Reorientation Phase Transition Research Articles

In-situ HRTEM observations of intermediate phase transformation in lattice reorientation in HCP rhenium

In-situ high-resolution transmission electron microscopy (HRTEM) is performed to investigate the deformation behavior of hexagonal close-packed rhenium (Re) which is compressed along the 〈11¯00〉 direction. Atomistic simulations are also conducted to better understand the deformation mechanisms. Two types of lattice reorientation are observed during compression. The first type involves the reorientation of one lattice by ∼90° around 〈112¯0〉, which is accomplished by the formation of an intermediate face-center-cubic (FCC) phase at the interface. This transformation sequence can be described as {11¯00}matrix→{111}FCC→(0001)twin. In the second type, a new grain is formed but does not satisfy any known twin relationship with the matrix, and an intermediate FCC phase is also formed. The transformation sequence can be described as {11¯01}matrix→{111}FCC→(0001)grain. Mechanisms responsible for the observed lattice reorientation and sequential phase transitions are analyzed by conducting lattice correspondence analyses on the simulation results. Strain accommodation is also analyzed to explain the mechanisms for lattice reorientation and the intermediate phase transformations. The results provide new insight into the deformation behavior of HCP metals.

Laser-driven first-order spin reorientation and Verwey phase transitions in magnetite Fe3O4 beyond the range of thermodynamic equilibrium

Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opportunities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partial or complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses. Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains undergoing the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.

Perspective on synthesis, structure, and magnetic properties of R–Fe–H hydrides

The structural and magnetic properties of the multicomponent R–Fe–H compounds with a high content of Fe and H are reported. The process of synthesis of the hydrides (R,R′)2Fe14BH5.5 [where R and R′ are light (Nd) and heavy (Dy, Ho, Er, Tm) rare earth metals, respectively] with a maximum hydrogen content is described in detail. The paper also provides insights into the synthesis of single-crystalline hydrides using the example of the R2(Fe,Co)14BH3 series. The hydrides (R,Nd)2Fe14BH5.5, R2(Fe,Co)14BH3, R2(Fe,Al)17H3 have a significantly increased volume as compared to the parent materials. High-field magnetization results of both parent and hydrogenated compounds at low temperatures are presented. Spin–reorientation phase transitions induced by an external magnetic field are observed. The parameter of the intersublattice exchange interaction and the influence of hydrogen on it are estimated within the framework of the mean field theory. The magnetocaloric effect of the compounds with a magnetic compensation point is studied with a special emphasis placed on the change of the sign of the effect.

Magnetic Domain Control of ErFeO3 by Intense Terahertz Free Electron Laser Pulses

The control of magnetic domain in correlated spin systems by optical pulses has been gaining importance in spintronics and related fields. In particular, ultrafast control of magnetic domain is one of the top areas of interest. Herein we demonstrate the magnetic domain control in ErFeO3 single crystal, a weak ferromagnet, induced by high-intensity terahertz (THz) pulse excitation from a free-electron laser. THz pulses with a central frequency of 4 THz and pulse energy in the order of 10 mJ were used. The microscopic images of the magnetic domain inversion induced by the THz irradiation were recorded by the Faraday-rotation method using a He–Ne probe laser. The experimental results of the THz pulse irradiation at room temperature and at low temperature around the spin reorientation phase transition are presented. We show that the THz pulse excitation can expand the minor magnetic domain toward the direction of the THz beam spot. The magnetic domain inversion is caused by the combination of depinning of the magnetic domain due to local heating and the entropic force owing to the thermal gradient induced by THz irradiation.

Magnetic order of Dy3+ and Fe3+ moments in antiferromagnetic DyFeO3 probed by spin Hall magnetoresistance and spin Seebeck effect

We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in single crystal of the rare-earth antiferromagnet DyFeO$_{3}$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe$^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the Dy$^{3+}$ moments up to room temperature. At and below the Morin temperature of 50$\,$K, the SMR monitors the spin-reorientation phase transition of Fe$^{3+}$ spins. Below 23$\,$K, additional features emerge that persist below 4$\,$K, the ordering temperature of the Dy$^{3+}$ magnetic sublattice. We conclude that the combination of SMR and SSE is a simple and efficient tool to study spin reorientation phase transitions and sublattice magnetizations.

Temperature tunable oscillator of THz-frequency signals based on the orthoferrite/heavy metal heterostructure

There is a considerable interest in development and realization of the terahertz-frequency range oscillators and detectors. In this work a concept of temperature tunable THz oscillator is theoretically developed. As an active element we suggest holmium orthoferrite near its reorientational phase transition. We show that the heating of the orthoferrite spin system decreases the threshold of the DC electric-current required for self-oscillations due to the anisotropy constant reduction. Such concept of the temperature-tuning of magnetic properties can be applied for wide range of ferri- or antiferromagnets. We discuss a possibility of applying orthoferrite/heavy metal heterostructure as continuously tunable oscillators and detectors of THz-frequency signals.

Dzyaloshinskii – Moriya interaction, epitaxial strains, phase transitions in multiferroics with cycloidal structure

In this paper, we explore magnetic phases and spin reorientation phase transitions (SRPT) in the BiFeO3 – like multiferroics. We consider the Dzyaloshinskii – Moriya exchange interaction (DMI) as the principle mechanism responsible for appearance of spin space modulated structures (SSMS) in BiFeO3 and focus the peculiar features of the DMI manifestation in single crystals and films. As an example we take a (110) – oriented BiFeO3 film whose thickness is much lower than the period of SSMS (dfilm≪λ) and construct phase diagram in terms “magnetic field H – perpendicular magnetic anisotropy (PMA) constant κ” accounting the PMA dependence on the strains, where the tensile strains correspond to the situation κ > 0; the compressive strains correspond to the situation κ < 0. Our findings shows that at the condition (dfilm≪λ) the SSMS modulated along the [001] axis with spin rotating in a plane (110) exists as the ground magnetic state in a definite range of the control parameters (H, ±κ). We show that magnetic field applied along the normal to the film along with strains leads to the transition into homogeneous magnetic state of the “easy plane” type at κ > 0 and tilted phase at κ < 0. As well as in the single crystals at the positive values κ > 0 phase transition from SSMS into homogeneous “easy plane” phase goes through the cone SSMS whose existence was recently experimentally confirmed [S. Kawachi, et al, Phys. Rev. Mater. 1, 024, 408 (2017)]. We believe that our findings performed for the (110) BiFeO3 film will be of interest for further experimental and theoretical research and the implementations of BiFeO3 based technologies.

Magnetic Domain-Wall Induced Electric Polarization in NdCrO3 Polycrystalline Ceramic.

We reported the magnetic, dielectric and magnetoelectric properties of NdCrO3 polycrystalline ceramics. Magnetization curves revealed two magnetic transitions at 227 K and 38 K, which corresponded to Cr3+ canted antiferromagnetic ordering and Cr3+ spin reorientation phase transition, respectively. At 11.5 K, a Schottky-type anomaly was observed, caused by Nd3+ ground doublet Zeeman splitting. High-temperature dielectric relaxation exhibited a type of thermally activated relaxation process, which mainly resulted from the Maxwell–Wagner effect. The spin-reorientation of Cr3+ ions and the Nd3+ ground doublet splitting were observed to be accompanied by an electric polarization. The polarization could be induced by the presence of the antiferromagnetic-type domain walls, which led to spatial inversion symmetry breaking.

Anomalies in the magnetic properties of some terbium-yttrium iron garnets at low temperatures

This work is aimed to study the influence of the replacement of the magnetic cations Tb3+ by diamagnetic cations Y3+ on the low-temperature point’, TB predicted by Belov for ferrimagnets with and without magnetic compensation point, Tcomp. Isothermal magnetizations, MT(H) are carried out on single crystals of two mixed terbium-yttrium ferrite garnets TbxY3-xFe5O12 (x = 1.98 and 0.37). The measurements are performed using DC magnetic field, H up to 80 kOe applied along the <111> and <100> crystallographic directions in the 1.5-300 temperature range. From the approximation to the second order of H, the expansion of MT(H) is analyzed on the basis of the T-dependences of the spontaneous magnetization, MS, the paraprocess magnetic susceptibility, χp and the parameter, |b| of the bH2 term. The manifestation of ferromagnetic-and antiferromagnetic-type low-temperature paraprocess and other concomitant effects is observed in the temperature dependences of the derivative, dMS/dT where MS exhibits Néel’s-type curves, i.e., N-type with Tcomp = 137 K for x = 1.98 and P-type without a Tcomp point for x = 0.37). It was established that low-T anomalies appear in the area of TB = 58 K reported before for the pure TbIG. The results obtained for x = 0.37 in the case of the free sphere show that |b| undergo changes at TSR1 = 40 K, TSR2 = 140 K, and TSR3 = 232 K. Thus, we suggest that as T increase, the previous spin reorientation phase transition occurs in a wide T-range, (40-232 K) from the easy axis of magnetization <100> via stable and intermediate low-symmetry angular phases <uuw> towards the high symmetry phase <111>.

Magnetic field tuning of spin resonance in TmFeO3 single crystal probed with THz transient

TmFeO3, a canted antiferromagnet, has two intrinsic spin resonance modes in the terahertz (THz) frequency regime: quasi-ferromagnetic (q-FM) mode and quasi-antiferromagnetic (q-AFM) mode. Both the q-FM and q-AFM modes show strong magnetic field and temperature dependence. Hereby, by employing THz time-domain spectroscopy combined with external magnetic field and low temperature system, we systematically investigated the magnetic field induced frequency shift of q-FM and q-AFM modes as well as the temperature driven spin reorientation phase transition in TmFeO3 single crystal. In contrast to the isotropic temperature dependent two-mode, the magnetic field dependence of two-mode is strongly anisotropic: the magnetic field applied along c-axis (a-axis) can harden (soften) the spin resonance frequency of q-FM mode for Γ4 phase of TmFeO3, and the field applied along b-axis shows negligible frequency shift for the q-FM mode, with the q-AFM mode relatively stable. The present study provides solid evidence that the magnetic anisotropy in rare earth orthoferrite plays a dominant role in the q-FM mode and the occurrence of spin reorientation phase transition. With the magnetic anisotropic energy obtained from the temperature dependent q-FM and q-AFM mode frequencies, we can predict both magnetic field and temperature dependence of spin resonance in TmFeO3 single crystal via phenomenological analysis.

Consequences of R3+ cationic radii on the dielectric and magnetic behavior of RCrO3 perovskites

In this study, the consequences of R3+ cationic radii on the dielectric, conductivity and magnetic response of selected RCrO3 perovskites are presented. The frequency-dependent real part of dielectric constant (e′) and loss tangent (tan δ) were studied at 300 K and observed that at low frequency the values are high. From the conductivity (σAC) studies, we found that values of frequency exponent (s) are in the range 0.21–0.51 for the measured frequency range, which clearly indicates that the conduction mechanism is due to small polaron hopping. The magnetic studies reveal that all the synthesized samples exhibit magnetic phase transition (paramagnetic to antiferromagnetic) below the critical Neel temperature (TN) which is associated to Cr3+–O2−–Cr3+ exchange interaction and the value of TN decreases uniformly with decreasing R3+ cationic radii. Some samples show interesting magnetic behavior, along with isothermal hysteresis curve proposing the spin reorientation phase transition of Cr3+ magnetic ordering.

Одноионный механизм слабого антиферромагнетизма и спин-флоп-переход в двухподрешеточном ферромагнетике

The ground state of a Heisenberg ferromagnet with noncollinear single-ion anisotropy axes of two sublattices is investigated in the external magnetic field applied in the anisotropy axes plane. The noncollinearity of local anisotropy axes leads to new effect - the first order spin reorientation phase transition of the spin-flop type. The transition field depends on the value of anisotropy and orientation of the sublattice axes. Stability analysis of magnetic states shown that the field induced transition is accompanied by the magnetization hysteresis. The dependencies of the spin-flop transition field, the magnetization jump value and the susceptibility on the anisotropy parameters are determined. The obtained results are used for explanation of the magnetization field dependence in the ferromagnetic crystal PbMnBO_4.

Terahertz spectroscopic characterization of spin mode and crystal-field transition in high-throughput grown &lt;inline-formula&gt;&lt;tex-math id="Z-20201027023844-1"&gt;\begin{document}$ {\bf Sm}_{ x}{\bf Pr}_{ 1– x}{\bf FeO_3} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200732_Z-20201027023844-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200732_Z-20201027023844-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; crystals

Terahertz (THz) transient has become an effective method to study the optical and electronic spin characteristics of the rare earth orthoferrites &lt;i&gt;R&lt;/i&gt;FeO&lt;sub&gt;3&lt;/sub&gt;. High-throughput grown crystal sample is sliced at different locations, then the continuously tunable rare earth elements co-doped single crystal Sm&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Pr&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; is studied with antiferromagnetic spin mode (qAFM) and crystal field transitions of rare earth ions under zero magnetic fields. Using THz time-domain spectroscopy, the qAFM resonance frequencies of Sm&lt;sub&gt;0.2&lt;/sub&gt;Pr&lt;sub&gt;0.8&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; and Sm&lt;sub&gt;0.4&lt;/sub&gt;Pr&lt;sub&gt;0.6&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; single crystals are located on the connection line of the qAFM frequencies of PrFeO&lt;sub&gt;3&lt;/sub&gt; (0.57 THz) and SmFeO&lt;sub&gt;3&lt;/sub&gt; (0.42 THz), therefore the frequency of qAFM increases linearly with doping concentration of Sm&lt;sup&gt;3+&lt;/sup&gt; ion increasing. The Sm&lt;sub&gt;0.4&lt;/sub&gt;Pr&lt;sub&gt;0.6&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; crystal undergoes a temperature-induced spin reorientation phase transition at about 160 K. When the crystal temperature is lower than 80 K, a wide band absorption peak of about 0.5 THz appears in the absorption spectrum of Sm&lt;sub&gt;0.2&lt;/sub&gt;Pr&lt;sub&gt;0.8&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt; due to the crystal field effect. Our results show that THz spectral data not only allow us to monitor the quality of rare earth orthoferrite crystals prepared by high throughput and analyze the rare earth elements of the sample, but also improve the ability to analyze the physical properties of the co-doped &lt;i&gt;R&lt;/i&gt;FeO&lt;sub&gt;3&lt;/sub&gt;.

Magnetic Properties in the Metallic Magnets with Large Anisotropy

We study low temperature properties in the metallic magnets, considering the itinerant electron mediated ferromagnetism. Applying the Monte Carlo simulations to the extended double exchange model, we discuss reorientation phase transition and anisotropy field for the metallic magnets.

Magnetocaloric effect in cubically anisotropic magnets

Magnetocaloric response in anisotropic single-crystals with cubic crystal symmetry is studied. As compared to isotropic magnets where only paramagnetic-to-ferromagnetic phase transition occurs at Curie temperature (TC=185 K) and magnetization increases monotonously with increasing field and/or decreasing temperature, the spin reorientation phase transitions may occur at lower temperatures (∼115 K) as the magnetic field gets oriented along one of the cubic anisotropy easy axes and induces nonmonotonic increases in magnetization with increasing field and temperature, leading to a spike-like entropy change (−ΔSM) and positive ΔSM at TC, or move the −ΔSM peak towards higher temperatures. Finally, the power laws of maximum −ΔSM and relative cooling power with the field and the double-shifted loop results evidence that only second-order phase transition occurs and the abnormal magnetization behavior completely comes from the spin order evolution due to energy competition. Therefore, the Maxwell relation is applicable. The results are also suitable for multiple anisotropy or polycrystalline materials with well-defined domains.

Magnetic, magneto-optical and specific heat studies of the low temperature anomalies in the magnetodielectric DyIG ferrite garnet

Abstract Measurements of magnetization and Cotton Mouton effect (CM) under external magnetic field (Hex) combined with specific heat experiments at Hex = 0, were performed on dysprosium iron garnet Dy3Fe5O12 (or DyIG) single crystals. The low-temperature dependences of some magnetic, magneto-optical and thermodynamic quantities, such as spontaneous magnetization, both magnetic and CM susceptibilities and the excess specific heat due to the contribution of the Dy3+ ions only, give rise to the appearance of two types of anomalies. The first one which is related to the ‘low temperature paraprocess’ effect, occurs in the region of the ‘Belov point’ TB predicted at 42 K. Whereas the second one, which is due to the previous first-order spin reorientation phase transition, takes place at TSR = 15 K. The easy axis of magnetization defined by the resultant iron sublattice magnetization vector MFe, which remains along the 〈1 1 1〉 direction above TSR, changes spontaneously below TSR and becomes aligned along the low symmetry angular 〈u u w〉 phase. The results are analyzed taking into account those reported earlier, such as the negative magnetodielectric property for Hex

Intrinsic evolution of novel (Nd, MM)2Fe14B-system magnetic flakes

The Nd-substituted (Nd x MM1−x )–Fe–B strip-casting flakes were prepared by induction melting in the vacuum furnace and then subsequently by strip-casting technology. The microstructure and magnetic properties of (Nd x MM1−x )–Fe–B alloys are related to the Nd substitution. 2:14:1 main phases and minor impure phases coexist in the MM–Fe–B flake. For example, La2O3 and CeFe2 impure phases are obviously detected in the x = 0 specimen. As an increase of the Ce concentration is inversely accompanied with the decrease of the Nd content (x) in (Nd x MM1−x )2Fe14B main phases (0 ≤ x ≤ 1), XRD analysis shows that the overall diffraction peaks of the main phases shift to right domestically because of smaller radius Ce4+. The melting point, spin reorientation phase transition temperature, Curie temperature, magneto-crystalline anisotropy field (at 300 K), and the magnetization (M 9T) for MM–Fe–B/(Nd0.4MM0.6)–Fe–B/(Nd0.7MM0.3)–Fe–B/Nd–Fe–B strip-casting alloys are 1376.15/1414.15/1439.15/1458.15 K, 74/113/124/135 K, 493.2/538.4/559.7/582.3 K, 48/55.2/64.4/70.1 kOe and 136.5/143.7/151.5/153.7 emu/g, respectively. Due to the varied composition of hard magnetic main phases, M 9T increases gradually with the increase of Nd content (x). SEM observation and EDX results demonstrate that more Nd and Pr elements aggregate into the 2:14:1 ferromagnetic phase, while less La and Ce elements are prone to the RE-rich region compared with the nominal ratio. As a result, the growth of M 9T becomes extraordinary under maximum external field 9 T, indicating that the (Nd0.7MM0.3)–Fe–B flake may display relatively good magnetic properties and those with higher Nd content have evident effect on magnetization, compositions, and microstructures of hard magnetic main phases. Therefore, practical application of (Nd x MM1−x )–Fe–B-sintered magnets will be very prospective.

Evolution of magnetic phases inSmCrO3: A neutron diffraction and magnetometric study

The classical belief about the mechanism of spin reorientation phase transition (SRPT) and ground-state magnetic structure in ${\mathrm{SmCrO}}_{3}$ has become intriguing because of inconsistent bulk magnetization observations. The presence of highly neutron-absorbing Sm atom has so far evaded the determination of microscopic magnetic structure. In the present report, we have utilized very high-energy ``hot neutrons'' to overcome the Sm absorption and to determine the thermal evolution of magnetic configurations. Unambiguously, three distinct phases are observed: the uncompensated canted antiferromagnetic structure ${\mathrm{\ensuremath{\Gamma}}}_{4}({G}_{x},{A}_{y},{F}_{z};{F}_{z}^{R})$ occurring below the N\'eel temperature (${T}_{N}=191$ K), the collinear antiferromagnetic structure ${\mathrm{\ensuremath{\Gamma}}}_{1}({A}_{x},{G}_{y},{C}_{z};{C}_{z}^{R})$ occurring below 10 K, and a nonequilibrium configuration with cooccurring ${\mathrm{\ensuremath{\Gamma}}}_{1}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$ phases in the neighborhood of the SRPT (10 K $\ensuremath{\le}T\ensuremath{\le}$ 40 K). In differing to the earlier predictions, we divulge the SRPT to be a discontinuous transition where chromium spins switch from the $a\ensuremath{-}b$ crystallographic plane to the $b\ensuremath{-}c$ crystallographic plane in a discrete manner with no allowed intermediate configuration. The canting angle of chromium ions in the $a\ensuremath{-}b$ plane is unusually not a thermal constant, rather it is empirically discerned to follow exponential behavior. The competition between magnetocrystalline anisotropy and free energy derived by isotropic and antisymmetric exchange interactions between different pairs of magnetic ions is observed to govern the mechanism of SRPT.

Investigation on ultrafast opto-magnetism in rare earth orthoferrites single crystals

The ReFeO3 compounds belong to non-collinear antiferromagnet with unique magnetic properties. A series of outstanding achievement about ultrafast opto-magnetism on rare-earth orthoferrites crystals have been reported since 2004, including ultrafast laser-induced spin reorientation phase transition and its coherent control by a single laser pulse. Ultrafast laser-induced spin reorientation phase transition is expected to have high impact on the development of novel devices based on ultrafast control of magnetic state of matter. As the continuous progress of THz technology, ultrafast optical modification of exchange interactions based on ReFeO3 single crystal will be significant to fundamental research in magnetism. In this paper, combined our investigation with international investigations on ReFeO3 crystal growth and ultrafast opto-magnetism, the authors introduce on the progress of ultrafast opto-magnetism in ReFeO3 crystals. Based on them, we make a perspective on ReFeO3 crystal growth and application.

Low-temperature absorption spectra and electron structure of HoFe3(BO3)4 single crystal

Polarized absorption spectra of HoFe3(BO3)4 single crystal in the range of 8500–24 500 cm−1 were studied as a function of temperature beginning from 2 K. The ground and excited electron states of Ho3+ were identified. The abrupt changes of the spectra at the reorientation phase transition at 4.7 K were observed. The exchange splitting of some excited states were revealed and measured. They changed at the reorientation phase transition. Several vibronic transitions were observed. The splitting of absorption lines corresponding to the C2 local symmetry of the Ho ion was not observed. Moreover, spectra of some absorption bands correspond to splitting in the cubic crystal field. There are some absorption lines, whose polarization cannot be explained both in D3 and C2 local symmetries. Some lines appear or disappear as a result of the transition from the easy axis to the easy plane state of the crystal. All these observations testify to the substantial changes of the local magnetic and structural properties in the excited states of the Ho3+ ion and to the strong influence of the magnetic moments orientation on the polarization of the electron transitions.