Physical properties of multiferroic hexagonal HoMnO3 thin films
TL;DR: In this paper, the magnetic and ferroelectric properties of hexagonally grown HoMnO3 thin films were investigated, and the magnetic measurements revealed bulk-like magnetic phase transitions with an additional spin-glass-like behavior feature below the Neel temperature.
Abstract: The authors investigated the magnetic and ferroelectric properties of hexagonally grown HoMnO3 thin films. The magnetic measurements revealed bulklike magnetic phase transitions with an additional spin-glass-like behavior feature below the Neel temperature. The ferroelectricity in the films was distinctly different from the suggested bulk behavior. Below 40K, the HoMnO3 films showed typical ferroelectric character: their remnant polarization and coercive field values at 20K were 3.7μC∕cm2 and 0.69MV∕cm. Above 40K, however, the films exhibited an unusual antiferroelectriclike behavior, with more pronounced features appearing at higher temperatures. These intriguing physical properties make HoMnO3 films a potential candidate material for numerous future applications.
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TL;DR: Giant flexoelectric effects on ferroelectric properties are discussed in this overview of recent experimental observations of oxide epitaxial thin films.
Abstract: Interfacial strain gradients in oxide epitaxial thin films provide an interesting opportunity to study flexoelectric effects and their potential applications. Oxide epitaxial thin films can exhibit giant and tunable flexoelectric effects, which are six or seven orders of magnitude larger than those in conventional bulk solids. The strain gradient in an oxide epitaxial thin film can generate an electric field above 1 MV m−1 by flexoelectricity, large enough to affect the physical properties of the film. Giant flexoelectric effects on ferroelectric properties are discussed in this overview of recent experimental observations.
64 citations
TL;DR: As Phase Transitions's 30th anniversary this year, it is extremely rewarding to see tangible evidence of the journal's continually increasing value to researchers in the field of transition.
Abstract: As we celebrate Phase Transitions's 30th anniversary this year, it is extremely rewarding to see tangible evidence of the journal's continually increasing value to researchers in the field. We are ...
51 citations
TL;DR: In this article, the authors review achievements on the growth of hexagonal and orthorhombic RMnO3 epitaxial thin films and the characterization of their magnetic and ferroelectric properties, discuss some challenging issues, and suggest some guidelines for future research and developments.
35 citations
TL;DR: The symmetry analysis shows that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T_{N} in hexagonal ferrites and manganites, because the K_{3} structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry.
Abstract: To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature (${T}_{N}$) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher ${T}_{N}$ in hexagonal ferrites and manganites, because the ${K}_{3}$ structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between ${T}_{N}$ and the tolerance factor and a power-law relation between ${T}_{N}$ and the ${K}_{3}$ distortion amplitude. Following the analysis, a record-high ${T}_{N}$ (185 K) among hexagonal ferrites was predicted in hexagonal ${\mathrm{ScFeO}}_{3}$ and experimentally verified in epitaxially stabilized films. These results add to the paradigm of spin-lattice coupling in antiferromagnetic oxides and suggests further tunability of hexagonal ferrites if more lattice distortion can be achieved.
29 citations
TL;DR: In this article, antiferroelectric-like double polarization hysteresis loops in multiferroic thin films below the ferroelectric Curie temperature were reported. But the authors did not consider the site preference of point defects.
Abstract: We report on antiferroelectriclike double polarization hysteresis loops in multiferroic ${\text{HoMnO}}_{3}$ thin films below the ferroelectric Curie temperature. This intriguing phenomenon is attributed to the domain pinning by defect dipoles which were introduced unintentionally during film growth process. Electron paramagnetic resonance suggests the existence of ${\text{Fe}}^{1+}$ defects in thin films and first-principles calculations reveal that the defect dipoles would be composed of oxygen vacancy and ${\text{Fe}}^{1+}$ defect. We discuss migration of charged point defects during film growth process and formation of defect dipoles along ferroelectric polarization direction, based on the site preference of point defects. Due to a high-temperature low-symmetry structure of ${\text{HoMnO}}_{3}$, aging is not required to form the defect dipoles in contrast to other ferroelectrics (e.g., ${\text{BaTiO}}_{3}$).
28 citations
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Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.
4,315 citations
TL;DR: This work identifies the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO3, using a combination of single-crystal X-ray diffraction, thorough structure analysis and first-principles density-functional calculations, and suggests an avenue for designing novel magnetic ferroelectrics.
Abstract: Understanding the ferroelectrocity in magnetic ferroelectric oxides is of both fundamental and technological importance. Here, we identify the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO3, using a combination of single-crystal X-ray diffraction, thorough structure analysis and first-principles density-functional calculations. The ferroelectric phase is characterized by a buckling of the layered MnO5 polyhedra, accompanied by displacements of the Y ions, which lead to a net electric polarization. Our calculations show that the mechanism is driven entirely by electrostatic and size effects, rather than the usual changes in chemical bonding associated with ferroelectric phase transitions in perovskite oxides. As a result, the usual indicators of structural instability, such as anomalies in Born effective charges on the active ions, do not hold. In contrast to the chemically stabilized ferroelectrics, this mechanism for ferroelectricity permits the coexistence of magnetism and ferroelectricity, and so suggests an avenue for designing novel magnetic ferroelectrics.
1,042 citations
TL;DR: A system whose magnetic phase can be controlled by an external electric field is reported: ferromagnetic ordering in hexagonal HoMnO3 is reversibly switched on and off by the applied field via magnetoelectric interactions.
Abstract: The quest for higher data density in information storage is motivating investigations into approaches for manipulating magnetization by means other than magnetic fields. This is evidenced by the recent boom in magnetoelectronics and 'spintronics', where phenomena such as carrier effects in magnetic semiconductors and high-correlation effects in colossal magnetoresistive compounds are studied for their device potential. The linear magnetoelectric effect-the induction of polarization by a magnetic field and of magnetization by an electric field-provides another route for linking magnetic and electric properties. It was recently discovered that composite materials and magnetic ferroelectrics exhibit magnetoelectric effects that exceed previously known effects by orders of magnitude, with the potential to trigger magnetic or electric phase transitions. Here we report a system whose magnetic phase can be controlled by an external electric field: ferromagnetic ordering in hexagonal HoMnO3 is reversibly switched on and off by the applied field via magnetoelectric interactions. We monitor this process using magneto-optical techniques and reveal its microscopic origin by neutron and X-ray diffraction. From our results, we identify basic requirements for other candidate materials to exhibit magnetoelectric phase control.
820 citations
TL;DR: In this article, the preparation, determination of optical properties, and approximate crystal structure of the ferroelectric manganese (III) trioxides of Ho, Er, Tm, Yb, Lu, and Y are reported.
Abstract: The preparation, determination of optical properties, and approximate crystal structure of the ferroelectric manganese (III) trioxides of Ho, Er, Tm, Yb, Lu, and Y are reported. The compounds crystallize in the hexagonal system, probable space group P6₃ cm, and their structure is characterized by unusual five- and sevenfold coordination polyhedra about manganese and rare-earth atoms, respectively. The unacceptable values assumed by the temperature factors of two oxygen ions throw doubt on the exact determination of atom positions by least- squares refinement. Possible causes of this difficulty are reviewed. (auth)
450 citations
TL;DR: In this paper, single crystals of hexagonal $R{\mathrm{MnO}}_{3}$ $(R=\mathrm {Y},$ Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration.
Abstract: Single crystals of hexagonal $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{Y},$ Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to ${T}_{\mathrm{N}}$ is $\ensuremath{\sim}10$ in $R{\mathrm{MnO}}_{3},$ indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below ${T}_{\mathrm{N}},$ indicating that a part of the spins are still fluctuating at $T\ensuremath{\ll}{T}_{\mathrm{N}}.$ It is also found that the dielectric anomaly at ${T}_{\mathrm{N}}$ is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.
399 citations