Hexagonal Manganites Research Articles

Spin wave and spin flip in hexagonal LuMnO3 single crystal

Manipulation and control of spin wave and spin flip are crucial for future developments of magnonic and spintronic devices. We present that the spin wave in hexagonal LuMnO3 single crystal can be selectively excited with laser polarization perpendicular to the c-axis of hexagonal LuMnO3 and photon energy ∼1.8 eV. The selective excitation of spin wave also suggests that the spin flip can be selectively controlled in hexagonal manganites. In addition, a microscopic model of the spin wave generation correlated with the four-spin-flip in hexagonal manganites is suggested to account for the line-shape of the observed spin wave.

Structural and electrical studies of sol–gel synthesized nanocrystalline hexagonal yttrium iron manganite ceramics

Hexagonal yttrium manganites, YMnO3, are interesting materials for their multiferroic behavior. Substituting suitable cations either at the Y-site or Mn-site offers great opportunities to produce a variety of manganites and tune their properties. Nanocrystalline yttrium iron manganites with the compositional formula Y[Formula: see text]Fe[Formula: see text]MnO3, [Formula: see text] = 0.0, 0.10, 0.15, 0.20 and 0.25, were synthesized by sol–gel autocombustion method. The prepared samples were heated at 1100[Formula: see text]C for 1 h. Another set of samples with compositional formula YFe[Formula: see text]Mn[Formula: see text]O3, [Formula: see text] = 0.0, 0.10, 0.15, 0.20 and 0.25, were also synthesized by the same method and heated at 1100[Formula: see text]C for 1 h. Various characterizations were done on these manganite systems synthesized by substituting iron at different sites. X-ray diffraction (XRD) technique studied the structure of the samples and analysis of XRD patterns confirmed the formation of hexagonal phase in the samples. Structural parameters such as lattice constants, crystallite size, theoretical density, etc. were determined using the XRD data. The unit cell dimensions have been found to agree with the standard data and the Debye–Scherrer crystallite size obtained from XRD data ranges from 42 nm to 77 nm. The room temperature frequency variations of electrical properties such as dielectric constant, dielectric loss and AC conductivity were measured in the range of 100 Hz–2 MHz and the variations showed a dispersive behavior for all the samples. The various measurements and the results obtained were studied and discussed in the paper.

Benefits of resource strategy for sustainable materials research and development

Material and product life cycles are based on complex value chains of technology-specific elements. Resource strategy aspects of essential and strategic raw materials have a direct impact on applications of new functionalized materials or the development of novel products. Thus, an urgent challenge of modern materials science is to obtain information about the supply risk and environmental aspects of resource utilization, especially at an early stage of basic research. Combining the fields of materials science, industrial engineering and resource strategy enables a multidisciplinary research approach to identify specific risks within the value chain, aggregated as the so-called ‘resource criticality’. Here, we demonstrate a step-by-step criticality assessment in the sector of basic materials research for multifunctional hexagonal manganite YMnO3, which can be a candidate for future electronic systems. Raw material restrictions can be quantitatively identified, even at such an early stage of materials research, from eleven long-term indicators including our new developed Sector Competition Index. This approach for resource strategy for modern material science integrates two objective targets: reduced supply risk and enhanced environmental sustainability of new functionalized materials, showing drawbacks but also benefits towards a sustainable materials research and development.

Spatial anisotropy of topological domain structure in hexagonal manganites

The domain structure of hexagonal manganites is simulated based on the phenomenological Ginzburg-Landau theory, and special attention is paid to the evolution of a topological vortex-antivortex pattern with the varying out-of-plane anisotropies of two stiffness parameters for the in-plane ($xy$-plane) trimerization amplitude $Q$ and out-of-plane ($z$-axis) polarization $P$. It is revealed that the topological domain structure can be remarkably modulated by the stiffness anisotropies. A larger stiffness for $Q$ along the $z$ axis causes the trajectory lines of the vortex nodes and antivortex nodes to be seriously stretched along the $z$ axis, eventually leading to the topological stripelike domain pattern. The larger stiffness for either $Q$ or $P$ along the $z$ axis makes the domain walls perpendicular to the $z$ axis wider, while the domain walls parallel to the $z$ axis remain less affected. The present work suggests that the topological domain structure may be controlled by some approaches (e.g., lattice strain) which can change the trimerization stiffness and polarization stiffness in hexagonal manganites.

Growth, domain structure, and magnetic properties of CaMnO3(110) and La0.7Ca0.3MnO3(110) layers synthesized on hexagonal YMnO3(0001)

Oxide heterostructures based on cubic perovskite have been studied widely but there has been little interest in the combination of different polymorphs, such as hexagonal and cubic manganites, into a single heterostructure.

Oxygen storage properties of hexagonal HoMnO3+δ.

Structural and oxygen content changes of hexagonal HoMnO3+δ manganite at the stability boundary in the perovskite phase have been studied by X-ray diffraction and thermogravimetry using in situ oxidation and reduction processes at elevated temperatures in oxygen and air. The oxygen storage properties during structural transformation between stoichiometric Hex0 and oxygen-loaded Hex1 phases, transition temperatures and kinetics of the oxygen incorporation and release are reported for materials prepared by the solid-state synthesis and high-impact mechanical milling. Long-term annealing experiments have shown that the Hex0 (δ = 0) → Hex1 (δ ≈ 0.28) phase transition is limited by the surface reaction and nucleation of the new phase for HoMnO3+δ 15MM. The temperatures of Hex0 ↔ Hex1 transitions have been established at 290 °C and 250 °C upon heating and cooling, respectively, at a rate of 0.1° min-1, also indicating that the temperature hysteresis of the transition could possibly be as small as 10 °C in the equilibrium. Ball-milling of HoMnO3+δ has only a small effect on improving the speed of the reduction/oxidation processes in oxygen, but importantly, allowed for considerable oxygen incorporation in air at a temperature range of 220-255 °C after prolonged heating. The Mn 2p XAS results of the Mn valence in oxygen loaded samples support the oxygen content determined by the TG method. The magnetic susceptibility data of the effective Mn valence gave inconclusive results due to dominating magnetism of the Ho3+ ions. Comparison of HoMnO3+δ with previously studied DyMnO3+δ indicates that a tiny increase in the ionic size of lanthanide has a huge effect on the redox properties of hexagonal manganites and that practical properties could be significantly improved by synthesizing the larger average size (Y,Ln)MnO3+δ manganites.

Interstitial oxygen as a source of p-type conductivity in hexagonal manganites.

Hexagonal manganites, h-RMnO3 (R=Sc, Y, Ho–Lu), have been intensively studied for their multiferroic properties, magnetoelectric coupling, topological defects and electrically conducting domain walls. Although point defects strongly affect the conductivity of transition metal oxides, the defect chemistry of h-RMnO3 has received little attention. We use a combination of experiments and first principles electronic structure calculations to elucidate the effect of interstitial oxygen anions, Oi, on the electrical and structural properties of h-YMnO3. Enthalpy stabilized interstitial oxygen anions are shown to be the main source of p-type electronic conductivity, without reducing the spontaneous ferroelectric polarization. A low energy barrier interstitialcy mechanism is inferred from Density Functional Theory calculations to be the microscopic migration path of Oi. Since the Oi content governs the concentration of charge carrier holes, controlling the thermal and atmospheric history provides a simple and fully reversible way of tuning the electrical properties of h-RMnO3.

New synthesis route for obtaining carbon-free hexagonal RE manganites via novel simple individual precursors. The interplay between magnetic and thermodynamic properties of hexagonal RMnO3 (R = Ho–Yb, Y)

New simple heterometallic ionic complexes [Mn(MeCN)6][R(NO3)5] (R=Ho (1), Er (2), Tm (3), Yb (4), Y (5)) were synthesised, structurally characterized and their thermal behavior was investigated. Complexes under study were proposed as precursors for corresponding pure, carbon-free rare-earth hexagonal manganites RMnO3 (P63cm). Procedures for the preparation of RMnO3 were optimized. The magnetic behavior of RMnO3 was studied. The Cp(T) dependences for RMnO3 in the range of 5–340K were first measured by adiabatic calorimetry, and the smoothed values of the standard thermodynamic functions Cp0(T), S0(T), Φ0(T), and ΔH0(T) were calculated for the first time. The interplay between magnetic and thermodynamic properties of RMnO3 was shown.

Magnetic structures and magnetoelastic coupling of Fe-doped hexagonal manganitesLuMn1−xFexO3(0≤x≤0.3)

We have studied the crystal and magnetic structures of Fe-doped hexagonal manganites LuMn1-xFexO3 (x = 0, 0.1, 0.2, and 0.3) by using bulk magnetization and neutron powder diffraction methods. The samples crystalize consistently in a hexagonal structure and maintain the space group P63cm from 2 to 300 K. The N\'eel temperature TN increases continuously with increasing Fe-doping. In contrast to a single {\Gamma}4 representation in LuMnO3, the magnetic ground state of the Fe-doped samples can only be described with a spin configuration described by a mixture of {\Gamma}3 (P63'cm') and {\Gamma}4 (P63'c'm) representations, whose contributions have been quantitatively estimated. The drastic effect of Fe-doping is highlighted by composition-dependent spin reorientations. A phase diagram of the entire composition series is proposed based on the present results and those reported in literature. Our result demonstrates the importance of tailoring compositions in increasing magnetic transition temperatures of multiferroic systems.

Modulating Magnetic Properties by Tailoring In-Plane Domain Structures in Hexagonal YMnO3 Films.

Periodic structures and the coupling of multiorder parameters in complex oxides heterojunctions can generate exotic properties, of interest both for fundamental researches and for device applications. Here, we report a self-assembling in-plane periodic domain structure, and the resulting rich magnetic states, in a h-YMnO3 thin film fabricated on c-face sapphire substrate. Detailed structural investigations at atomic-level reveal the fashion of alternating domains under tensile or compressive strains separated by a boundary region. Tuned by this in-plane domain structure, the abnormal magnetic properties, such as the ferromagnetic enhancement and the unexpected spin glass state (below ∼38 K), are realized. Moreover, the existence of ferroelectric polarization is confirmed by scanning transmission electron microscopy, which brings in the chances of magnetoelectric coupling effect. These results manifest the close connections between the magnetic properties and such in-plane microstructures, suggesting the possibility of tuning the coupling effects via strain engineering in the hexagonal manganite film.

Homotopy-Theoretic Study &Atomic-Scale Observation of Vortex Domains in Hexagonal Manganites.

Essential structural properties of the non-trivial “string-wall-bounded” topological defects in hexagonal manganites are studied through homotopy group theory and spherical aberration-corrected scanning transmission electron microscopy. The appearance of a “string-wall-bounded” configuration in RMnO3 is shown to be strongly linked with the transformation of the degeneracy space. The defect core regions (~50 Å) mainly adopt the continuous U(1) symmetry of the high-temperature phase, which is essential for the formation and proliferation of vortices. Direct visualization of vortex strings at atomic scale provides insight into the mechanisms and macro-behavior of topological defects in crystalline materials.

Phase transformations in HoMnO3 oxide at high temperatures

Hexagonal holmium manganite is investigated by means of high-temperature X-ray radiography and thermal analysis. At 1200°C, the manganite transitions from the structural P63cm modification into the P63/mmc form. Mechanoactivation in a high-energy ball mill results in the emergence of the orthorhombic phase of HoMnO3, which undergoes the Jahn–Teller transition in the temperature range of 1000–1160°C.

Domain wall conductivity in semiconducting hexagonal ferroelectric TbMnO3 thin films

Although enhanced conductivity of ferroelectric domain boundaries has been found in BiFeO3 and Pb(Zr,Ti)O3 films as well as hexagonal rare-earth manganite single crystals, the mechanism of the domain wall conductivity is still under debate. Using conductive atomic force microscopy, we observe enhanced conductance at the electrically-neutral domain walls in semiconducting hexagonal ferroelectric TbMnO3 thin films where the structure and polarization direction are strongly constrained along the c-axis. This result indicates that domain wall conductivity in ferroelectric rare-earth manganites is not limited to charged domain walls. We show that the observed conductivity in the TbMnO3 films is governed by a single conduction mechanism, namely, the back-to-back Schottky diodes tuned by the segregation of defects.

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.

Electronic and crystal structure changes induced by in-plane oxygen vacancies in multiferroicYMnO3

The widely spread oxygen vacancies $({V}_{\mathrm{O}})$ in multiferroic materials can strongly affect their physical properties. However, their exact influence has rarely been identified in hexagonal manganites. Here, with the combined use of transmission electron microscopy (TEM) and first-principles calculations, we have systematically studied the electronic and crystal structure modifications induced by ${V}_{\mathrm{O}}$ located at the same Mn atomic plane (in-plane ${V}_{\mathrm{O}})$. Our TEM experiments reveal that the easily formed in-plane ${V}_{\mathrm{O}}$ not only influence the electronic structure of $\mathrm{YMn}{\mathrm{O}}_{3}$ but alter the in-plane Wyckoff positions of Mn ions, which may subsequently affect the intraplane and interplane exchange interaction of Mn ions. The ferroelectricity is also impaired due to the introduction of ${V}_{\mathrm{O}}$. Further calculations confirm these electronic and structural changes and modifications. Our results indicate that the electronic and crystal structure of $\mathrm{YMn}{\mathrm{O}}_{3}$ can be manipulated by the creation of ${V}_{\mathrm{O}}$.

ChemInform Abstract: Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films Having Narrow Band Gaps.

AbstractEpitaxial hexagonal manganite thin films h‐LnMnO3 (Ln: Lu, Y) are grown on a Pt (111) film on Al2O3 (0001) substrate by pulsed laser deposition (850 °C substrate temperature, 1.5 J/cm2, 3 Hz).

Evolution of the statistical distribution in a topological defect network.

The complex networks of numerous topological defects in hexagonal manganites are highly relevant to vastly different phenomena from the birth of our cosmos to superfluidity transition. The topological defects in hexagonal manganites form two types of domain networks: type-I without and type-II with electric self-poling. A combined phase-field simulations and experimental study shows that the frequencies of domains with N-sides, i.e. of N-gons, in a type-I network are fitted by a lognormal distribution, whereas those in type-II display a scale-free power-law distribution with exponent ∼2. A preferential attachment process that N-gons with a larger N have higher probability of coalescence is responsible for the emergence of the scale-free networks. Since the domain networks can be observed, analyzed, and manipulated at room temperature, hexagonal manganites provide a unique opportunity to explore how the statistical distribution of a topological defect network evolves with an external electric field.

B23-O-08Microstructures in improper ferroelectric compounds revealed by electron microscopy

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

B23-O-07Multivariate Statistical Analysis of EMCD Spectra measured at the Fe/MgO Interface

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films Having Narrow Band Gaps

Ferroelectric photovoltaics (FPVs) are being extensively studied owing to their anomalously high photovoltages, coupled with reversibly switchable photocurrents. However, FPVs suffer from their extremely low photocurrents, which is primarily due to their wide band gaps. Herein, we present a new class of FPV by demonstrating (i) a nearly optimum band gap of ∼1.55 eV and (ii) the ferroelectric polarization switching in the epitaxial hexagonal manganite thin films, h-RMnO3, where R = Lu and Y. According to the thickness-dependent photovoltaic measurements, the ITO/h-LuMnO3/Pt solar cell shows a power conversion efficiency of ∼0.11% when the thickness of the h-LuMnO3 layer is ∼150 nm. We have shown that the PCE is 1–3 orders higher than those of classical FPVs such as undoped Pb(Zr,Ti)O3 and BiFeO3 under the standard AM 1.5G illumination. We have further elucidated that the switchable photovoltaic effect dominates over the nonferroelectric internal field effect.