Hexagonal RMnO3 Research Articles

Multiferroic RMnO3 thin films

Multiferroic materials have received an astonishing attention in the last decades due to expectations that potential coupling between distinct ferroic orders could inspire new applications and new device concepts. As a result, a new knowledge on coupling mechanisms and materials science has dramatically emerged. Multiferroic RMnO3 perovskites are central to this progress, providing a suitable platform to tailor spin–spin and spin–lattice interactions. With views towards applications, the development of thin films of multiferroic materials have also progressed enormously and nowadays thin-film manganites are available, with properties mimicking those of bulk compounds. Here we review achievements on the growth of hexagonal and orthorhombic RMnO3 epitaxial thin films and the characterization of their magnetic and ferroelectric properties, we discuss some challenging issues, and we suggest some guidelines for future research and developments. Les matériaux multiferroïques ont reçu une attention étonnante au cours des dernières décades liée à la possibilité de couplage entre les ordres ferroïques et à son potentiel pour de nouvelles applications et de nouveaux concepts de composants. De ce fait, une nouvelle connaissance des mécanismes de couplage et de la science des matériaux a émergé. Les pérovskites multiferroïques RMnO3 sont au centre de ces progrès, en ce sens qu'elles fournissent une plateforme adaptée pour façonner les interactions spin–spin et spin–réseau. En ce qui concerne les applications, le développement de films minces de matériaux multiferroïques a aussi énormément progressé, et de nos jours des films minces de manganites avec des propriétés similaires à celles des matériaux massifs existent. Nous passons en revue ici les résultats obtenus dans le domaine de la croissance de couches minces épitaxiés de RMnO3 hexagonal et orthorhombique et de la caractérisation de leurs propriétés magnétiques et ferroélectriques. Nous discutons certains enjeux et proposons quelques idées pour des recherches et développements futurs.

Observation of vortex domain structures in multiferroic hexagonal manganites RMnO3by transmission electron microscopy

Multiferroic hexagonal manganite RMnO3 (R = rare-earth elements) shows improper ferroelectricity accompanied by tilting of MnO5 hexahedra as the primary order parameter. The ferroelectricity is originated from displacements of rare-earth ions along c-axis triggered by the MnO5 hexahedra tilting. Although coupling between ferroelectric and antiferromagnetic domains below the magnetic transition temperature of ∼90 K has been reported from previous work[1], the relationship between the ferroelectric domains and structural domains due to the MnO5 hexahedra tilting has not been well-studied. In this talk, we will report our studies on unique patterns of ferroelectric antiphase domains with a vorticity in hexagonal RMnO3, obtained from the results of transmission electron microscopy [2].The electron diffraction patterns obtained at room temperature exhibit superlattice reflection spots due to the MnO5 hexahedra tilting and displacements of rare-earth ions along c-axis, in addition to the fundamental reflections associated with the high symmetry structure with the space group of P63/mmc. Unique antiphase/ferroelectric "cloverleaf-like" domain patterns are clearly observed in dark-field images taken using superlattice spots. The fundamental and superlattice dark-field imaging combined with high-resolution imaging clearly demonstrates that in the cloverleaf-like domain patterns the antiphase and ferroelectric domains arrange periodically with certain rotation direction. In addition, there exist two types of cloverleaf-like domain patterns with the opposite rotations next to each other in the superlattice dark-field images. These results indicate that the cloverleaf-like domain patterns can be considered as the aggregation of vortices and antivortices consisting of ferroelectric and antiphase domains.

New RMnO3+δ (R=Y, Ho; δ≈0.35) phases with modulated structure

A series of hexagonal RMnO3 (R=Y, Ho–Lu) compounds are first annealed in various highly oxidizing conditions to show that for the largest R constituents, Y and Ho, it is possible to load the phases with large amounts of excess oxygen. Within the oxygen-annealed samples we identify new heavily-oxygenated phases, that is, YMnO3.35 and HoMnO3.34. The former is then thoroughly characterized by thermogravimetric analysis, X-ray diffraction and electron diffraction. These studies evidence a hexagonal sub-cell (aS≈3.578Å (aH/3), cS≈11.18Å, space group P63mc) and satellites, associated with two modulation vectors q→1⁎≈0.41a→S⁎+0.41b→S⁎+1/3c→S⁎andq→2⁎=(1/2). The results are interpreted by the formation of a modulated composite structure, resulting from the evolution of the hexagonal parent YMnO3+δ structure toward a reduced pyrochlore-type structure Y2Mn2O7−x, in long- and short-range ordering.

Hydrothermal synthesis and magnetic properties of Fe3+-doped multiferroic hexagonal rare-earth manganates

Hexagonal RMn1−xFexO3 (R=Er, Tm, Yb and Lu; x=0, 0.1, 0.3 and 0.5) were successfully synthesized via the low-temperature hydrothermal technique. Fe3+ doping was used to elevate the low antiferromagnetic transition temperature TN which is a fatal weakness for hexagonal manganates RMnO3 to be used as multiferroic materials. The structures, compositions, morphologies and magnetic properties of the samples and the valences of Mn and Fe were studied via the methods of XRD, Rietveld refinement, ICP-MS, EDS, SEM, SQUID and XPS. ErMnO3 and RMn1−xFexO3 (R=Tm, Yb and Lu; x=0, 0.1, 0.3 and 0.5) were all of pure hexagonal structure while ErMn1−xFexO3 (x=0.1, 0.3 and 0.5) comprised hexagonal and orthorhombic structures. As for the undoped RMnO3 (R=Er, Tm, Yb and Lu), the cell parameters a, c and cell volume V decreased while the antiferromagnetic transition temperatures TN rose with the decrease of R3+ radius. The antiferromagnetic transition temperatures TN were elevated by Fe3+ doping and a linear relation was observed between ΔTN and Fe doping concentration x. A percentage of Fe3+ doping elevated TN by 0.3–0.5K, and the elevation was more obvious when the R3+ ionic radius was smaller.

Hidden order in hexagonalRMnO3multiferroics (R=Dy–Lu, In, Y, and Sc)

Hexagonal RMnO3 manganites are improper ferroelectrics in which the electric polarization is a by-product of the tripling of the unit cell. In YMnO3, there is a second transition at ~ 920K whose nature remains unexplained. We argue that this transition can be seen as a sort of hidden order in which a residual symmetry displayed by the trimerization order parameter is spontaneously broken. This additional order gives rise to twelve structural domains instead of six, and structural domain boundaries that can be either ferroelectric or non-ferroelectric domain walls.

Unfolding of vortices into topological stripes in a multiferroic material.

Multiferroic hexagonal RMnO(3) (R=rare earths) crystals exhibit dense networks of vortex lines at which six domain walls merge. While the domain walls can be readily moved with an applied electric field, the vortex cores so far have been impossible to control. Our experiments demonstrate that shear strain induces a Magnus-type force pulling vortices and antivortices in opposite directions and unfolding them into a topological stripe domain state. We discuss the analogy between this effect and the current-driven dynamics of vortices in superconductors and superfluids.

マルチフェロイック六方晶マンガン酸化物のボルテックス分域構造と新奇物性

マルチフェロイック六方晶マンガン酸化物のボルテックス分域構造と新奇物性

Anisotropy of optical properties of hexagonal RMnO3 Manganites (R = Ho, Er, Tm, Yb)

The evolution of real and imaginary parts of the complex permittivity of hexagonal single crystals of RMnO3 manganites (R = Ho, Er, Tm, Yb) differing in the radius of rare-earth ion r R has been studied using spectroscopic ellipsometry in the range of 0.5–5.0 eV at room temperature. Spectra of dielectric functions show a strong polarization dependence. The optical-absorption edge for polarization E ⊥ c is determined by the intense narrow peak at 1.6 eV, whereas for polarization E ‖ c, the peak is shifted toward high energies by 0.15–0.20 eV and its intensity is suppressed greatly. It has been shown that, when r R decreases, the energy position of the intense peak at 1.6 eV in the spectrum of the imaginary part of the dielectric function for polarization E ⊥ c shifts toward low energies by no more than 0.1 eV, which reflects changes in the local surroundings of the Mn3+ ion. For the both polarizations, a broad absorption band with the center at 2.4 eV has been revealed; the band was detected earlier in the antiferromagnetic phase in nonlinear spectra upon the optical generation of the second harmonic. Spectra of permittivity have been analyzed within available concepts on the electronic structure of hexagonal RMnO3 compounds and have been compared with corresponding spectra of previously studied orthorhombic RMnO3 compounds.

Weak Ferromagnetic Transition with a Dielectric Anomaly in Hexagonal Lu0.5Sc0.5FeO3

Lu1-xScxFeO3 (0 ≤ x ≤ 1) was synthesized by a conventional solid-state reaction. The hexagonal phase appeared at 0.4 ≤ x ≤ 0.6, between the perovskite phase (0 ≤ x ≤ 0.3) and the bixbyite phase (0.7 ≤ x ≤ 1). Structural, magnetic, and dielectric properties of hexagonal Lu0.5Sc0.5FeO3 were investigated. Synchrotron X-ray diffraction measurements revealed that the crystal structure of Lu0.5Sc0.5FeO3 is isomorphic to hexagonal ferroelectrics RMnO3 (R = rare earth ion) with a polar space group of P63cm. A weak ferromagnetic transition with a dielectric anomaly occurred at a much higher temperature (162 K) than those in hexagonal RMnO3. Although remanent magnetization was observed below the transition temperature, it decreased to almost zero at 10 K. These results indicate a strong antiferromagnetic interaction between ground-state Fe(3+) ions on the triangular lattice.

Direct Observation of Multiferroic Vortex Domains in YMnO3

Topological vortices with swirling ferroelectric, magnetic and structural anti-phase relationship in hexagonal RMnO3 (R = Ho to Lu, Y, and Sc) have attracted much attention because of their intriguing behaviors. Herein, we report the structure of multiferroic vortex domains in YMnO3 at atomic scale using state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM). Two types of displacements were identified among six domain walls (DWs); six translation-ferroelectric domains denoted by α+, γ−, β+, α−, γ+ and β−, respectively, were recognized, demonstrating the interlocking nature of the anti-vortex domain. We found that the anti-vortex core is about four unit cells wide. In addition, we reconstructed the vortex model with three swirling pairs of DWs along the [001] direction. These results are very critical for the understanding of topological behaviors and unusual properties of the multiferroic vortex.

Complex structural-ferroelectric domain walls in thin films of hexagonal orthoferrites RFeO3 (R = Lu, Er)

Hexagonal orthoferrites have recently attracted much attention as possible high-temperature ferromagnetic ferroelectrics. The ferroelectric domain structure of hexagonal RMnO3, their antiferromagnetic structural analogies, has recently shown an atypical and complicated behavior. Hexagonal RFeO3 are expected to exhibit similar domain structure that should coexist with weak ferromagnetic order and may represent a material with a unusual magnetoelectric interaction. In this report, we discuss microscopic ferroelectric domain structure of hexagonal orthoferrites in a thin-film state and demonstrate a distinct and unusual improper ferroelectric behavior of these oxide materials.

Mutual induction of magnetic 3dand 4forder in multiferroic hexagonal ErMnO3

The complex interplay between the 3d and 4f moments in hexagonal ErMnO3 is investigated by magnetization, optical second harmonic generation, and neutron-diffraction measurements. We revise the phase diagram and provide a microscopic model for the emergent spin structures with a special focus on the intermediary phase transitions. Our measurements reveal that the 3d exchange between Mn^{3+} ions dominates the magnetic symmetry at 10 K < T < T_N with Mn^3+ order according to the Gamma_4 representation triggering 4f ordering according to the same representation on the Er^{3+}(4b) site. Below 10 K the magnetic order is governed by 4f exchange interactions of Er^{3+} ions on the 2a site. The magnetic Er^{3+}(2a) order according to the representation Gamma_2 induces a magnetic reorientation (Gamma_4 --> Gamma_2) at the Er^{3+}(4b) and the Mn^{3+} sites. Our findings highlight the fundamentally different roles the Mn^{3+}, R^{3+}(2a), and R^{3+}(4b) magnetism play in establishing the magnetic phase diagram of the hexagonal RMnO3 system.

Critical behavior of multiferroic hexagonal RMnO3

AbstractUsing a microscopic model and a Green's function technique we have studied the critical behavior of some multiferroics such as hexagonal RMnO3. The temperature dependence and the external field dependence of the magnetization and susceptibility are determined. The critical exponents β and γ are calculated. Applying the scaling laws α, δ, and ν are also obtained. The critical exponents are in very good agreement with the existing experimental data.

Ellipsometric Study of Anisotropy Permittivity of Hexagonal Manganites RMnO3 (R = Ho, Tm, Yb)

The optical properties of single crystals of the hexagonal manganites RMnO3 (R = Ho, Tm, Yb) have been investigated by spectroscopic ellipsometry in the spectral range 0.6-5.0 eV. The measurements show a strong anisotropy of the diagonal elements of the dielectric tensor. It has been demonstrated that the optical absorption edge for the polarization Ec was determined by the intense band of O(2p)Mn (3d) transition centered near 1.6 eV, whereas this band for the polarization Ec was strongly suppressed and shifted toward higher energies by 0.15-0.20 eV. The spectral dependences of the ε1(E) and ε2(E) for HoMnO3, TmMnO3 and YbMnO3 single crystals were qualitatively similar. The broad absorption band at E&lt;1.0 eV clearly appears at the energies below the fundamental absorption edge of all of study compounds. It is possible, the origin of the near-infrared ε2(E) absorption is associated with 4f-4f transitions of R3+ ions.

Ferroelectric and Structural Antiphase Domains in Hexagonal RMnO3

ABSTRACTWe have investigated characteristic ferroelectric and structural antiphase domain structures in single crystals of hexagonal RMnO3 (R=Y, Ho, Lu, and Yb) by obtaining various electron diffraction patterns, dark-filed images and high-resolution lattice images. In the ferroelectric phase of RMnO3 characteristic domain structures consisting of six ferroelectric and structural antiphase domains, which can be identified as the “cloverleaf” pattern, is found in the (110) plane, in addition to the (001) plane, and are inherent to the ferroelectric phase of hexagonal RMnO3. In domain configuration with the cloverleaf pattern in the (110) plane, the structural antiphase boundaries are inclined to be parallel to the [001] direction.

Second harmonic generation spectroscopy in magnetic and multiferroic materials

Spectroscopic studies of magnetically ordered materials, multiferroic insulators, and magnetic semiconductors reveal several previously unexplored mechanisms of optical second harmonic generation (SHG). In contrast to non-magnetic materials, SHG in such systems is determined by arrangements of both charges and spins. In general, the nonlinear optical susceptibility arises from electric-dipole contributions in the case of noncentrosymmetric crystal structures and from magnetic-dipole contributions in the case of centrosymmetric crystal structures. We show that both these mechanisms are efficient in magnetic materials. Obeying different selections rules, SHG spectroscopy may provide new information in comparison to linear optics. The observation of resonance SHG at d–d transitions in antiferromagnetic oxides Cr2O3, NiO, CuB2O4 is discussed. The SHG spectroscopy was successfully applied for distinguishing subtle variations of magnetic structures in hexagonal manganites RMnO3. Giant SHG in the region of charge-transfer transitions was observed in multiferroic manganite TbMn2O5. Spin-induced SHG was observed near the band gap in centrosymmetric magnetic semiconductors EuTe and EuSe. One of the most striking potentials of SHG is visualization of antiferromagnetic 180° domain structures which are indistinguishable in linear optics.

Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3

Measurements of the structure of hexagonal RMnO3 [R = rare earths (Ho) and Y] for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes, or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho–O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ∼900 K there are large displacements of the Ho, O3, and O4 ions along the z axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to toward central points between pairs of Ho ions on the z axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The Born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures.

Weak ferromagnetism in hexagonal orthoferrites RFeO3 (R = Lu, Er-Tb)

Hexagonal orthoferrites of rare earths RFeO3 (R = Lu, Er-Tb) were grown epitaxially on (111)ZrO2(Y2O3) substrates using metal-organic chemical vapour deposition. Temperature and field dependences of magnetization were measured and analyzed for all samples and revealed weak ferromagnetic behavior below T = 120–140 K. The difference in electronic structure along with a distinct similarity in the crystal structure of hexagonal manganites RMnO3 and hexagonal orthoferrites RFeO3 are brought into focus in order to explain the results. Hexagonal orthoferrites are regarded as a promising family of multiferroics.

Spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films

We present the results of an optical method of quantitatively estimating the spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films. The two in-plane (a-b plane) spin exchange integrals J1 (intratrimer Mn-Mn interaction) and J2 (intertrimer Mn-Mn interaction) are deduced from the magnon scattering peak wavenumbers. We found that J2 decreases systematically when the R ionic radius increases, while J1 is nearly independent of R ionic radius, contrary to the expectation in single crystals. We show that the R dependence of J1 could be understood in terms of the stress in the thin films. Our result indicates that the stress has stronger effect on the atomic displacement of the intratrimer Mn-Mn distance than the intertrimer Mn-Mn distance.

Synthesis and oxygen content dependent properties of hexagonal DyMnO 3+ δ

Oxygen deficient polycrystalline samples of hexagonal P6 3 cm (space group #185) DyMnO 3+ δ ( δ<0) were synthesized in Ar by intentional decomposition of its perovskite phase obtained in air. The relative stability of these phases is in accord with our previous studies of the temperature and oxygen vacancy dependent tolerance factor. Thermogravimetric measurements have shown that hexagonal samples of DyMnO 3+ δ (0≤ δ≤0.4) exhibit unusually large excess oxygen content, which readily incorporates on heating near 300 °C in various partial-pressures of oxygen atmospheres. Neutron and synchrotron diffraction data show the presence of two new structural phases at δ≈0.25 (Hex 2) and δ≈0.40 (Hex 3). Rietveld refinements of the Hex 2 phase strongly suggest it is well modeled by the R3 space group (#146). These phases were observed to transform back to P6 3 cm above ∼350 °C when material becomes stoichiometric in oxygen content ( δ=0). Chemical expansion of the crystal lattice corresponding to these large changes of oxygen was found to be 3.48×10 −2 mol −1. Thermal expansion of stoichiometric phases were determined to be 11.6×10 −6 and 2.1×10 −6 K −1 for the P6 3 cm and Hex 2 phases, respectively. Our measurements also indicate that the oxygen non-stoichiometry of hexagonal RMnO 3+ δ materials may have important influence on their multiferroic properties.