High Magnetic Ordering Temperature Research Articles

Magnetic properties and linear negative magnetocapacitance in rutile like LiFe2SbO6

Abstract We report the high-temperature magnetic ordering and the observation of linear negative magnetocapacitance mediated by a low-temperature magnetic phase crossover in the compound LiFe2SbO6. The combined observations of the absence of a second harmonic generation (SHG) signal, and Rietveld refinements of X-ray and neutron powder diffraction (NPD) patterns of the sample confirm that LiFe2SbO6 crystalizes in a centrosymmetric Pnnm structure. The DC magnetization measurements reveal that an antiferromagnetic ordering sets in at high temperature (TN= 850). The magnetic anomaly in the DC magnetization at a lower temperature T = 13 K, is corroborated by AC susceptibility and the specific heat measurements. We observe a linear negative magnetocapacitance below T = 13 K, demonstrating an indirect magneto-dielectric coupling. Further, the neutron powder diffraction study reveals a collinear antiferromagnetic ordering with a wavevector k = (0, 0, 0).

Switching of magnetoelectric states in the Y-type hexaferrite Ba0.5Sr1.5CoMgFe11AlO22

The multiferroic Y-type hexaferrites BaxSr2−xMe2Fe12−yAlyO22 (Me = Zn2+, Co2+, Mg2+, etc.) have attracted much attention due to their giant magnetoelectric (ME) effect up to room temperature and low modulated magnetic field by the chemical doping control of the complex magnetic phases. However, the research of substitution between the Me ions is rare. As doping at the Me ion site can combine the advantages of both, e.g., higher magnetic ordering temperature in Co2 and stronger ME coefficient in Mg2 Y-type hexaferrites, herein, we report the stability and switching of magnetoelectric states in the Y-type hexaferrites Ba0.5Sr1.5CoMgFe11AlO22 single crystals. Our results demonstrate that substituting half of the Mg2+ with Co2+ enhances the transition temperature of the alternating longitudinal conical phase to proper screw spin order up to room temperature compared to those Mg2 Y-type hexaferrites. Simultaneous occurrence of in-plane and out-of-plane ferroelectric polarization is observed, alongside comparable spontaneous magnetization. It was found that the in-plane spin-driven polarization can be reversible below 50 K with a substantial ME coefficient α = −8000 ps/m but becomes irreversible at 100 K. This reversal in the sign of the ME coefficient signifies the transition between two distinct ME states at high temperature. The reversibility and irreversibility of spin-induced polarization are discussed within the framework of free energy based on the ferroelectric phase, which prevail in numerous Y-type hexaferrites. Our results provide insights into understanding the role of the Me ions in the magnetoelectric coupling in Y-type hexaferrites.

Designing Magnetic Properties in CrSBr through Hydrostatic Pressure and Ligand Substitution

AbstractMagnetic van der Waals (vdW) materials are a promising platform for producing atomically thin spintronic and optoelectronic devices. The A‐type antiferromagnet CrSBr has emerged as a particularly exciting material due to its high magnetic ordering temperature, semiconducting electrical properties, and enhanced chemical stability compared to other vdW magnets. Exploring mechanisms to tune its magnetic properties will facilitate the development of nanoscale devices based on vdW materials with designer magnetic properties. Here it is investigated how the magnetic properties of CrSBr change under pressure and ligand substitution. Pressure compresses the unit cell, increasing the interlayer exchange energy while lowering the Néel temperature. Ligand substitution, realized synthetically through Cl alloying, anisotropically compresses the unit cell and suppresses the Cr‐halogen covalency, reducing the magnetocrystalline anisotropy energy and decreasing the Néel temperature. A detailed structural analysis combined with first‐principles calculations reveals that alterations in the magnetic properties are intricately related to changes in direct Cr–Cr exchange interactions and the Cr–anion superexchange pathways. Further, it is demonstrated that Cl alloying enables chemical tuning of the interlayer coupling from antiferromagnetic to ferromagnetic, which is unique among known two‐dimensional magnets.

Permanent Porosity in the Room-Temperature Magnet and Magnonic Material V(TCNE)2.

Materials that simultaneously exhibit permanent porosity and high-temperature magnetic order could lead to advances in fundamental physics and numerous emerging technologies. Herein, we show that the archetypal molecule-based magnet and magnonic material V(TCNE)2 (TCNE = tetracyanoethylene) can be desolvated to generate a room-temperature microporous magnet. The solution-phase reaction of V(CO)6 with TCNE yields V(TCNE)2·0.95CH2Cl2, for which a characteristic temperature of T* = 646 K is estimated from a Bloch fit to variable-temperature magnetization data. Removal of the solvent under reduced pressure affords the activated compound V(TCNE)2, which exhibits a T* value of 590 K and permanent microporosity (Langmuir surface area of 850 m2/g). The porous structure of V(TCNE)2 is accessible to the small gas molecules H2, N2, O2, CO2, ethane, and ethylene. While V(TCNE)2 exhibits thermally activated electron transfer with O2, all the other studied gases engage in physisorption. The T* value of V(TCNE)2 is slightly modulated upon adsorption of H2 (T* = 583 K) or CO2 (T* = 596 K), while it decreases more significantly upon ethylene insertion (T* = 459 K). These results provide an initial demonstration of microporosity in a room-temperature magnet and highlight the possibility of further incorporation of small-molecule guests, potentially even molecular qubits, toward future applications.

Elemental compositional modeling of magnetic ordering temperature for spinel ferrite magnetocaloric compounds using intelligent algorithms

Spinel ferrite recently attracted attention for possible application in magnetic refrigeration due to its noticeable high magnetocaloric effect and tunable magnetic ordering temperature around room temperature. Being a magnetic semiconductor, the material has enjoyed wider application in different practical domains such as drug delivery, humidity sensor, photo-catalyst, high density data storage, magnetic resonance imaging and magnetic cooling among others. However, simplicity of its preparation and excellent cost effectiveness as compared to the existing magnetocaloric-based materials further contribute to its suitability for attaining magnetic cooling. Effective utilization of this material for magnetic cooling requires precise measurement of its magnetic ordering temperature (MOT) which requires laborious experimental procedures and sophisticated equipment. This work addresses the challenges by employing elemental compositions of spine ferrite in developing hybrid models for predicting MOT using hybrid genetic-based support vector regression algorithm (GBSVRA) and extreme learning machine (ELM). The developed ELM-SN model with sine activation function performs better than hybrid GBSVRA and ELM-SG (with sigmoid activation function) model with performance improvement of 42.63% and 38.78%, respectively, through RMSE performance yardstick, while the ELM-SG model outperforms hybrid GBSVRA model with performance enhancement of 2.87% when validated using testing dataset. The developed ELM-SN model further outperforms other two developed models using other performance metrics. Harnessing the potentials of the presented models would strengthen precise, effective and quick tuning of spinel ferrite MOT for achieving magnetic cooling without experimental cost and difficulties.

A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials

Two-dimensional (2D) nonlayered materials have recently provoked a surge of interest due to their abundant species and attractive properties with promising applications in catalysis, nanoelectronics, and spintronics. However, their 2D anisotropic growth still faces considerable challenges and lacks systematic theoretical guidance. Here, we propose a general thermodynamics-triggered competitive growth (TTCG) model providing a multivariate quantitative criterion to predict and guide 2D nonlayered materials growth. Based on this model, we design a universal hydrate-assisted chemical vapor deposition strategy for the controllable synthesis of various 2D nonlayered transition metal oxides. Four unique phases of iron oxides with distinct topological structures have also been selectively grown. More importantly, ultra-thin oxides display high-temperature magnetic ordering and large coercivity. MnxFeyCo3-x-yO4 alloy is also demonstrated to be a promising room-temperature magnetic semiconductor. Our work sheds light on the synthesis of 2D nonlayered materials and promotes their application for room-temperature spintronic devices.

Chemical Tuning Meets 2D Molecular Magnets.

2D magnets provoke a surge of interest in large anisotropy in reduced dimensions and are promising for next-generation information technology where dynamic magnetic tuning is essential. Until recently, the crucial metal-organic magnet Cr(pyz)2 ·xLiCl·yTHF with considerable high coercivity and high-temperature magnetic order opens up a new platform to control magnetism in metal-organic materials at room temperature. Here, an in-situ chemical tuning route is reported to realize the controllable transformation of low-temperature magnetic order into room-temperature hard magnetism in Cr(pyz)2 ·xLiCl·yTHF. The chemical tuning via electrochemical lithiation and solvation/desolvation exhibits continuously variable magnetic features from cryogenic magnetism to the room-temperature optimum performance of coercivity (Hc ) of 8500Oe and energy product of 0.6 MGOe. Such chemically flexible tunability of room-temperature magnetism is ascribed to the different degrees of lithiation and solvation that modify the stoichiometry and Cr-pyrazine coordination framework. Furthermore, the additively manufactured hybrid magnets show air stability and electromagnetic induction, providing potential applications. The findings here suggest chemical tuning as a universal approach to control the anisotropy and magnetism of 2D hybrid magnets at room temperature, promising for data storage, magnetic refrigeration, and spintronics.

Ultrathin ferrite nanosheets for room-temperature two-dimensional magnetic semiconductors

The discovery of magnetism in ultrathin crystals opens up opportunities to explore new physics and to develop next-generation spintronic devices. Nevertheless, two-dimensional magnetic semiconductors with Curie temperatures higher than room temperature have rarely been reported. Ferrites with strongly correlated d-orbital electrons may be alternative candidates offering two-dimensional high-temperature magnetic ordering. This prospect is, however, hindered by their inherent three-dimensional bonded nature. Here, we develop a confined-van der Waals epitaxial approach to synthesizing air-stable semiconducting cobalt ferrite nanosheets with thickness down to one unit cell using a facile chemical vapor deposition process. The hard magnetic behavior and magnetic domain evolution are demonstrated by means of vibrating sample magnetometry, magnetic force microscopy and magneto-optical Kerr effect measurements, which shows high Curie temperature above 390 K and strong dimensionality effect. The addition of room-temperature magnetic semiconductors to two-dimensional material family provides possibilities for numerous novel applications in computing, sensing and information storage.

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

First-principles study on giant magneto-optical Kerr effect in double perovskites Sr2BB′O6 (B = Cr, Mo, B′ = W, Re, Os)

Sr2CrB′O6 (B′ ​= ​W, Re, Os) are famous double perovskite oxides because of their high magnetic ordering temperature Tc and half metallic property. Based on them, Sr2MoB′O6 were designed, and were also reported to be half metals and possess Tc well above the room temperature. In this work, we systematically investigated the magneto-optical Kerr effect of Sr2BB′O6 (B = Cr, Mo, B′ ​= ​W, Re, Os) by the density functional theory. All these compounds exhibit remarkable magneto-optical Kerr effect in the infrared to visible spectral ranges. The maxima of Kerr rotation for half metals Sr2CrWO6, Sr2CrReO6, and Sr2MoOsO6 can reach up to −5.02°, −2.56°, and 4.45°, respectively. The characteristics of the Kerr spectra in lower and higher energy region are dominated by two different mechanisms, respectively. Our work provides a theoretical basis for designing new magneto-optical materials.

Exploring the heavy transition metal trihalide family: Two-dimensional magnetic materials with tunable band gap, huge magnetic anisotropy, and high-temperature magnetic ordering

In light of the forecast of recent topics of magnetic van der Waals materials [Burch et al., Nature (London) 563, 47 (2018); Gong and Zhang, Science 363, 706 (2019)], it was encouraged to explore new magnetic two-dimensional (2D) materials that contain heavy atoms so as to elevate the magnetic transition temperature. Here, relativistic density functional theory calculations, combined with ab initio molecular dynamics simulations, lattice dynamical calculations, and classical Monte Carlo (MC) simulations, were performed on transition metal trihalide (${\mathrm{TMI}}_{3}$; TM = La, Hf, Ta, W, Re, Os, Ir, Pt) layers, for which heavy $5d$-block and $5p$-block elemental atoms were considered owing to their strong spin-orbit coupling. We identified that these newly designed layers are energetically, dynamically, thermally, and thermomagnetically stable at room temperature. Our investigations revealed that most of them possess the intrinsic semiconducting characteristic with tunable band gap, diverse magnetic properties (ferromagnetic, antiferromagnetic, and nonferromagnetic orders), and large magnetic anisotropy energy (generally larger than 1.9 meV/f.u. and up to 29 meV/f.u.). Based on MC simulations, we proposed that ${\mathrm{TaI}}_{3}$, ${\mathrm{OsI}}_{3}$, and ${\mathrm{PtI}}_{3}$ layers have high Curie temperatures of 575, 225, and 170 K, respectively, which are much larger than those of most magnetic 2D materials such as the notable ${\mathrm{CrI}}_{3}$ (45 K), ${\mathrm{CrGeTe}}_{3}$ (28 K), and ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ layers (130 K). Importantly, our calculations evidenced the high stability of the ${\mathrm{TaI}}_{3}$ layer under ambient conditions, large magnetic moment of $2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, large magnetic anisotropy energy of $\ensuremath{-}7.3$ meV/f.u., and high Curie temperature of 575 K, making it possible to experimentally study this layer by more ex situ measurements and even to utilize it in future electronic devices.

Unconventional ferrimagnetism and enhanced magnetic ordering temperature in monolayer CrCl3 by introducing O impurities and Cl vacancies

Among chromium trihalides, a specific group of layered van der Waals magnetic materials, chromium trichloride (CrCl3) is the only system relatively stable under ambient conditions. This is also observed in reduced dimensionality where the emergence of extrinsic long-range ordered oxidized and Cl-vacancy-defective CrCl3 phases is experimentally reported. In this work, the magnetic properties of such two-dimensional (2D) systems are studied using density functional theory (DFT) calculations, including the electron-electron (U) repulsion interactions, and Monte Carlo (MC) simulations. Once the Cl vacancies are introduced, the results indicate that the monolayer CrCl3 has a magnetic moment that is enhanced linearly (up to 3.14 µ B /Cr) in the (1%–10%) vacancy concentration range. This determines a strengthening of the ferromagnetic state and a two-fold increase of the Curie temperature (up to 146 K) as valuated from MC simulations. More interestingly, once oxygen extrinsic impurities are considered, the monolayer CrCl3 structure is hybridized forming a stable ordered phase (O–CrCl3) with oxygen atoms allocated on the Cr atomic layer in the center of the honeycomb ring formed by Cr atoms. The magnetic moments of the O–CrCl3 system are localized on both Cr and O atoms, with oxygen antiferromagnetically coupled to chromium, resulting in a 2D ferrimagnetic hexagonal lattice system with an average magnetic moment of 2.14 µ B /Cr and a high magnetic ordering temperature (110 K) predicted with DFT in the mean field approach.

Strain‐Induced Magnetism in MSi2N4 (M = V, Cr): A First‐Principles Study

AbstractThe discovery of ferromagnetic 2D van der Waals (vdW) materials has opened up opportunities for exploring and harnessing magnetism in low‐dimensional limit, and developing innovative paper‐like spintronic devices. Stress engineering demonstrates unique advantages in regulating material properties, especially for 2D materials. With the aid of first‐principles calculations, the effectiveness of strain engineering for modulating the electronic and magnetic properties of MSi2N4 (M = V, Cr) monolayers from a new family of layer‐structured 2D vdW materials is explored. Their magnetic configurations are effectively tuned via the uniaxial and biaxial tensile strain of in‐plane lattice vectors. VSi2N4 exhibits half‐metallic ferromagnetic character, whereas CrSi2N4 is prone to be antiferromagnetic (AFM), which is rationalized from the competition between the direct metal–metal AFM interaction and the 900 ferromagnetic superexchange. More intriguingly, VSi2N4 demonstrates a high magnetic ordering temperature of 285 K, which is close to room temperature. The predicted appealing properties of VSi2N4 under strain await experimental confirmation.

Effects of Y- and La-doping on the magnetic ordering, Kondo effect, and spin dynamics in Ce1−x M x Ru2Al10

The influence of Y- and La-substitution for Ce on the competing Kondo effect and magnetic ordering, as well as on spin dynamics in the Kondo semiconductor CeRu2Al10 have been investigated by means of thermal, electronic, and magnetic properties. The parent compound CeRu2Al10 is known to be a controversial antiferromagnet with high magnetic ordering temperature T 0 = 27 K. A small negative chemical pressure caused by La-doping results rapid suppression of T 0 and spin gap energy Δ m , compared to a small positive pressure caused by Y-doping. Upon Y- and La-doping, the electrical resistivity ρ(T) illustrates the evolution from dense Kondo semiconductor to incoherent single-ion Kondo behaviour, and hence weakens the c–f hybridization and thus lowers the Kondo temperature. The 5% Y- and La-doped compounds show enormous enhancement in the thermoelectric power and complex behaviour in the Hall resistivity below T 0 due to an abrupt change in charge carrier mobility with temperature. The magnetic contribution to electrical resistivity ρ mag(T) (≳50% La) and specific heat C P(T)/T (≳70% La) evidence non-Fermi-liquid behaviour at low temperature in the La-doped system, due to interplay of atomic disorder with spin-fluctuation. Application of magnetic field suppresses the spin-fluctuation in C P(T)/T and eventually emerges to Fermi-liquid state in the 95% La-doped compound in 9 T. The magnetic phase diagram illustrates that the strength of the Kondo interaction in the doped systems are primarily controlled by the effect of volume change as described by the compressible Kondo lattice model. We ascribe the fascinating observation of T K ≃ 4T 0 to anisotropy in the single-ion crystal electric field in the presence of strong anisotropic c–f hybridization.

Thickness and Spin Dependence of Raman Modes in Magnetic Layered Fe3GeTe2

Abstract2D layered Fe3GeTe2 has attracted increasing attention due to its high magnetic ordering temperature and novel physical properties. Lattice dynamics is a fundamental property of Fe3GeTe2, and its relationships with the number of layers and interlayer spin ordering have not yet been explored in depth. Here, by first‐principles density functional theory calculations, the phonon vibrations and Raman intensities of Fe3GeTe2 are systematically studied from the bulk to monolayer structures. Furthermore, the spin‐phonon coupling effect is investigated by considering different interlayer magnetic orderings: ferromagnetic and antiferromagnetic. It is found that the frequencies of Raman modes in Fe3GeTe2 exhibit considerable dependence on the layer number and spin ordering. The results not only reveal the notable spin‐phonon interactions in Fe3GeTe2, but also demonstrate that Raman modes can be utilized for characterizing the sample thickness and interlayer spin ordering in this 2D magnet.

Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper

Rhombohedral-stacked few-layer graphene (FLG) displays peculiar electronic properties that could lead to phenomena such as high-temperature superconductivity and magnetic ordering. To date, experimental studies have been mainly limited by the difficulty in isolating rhombohedral FLG with thickness exceeding 3 layers and device-compatible size. In this work, we demonstrate the synthesis and transfer of rhombohedral graphene with thickness up to 9 layers and areas up to ∼50 μm2. The domains of rhombohedral FLG are identified by Raman spectroscopy and are found to alternate with Bernal regions within the same crystal in a stripe-like configuration. Near-field nano-imaging further confirms the structural integrity of the respective stacking orders. Combined spectroscopic and microscopic analyses indicate that rhombohedral-stacking formation is strongly correlated to the underlying copper step-bunching and emerges as a consequence of interlayer displacement along preferential crystallographic orientations. The growth and transfer of rhombohedral FLG with the reported thickness and size shall facilitate the observation of predicted unconventional physics and ultimately add to its technological relevance.

Electronic phase-crossover and room temperature ferromagnetism in a two-dimensional (2D) spin lattice.

Tuning of system properties such as electronic and magnetic behaviour through various engineering techniques is necessary for optoelectronic and spintronic applications. In our current work, we employ first-principles methodologies along with Monte-Carlo simulations to comprehensively study the electronic and magnetic behaviour of 2-dimensional (2D) Cr2Ge2Te6 (Tc = 61 K), uncovering the impact of strain and electric field on the material. In the presence of strain, we were able to achieve high temperature magnetic ordering in the layer along with observable phase crossover in the electronic state of the system, where the system exhibited transference from semiconducting to half-metallic state. Finally, on coupling strain and electric field remarkable increase in Curie temperature (Tc) ∼ 331 K (above 5-fold enhancement from pristine configuration) was observed, which is very well above room temperature. Our inferences have shed light on a relatively new type of coupling method involving strain and electric field which may have tremendous implications in the development of 2D spintronic architecture.

Remarkable magnetoelectric effect in single crystals of honeycomb magnet Mn4Nb2O9

Linear magnetoelectrics refer to those compounds in which ferroelectric (FE) polarization can be generated by applying the magnetic field. This scenario opens an additional avenue toward high-temperature magnetoelectric (ME) coupling that is achievable in a large class of relatively weak frustrated magnetic systems such as honeycomb antiferromagnets. It is, thus, urgent to unveil the physics underlying the linear ME coupling in these linear ME materials. We grow the single crystals of Mn4Nb2O9, a linear ME candidate with high magnetic ordering temperature, and carry out a set of structural, magnetic, and ME characterizations. An antiferromagnetic ordering with [001]-oriented moments at the Neel point TN = 109 K is identified together with magnetic field driven large electric polarization emerging at TN, due to the strong exchange striction dependent mechanism. The measured ME coupling tensor α fits well the magnetic symmetry −3′m′, consistent with the linear ME scenario. Furthermore, remarkable responses of FE polarization and magnetization to the magnetic field and electric field, respectively, are demonstrated.

High-temperature short-range order in Mn3RhSi

Conventional phase transitions are well understood in terms of the order parameter, based on the Landau–Ginzburg–Wilson theory. However, unconventional magnetic orders have been observed in clean systems such as MnSi. The unconventional magnetic orders of conduction electrons in the metallic phase has been observed for high-temperature superconductors and heavy fermion compounds. However, these unconventional magnetic orders have been limited to relatively low temperatures as quantum phase transitions. Here high-temperature magnetic short-range order is observed as one of the unconventional magnetic orders at temperatures up to 720 K in a noncentrosymmetric intermetallic antiferromagnet Mn3RhSi with a well-ordered lattice. The magnetic Mn ions form a hyperkagome network of corner-sharing triangles, where the spins are geometrically frustrated. The spin network is equivalent to that of a spin liquid and non-Fermi-liquid material, β-Mn. Our observation indicates that a metallic phase with magnetic short-range order exists at high temperatures.

Spontaneous Magnetization and Optical Activity in the Chiral Series {(L-proline)nV[Cr(CN)6]x} (0 < n < 3)

The incorporation of the natural amino acid L-proline in the synthesis to vanadium-chromium Prussian blue derivatives results in materials exhibiting magnetic ordering including chiral magnetic centers. Although the amorphous nature of these materials makes difficult to assess the structural features of these proline-containing compounds, magnetic and spectroscopic data confirms their multifunctionality. They exhibit high-temperature magnetic ordering (Tc < 255 K) and a circular dichroic signal, representing the molecule-based chiral magnets with the highest ordering temperatures reported to date. In addition, the presence of chiral L-proline (or D-proline) has additional benefits, including higher redox stability and the appearance of magnetic hysteresis. The latter was not observed in the parent compounds, the series of room temperature molecule-based magnets V[Cr(CN)6]x.