High Curie Temperature Ferromagnetism Research Articles

CrO<sub>2</sub> monolayer: a two-dimensional ferromagnet with high Curie temperature and half-metallicity

Owing to the complete spin-polarization of electronic states near Fermi energy, half-metallic ferromagnets, especially two-dimensional half-metallic ferromagnets, have garnered significant attention in the field of spintronics. However, the practical applications of these materials are greatly hindered by their low Curie temperatures. Therefore, the exploration of high Curie temperature half-metallic ferromagnets poses a necessary and challenging task. In this study, we predict a two-dimensional transition metal oxide, CrO<sub>2</sub> monolayer, and employ first-principles calculations to investigate the crystal structure, electronic properties, magnetic ground state, and ferromagnetic phase transition. The calculations of phonon spectrum, elastic constant, and molecular dynamics simulations indicate that CrO<sub>2</sub> monolayer is dynamically, mechanically, and thermally stable. The convex hull diagram of Cr-O systems shows that the hull energy of the predicted CrO<sub>2</sub> layer is only 0.18 eV, further confirming the structural stability and large possibility for experimental fabrication. More importantly, the electronic and magnetic properties of CrO<sub>2</sub> monolayer demonstrate that it is a two-dimensional ferromagnetic half-metal with wide band gap. Five d suborbitals are divided into E<sub>g</sub> and T<sub>2g</sub> orbitals because of the crystal field of Cr atom in the center of O tetrahedron, and the spin-polarizations of E<sub>g</sub> orbitals make a major contribution to the moment around Cr atom. The ferromagnetic coupling along Cr-O-Cr chain is dominated by the superexchange interaction bridged by O 2p orbitals, similar to the typical Mn-O-Mn superexchange model. The magnetic behavior of the Cr spin lattice in a CrO<sub>2</sub> monolayer is described by a two-dimensional Heisenberg model, in which the exchange coupling anisotropy is ignored and the single ion anisotropy is the main consideration. By solving the Heisenberg model through using the Monte Carlo simulation method, the Curie temperature is determined to be over 400 K. The high Curie temperature ferromagnetism is rare in two-dimensional ferromagnetic materials and even rarer in semi-metallic materials, which makes it an ideal material for fabricating spintronic devices and studying spin quantum effects.

High Curie temperature and half-metallic ferromagnetism in Cr- and V-doped ZnSe in wurtzite phase: First-principles study

The electronic and magnetic properties of Cr-, V-doped, and co-doped ZnSe in the wurtzite structure have been calculated within LSDA + U method. The ab-initio calculations are carried out using Atomistic ToolKit code, based on the density functional theory. The band structure and density of states calculations showed the half-metallic behavior for 12.5%, 6.25%, 3.125%, 1.5625% concentrations in ZnSe:Cr, ZnSe:V and ZnSe:Cr,V. The magnetic moments of Zn1-xCrxSe and Zn1-xVxSe supercells are 4 and 3 μB, respectively. Co-doping greatly raises the ferromagnetism and the value of total magnetic moment of the supercell is 7 μB. The defect formation energies were calculated and obtained that ferromagnetic state is more stable than antiferromagnetic state. The Curie temperatures are estimated for Zn1-xCrxSe and Zn1-xVxSe supercells for all studies concentrations. The results indicate that these materials are high Curie temperature DMSs. First-principles studies show that Cr- and V-doped ZnSe are promising materials for spintronics.

The high Curie temperature and long-range ferromagnetism in Mn-doped 3C-SiC: a study using first-principles calculation

The high Curie temperature and long-range ferromagnetism in Mn-doped 3C-SiC: a study using first-principles calculation

High Curie Temperature Ferromagnetism and High Hole Mobility in Tensile Strained Mn‐Doped SiGe Thin Films

AbstractDiluted magnetic semiconductors based on group‐IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn‐doped SiGe thin films are first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples become ferromagnetic semiconductors, in which the Curie temperature increases with increasing Mn doping concentration and reaches 280 K with 5% Mn concentration. The data suggest that the ferromagnetism comes from the hole‐mediated process and is enhanced by the tensile strain in the SiGe crystals. Meanwhile, the Hall effect measurement up to 33 T to eliminate the influence of anomalous Hall effect reveals that the hole mobility of the annealed samples is greatly enhanced and the maximal value is ≈1000 cm2 V−1 s−1, owing to the tensile strain‐induced band structure modulation. The Mn‐doped SiGe thin films with high Curie temperature ferromagnetism and high hole mobility may provide a promising platform for semiconductor spintronics.

Families of magnetic semiconductors — an overview

The interplay of magnetic and semiconducting properties has been in the focus for more than a half of the century. In this introductory article we briefly review the key properties and functionalities of various magnetic semiconductor families, including europium chalcogenides, chromium spinels, dilute magnetic semiconductors, dilute ferromagnetic semiconductors and insulators, mentioning also sources of non-uniformities in the magnetization distribution, accounting for an apparent high Curie temperature ferromagnetism in many systems. Our survey is carried out from today's perspective of ferromagnetic and antiferromagnetic spintronics as well as of the emerging fields of magnetic topological materials and atomically thin 2D layers.

Defect-related high temperature ferromagnetism in mechanically milled hexagonal boron nitride nanoplates

We report a high Curie temperature ferromagnetism in milled hexagonal boron nitride (h-BN) nanoplates. The nanoplates with different sizes were synthesized via high-energy ball milling method. The magnetization versus magnetic field curves clearly indicate that the milled h-BN nanoplates exhibit distinct ferromagnetism, and their saturation magnetization increases monotonically from 0.015 to 0.032 emu/g with increasing of milling time at room temperature, while the bulk BN powders present diamagnetism. Besides, the temperature-dependent magnetization curve shows that the Curie temperature of the milled h-BN nanoplates is greater than 900 K. Meanwhile, Combined with the results of X-ray diffraction, high-resolution transmission electron microscope, X-ray photoelectron spectroscopy, Raman, and Electron spin resonance, the ferromagnetic behaviors of milled h-BN samples are attributed to nitrogen defects on the surface. Furthermore, the nitrogen defects on surface of h-BN can induce ferromagnetic states, which can be confirmed by the first principle calculation. The performance of high temperature ferromagnetism in h-BN material makes it promising candidates for application in spintronic devices.

First-principles prediction of the control of magnetic properties in Fe-doped GaSb and InSb

Recently, Fe-doped semiconductors have been attracting much attention as ferromagnetic semiconductors due to the possibility that they may exhibit high Curie temperatures and low power consumption and that they may be useful for high-speed spin devices. High Curie temperature ferromagnetism has been observed in Fe-doped InAs, from which both n- and p-type ferromagnetic semiconductors can be fabricated. In order to obtain a higher Curie temperature than that of (In, Fe)As, we have focused on GaSb and InSb as host semiconductors. We have investigated their electronic structures, magnetic properties, and structural stability by using the Korringa-Kohn-Rostoker Green's function method within density functional theory. We have found that (Ga, Fe)Sb and (In, Fe)Sb show complex magnetic properties, which are determined by the correlation between magnetic exchange coupling constants and chemical pair interactions. Isoelectronic Fe-doped GaSb and InSb have strong antiferromagnetic interactions due to the super-exchange mechanism. By shifting the Fermi level–i.e., by n- or p-type doping–(Ga, Fe)Sb and (In, Fe)Sb can be made to undergo a magnetic transition from antiferromagnetic to ferromagnetic ordering. This transition can be well understood in terms of the Alexander-Anderson-Moriya mechanism. Our calculations indicate the possibility of manipulating (Ga, Fe)Sb and (In, Fe)Sb to achieve high Curie temperatures.

Electronic structure, stability, and magnetism of rutile-type ultrathin TiO2 nanotubes

In this study, based on density functional theory, we predicted the electronic structure, stability, and magnetism of ultrathin rutile-type TiO2 nanotubes (u-TiO2NTs) with different curvatures. The curvature energy calculations showed that the (m, 0) u-TiO2NTs follow the classical elasticity theory. As the diameter of the NTs increases, the wall thickness becomes thinner, and the binding energy and band gap increase until saturation. For the (10, 0) u-TiO2NT, the neutral Ti vacancies (VTi0) could induce a large magnetic moment and long-range ferromagnetic coupling, whereas the neutral O vacancies (VO0) cannot. In addition to calculations of the vacancy formation energy, we investigated the magnetism and magnetic coupling of the (10, 0) u-TiO2NT with VTi− or VO+. We found that the introduction of VTi− vacancies led to unstable ferromagnetic coupling in the NTs, whereas the VO+ vacancies preferred to couple in a stable ferromagnetic manner. Our results suggest a possible route for obtaining high Curie temperature ferromagnetism in TiO2 materials.

Investigation on electronic and magnetic properties of Co and Mn in ZnO with different doping types

The magnetic moments of five different doping configurations of Co, Mn, and Co-Mn codoped ZnO were investigated by using first-principles based on density functional theory (DFT), respectively. The total energy, electronic structure, density of state (DOS), partial density of states (PDOS), spatial spin density distribution and Curie temperature of doping ZnO were also studied and analyzed. The energy calculation results show that Co doped ZnO performance ferromagnetism and antiferromagnetism, Mn doped ZnO exhibits only antiferromagnetism, and the Curie temperature of both Mn and Co doping cannot reach the room temperature, but Co-Mn codoping can obtain a high Curie temperature and better ferromagnetism. The electronic structures illustrate that compact structures show better room temperature ferromagnetism. The DOS and PDOS illustrate the magnetic moment is mainly consisted of exchange interaction between Co and Mn, and there are a little exchange interaction between Mn and O atoms. It can be inferred that Co-Mn codoping is a better dopant than Co and Mn in the field of room temperature ferromagnetic materials.

Clustering and spin interactions in Fe-doped diamond

The origin of high Curie temperature ferromagnetism in dilute magnetic semiconductors and oxides has often been attributed to clustering and crystallographic phase separation of magnetic atoms, which may have a detrimental impact on the properties of the host material for target applications. We present Density Functional Theory calculations on the stability and magnetic interactions of embedded Fe+1 ions in diamond by considering various possible cluster configurations. We find that Fe ions have a strong tendency to form embedded clusters in diamond, with larger cluster sizes (n>3) suppressing the induced spin moment. Also, we find that the electronic structure of the stable embedded Fe+1 clusters is insulating, in contrast to homogeneous distribution, where a half metallic character with 100% spin polarisation at the Fermi level has previously been predicted. These results present important implications to the understanding of the properties of transition metal dopants in diamond, as well as in other dilute magnetic semiconductors where the effect of aggregation of dopants has generally been neglected.

The origin of magnetism induced by intrinsic defects in anatase-type ultrathin TiO2 nanotube

The structural, electronic and magnetic properties of intrinsic defects in anatase-type ultrathin TiO2 nanotube have been investigated systematically by first-principles calculations. The neutral Ti vacancies (VTi0) could induce a large magnetic moments and long-range ferromagnetic coupling, nevertheless, it is difficult to create a significant amount of VTi0 in nanotube due to its higher formation energy. The neutral oxygen vacancies (VO0) are more easily to form than VTi0 vacancies, and the type of their magnetic coupling is related not only to the kinds of oxygen vacancies but also to the distance of vacancies, and their antiferromagnetic couplings are stable in most cases. Thus, the monovalence positively- and negatively-charged vacancy defects (VO+, VO-, VTi+, and VTi-) in nanotube are predicted. By comparing the formation energies of these neutral and charged vacancies, the VO+ vacancies are more easily formed in nanotube under O-poor condition, and their ferromagnetic couplings are energetically favorable. The VTi- vacancies are more stable under O-rich condition, and they prefer to couple in a antiferromagnetic way. Our work offers a possible route toward high Curie temperature ferromagnetism in TiO2 materials.

Probing the Role of Dopant Oxidation State in the Magnetism of Diluted Magnetic Oxides Using Fe-Doped In2O3 and SnO2 Nanocrystals

Investigation of the origin of high-Curie temperature ferromagnetism in diluted magnetic oxides has become one of the focal points of research on solid-state magnetism. While several possible mechanisms have been proposed theoretically, broader experimental evidence is still lacking. Here we report a comparative study of the electronic structure and magnetic properties of colloidal Fe-doped In2O3 and SnO2 nanocrystals, as building blocks for grain-boundary-rich diluted magnetic oxide films. The dopant ions in both nanocrystal host lattices are principally in 3+ oxidation state, with possibly a minor presence of Fe2+ in In2O3, and no conclusive evidence of the presence of Fe2+ in SnO2 nanocrystals. Subsequently, we found that Fe-doped In2O3 nanocrystalline films exhibit only minor ferromagnetic ordering (with a magnetic moment of less than ca. 0.1 μB/Fe) and decreasing saturation magnetization with increasing doping concentration at room temperature. The saturation magnetic moment of Fe-doped SnO2 nanocrys...

Strain Manipulated Magnetic Properties in ZnO and GaN Induced by Cation Vacancy

The effects of isotropic strains on the magnetic properties in ZnO and GaN induced by cation vacancies are comparatively investigated by density functional theory calculations. The magnetic moments and the couplings between vacancies in different charged states are calculated as a function of strains. The modulation of strain on the magnetic properties relies on the materials and the charge states of cation vacancies in them. As the occurrence of charge transfer in ZnO:V Zn under compression, the coupling between $$V_{\rm{Zn}}^{0} $$ is antiferromagnetic (AFM) and it could be stabilized by strains. Tensions can strengthen the ferromagnetic (FM) coupling between $$V_{\rm{Zn}}^{0} $$ but weaken that of $$V_{\rm{Ga}}^{ - } $$ . The neutral V Ga are always AFM coupling under strains from −6 to +6% and could be stabilized by compressions. The interactions between $$V_{\rm{Ga}}^{ - } $$ are always FM with ignorable variations under strains; however, the FM couplings between $$V_{\rm{Ga}}^{2 - } $$ could be strengthened by compressions. These varying trends of magnetic coupling under strains are interpreted by the band coupling models. Therefore, strain-engineering provides a route to manipulate and design high Curie temperature ferromagnetism derived and mediated by intrinsic defect for spintronic applications.

Realization of high Curie temperature ferromagnetism in atomically thin MoS2 and WS2 nanosheets with uniform and flower-like morphology.

High Curie temperature ferromagnetism has been realized in atomically thin MoS2 and WS2 nanosheets. The ultrathin nanosheet samples were prepared via a novel, simple and efficient chemical vapor deposition method; different kinds of transition metal disulfides (MoS2 and WS2) could be obtained by sulphuring the corresponding cation sources (MoO3 and WCl6). Through related morphological and structural characterization, we confirm that large-area, uniform, few-layer MoS2 and WS2 nanosheets were successfully synthesized by this method. Both nanosheet samples exhibit distinct ferromagnetic behavior. By careful measurement and fitting of the magnetization of MoS2 and WS2 samples at different temperatures, we deconstruct the magnetization into its diamagnetic, paramagnetic and ferromagnetic contributions. The ferromagnetic contributions persist until 865 K for MoS2 and 820 K for WS2. We attribute the observed ferromagnetic properties to the defects and dislocations produced during the growth process, as well as the presence of edge spins at the edge of the nanosheets.

The study of high Curie temperature ferromagnetism properties in Mn-doped SiC thin film

Mn-doped 3C-SiC film has been prepared onto the Si (111) substrate by employing a molecular beam epitaxy method. The experimental analysis establishes that the prepared sample shows the ferromagnetic property with a relatively high Curie temperature (Tc) of 355K, which is an exciting phenomenon on account of the scarceness in the SiC-based diluted magnetic semiconductor. The analysis derived from the X-ray diffraction and absorption spectroscopy patterns indicates that Mn atoms should react with Si atoms and then form Mn4Si7 compounds. Combined with the theoretical simulation, it is speculated that a new alloy phase of Mn4Si7Cx maybe appear, which should be responsible for the exceptionally high Tc ferromagnetic behavior in the sample.

Formation of different magnetic phases and high Curie temperature ferromagnetism in Fe57-implanted ZnO film

We investigated magnetic properties of ZnO thin film implanted with Fe 57 ions to the fluence of 1.00×10 17 ions/cm 2 . Both vibrating sample magnetometry and magneto-optical Kerr effect measurements revealed strong room temperature ferromagnetism with similar hysteresis loops. Temperature dependent measurements showed a very high Curie temperature around 850 K. Conversion electron Mössbauer spectroscopy experiments proved the existence of paramagnetic Fe +3 and Fe +2 ions and also the presence of substitutional Fe atoms in the hexagonal ZnO crystal resulting in intrinsic ferromagnetic order. • The implanted iron atoms form different magnetic phases in the host ZnO film. • Mössbauer data proved the existence of paramagnetic Fe +3 and Fe +2 ions and substitutional Fe atoms. • The observed RT ferromagnetism is attributed to the iron atoms occupying ZnO crystal lattice. • Temperature dependent measurements showed a very high T C around 850 K.

Possible ferromagnetism in Cd-doped TiO2: A first-principles study

The magnetic properties of Cd-doped TiO2 have been investigated by first-principles calculations. It is found that the doped system favors the spin-polarized state and high Curie-temperature ferromagnetism can be expected in it. The ferromagnetism can be attributed to the p-d hybridization between Cd and its surrounded oxygen atoms. Cd atoms do not tend to form clusters in TiO2. The doped system can be favorably synthesized in oxygen-rich condition. Moreover, Ti vacancies are much easier to form than oxygen vacancies in the doped system. We find that oxygen vacancies are harmful to the ferromagnetism of the doped system while Ti vacancies are beneficial to the stability of ferromagnetism.

The magnetism study of N-doped diamond

We perform the first-principles calculations to investigate the roles of C vacancy and nitrogen impurity in the magnetic properties of diamond. The coupling is ferromagnetic between the C vacancies in -2e charged state, whereas they prefer to interact antiferromagnetically in -e charged state. Substituting C with N atoms can manipulate the charge states of C vacancies and the magnetic interactions between them. Our work offers a possible route toward high Curie temperature ferromagnetism in metal-free diamond.

Effect of Ni doping on the microstructure and high Curie temperature ferromagnetism in sol–gel derived titania powders

Undoped, 0.05 and 0.5mol% Ni-doped TiO2 powders were prepared by a modified sol–gel route. The doping effects on the microstructure and magnetism for the powdered samples have been systematically investigated. Doping of Ni in TiO2 inhibited rutile crystal growth. The probable reason for this is discussed on the basis of band calculation based analysis of electronic structures of 3d transition metal-doped TiO2 and the energetic, transformation kinetics and phase stability of anatase over rutile as the function of particle size. Room temperature ferromagnetism (RTFM) with the saturation magnetization of 12memug−1 and Curie temperature as high as 820K is observed only in case of 0.05mol% Ni:TiO2 powdered sample, whereas undoped TiO2 was diamagnetic and 0.5mol% Ni:TiO2 was paramagnetic in nature. The role of any magnetic impurity or any Ni metal in the origin of the RTFM has been ruled out by energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analysis, whereas magnetic force microscopy (MFM) established the presence of magnetic domains, supporting the intrinsic diluted magnetic semiconductor behavior. The observed ferromagnetism has been attributed to the spin ordering through exchange interaction between holes trapped in oxygen orbitals adjacent to Ni substitutional sites.

Nanoscale inhomogeneities: A new path toward high Curie temperature ferromagnetism in diluted materials

Room temperature ferromagnetism has been one of the most sought after topics in today's emerging field of spintronics. It is strongly believed that defect- and inhomogeneity- free sample growth should be the optimal route for achieving room-temperature ferromagnetism and huge efforts are made in order to grow samples as "clean" as possible. However, until now, in the dilute regime it has been difficult to obtain Curie temperatures larger than that measured in well annealed samples of (Ga,Mn)As ($\sim$190 K for 12% doping). In the present work, we propose an innovative path to room-temperature ferromagnetism in diluted magnetic semiconductors. We theoretically show that even a very small concentration of nanoscale inhomogeneities can lead to a tremendous boost of the critical temperatures: up to a 1600% increase compared to the homogeneous case. In addition to a very detailed analysis, we also give a plausible explanation for the wide variation of the critical temperatures observed in (Ga,Mn)N and provide a better understanding of the likely origin of very high Curie temperatures measured occasionally in some cases. The colossal increase of the ordering temperatures by nanoscale cluster inclusions should open up a new direction toward the synthesis of materials relevant for spintronic functionalities.