Rare-earth Orthoferrites Research Articles

Magnon and electromagnon excitations in multiferroicDyFeO3

The existence of electric dipole active magnetic excitations, or electromagnons, in rare-earth orthoferrites was theoretically predicted more than two decades ago but their experimental observation has not been confirmed until now. In this work, magnetic modes in DyFeO${}_{3}$ single crystals were studied at low temperatures and in external magnetic fields ($H$) by means of synchrotron radiation-based far-infrared spectroscopy. The authors report observation of two electromagnons, which appear in the spectra at 20 cm${}^{-1}$ and 50 cm${}^{-1}$ only below the magnetic ordering temperature ${T}_{N}$ of Dy${}^{3+}$ of 4.2 K. It is demonstrated that electromagnons provide a significant contribution to the static electric permittivity along the $c$ axis ${\ensuremath{\epsilon}}_{c}$(0). Both the frequency of the electromagnons and ${\ensuremath{\epsilon}}_{c}$(0,$H$) manifest a strong hysteresis upon cycling of the external magnetic field.

Enhanced spin – Reorientation temperature and origin of magnetocapacitance in HoFeO3

We report on the increase in the spin reorientation temperature in HoFe0.5Cr0.5O3 compound by isovalent substitution (Cr3+) at the Fe-site and the magnetocapacitance in the HoFeO3 compound. Spin reorientation transition is evident around 50K and 150K for the x=0 and x=0.5 compounds respectively. The increase in the spin reorientation transition temperature in case of x=0.5 compound can be attributed to the domination of the Ho3+–Fe3+ interaction over the Fe3+–Fe3+ interaction. Decrease in Néel temperature from 643K (x=0) to 273K (x=0.5) can be ascribed to the decrease in the interaction between antiferromagnetically aligned Fe3+ moments as a result of the dilution of the Fe3+ moments with the Cr3+ addition. From the magnetization M vs. magnetic field H variation it is evident that the coercivity, HC decreases for x=0.5 compound, hinting the magnetic softening of the HoFeO3 compound. Observed magnetocapacitance could be due to lossy dielectric mechanism in the present compound. Indeed, present results would be helpful in understanding the physics behind rare-earth orthoferrites.

Origin of the magnetization and compensation temperature in rare-earth orthoferrites and orthochromates

We consider orthoferrite and orthochromite perovskites with the formula $\mathit{RM}{\mathrm{O}}_{3}$, where $R$ is a lanthanide and $M$ is Fe or Cr. We identify an atomistic interaction that couples the magnetic moments of the $R$ and $M$ cations with the oxygen octahedral rotations characterizing these crystals. We show that this interaction results in an effective magnetic field acting on the $R$ atoms, which in turn explains several intriguing features of these compounds, such as the existence (or absence) of a net magnetization associated with the $R$ sublattice and its crystallographic direction. Further, considered together with the couplings described by L. Bellaiche et al. [J. Phys.: Condens. Matter 24, 312201 (2012)], this interaction explains why some $\mathit{RM}{\mathrm{O}}_{3}$ compounds display a compensation temperature at which the magnetization cancels and changes sign, while others do not.

Tailoring the magnetic and magnetoelectric properties of rare earth orthoferrites for room temperature applications

The M(T) curve shows negative magnetization value for samples containing Er ions. The dielectric constant value does not show any anomaly around the magnetic transition temperature. The temperature at which maximum MDE effect observed corresponds to dielectric loss maxima.

Properties of rare-earth orthoferrites perovskite driven by steric hindrance

REFeO3 (RE = La, Pr, Nd, Sm) perovskites present interesting properties and are studied for their potential applications as smart devices and the requirement of miniaturization and low power consumption in such electronic components has led to the development of thin films. The understanding of crystallization mechanisms and the characterization of such perovskites are thus important to understand their properties. In this study, we purpose a systematic comparison of structural, optical and electrical properties of REFeO3 thin films, and we investigate the direct link between properties and crystallographic distortion. The as-deposited films are annealed in air under XRD measurements in order to investigate their complete crystallization kinetic, using Johnson Mehl Avrami Kolmogorov model. TEM observations of films are in agreement with this model. Then, optical and electrical characterization of films were performed by using UV–visible spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), and 4-probes measurements. We found that the activation energy of crystallization, the infrared vibration mode position and the activation energy of conduction mechanism are driven by distortion of the perovskite, which can be described by the Goldschmidt tolerance factor. All the studied properties are shifted due to decrease of RE ionic radius and the direct relationship between structural distortion and physical properties shifting is discussed.

Controllable Assembly of Hierarchical Macroporous-Mesoporous LnFeO₃ and Their Catalytic Performance in the CO + NO Reaction.

A new synthesis strategy to prepare hierarchical macroporous-mesoporous materials employing poly(ethylene oxide)-poly(phenylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) as a single template and an acid adjusting agent was reported. There is a hierarchical structure including macropores with a size of 50-100 nm and mesopores in the macroporous walls with a size of 3-5 nm. The macroporous walls are composed of rare earth orthoferrite nanoparticles with a size of 5-10 nm. These hierarchically porous materials show high catalytic activities for the CO + NO reaction, and NO can be fully converted to N2 at temperatures as low as 350 °C, indicating their potential in the catalytic conversion of automotive exhaust gas and other catalysis-related fields. This synthesis strategy is a facile method for the preparation of hierarchical porous materials and may give us a guideline for the synthesis of functional materials with further catalytic applications.

Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: beyond the rigid band approximation

The exchange interaction among electrons is one of the most fundamental quantum mechanical interactions in nature and underlies any magnetic phenomena from ferromagnetic ordering to magnetic storage. The current technology is built upon a thermal or magnetic field, but a frontier is emerging to directly control magnetism using ultrashort laser pulses. However, little is known about the fate of the exchange interaction. Here we report unambiguously that photoexcitation is capable of quenching the exchange interaction in all three 3d ferromagnetic metals. The entire process starts with a small number of photoexcited electrons which build up a new and self-destructive potential that collapses the system into a new state with a reduced exchange splitting. The spin moment reduction follows a Bloch-like law as , where ΔE is the absorbed photon energy and β is a scaling exponent. A good agreement is found between the experimental and our theoretical results. Our findings may have a broader implication for dynamic electron correlation effects in laser-excited iron-based superconductors, iron borate, rare-earth orthoferrites, hematites and rare-earth transition metal alloys.

The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO3

Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO3, have been reinvestigated in detail. The spin-reorientation transition of the Fe3+ magnetic phase and the low-temperature magnetic ordering of Er3+ were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe3+ align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Neel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe3+ moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er3+ moments align antiferromagnetically with a Cy mode below...

Terahertz probes of magnetic field induced spin reorientation in YFeO3 single crystal

Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO3 single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO3 (R = Y3+ or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching in the orthoferrite antiferromagnetic systems.

Pre and Post Effect of Swift Heavy Ion Irradiation on Infrared Spectral Evolution of Y<sub>3+</sub><i><sub>x</sub></i>Fe<sub>5-</sub><i><sub>x</sub></i>O<sub>12</sub> (<i>x</i> = 0.0 - 1.0) System

Present work aims to investigate the effect of Y3+ - substitution and 50 MeV, Li3+ ion irradiation (fluence: 5 x 1013 ions/cm2) on infrared spectral evolution of Y3+xFe5-xO12 (x = 0.0, 0.2 0.4, 0.6, 0.8 and 1.0) garnet system. Infrared absorption spectra for x = 0.0 – 0.6 compositions show three absorption bands while x = 0.8 – 1.0 compositions are characterized by two absorption bands, identical with IR spectra of rare earth ortho ferrites. The position and intensity of bands are found to be influenced by Y3+ - substitution. The intensity of absorption bands for x = 0.0 – 0.4 compositions is found to increase, while for x = 0.6 – 1.0 compositions no effect of swift heavy ion irradiation has been observed. The results have been explain in the light of SHII induced defect states, partial removal of un-wanted YFeO3 phase and change in structural parameters. It seems that YFeO3 phase is irradiation hard phase.

Pre and Post Effect of Swift Heavy Ion Irradiation on Infrared Spectral Evolution of Y<sub>3+</sub><i><sub>x</sub></i>Fe<sub>5-</sub><i><sub>x</sub></i>O<sub>12</sub> (<i>x</i> = 0.0 - 1.0) System

Present work aims to investigate the effect of Y3+ - substitution and 50 MeV, Li3+ ion irradiation (fluence: 5 x 1013 ions/cm2) on infrared spectral evolution of Y3+xFe5-xO12 (x = 0.0, 0.2 0.4, 0.6, 0.8 and 1.0) garnet system. Infrared absorption spectra for x = 0.0 – 0.6 compositions show three absorption bands while x = 0.8 – 1.0 compositions are characterized by two absorption bands, identical with IR spectra of rare earth ortho ferrites. The position and intensity of bands are found to be influenced by Y3+ - substitution. The intensity of absorption bands for x = 0.0 – 0.4 compositions is found to increase, while for x = 0.6 – 1.0 compositions no effect of swift heavy ion irradiation has been observed. The results have been explain in the light of SHII induced defect states, partial removal of un-wanted YFeO3 phase and change in structural parameters. It seems that YFeO3 phase is irradiation hard phase.

Electrical and Magnetic Properties of Some Rare Earth Orthoferrites (RFeO3 where R = Y, Ho, Er) Systems

The electrical and magnetic characteristics of some rare earth orthoferrites (RFeO3, where R = Y, Ho & Er) have been discussed. These two characteristics suggest that such system can be categorized under multiferroic with different Curie and Neel temperature. The electrical characteristics shows relaxation phenomenon with Curie temperature varies with change in frequency. The Curie temperature is in the range of 340–480°C, whereas Neel temperature is about 370°C.

Creating multiferroics with large tunable electrical polarization from paraelectric rare-earth orthoferrites

The quest for materials possessing both a magnetic ordering temperature above room temperature and a large electrical polarization is an important research direction in order to design novel spintronic and memory devices. Up to now, BiFeO3 and related systems are the only known compounds simultaneously possessing such characteristics. Here, first-principles calculations predict that another family of materials, namely epitaxial films made of rare-earth orthoferrites (RFeO3), can also exhibit such desired features. As a matter of fact, applying a large enough strain to these compounds, which are nominally paraelectric and have a high magnetic transition temperature, is predicted to render them ferroelectric, and thus multiferroic. At high compressive strain, the resulting ferroelectric phase of RFeO3 systems having large rare-earth ions is even a tetragonal state characterized by a giant polarization and axial ratio. For large tensile strain, two striking inhomogenous ferroelectric phases—including one never observed before in any perovskite—are further predicted as having significant polarization. A multiphase boundary also occurs, which may lead to optimization of properties or unusual features. Finally, many quantities, including electrical polarization and magnetic ordering temperature, are tunable by varying the epitaxial strain and/or chemical pressure.

Twofold spin reorientation and field-induced incomplete phase transition in single-crystalDy0.5Pr0.5FeO3

We report an intriguing twofold spin reorientation transition of type ${\ensuremath{\Gamma}}_{4}({G}_{x},{A}_{y},{F}_{z})\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}({A}_{x},{G}_{y},{C}_{z})\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{2}({F}_{x},{C}_{y},{G}_{z})$ for the ${\mathrm{Fe}}^{3+}$ magnetic sublattice near spin reorientation temperatures ${T}_{\mathrm{SR}1}\phantom{\rule{0.28em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.28em}{0ex}}77 \mathrm{K}$ and ${T}_{\mathrm{SR}2}\phantom{\rule{0.28em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.28em}{0ex}}45 \mathrm{K}$ in a rare-earth orthoferrite ${\mathrm{Dy}}_{0.5}{\mathrm{Pr}}_{0.5}{\mathrm{FeO}}_{3}$ single crystal. Magnetic-field-induced incomplete spin-configuration transitions (${\ensuremath{\Gamma}}_{4}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\Gamma}}_{41}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\Gamma}}_{42}$ for $H =\phantom{\rule{0.28em}{0ex}}20\phantom{\rule{0.28em}{0ex}}\mathrm{kOe}$ and ${\ensuremath{\Gamma}}_{4}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{42}$ for $H =\phantom{\rule{0.28em}{0ex}}40\phantom{\rule{0.28em}{0ex}}\mathrm{kOe}$) were observed by measurement of magnetization as a function of temperature. The spin reorientation temperature of a ${\mathrm{Dy}}_{0.5}{\mathrm{Pr}}_{0.5}{\mathrm{FeO}}_{3}$ single crystal can be controlled by changing the magnitude of the applied magnetic field. We also show that spin reorientation of the ${\ensuremath{\Gamma}}_{1}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{4}$ type between ${T}_{\mathrm{SR}2}$ and ${T}_{\mathrm{SR}1}$ can be induced by an applied magnetic field along the $c$ axis. The origin of the intriguing magnetic behavior is ascribed to the anisotropic effective field whose strength is determined by the interactions with ${R}^{3+}$($R =\phantom{\rule{0.28em}{0ex}}\mathrm{Dy},\phantom{\rule{0.28em}{0ex}}\mathrm{Pr}$) spins and can be modified by the external applied magnetic field. It provides deeper insight into the ${\mathrm{Fe}}^{3+}\ensuremath{-}{R}^{3+}$ magnetic interaction which dominates the sophisticated magnetic phase transitions in the rare-earth orthoferrites.

Selected multiferroic perovskite oxides containing rare earth and transition metal elements

Multiferroic materials are currently the subject of intensive research worldwide, because of both their fundamental scientific problems and also possible technological applications. Among a number of candidates in the laboratories, compounds consisting of rare earth and transition metal perovskite oxides have very unusual structural and physical properties. In contrast to the so-called type I multiferroics, ferroelectricity may be induced by magnetic ordering or by applying external fields. In this review, the recent progress on the experimental and theoretical studies of some selected type II multiferroics is presented, with a focus on the perovskite oxides containing rare earth and transition metal elements. The rare earth orthoferrite crystals, rare earth titanate strained film, and rare earth-based superlattices are systematically reviewed to provide a broad overview on their promising electric, magnetic, and structural properties. The recent experimental advances in single-crystal growth by optical floating zone method are also presented. First-principles investigations, either supported by experimental results or awaiting for experimental verifications, are shown to offer useful guidance for the future applications of unconventional multiferroics.

On the oxidation state of ‘Fe’ in LaFe1−xNixO3

The electrical and magnetic properties of rare earth orthoferrites are strongly dependent upon the electronic structure of Fe cations. This study investigates the electronic structure of Fe as a function of composition in LaFe1−xNixO3 using Fe K-edge X-ray absorption spectroscopy at room temperature. Analysis of the Fe K-edge X-ray absorption spectra indicates the presence of Fe3+ and Fe4+ in LaFe1−xNixO3. The concentration of Fe4+ increases with increasing Ni content in LaFe1−xNixO3. Variations in the oxidation state of Fe have been correlated to those in the electrical and magnetic characteristics of LaFe1−xNixO3.

Temperature induced spin switching in SmFeO3 single crystal.

The prospect of controlling the magnetization (M) of a material is of great importance from the viewpoints of fundamental physics and future applications of emerging spintronics. A class of rare-earth orthoferrites RFeO3 (R is rare-earth element) materials exhibit striking physical properties of spin switching and magnetization reversal induced by temperature and/or applied magnetic field. Furthermore, due to the novel magnetic, magneto-optic and multiferroic properties etc., RFeO3 materials are attracting more and more interests in recent years. We have prepared and investigated a prototype of RFeO3 materials, namely SmFeO3 single-crystal. And we report magnetic measurements upon both field cooling (FC) and zero-field cooling (ZFC) of the sample, as a function of temperature and applied magnetic field. The central findings of this study include that the magnetization of single-crystal SmFeO3 can be switched by temperature, and tuning the magnitude of applied magnetic field allows us to realize such spin switching even at room temperature.

Pressure-induced magnetic, structural, and electronic phase transitions in LaFeO3: A density functional theory (generalized gradient approximation) + U study

We have investigated the behavior of orthoferrite LaFeO3 at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO3 in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO3 and other rare-earth orthoferrites.

Terahertz spectroscopic observation of spin reorientation induced antiferromagnetic mode softening in DyFeO3 ceramics

Dysprosium orthoferrite has been found to undergo the Γ4(Gx,Fz)→Γ1(Gy) spin reorientation transition (Morin transition) at about 37K. In this study, the Morin transition in DyFeO3 ceramic is investigated using a temperature-tunable terahertz spectroscopy system. It is experimentally demonstrated that the DyFeO3 ceramics exhibit obvious terahertz absorption and emission at the antiferromagnetic resonant frequency. It is also shown that the lifetime of magnetic dipole emission remarkably shortens and the quasi-antiferromagnetic mode softens below 198K, which can be ascribed to the decreased terahertz absorption and reduced internal magnetic field induced by the spin reorientation, respectively. This contributes to the research of rare earth orthoferrites׳ terahertz electromagnetic properties.

ChemInform Abstract: Novel Features of Multiferroic and Magnetoelectric Ferrites and Chromites Exhibiting Magnetically Driven Ferroelectricity

AbstractReview: 86 refs.