structure with transparent Al-doped ZnO (AZO) as top electrodes are explored. It is revealed that the photocurrent will first increase and then decrease with increasing film thickness. The highest photoconductivity gain is ~387.4 observed at +4 V bias for the device with 160 nm YMO film. These results provide a guideline for hexagonal rare-earth manganites to work for UV photodetector application.

Low-energy structural dynamics of ferroelectric domain walls in hexagonal rare-earth manganites.

Domain walls (DWs) in ferroic materials, across which the order parameter abruptly changes its orientation, can host emergent properties that are absent in the bulk domains. Using a broadband (106 to 1010 Hz) scanning impedance microscope, we show that the electrical response of the interlocked antiphase boundaries and ferroelectric DWs in hexagonal rare-earth manganites (h-RMnO3) is dominated by the bound-charge oscillation rather than free-carrier conduction at the DWs. As a measure of the rate of energy dissipation, the effective conductivity of DWs on the (001) surfaces of h-RMnO3 at gigahertz frequencies is drastically higher than that at dc, whereas the effect is absent on surfaces with in-plane polarized domains. First-principles and model calculations indicate that the frequency range and selection rules are consistent with the periodic sliding of the DW around its equilibrium position. This acoustic wave-like mode, which is associated with the synchronized oscillation of local polarization and apical oxygen atoms, is localized perpendicular to the DW but free to propagate along the DW plane. Our results break the ground to understand structural DW dynamics and exploit new interfacial phenomena for novel devices.

Pressure and strain effects of hexagonal rare-earth manganites: a first-principles study

We have investigated the structural, electrical and magnetic properties as well as the phonon modes of hexagonal rare-earth manganites (RMnO3, R = Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu) under chemical pressure, hydrostatic pressure and epitaxial strain by first-principles calculations. The magnetic ground state of RMnO3 is found to have magnetic configuration and to be stable under all considered external conditions. In contrast, the K3 phonon mode, which is the primary order parameter and responsible for the ‘improper ferroelectricity’, is greatly influenced by pressure and epitaxial strain. Consequently, the electric polarization is enhanced by 56.7% when the chemical pressure increases from R = Pr to R = Lu. The hydrostatic pressure can also improve the polarization to a certain degree, e.g. by 14.7% from 0 GPa to 40 GPa in LuMnO3. Finally, the dependence of polarization on the epitaxial strain is also given, revealing that the compressive strain could promote the ferroelectricity while tensile strain will suppress it.

Hexagonal rare-earth manganites as promising photovoltaics and light polarizers

Ferroelectric materials possess a spontaneous electric polarization and may be utilized in various technological applications ranging from non-volatile memories to solar cells and light polarizers. Recently, hexagonal rare-earth manganites, h-RMnO$_3$ (R is a rare-earth ion) have attracted considerable interest due to their intricate multiferroic properties and improper ferroelectricity characterized by a sizable remnant polarization and high Curie temperature. Here, we demonstrate that these compounds can serve as very efficient photovoltaic materials and, in addition, possess remarkable optical anisotropy properties. Using first-principles methods based on density-functional theory and considering h-TbMnO$_3$ as a representative manganite, we predict a strong light absorption of this material in the solar spectrum range, resulting in the maximum light-to-electricity energy conversion efficiency up to 33%. We also predict an extraordinary optical linear dichroism and linear birefringence properties of h-TbMnO$_3$ in a broad range of optical frequencies. These results uncover the unexplored potential of hexagonal rare-earth manganites to serve as photovoltaics in solar cells and as absorptive and birefringent light polarizers.

Magnetic charges and magnetoelectricity in hexagonal rare-earth manganites and ferrites

Magnetoelectric (ME) materials are of fundamental interest and show broad potential for technological applications. Commonly the dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$. Our previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites $R$MnO$_3$ and ferrites $R$FeO$_3$ ($R$ = Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic and structural degrees of freedom, with the transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a 120$^\circ$ noncollinear spin arrangement. Here we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites, clarifying the conditions under which exchange striction leads to an enhanced $Z^{\rm m}$ values and anomalously large in-plane spin-lattice ME effects.

Direct observation of interlocked domain walls and topological four-state vortex-like domain patterns in multiferroic YMnO3 single crystal

Using the advanced spherical aberration-corrected high angle annular dark field scanning transmission electron microscope imaging techniques, we investigated atomic-scale structural features of domain walls and domain patterns in YMnO3 single crystal. Three different types of interlocked ferroelectric-antiphase domain walls and two abnormal topological four-state vortex-like domain patterns are identified. Each ferroelectric domain wall is accompanied by a translation vector, i.e., 1/6[210] or −1/6[210], demonstrating its interlocked nature. Different from the four-state vortex domain patterns caused by a partial edge dislocation, two four-state vortex-like domain configurations have been obtained at atomic level. These observed phenomena can further extend our understanding of the fascinating vortex domain patterns in multiferroic hexagonal rare-earth manganites.

Direct visualization of magnetoelectric domains

The coupling between the magnetic and electric dipoles in multiferroic and magnetoelectric materials holds promise for conceptually novel electronic devices. This calls for the development of local probes of the magnetoelectric response, which is strongly affected by defects in magnetic and ferroelectric ground states. For example, multiferroic hexagonal rare earth manganites exhibit a dense network of boundaries between six degenerate states of their crystal lattice, which are locked to both ferroelectric and magnetic domain walls. Here we present the application of a magnetoelectric force microscopy technique that combines magnetic force microscopy with in situ modulating high electric fields. This method allows us to image the magnetoelectric response of the domain patterns in hexagonal manganites directly. We find that this response changes sign at each structural domain wall, a result that is corroborated by symmetry analysis and phenomenological modelling, and provides compelling evidence for a lattice-mediated magnetoelectric coupling. The direct visualization of magnetoelectric domains at mesoscopic scales opens up explorations of emergent phenomena in multifunctional materials with multiple coupled orders.

Optical Spectroscopy of Multiferroic Rare-Earth Manganites

Hexagonal rare-earth manganites (RMnO3 with R = Er, Tm, Yb) have been studied by high resolution absorption spectroscopy in the infrared spectral range around the Néel-temperature (TN ≈ 80 K). Series of transitions have been assigned for the R3+ rare-earth ions in trigonal crystal field. Some of the Stark components of the rare-earth multiplets split below the Néel-temperature due to magnetic ordering. The Néel-temperature for the three compounds investigated decreases with increasing lattice constant (a).

Magnetic order of the hexagonal rare-earth manganite Dy0.5Y0.5MnO3

Hexagonal Dy(0.5)Y(0.5)MnO3, a multiferroic rare-earth manganite with geometrically frustrated antiferromagnetism, has been investigated with single-crystal neutron diffraction measurements. Below 3.4 K magnetic order is observed on both the Mn (antiferromagnetic) and Dy (ferrimagnetic) sublattices that is identical to that of undiluted hexagonal DyMnO3 at low temperature. The Mn moments undergo a spin reorientation transition between 3.4 K and 10 K, with antiferromagnetic order of the Mn sublattice persisting up to 70 K; the antiferromagnetic order in this phase is distinct from that observed in undiluted (h)DyMnO3, yielding a qualitatively new phase diagram not seen in other hexagonal rare-earth manganites. A magnetic field applied parallel to the crystallographic c axis will drive a transition from the antiferromagnetic phase into the low-temperature ferrimagnetic phase with little hysteresis.

Antiferromagnetic ordering in RMnO 3 rare-earth manganites observed by optical spectroscopy

The optical absorption spectra of hexagonal rare-earth manganites (RMnO 3 with R = Er, Tm, Yb) have been measured in the 5000–13,000 cm −1 infrared spectral range as a function of temperature between 8 and 300 K. A series of transitions have been assigned for the trivalent rare-earth ions in trigonal crystal field. Some of the Stark components of the rare-earth multiplets split below the Néel-temperature due to antiferromagnetic ordering. The Néel-temperature for the three compounds investigated decreases with increasing ionic radius of the rare-earth ions.

Optical spectroscopy of charge transfer transitions in multiferroic manganites, ferrites, and related insulators

The results of theoretical and experimental studies of charge transfer (CT) transitions in multiferroic manganites, ferrites, and related insulators are reviewed. Starting with a simple cluster model approach one-center p-d and two-center d-d CT transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom are all addressed. Optical ellipsometry data in the spectral range of 0.6–5.8eV in perovskite and hexagonal rare-earth manganites RMnO3 and in orthorhombic manganites RMn2O5 are analyzed. Two groups of iron oxides, where Fe3+ ions occupy either only octahedral positions (BiFeO3, orhoferrites RFeO3 et al.) and materials with Fe3+ ions both in octahedral and tetrahedral positions (hematite α-Fe2O3, garnets RFe5O12, lithium ferrite LiFe5O8, Ca2Fe2O5 et al.), are discussed.

Thermal transport by lattice excitations in hexagonal rare-earth manganites

Thermal transport mechanism in a ferroelectricity-magnetism-coexisting system, hexagonal YMnO3 is investigated. In addition to the acoustic phonon contribution, heat transport induced by optical phonon is taken into account to explain the excess thermal conductivity above TN. Magnetoelectric coupling effect on both acoustic and optical phonons is embodied by change in Debye and Einstein temperatures, respectively. Magnon-phonon resonant interaction is considered as one of the scattering mechanisms that correct relaxation time of acoustic phonon thermal conductivity. Our theoretical fitting is in good agreement with the experimental data.

Thin films and superlattices of multiferroic hexagonal rare earth manganites

Hexagonal YMnO3 and HoMnO3 as well as (YMnO3/HoMnO3)15 superlattices were grown on (111) ZrO2 (Y2O3) and (111) Pt/TiO2/SiO2/p-type (100)Si substrates. Epitaxial stabilization was used on the same substrates to grow DyMnO3 or TbMnO3 (which normally crystallize in an orthorhombic, perovskite-type, structure). All heterostructures were obtained oriented, with the c-axis of the hexagonal cell perpendicular to the substrate plane. This orientation is desired since it is the direction of the ferroelectric polarization in hexagonal manganites. The strain state of the films grown on conductive Pt electrodes, tracked by the evolution of the c parameter as a function of film thickness, was found to be completely different than the one obtained on YSZ. Such a study is important for future strain engineering in multilayers. High-resolution transmission electron microscopy on superlattices grown on (111)Pt indicates a high crystalline quality along the c-axis and sharp interfaces.