High-temperature Ferromagnetism Research Articles

Robust Ferromagnetism in Hexagonal Honeycomb Transition Metal Nitride Monolayer.

Two-dimensional intrinsic magnetic materials with high Curie temperature are promising candidates for next-generation spintronic devices. In this work, we design two kinds of two-dimensional transition metal nitrides, VN2 and FeN2, both with a hexagonal honeycomb lattice. Based on the formation energy, and phonon spectra calculations as well as the molecular dynamics simulations, their structural stability is demonstrated. Then, we determine the ferromagnetic ground states of VN2 and FeN2 monolayers through the energy calculations, and the Curie temperatures of 222 K and 238 K are estimated by solving the Heisenberg model using the Monte Carlo simulation method. Hence, the VN2 and FeN2 monolayers are demonstrated to be new two-dimensional ferromagnetic materials with high temperature ferromagnetism or large-gap half-metallicity.

Two-dimensional monolayer CrGaS3: A ferromagnetic semiconductor with high Curie temperature and tunable magnetic anisotropy

Two-dimensional (2D) intrinsic ferromagnetic (FM) materials are promising candidates for fabricating next generation high-performance spintronic devices. However, all experimentally verified 2D FM semiconductors have Curie temperature (Tc) far below room temperature, which hinders their practical applications. Based on first-principles calculations, a stable and previously undiscovered 2D CrGaS3 structure is predicted, which is a semiconductor with an indirect bandgap of 1.99 eV and displays out-of-plane magnetic anisotropy. More importantly, it exhibits high-temperature ferromagnetism, with Tc ranging between 520 and 814 K. The high Tc is attributed to the presence of both direct-exchange and super-exchange interactions that are ferromagnetic, along with the eg-px/py-eg super exchange having a zero virtual exchange gap. Furthermore, it has been observed that the magnetic anisotropy can be tuned by external strain. These findings indicate its potential as a promising candidate for the rapid development of 2D spintronic applications.

Dual-component anomalous Hall effect in a helical spin-spiral metamagnet

We report a dual-component anomalous Hall effect (AHE) in polycrystalline Fe3Ga4 thin films grown on STO (001) and Al2O3 substrates. Systematic magnetic and magnetotransport measurements reveal an AHE consisting of positive and negative contributions that coexist across a wide range of temperatures and magnetic phases. We find that both magnitudes are nearly equal in the low-temperature ferromagnetic (FM) phase, but that their relative ratio is reduced upon heating through the antiferromagnetic helical spin-spiral state where they compete with metamagnetism and topological Hall effects, maintaining finite values at least up to the high-temperature FM phase.

Strong Room-Temperature Ferromagnetism in Ultrathin NiOOH Nanosheets through Surfactant Manipulation.

Two-dimensional (2D) ferromagnetic (FM) materials with nanoscale thickness and spontaneous net magnetization have emerged as a promising class of functional materials for applications in next-generation spintronics, quantum processing, and data storage devices. However, most 2D materials exhibit weak FM even at low temperatures, limiting their potential applications in many technological fields. The fabrication of strong room-temperature FM 2D materials is highly desirable for the development of practical applications. Here, we demonstrate an ionic layer epitaxy strategy to synthesize few-layered NiOOH nanosheets with strong room-temperature FM and a saturation magnetization up to 409.86 emu cm-3 at 300 K. The results are consistent with the ab initio predictions of a stable FM NiOOH nanolayer structure with an FM configuration. The FM strength of the NiOOH nanosheets can be tuned by controlling the surfactant monolayer density and annealing. This work offers a promising strategy for achieving strong high-temperature FM in 2D materials for spintronic applications.

Double-Layer Kagome Metals Pt3Tl2 and Pt3In2

The connectivity and inherent frustration of the kagome lattice can produce interesting electronic structures and behaviors in compounds containing this structural motif. Here we report the properties of Pt3X2 (X = In and Tl) that adopt a double-layer kagome net structure related to that of the topologically nontrivial high-temperature ferromagnet Fe3Sn2 and the density wave hosting compound V3Sb2. We examined the structural and physical properties of single crystal Pt3Tl2 and polycrystalline Pt3In2 using X-ray and neutron diffraction, magnetic susceptibility, heat capacity, and electrical transport measurements, along with density functional theory calculations of the electronic structure. Our calculations show that Fermi levels lie in pseudogaps in the densities of states with several bands contributing to transport, and this is consistent with our Hall effect, magnetic susceptibility, and heat capacity measurements. Although electronic dispersions, characteristic of simple kagome nets with nearest-neighbor hopping, are not clearly seen, likely due to the extended nature of the Pt 5d states, we do observe moderately large and non-saturating magnetoresistance values and quantum oscillations in the magnetoresistance and magnetization associated with the kagome nets of Pt.

Photo-induced high-temperature ferromagnetism in YTiO3

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases1–7. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic and chemical constraints8. Here we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO3, a material that shows only partial orbital polarization, an unsaturated low-temperature magnetic moment and a suppressed Curie temperature, Tc = 27 K (refs. 9–13). The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to Tneq > 80 K, nearly three times the thermodynamic transition temperature. We interpret these effects as a consequence of the light-induced dynamical changes to the quasi-degenerate Ti t2g orbitals, which affect the magnetic phase competition and fluctuations found in the equilibrium state14–20. Notably, the light-induced high-temperature ferromagnetism discovered in our work is metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.

Interfacial engineering of ferromagnetism in wafer-scale van der Waals Fe4GeTe2 far above room temperature

Despite recent advances in exfoliated vdW ferromagnets, the widespread application of 2D magnetism requires a Curie temperature (Tc) above room temperature as well as a stable and controllable magnetic anisotropy. Here we demonstrate a large-scale iron-based vdW material Fe4GeTe2 with the Tc reaching ~530 K. We confirmed the high-temperature ferromagnetism by multiple characterizations. Theoretical calculations suggested that the interface-induced right shift of the localized states for unpaired Fe d electrons is the reason for the enhanced Tc, which was confirmed by ultraviolet photoelectron spectroscopy. Moreover, by precisely tailoring Fe concentration we achieved arbitrary control of magnetic anisotropy between out-of-plane and in-plane without inducing any phase disorders. Our finding sheds light on the high potential of Fe4GeTe2 in spintronics, which may open opportunities for room-temperature application of all-vdW spintronic devices.

Parallel Aluminum-Cobalt Oxide Nanosheet Arrays with High-Temperature Ferromagnetism.

Parallel nanomaterials possess unique properties and show potential applications in industry. Whereas, vertically aligned 2D nanomaterials have plane orientations that are generally chaotic. Simultaneous control of their growth direction and spatial orientation for parallel nanosheets remains a big challenge. Here, a facile preparation of vertically aligned parallel nanosheet arrays of aluminum-cobalt oxide is reported via a collaborative dealloying and hydrothermal method. The parallel growth of nanosheets is attributed to the lattice-matching among the nanosheets, the buffer layer, and the substrate, which is verified by a careful transmission electron microscopy study. Furthermore, the aluminum-cobalt oxide nanosheets exhibit high-temperature ferromagnetism with a 919 K Curie temperature and a 5.22 emu g-1 saturation magnetization at 300 K, implying the potential applications in high-temperature ferromagnetic fields.

Room-Temperature Intrinsic Ferromagnetic Chromium Tellurium Compounds with Thickness-Tunable Magnetic Texture.

2D ferromagnetic chromium tellurides exhibit intriguing spin configurations and high-temperature intrinsic ferromagnetism, providing unprecedented opportunities to explore the fundamental spin physics and build spintronic devices. Here, a generic van der Waals epitaxial approach is developed to synthesize the 2D ternary chromium tellurium compounds with thicknesses down to mono-, bi-, tri-, and few-unit cells(UC). The Mn0.14 Cr0.86 Te evolves from intrinsic ferromagnetic behavior in bi-UC, tri-UC, and few-UC to temperature-induced ferrimagnetic behavior as the thickness increases, resulting in a sign reversal of the anomalous Hall resistance. Temperature- and thickness-tunable labyrinthine-domain ferromagnetic behaviors are derived from the dipolar interactions in Fe0.26 Cr0.74 Te and Co0.40 Cr0.60 Te. Furthermore, the dipolar-interaction-induced stripe domain and field-induced domain wall (DW) motion velocity are studied, and multibit data storage is realized through an abundant DW state. The magnetic storage can function in neuromorphic computing tasks, and the pattern recognition accuracy can reach up to 97.93%, which is similar to the recognition accuracy of ideal software-based training (98.28%). Room-temperature ferromagnetic chromium tellurium compounds with intriguing spin configurations can significantly promote the exploration of the processing, sensing, and storage based on 2D magnetic systems.

Orbital selective coupling in CeRh3B2 : Coexistence of high Curie and high Kondo temperatures

We investigated the electronic structure of the enigmatic CeRh$_3$B$_2$ using resonant inelastic scattering and x-ray absorption spectroscopy in combination with $ab$ $initio$ density functional calculations. We find that the Rh 4$d$ states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4$f$ crystal-field strength is too small to explain the strong reduction of the Ce magnetic moment. The data reveal instead the presence of two different active Ce 4$f$ orbitals, with each coupling selectively to different bands in CeRh$_3$B$_2$. The inter-site hybridization of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5$d$ band combined with the intra-site Ce 4$f$-5$d$ exchange creates the strong ferromagnetism, while hybridization between the |J=5/2,Jz=+/-5/2> and the B $sp$ in the $ab$-plane contributes to the Kondo interaction which causes the moment reduction. This orbital selective coupling explains the unique and seemingly contradictory properties of CeRh$_3$B$_2$.

Scaling analysis of anomalous Hall resistivity and magnetoresistance in the quasi-two-dimensional ferromagnet Fe3GeTe2

${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ is a two-dimensional van der Waals bonded layered compound that shows high-temperature itinerant ferromagnetism. We study aspects of the scattering mechanism in the single crystal of high-${T}_{\text{C}}\phantom{\rule{4pt}{0ex}}{\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ via resistivity, magnetotransport, and Hall effect measurements. The quadratic temperature dependence of electrical resistivity below the ${T}_{\text{C}}$ (210 K) points towards the dominance of electron-magnon scattering. A nonsaturating positive magnetoresistance (MR) is observed at low temperatures when the magnetic field is applied parallel to the sample plane. The linear negative MR at high fields for $T<{T}_{\text{C}}$ corroborates to the suppression in the magnon population due to the damping of spin waves. In the high-temperature regime ($T>{T}_{\text{C}}$), MR can be described by the scattering from spin fluctuations using the Khosla and Fischer model. Isothermal Hall resistivity curves unveil the presence of anomalous Hall resistivity. The correlation between magnetoresistance and side jump mechanism further reveals that the electron-magnon scattering is responsible for the side jump contribution to the anomalous Hall effect. Our results provide a clear understanding of the role of electron-magnon scattering on the temperature-driven evolution of the anomalous Hall effect that rules out its origin to be the topological band structure.

Above-Room-Temperature Ferromagnetism in Thin van der Waals Flakes of Cobalt-Substituted Fe5GeTe2

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying the fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling thin and highly tunable spintronic devices. To realize high-quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here, the study of exfoliated flakes of cobalt-substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm corresponding to 16 layers. This result expands the portfolio of thin room-temperature 2D magnet flakes exfoliated from robust single crystals that reach a thickness regime relevant to practical spintronic applications. The Curie temperature Tc of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed, which predicted a high value of Tc of ∼270 K in CFGT monolayers. Pathways toward further enhancing monolayer Tc are discussed. These results support CFGT as a promising platform for realizing high-quality room-temperature 2D magnet devices.

Computational discovery of diverse functionalities in two-dimensional square disulfide monolayers: auxetic behavior, high curie temperature ferromagnets, electrocatalysts, and photocatalysts

By means of systematic density functional theory (DFT) computations, we identified 12 stable two-dimensional (2D) disulfide monolayers featuring a square lattice from a pool of 68 candidate monolayers.

Predicted novel two-dimensional ferromagnetic VO2 with high Curie temperature and ferroelasticity

A two-dimensional VO2 cleaved from its bulk VO2 counterpart with high Curie temperature and magnetoelastic coupling is predicted from first-principles calculations.

Synthesis and high-temperature ferromagnetism of Fe-doped SiGe diluted magnetic semiconductor thin films.

Fe-doped SiGe (Si0.25Ge0.75:Fex, x = 0.01, 0.025, and 0.05) thin films were prepared by radio frequency magnetron sputtering and subsequent rapid thermal annealing on a Ge (100) substrate and their structural, magnetic and magneto-transport properties were investigated. Structural characterization using AFM, SEM, XRD, and HRTEM shows that the obtained samples are polycrystalline and their lattice constants increase with the Fe concentration. Analysis of their electronic and spintronic states using XPS and XMCD reveals that Fe dopants mainly exist as substitutional Fe2+ ions in the SiGe lattice, providing both local magnetic moments and hole carriers. Furthermore, magnetization measurements indicate that all the samples exhibit ferromagnetism, and their Curie temperature increases with the Fe concentration up to 294 K; meanwhile, magneto-transport measurements reveal a giant magnetoresistance (GMR) effect of over 800% and an anomalous Hall effect (AHE), as well as semiconducting behaviors, in the samples. Further analysis suggests that the ferromagnetism comes from a hole-mediated process originating from substitutional Fe dopants in the SiGe matrix and this is enhanced by the tensile strain in the films. The synthesis and high-temperature ferromagnetism of Fe-doped SiGe thin films may play a key role in group IV-based spintronic applications.

High-temperature ferromagnetism and strong π-conjugation feature in two-dimensional manganese tetranitride

Two-dimensional (2D) magnetic materials have attracted tremendous research interest because of the promising application in the next-generation microelectronic devices. Here, by the first-principles calculations, we propose a two-dimensional ferromagnetic material with high Curie temperature, manganese tetranitride MnN4 monolayer, which is a square-planar lattice made up of only one layer of atoms. The structure is demonstrated to be stable by the phonon spectra and the molecular dynamic simulations, and the stability is ascribed to the π–d conjugation between π orbital of N=N bond and d orbital of Mn. More interestingly, the MnN4 monolayer displays robust 2D ferromagnetism, which originates from the strong exchange couplings between Mn atoms due to the π–d conjugation. The high critical temperature of 247 K is determined by solving the Heisenberg model using the Monte Carlo method.

Emergent Magnetic States and Tunable Exchange Bias at 3d Nitride Heterointerfaces.

Interfacial magnetism stimulates the discovery of giant magnetoresistance (MR) and spin-orbital coupling across the heterointerfaces, facilitating the intimate correlation between spin transport and complex magnetic structures. Over decades, functional heterointerfaces composed of nitrides have seldom been explored due to the difficulty in synthesizing high-quality nitride films with correct compositions. Here, the fabrication of single-crystalline ferromagnetic Fe3 N thin films with precisely controlled thicknesses is reported. As film thickness decreases, the magnetization dramatically deteriorates, and the electronic state changes from metallic to insulating. Strikingly, the high-temperature ferromagnetism is maintained in a Fe3 N layer with a thickness down to 2u.c. (≈8Å). The MR exhibits a strong in-plane anisotropy; meanwhile, the anomalous Hall resistivityreverses its sign when the Fe3 N layer thickness exceeds 5u.c. Furthermore, a sizable exchange bias is observed at the interfaces between a ferromagnetic Fe3 N and an antiferromagnetic CrN. The exchange bias field and saturation moment strongly depend on the controllable bending curvature using the cylinder diameter engineering technique, implying the tunable magnetic states under lattice deformation. This work provides a guideline for exploring functional nitride films and applying their interfacial phenomena for innovative perspectives toward practical applications.

Interlayer Coupling Induced Sharp Increase of the Curie Temperature in a Two-Dimensional MnSn Multilayer.

The MnSn monolayer synthesized recently is a novel two-dimensional ferromagnetic material with a hexagonal lattice, in which three Mn atoms come together to form a trimer, making it remarkably different from other magnetic two-dimensional materials. Most impressively, there occurs a sharp increase of the Curie temperature from 54 to 225 K when the number of layers increases from 1 to 3. However, no quantitative explanation has been reported in previous studies. Herein, by means of the first-principles calculation method and the Monte Carlo method, we demonstrate that strong interlayer ferromagnetic coupling plays an essential role in enhancing its critical temperature, which acts as a magnetic field to stabilize the ferromagnetism in MnSn multilayers. Our work not only explains the sharp increase of the Curie temperature of the MnSn film in experiments but also reveals that the interlayer coupling is a new routine to achieve high-temperature ferromagnetism in two-dimensional materials.

Anomalous resistivity upturn in the van der Waals ferromagnet Fe5GeTe2

FenGeTe2 (n = 3, 4, and 5) has recently attracted increasing attention due to its two-dimensional van der Waals characteristic and high temperature ferromagnetism, which makes promises for spintronic devices. A Fe(1) split site is an important structural characteristic of Fe5GeTe2, which makes it very different from other FenGeTe2 (n = 3 and 4) systems. The local atomic disorder and short-range order can be induced by the split site. In this work, high-quality van der Waals ferromagnet Fe5GeTe2 single crystals were grown to study low-temperature transport properties. We found a resistivity upturn below 10 K. The temperature and magnetic field dependence of the resistivity are in good agreement with a combination of the theory of disorder-enhanced three-dimensional electron–electron and single-channel Kondo effect. The Kondo effect exists only at low magnetic fields B<3 T, while electron–electron interaction dominates the appearance for the low-temperature resistivity upturn. We believe that the enhanced three-dimensional electron–electron interaction in this system is induced by the local atomic structural disorder due to the split site of Fe(1). Our results indicate that the split site of Fe plays an important role for the exceptional transport properties.

Prediction of single-atom-thick transition metal nitride CrN4 with a square-planar network and high-temperature ferromagnetism

Single-atom-thick two-dimensional (2D) materials such as graphene usually have a hexagonal lattice while the square-planar lattice is uncommon in the family of 2D materials. Here, we demonstrate that single-atom-thick transition metal nitride ${\mathrm{CrN}}_{4}$ monolayer is a stable free-standing layer with a square-planar network. The stability of square-planar geometry is ascribed to the combination of N=N double bond, Cr-N coordination bond, and $\ensuremath{\pi}\ensuremath{-}d$ conjugation, in which the double $\ensuremath{\pi}\ensuremath{-}d$ conjugation is rarely reported in previous studies. This mechanism is entirely different from that of the reported 2D materials, leading to lower formation energy and more robust stability than the synthesized $g\ensuremath{-}{\mathrm{C}}_{3}{\mathrm{N}}_{4}$ monolayer. On the other hand, the ${\mathrm{CrN}}_{4}$ layer has a ferromagnetic (FM) ground state, in which the FM coupling between two Cr atoms is mediated by electrons of the half-filled large $\ensuremath{\pi}$ orbitals from $\ensuremath{\pi}\ensuremath{-}d$ conjugation. The high-temperature ferromagnetism in ${\mathrm{CrN}}_{4}$ monolayer is confirmed by solving the Heisenberg model with the Monte Carlo method.