Room Temperature Ferromagnetism Research Articles

Two-dimensional Janus SVAN2 (A = Si, Ge) monolayers with intrinsic semiconductor character and room temperature ferromagnetism: tunable electronic properties via strain and an electric field.

In the context of developing next-generation information technology, two-dimensional materials with inherent ferromagnetism, a Curie temperature above room temperature, and significant magnetic anisotropy hold great promise. In this work, we employed first-principles calculations to investigate a novel two-dimensional Janus structure, namely SVAN2 (A = Si, Ge). Our findings reveal that these structures are not only dynamically and thermally stable, but also exhibit semiconductor properties alongside their ferromagnetic states. The Janus SVSiN2 monolayer exhibits an in-plane easy axis, while the SVGeN2 monolayer shows an out-of-plane easy axis, both characterized by a significant magnetic anisotropy energy (129 and 172 μeV, respectively). Notably, through Monte Carlo simulation, we found that the Curie temperature of the SVSiN2 monolayer is 330 K, which is higher than room temperature. Finally, by applying biaxial strain and an external electric field, we successfully regulated the electronic properties of the SVAN2 (A = Si, Ge) monolayers, enabling a transition from semiconductor to half-metallic behavior. These remarkable electronic and magnetic properties make the Janus SVAN2 (A = Si, Ge) monolayers promising candidate materials for spin electron applications.

Scrolling reduced graphene oxides to induce room temperature magnetism via spatial coupling of defects.

Due to its intriguing features and numerous applications, graphene has garnered a lot of interest in recent years. However, it is still very difficult to create graphene-based room-temperature magnets without transition metals or rare earth elements since pristine graphene is inherently diamagnetic due to the delocalized π bonding network. Herein, room-temperature ferromagnetism with a saturation magnetization of 0.93 emu g-1 (300 K) is achieved in defect-rich-reduced graphene oxide (DR-rGO) nanoscrolls by creating a spatial coupling of defects. The experiments and DFT calculations verify that spatial coupling of defects could enhance Rudermann-Kittel-Kasuya-Yosida interactions to induce magnetism in graphene. It displays high-efficiency electromagnetic wave absorption performance with a minimal reflection loss of -62.1 dB and an effective absorption bandwidth of 7.8 GHz (3.0 mm) thanks to greatly improved magnetism. This breakthrough serves as a building block for the creation of room-temperature magnetic carbon materials and expands their applications in many pertinent domains.

High spin polarization, large perpendicular magnetic anisotropy and room-temperature ferromagnetism by biaxial strain and carrier doping in Janus MnSeTe and MnSTe.

The emerging two-dimensional (2D) Janus systems with broken symmetry provide a new platform for designing ultrathin multifunctional spintronic materials. Recently, based on experimental monolayer MnSe2, ferromagnetism was predicted in Janus MnXY (X ≠ Y = S, Se, Te) monolayers; however, they exhibit low Curie temperatures and small magnetic anisotropic energies. To improve the Curie temperature and magnetic anisotropy, herein, we systemically explore the stability and electronic and magnetic properties of Janus MnSeTe and MnSTe monolayers under strain and carrier-doping using first-principles calculations and Monte Carlo simulations. It is found that both MnSeTe and MnSTe monolayers possess robustly high spin polarization with rational strain and carrier-doping. Both tensile strain and hole doping strengthen the ferromagnetic super-exchange interactions of the two nearest Mn atoms mediated by chalcogen atoms and exceedingly improve the perpendicular magnetic anisotropic energies (by up to 3.1 meV per f.u. for MnSeTe and 2.0 meV per f.u. for MnSTe). The Te-5p intraorbital hybridizations contributed to the main magnetic anisotropy. More remarkably, the tensile strain and hole doping collectively increase the Curie temperatures of MnSeTe and MnSTe to above and near room temperature (345 and 290 K, respectively). The present study reveals that Janus MnSeTe and MnSTe monolayers with robustly high spin polarization, room-temperature ferromagnetism and large perpendicular magnetic anisotropy are promising candidates for ultrathin multifunctional spintronic materials. This study will be of great interest for further experimental and theoretical explorations of 2D Janus manganese dichalcogenides.

Piezoelectricity and valley polarization in a semilithiated 2H-TiTe2 monolayer with near room-temperature ferromagnetism.

Two-dimensional ferromagnetic semiconductors with coupled valley physics and piezoelectric responses offer unprecedented opportunities to miniaturize low-power multifunctional integrated devices. Prompted by epitaxial fabrication of nonmagnetic 2H-TiTe2 monolayer on the Au(111) substrate, we predict through both density functional theory and Monte Carlo simulations that the semilithiated 2H-TiTe2 monolayer (Li@2H-TiTe2) is a stable near room-temperature semiconducting ferromagnet. Under an out-of-plane magnetization, Li@2H-TiTe2 exhibits a clean valley polarization up to 160 meV in its conduction band and a valley-contrasting Berry curvature due to the broken inversion and time-reversal symmetries, in favor of achievable anomalous valley Hall effect. Alternatively, the simultaneous charge, spin, valley Hall currents can be realized as well in the ferromagnetic system with circularly polarized light. Furthermore, the missing mirror symmetry generates a scarce vertical piezoelectricity as large as 0.89 pm V-1. These findings indicate that asymmetric surface functionalization by Li deposition on the 2H-TiTe2 monolayer opens up a vital avenue to predesign superior and tailored multifunctional materials.

The effect of Tb doping on the structural and magnetic properties of ZnO nanoparticles

Rare earth ions when incorporated in a semiconductor oxide lattice at nanoscale regime can affect the structural, optical as well as the magnetic properties of semiconductor host lattice. The change observed in the properties of the host lattice is due to the interaction between the dopant ions and host crystal. The structural and the morphological properties of Tb3+doped ZnO nanoparticles were studied using XRD, X-ray photoelectron spectroscopy and AFM studies. These experiments were performed on the doped nanoparticles to analyze the defects generated in the ZnO host lattice when a rare earth ion having ionic radii greater than that of Zn ion is doped in the ZnO nanoparticles. EDX measurement was used to estimate the elemental composition. An analysis of the XRD peak profile of the undoped and doped ZnO nanoparticle samples were done to evaluate the dislocation densities and the surface area. Both the parameters show an increase up to a certain concentration of Tb, which reveals the increase in the surface defects when Tb is introduced in ZnO host lattice. The strain calculated using Uniform deformation model from the XRD data also show an increase in the stress with the increase in Tb concentration which confirms the deformation of the host lattice. The magnetic study performed on the Pure ZnO nanoparticles shows a diamagnetic behaviour, while, Tb-doped ZnO nanoparticles exhibits room temperature ferromagnetism. The correlation between defects generated upon Tb-doping to the observed ferromagnetism, in the synthesized nanoparticles, has been reported.

Strong spin-lattice entanglement in cobaltites

Strongly correlated electronic system contains strong coupling among multi-order parameters and is easy to efficiently tune by external field. Cobaltite (LaCoO<sub>3</sub>) is a typical multiferroic (ferroelastic and ferromagnetic) material, which has been extensively investigated over decades. Conventional research on cobaltites has focused on the ferroelastic phase transition and structure modulation under stress. Recently, researchers have discovered that cobaltite thin films undergo a paramagnetic-to-ferromagnetic phase transition under tensile strain, however, its origin has been controversial over decades. Some experimental evidence shows that stress leads the valence state of cobalt ions to decrease, and thus producing spin state transition. Other researchers believe that the stress-induced nano-domain structure will present a long-range ordered arrangement of high spin states, which is the main reason for producing the ferromagnetism of cobalt oxide films. In this paper, we review a series of recent researches of the strong correlation between spin and lattice degrees of freedom in cobalt oxide thin films and heterojunctions. The reversible spin state transition in cobalt oxide film is induced by structural factors such as thin-film thickness, lattice mismatch stress, crystal symmetry, surface morphology, interfacial oxygen ion coordination, and oxygen octahedral tilting while the valence state of cobalt ions is kept unchanged, and thus forming highly adjustable macroscopic magnetism. Furthermore, the atomic-level precision controllable film growth technology is utilized to construct single cell layer cobaltite superlattices, thereby achieving ultra-thin two-dimensional magnetic oxide materials through efficient structure regulation. These advances not only clarified the strong coupling between lattice and spin order parameters in the strongly correlated electronic system, but also provided excellent candidate for the realization of ultra-thin room temperature ferromagnets that are required by oxide spintronic devices.

SYNTHESIS, ELECTRONIC STRUCTURE, AND MAGNETIC PROPERTIES OF NANOCRYSTALLINE OXYGEN-DEFICIENT TiO2–δ(B)

Nanocrystalline TiO2 polymorphs with nonstoichiometry, i.e. having intrinsic and/or impurity structural defects, attract considerable interest due to enhanced optical and catalytic activity, improved electronic properties, and existence of room-temperature ferromagnetism. Herein, due to combination of hydrothermal technology, ion exchange process, and high-temperature vacuum annealing, a gray-colored bronze titanium dioxide with a belt-like nanostructure has been successfully shynthesized. The electron paramagnetic resonance data indicate that coloring of bronze polymorph during vacuum annealing is caused by the appearance of F-centers within a crystal lattice (electron trapped in the anionic vacancies). X-ray diffraction, electron-microscopic studies, and adsorption measurements has no reveal obvious changes in phase composition, microstructure, and texture during vacuum heat treatment (in comparison with bronze TiO2 treteated under air). Spectrophotometry experiments illustrate a "narrowing" of the TiO2(B) band gap from 3.23 to 3.04 eV and an evolution of the optical absorption in the visible and near infrared ranges due to appearance of oxygen vacancies. The study of magnetic characteristics showed that TiO2–δ(B) has a ferromagnetic ordering at room temperature, and the nature of ferromagnetism in it is clearly defective (magnetic properties are determined by the exchange interaction between electrons located in the vacancy zone). At low temperatures (4 K) the magnetic properties of oxygen-deficient bronze titanium dioxide become similar to those for quasi-stoichiometric TiO2(B) prepared through annealing under oxidizing atmosphere (air).

First-principles study of room-temperature ferromagnetism in transition-metal doped H-SiNWs.

Hydrogen-saturated silicon nanowires (H-SiNWs) are the most attractive materials for nanoelectronics due to their special tunable electronic properties. The incorporation of magnetism in H-SiNWs can be extremely beneficial for a wide range of emerging spintronic devices, which can offer a more effective way to control spin. Here, we investigate the energetic stability, electronic properties, and magnetic properties of transition metal (TM), i.e., Fe and Mn doped Hydrogen-saturated silicon nanowires (TM:H-SiNWs) that have a diameter of 1 nm directed in (100), (110), and (111) facets using spin-polarized density functional theory (DFT). The calculations showed that the TM-doped H-SiNWs (TM:H-SiNWs) convince the electronic and magnetic alterations of H-SiNWs semiconductors. It can be ascertained that the total magnetization of the studied configurations is contributed by the hybridization between a localized p orbital of Si and a d orbital of the TM atoms. In addition, we report the Curie temperature of the TM:H-SiNWs using a mean-field approximation and a Monte Carlo simulation based on the Ising model. We obtain the above room temperature ferromagnetism in the (100) and (111) direction-oriented Mn:H-SiNWs. This study provides an in-depth knowledge of the properties of TM-doped H-SiNWs and can be used as a reference in silicon-based spintronic devices.

Large-scale fabrication and Mo vacancy-induced robust room-temperature ferromagnetism of MoSe2 thin films.

Molybdenum selenide (MoSe2) has recently attracted particular attention due to its room-temperature ferromagnetism (RTFM) and related spintronic applications. However, not only does the FM mechanism of MoSe2 remain controversial, but also the synthesis of MoSe2 thin films with robust RTFM is still an unmet challenge. Here it is shown that using polymer-assisted deposition under appropriate growth conditions, large-scale (4 cm × 4 cm) synthesis of MoSe2 thin films with robust RTFM and a smooth surface (roughness average ∼0.22 nm) is possible. A new record-high saturation magnetization (6.69 emu g-1) is achieved in the prepared MoSe2 thin films, about 5 times the previously reported record (1.39 emu g-1) obtained in 2H-MoSe2 nanoflakes. Meanwhile, the coercivity of the MoSe2 films can be tuned down to a new record-low value (5.0 Oe), one-tenth of the previously reported record. Notably, detailed analysis combining the experimental findings and calculation results shows that the robust RTFM mainly comes from the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction between the magnetic moments induced by abundant Mo vacancies (VMos) in the MoSe2 films. Our results give insights into the large-scale production and robust RTFM of MoSe2 thin films and may provide a platform for designing and fabricating spintronic materials and devices based on transition-metal dichalcogenides.

DFT calculations of Cu doped TiO2 ferromagnetic study

There is still some controversy about the room-temperature ferromagnetism generated by transition metal-doped TiO2 and its magnetic generation. In this paper, the samples with different doping ratio were prepared to verify the magnetism of doped TiO2, and then the geometry of copper-doped Ti16O32 supercell system was optimized by using the overall energy density function theory (DFT) and generalized gradient approximation (GGA). The results show that the doped energy band diagram shows electron self-selective cleavage near the Fermi energy level and the addition of Cu atoms does produce ferromagnetism. The magnitude of the induced magnetic moment is related to the distance of Cu atoms and oxygen vacancies. The system with a doped Cu-Cu distance of 3.82 A is more stable, and the ferromagnetism of the doped phase is more stable and lower 29 meV than the antiferromagnetic state in the absence of oxygen vacancies (Cu2Ti14O32), while a magnetic burst occurs in Cu2Ti14O30 after the formation of oxygen vacancies. Magnetic Cu-TiO2 may become a promising adsorbent to be applied in the field of pollutant control.

Supercritical CO2-induced room-temperature ferromagnetism in two-dimensional MoO3−x

With the use of supercritical CO2, two-dimensional defective MoO3−xcan achieve ideal ferromagnetic responses with the Curie temperature reaching over 380 K.

Band Gap Tuning in Transition Metal and Rare-Earth-Ion-Doped TiO2, CeO2, and SnO2 Nanoparticles

The energy gap Eg between the valence and conduction bands is a key characteristic of semiconductors. Semiconductors, such as TiO2, SnO2, and CeO2 have a relatively wide band gap Eg that only allows the material to absorb UV light. Using the s-d microscopic model and the Green's function method, we have shown two possibilities to reduce the band-gap energy Eg-reducing the NP size and/or ion doping with transition metals (Co, Fe, Mn, and Cu) or rare earth (Sm, Tb, and Er) ions. Different strains appear that lead to changes in the exchange-interaction constants, and thus to a decrease in Eg. Moreover, the importance of the s-d interaction, which causes room-temperature ferromagnetism and band-gap energy tuning in dilute magnetic semiconductors, is shown. We tried to clarify some discrepancies in the experimental data.

( Fe1−xNix)5GeTe2 : An antiferromagnetic triangular Ising lattice with itinerant magnetism

Based on first-principles calculations, an antiferromagnetic Ising model on a triangular lattice has been proposed to interpret the order of Fe$(1)$-Ge pairs and the formation of $\sqrt{3}$ $\times $ $\sqrt{3}$ superstructures in the Fe$_5$GeTe$_2$ (F5GT), as well as to predict the existence of similar superstructures in Ni doped F5GT (Ni-F5GT). Our study suggests that F5GT systems may be considered as a structural realization of the well known antiferromagnetic Ising model on a triangular lattice. Based on the superstructures, a Heisenberg-Landau Hamiltonian, taking into account both Heisenberg interactions and longitudinal spin fluctuations, is implemented to describe magnetism in both F5GT and Ni-F5GT. We unveil that frustrated magnetic interactions associated with Fe(1), tuned by a tiny Ni doping ($x \leq 0.2 $), is responsible for the experimentally observed enhancement of the $T_c$ to 478 K in Ni-F5GT. Our calculations show that at low doping levels, monolayer Ni-F5GT has almost the same magnetic phase diagram as that of the bulk, which indicates a pervasive beyond room temperature ferromagnetism in this Ni doped two-dimensional system.

Enhanced Room Temperature Ferromagnetism in Highly Strained 2D Semiconductor Cr2Ge2Te6

Emergent magnetism in van der Waals materials offers exciting opportunities in fabricating atomically thin spintronic devices. One pertinent obstacle has been the low transition temperatures (Tc) inherent to these materials, precluding room temperature applications. Here, we show that large structural gradients found in highly strained nanoscale wrinkles in Cr2Ge2Te6 (CGT) lead to significant increases of Tc. Magnetic force microscopy was utilized in characterizing multiple strained CGT nanostructures leading to experimental evidence of elevated Tc, depending on the strain percentage estimated from finite element analysis. Our findings are further supported by ab initio and DFT studies of the strained material, which indicates that strain directly augments the ferromagnetic coupling between Cr atoms in CGT, influenced by superexchange interaction; this provides strong insight into the mechanism of the enhanced magnetism and Tc.

Influence of surface roughness on magnetic properties of CoTbNi ternary alloy films

Rare earth-transition metal (RE-TM) amorphous alloys films with promising tunable perpendicular magnetic anisotropy are projected to be exploited for magnetic recording media and spintronic applications. Here variation of magnetic property of thin films of Co0·63Tb0·11Ni0.26, Co0·41Tb0·14Ni0.45 and Co0·32Tb0·08Ni0.60 ternary alloy fabricated on Si-substrate using dc magnetron sputtering at room temperature were studied. The x-ray diffraction pattern data confirms the amorphous nature of these alloy films. The average surface roughness (Sa and Ra) of the films is found to vary significantly with Ni content of the films. The room temperature field dependent magnetization measurement shows the room temperature ferromagnetism. The saturation magnetization and coercivity of the M-H curves are found to vary with Ni content as well as surface roughness of the films. However, the nature of variation of saturation magnetization and coercive field is in accordance with variation of surface roughness, indicating that film surface roughness play a vital role in tuning the magnetic properties of these thin films. Such tunable magnetic properties of these amorphous films make them good candidate for magnetic storage media applications.

Room temperature ferromagnetism in metal oxides for spintronics: a comprehensive review

Room temperature ferromagnetism in metal oxides for spintronics: a comprehensive review

Atomic arrangement of van der Waals heterostructures using X-ray scattering and crystal truncation rod analysis

Vanadium diselenide (VSe2) has intriguing physical properties such as unexpected ferromagnetism at the two-dimensional limit. However, the experimental results for room temperature ferromagnetism are still controversial and depend on the detailed crystal structure and stoichiometry. Here we introduce crystal truncation rod (CTR) analysis to investigate the atomic arrangement of bilayer VSe2 and bilayer graphene (BLG) heterostructures grown on a 6H–SiC(0001) substrate. Using non-destructive CTR analysis, we were able to obtain electron density profiles and detailed crystal structure of the VSe2/BLG heterostructures. Specifically, the out-of-plane lattice parameters of each VSe2 layer were modulated by the interface compared to that of the bulk VSe2 1T phase. The atomic arrangement of the VSe2/BLG heterostructure provides deeper understanding and insight for elucidating the magnetic properties of the van der Waals heterostructure.

Layer-Number-Independent Two-Dimensional Ferromagnetism in Cr3Te4

In a conventional magnetic material, a long-range magnetic order develops in three dimensions, and reducing a layer number weakens its magnetism. Here we demonstrate anomalous layer-number-independent ferromagnetism down to the two-dimensional (2D) limit in a metastable phase of Cr3Te4. We fabricated Cr3Te4 thin films by molecular-beam epitaxy and found that Cr3Te4 could host two distinct ferromagnetic phases characterized with different Curie temperatures (TC). One is the bulk-like "high-TC phase" showing room-temperature ferromagnetism, which is consistent with previous studies. The other is the metastable "low-TC phase" with TC ≈ 160 K, which exhibits a layer-number-independent TC down to the 2D limit in marked contrast with the conventional high-TC phase, demonstrating a purely 2D nature of its ferromagnetism. Such significant differences between two distinct phases could be attributed to a small variation in the doping level, making this material attractive for future ultracompact spintronics applications with potential gate-tunable room-temperature 2D ferromagnetism.

Room Temperature Ferromagnetism in Hydrogenated Janus CrSSe Monolayer Using Quantum Monte Carlo Simulation

Here, using first-principles calculations based on density-functional theory, we propose a novel member of the two-dimensional (2D) transitional metal dichalcogenide family known as a Janus CrSSe monolayer (denoted as CrSSe-ML). The 2D CrSSe-ML has a hexagonal crystal structure. The calculated phonon band structure suggests Janus CrSSe-ML is dynamically stable, and formation energy indicates thermodynamic stability. At the equilibrium lattice constant, it is reported that 2D Janus CrSSe-ML is a non-magnetic semiconductor with a direct bandgap of 0.93 eV (K–K). We found that compressive biaxial strain induces no local magnetic moment, while in contrast the tensile biaxial strain of 12% induces a magnetic moment of 2 μB and half-metallicity which is robust up to 20%. Furthermore, we studied the ferromagnetic (FM) and antiferromagnetic (AFM) coupling between Cr atoms in CrSSe-ML and found that the FM state is favorable over AFM by the amount of energy of 0.007 meV, which is less than 0.03 eV, so no room-temperature ferromagnetism is possible. At the end, we added the H atom at the most preferable Se top-site in a Janus CrSSe-ML and found that hydrogenated Janus CrSSe-ML is magnetic and half-metallic at the equilibrium lattice constant. We further studied FM and AFM calculation by considering fully hydrogenated 4 × 4 × 1 supercell of a Janus CrSSe-ML. We used quantum Monte Carlo simulations (MC) to estimate the Curie temperature (Tc) of hydrogenated CrSSe-ML under normal conditions. The calculated value of Tc is 553.96 K using the quantum MC simulations. Our calculations predicted that the 2D Janus CrSSe-ML material is a promising candidate for spintronic device applications above room temperature.

Induction of stable, room-temperature ferromagnetism in WO3 by doping with K, Li, and Na: Theoretical investigations

Inducing ferromagnetism in non-magnetic materials via doping with non-transition metals can provide persistent and stable magnetism toward spintronics application. Using density functional theory, we predict induced ferromagnetism (5 µB) in 3.13 at.% Li-, Na-, and K-doped monoclinic WO3. The strength of induced magnetism is influenced by the concentration of dopants. The induced magnetism is primarily contributed by the 2p orbital of O atom in doped-WO3, substantiated by spin-density analysis and Bader charge portioning. Cation substitutional doping, i.e., replacing a W+6 with monovalent alkaline metals like Li+, Na+, or K+, tends to inject five holes in the resultant system. The Curie temperatures computed using the mean field approximation exceeds room temperature for all the three systems, indicating persistent magnetism at room temperature. The stability of the magnetism at room temperature was confirmed via ab initio molecular dynamics simulations. The probable mechanism of magnetism and aspects related to the experimental feasibility of these systems have been studied. This work presents another avenue for exploration for the preparation of dilute magnetic semiconductors toward spintronics applications.