Intrinsic Ferromagnetic Semiconductors Research Articles

First principles study of electrical and magnetic properties of two-dimensional ferromagnetic semiconductors CrI<sub>3</sub> adsorbed by atoms

Recent experimental discovery of intrinsic ferromagnetism (FM) in chromium triiodide (CrI<sub>3</sub>) monolayer opens a new way to low-dimensional spintronics. Two-dimensional (2D) CrI<sub>3</sub> monolayer is of great significance for its magnetic and electronic properties. Generally, surface atomic adsorption is an effective way to modify the physical properties of layered magnetic materials. Here in this work, we use the first-principles method based on density functional theory (DFT) to systematically study the electronic structure and magnetic properties of 2D CrI<sub>3</sub> monolayers that have adsorbed other metal atoms (specifically, alkali (alkaline earth) metal (Li, K and Mg), transition metal (Ti, V, Mn, Fe, Co and Ni) and non-metal (N, P, O and S) atoms). Our results show that the metal atoms tend to be adsorbed in the center of the ring formed by the six I atoms and stay at the same height as Cr atoms, while the positions of the optimized non-metal atoms are in the ring formed by the six I atoms and depend on the type of the atoms. The adsorption of atoms (except for Ti and Mn atoms) does not change the intrinsic ferromagnetic semiconducting properties of CrI<sub>3</sub> monolayer. The CrI<sub>3</sub> monolayers with Ti or Mn adsorption are antiferromagnetic semiconductors. Moreover, we find that the adsorption of different atoms regulates the local magnetic moments of Cr atoms. The adsorption of metal atoms increases the local magnetic moments of Cr atoms, but not exceeding 4<i>μ</i><sub>B</sub>. However, the adsorption of non-metallic atoms makes the local magnetic moments of Cr atoms diversified. The adsorption of O and N atoms retain the local magnetic moment of Cr atoms, while the adsorption of P and S atoms increase the local magnetic moment. By combining the projected density of states, we analyze in detail the local magnetic moments of Cr atoms. The increase of the local magnetic moments of Cr atoms is directly related to the charges transferring. Our results provide new ideas for regulating the performance of the magnetism of 2D intrinsic ferromagnetic semiconductor CrI<sub>3</sub>, which will have potential applications in the spintronics in the future.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d 5 − d 3 system can be significantly strengthened by reducing the virtual exchange gap (G ex) between occupied and empty e g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO3 (R = Sc, Y, and Lu) compounds with the T C that is as high as ∼1510 K (YMnO3). The oxygen breathing motions open a band gap and create an unusual Mn2+/Mn4+ charge ordering of the Mn-d electrons, which play an important role in altering the G ex. Interestingly, the rhombohedral RMnO3 compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO3. These results not only deepen the understandings of magnetic couplings in d 5 − d 3 system, but also provide a way to design room-temperature FM–FE multiferroics.

Magnetoresistance of epitaxial GdN films

We report magnetoresistance measurements on epitaxial films of the intrinsic ferromagnetic semiconductor GdN electron doped with ∼1020cm−3 to ∼1021cm−3. The magnetoresistance across the temperature range of 10–300 K is dominated by a reduction of spin-disorder scattering in the presence of a magnetic field, imposing a resistance reduction of 27% in a field of 8 T. We show that the magnetoresistance closely follows the magnetic disorder as signaled by the departure of the magnetization from its fully saturated value Ms of 7μB/Gd3+.

Intrinsic Ferromagnetic Semiconductors in Two-Dimensional Alkali-Based Chromium Chalcogenides

Two-dimensional (2D) intrinsic ferromagnetic semiconductors (FMSs) received huge attention due to their fascinating electron charge and spin properties, which are promising candidates for spintroni...

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (Tc), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding Tc is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

High-temperature intrinsic ferromagnetism in heavily Fe-doped GaAs layers

The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor (DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs substrates using a pulsed laser deposition in a vacuum. The transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy investigations revealed that the conductive layers obtained at 180 and 200 ºC are epitaxial, do not contain any second-phase inclusions, but contain the Fe-enriched columnar regions of overlapped microtwins. The TEM investigations of the non-conductive layer obtained at 250 ºC revealed the embedded coherent Fe-rich clusters of GaAs:Fe DMS. The x-ray photoelectron spectroscopy investigations showed that Fe atoms form chemical bonds with Ga and As atoms with almost equal probability and thus the comparable number of Fe atoms substitute on Ga and As sites. The n-type conductivity of the obtained conductive GaAs:Fe layers is apparently associated with electron transport in a Fe acceptor impurity band within the GaAs band gap. A hysteretic negative magnetoresistance (MR) was observed in the conductive layers up to room temperature (RT). MR measurements point to the out-of-plane magnetic anisotropy of the conductive GaAs:Fe layers related to the presence of the columnar regions. The studies of the magnetic circular dichroism confirm that the layers obtained at 180, 200 and 250 ºC are intrinsic ferromagnetic semiconductors and the Curie point can reach up to at least RT in case of the conductive layer obtained at 200 ºC. It was suggested that in heavily Fe-doped GaAs layers the ferromagnetism is related to the Zener double exchange between Fe atoms with different valence states via an intermediate As and Ga atom.

Flat-band splitting induced tunable magnetism in defective CrI3 monolayer

Chromium triiodide (CrI3) monolayer, as a two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductor, creates a large number of opportunities for the spintronic application of atomically thin magnets. Using density functional theory, we studied the electronic structures and carrier doping of CrI3 monolayer with vacancy defects. The energy band structures show that the flat-bands near the Fermi level in the spin-up conduction bands split into isolated flat-bands for the CrI3 monolayer with I vacancies, regardless of the vacancy concentration, whereas these flat-bands cross together in the perfect case. Further analysis suggests that the separation of the flat-bands results from the reduction of the Cr–I interaction induced by I vacancies. The introduction of I vacancies not only preserves the semiconducting feature, but also enhances the total magnetic moments of I-vacancy-contained CrI3, and the enhancement of magnetic moments is closely related with the number of vacancies. With the help of the flat-band splitting, the carrier doping could be used to further tune the magnetic moments, and simultaneously maintain the semiconducting characteristic in the I-vacancy-contained CrI3. Thus, our findings extend the realistic application of CrI3 monolayer in spintronics.

Magnon-bipolar carrier drag thermopower in antiferromagnetic/ferromagnetic semiconductors: Theoretical formulation and experimental evidence

Quantized spin-wave known as magnon, a bosonic quasiparticle, can drag electrons or holes via sd exchange interaction and boost the thermopower over the conventional diffusive thermopower. P-type magnon-drag thermopower has been observed in both ferromagnetic and antiferromagnetic metallic and degenerate semiconductors. However, it has been less reported for intrinsic or n-type magnetic semiconductors; therefore, the impact of magnon-bipolar carrier drag on thermopower has remained unexplored. Here, a theoretical model for magnon-bipolar carrier drag thermopower is derived based on the magnon-carrier interaction lifetimes. The model predicts that the bipolar carrier drag thermopower becomes independent of both the carrier and magnon relaxation times. A proof of concept experiment is presented that confirms this prediction. We also report the observation of magnon-carrier drag thermopower in n-type and intrinsic ferromagnetic semiconductors experimentally. The p-type antiferromagnetic MnTe is doped with different amounts of Cr to produce non-degenerate and n-type semiconductors of various carrier concentrations. Cr dopants have a donor nature and create ferromagnetic-antiferromagnetic clusters due to the Cr3+ oxidation state. Heat capacity measurements confirm the presence of magnons in Cr-doped MnTe. It is shown that the magnon-drag thermopower is significantly reduced for 3%-5% Cr-doped samples due to bipolar drag effects and becomes negative for 14% and 20% Cr-doped MnTe due to dominant magnon-electron drag thermopower.

High-temperature ferromagnetic semiconductors: Janus monolayer vanadium trihalides

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are expected to stand out in the spintronic field. Recently, the monolayer VI$_{3}$ has been experimentally synthesized but the weak ferromagnetism and low Curie temperature ($T_C$) limit its potential application. Here we report that the Janus structure can elevate the $T_C$ to 240 K. And it is discussed that the reason for high $T_C$ in Janus structure originates from the lower virtual exchange gap between $t_{2g}$ and $e_{g}$ states of nearest-neighbor V atoms. Besides, $T_C$ can be further substantially enhanced by tensile strain due to the increasing ferromagnetism driven by rapidly quenched direct exchange interaction. Our work supports a feasible approach to enhance Curie temperature of monolayer VI$_{3}$ and unveils novel stable intrinsic FM semiconductors for realistic applications in spintronics.

Magnetic two‐dimensional layered crystals meet with ferromagnetic semiconductors

AbstractThe existence of intrinsic ferromagnetic semiconductors (FMSs) in two‐dimensional (2D) materials has been a long‐term concern and pursuit. Recent breakthroughs in the 2D FMSs, such as CrGeTe3 and CrX3 (X = Cl, Br, I) from bulk down to monolayer, have stimulated intensive researches on new physical phenomena and creative concepts. This minireview mainly summarizes recent progress of 2D intrinsic FMSs in theoretical side, and focuses on the ongoing strategies proposed to enhance ferromagnetism, involving the mechanisms of magnetic exchange interaction and the significance of magnetic anisotropy. Meanwhile, spin‐related multifunctionality with ultrathin FMSs and their van de Waals heterostructures in magnetoelectric, valleytronic, and nondissipative electronic technology are introduced, as well as the current challenges and the prospects in this field are discussed.image

Theoretical study of the structure and magnetism of Ga1−xVxSb compounds for spintronic applications

In this paper, the structural, electronic, and magnetic properties of Ga1−xVxSb compounds with various vanadium (V) contents (x) ranging from dilute doping to the extreme doping limit were systematically investigated by first-principles calculations. The results show that V atoms prefer to substitute for gallium (Ga) atoms, and the formation energy is lower under Sb-rich growth conditions than under Ga-rich growth conditions. Meanwhile, the SbGa antisite defects effectively decrease the energy barrier of the substitution process from 0.85 to 0.53 eV. The diffusion of V atoms in the GaSb lattice occurs through metastable interstitial sites with an energy barrier of 0.6 eV. At a low V concentration (x = 0.0625), V atoms prefer a homogeneous distribution with antiferromagnetic coupling among the V atoms. However, when x increases above 0.5, the magnetic coupling among V atoms changes to ferromagnetic coupling due to the enhanced superexchange interaction between the eg and t2g states of neighboring V atoms. At the extreme doping limit of x = 1.00, zinc blende VSb along with its analogs VAs and VP is an intrinsic ferromagnetic semiconductor that exhibits a large change in light absorption at the Curie temperature. The results indicate that Ga1−xVxSb compounds provide a platform to design next-generation electronic, spintronic, and optoelectronic devices.

Enhanced magnetic anisotropy and Curie temperature of the NiI2 monolayer by applying strain: a first-principles study.

Two-dimensional (2D) intrinsic ferromagnetic semiconductors with high magnetic anisotropy (MA) and Curie temperature (TC) are desirable for low-dimensional spintronic applications. We present here the structural stability, MA and TC of the semiconducting NiI2 monolayer under strain from -4% to 4% using first-principles calculations. The unstrained NiI2 monolayer exhibits an in-plane magnetic anisotropy energy of -0.11 meV per unit cell and a TC of 79 K. Most noteworthily, the in-plane MA and TC of the NiI2 monolayer are simultaneously enhanced under compressive strain; meanwhile, the NiI2 monolayer is still stable. In particular, when the compressive strain reaches -4%, the in-plane MA is more than three times higher than that in the unstrained system. Based on the second-order perturbation theory of spin-orbit coupling, the density of states and the orbital magnetic anisotropy contributions are analyzed, indicating that the compressive strain effect originates from the increase of the negative contribution from the spin-orbit coupling interaction between the opposite spin py and px orbitals of the I atom. This study provides a promising route for exploring new 2D ferromagnetic semiconductors with higher MA and TC.

4f conduction in the magnetic semiconductor NdN

We report the growth of films of the intrinsic ferromagnetic semiconductor NdN, and an investigation of their optical and transport properties. There is clear evidence of a strong anomalous Hall effect as expected from a 4f conduction channel, supported by an optical absorption into a 4f or 4f/5d hybridized tail at the base of the conduction band. The results reveal a heavy-fermion 4f/5d band lying where it can be occupied at controllable levels with nitrogen-vacancy donors.

Ultra-High-Temperature Ferromagnetism in Intrinsic Tetrahedral Semiconductors.

Ferromagnetic semiconductors exhibit novel spin-dependent optical, electrical, and transport properties, which are promising for next-generation highly functional spintronic devices. However, the possibility of practical applications is hindered by their low Curie temperature. Currently, whether semiconducting ferromagnetism can exist at room temperature is still unclear because of the absence of a solid physical mechanism. Here, on the basis of tight-binding model analysis and first-principles calculations, we report that ferromagnetism in a tetrahedral semiconductor originating from superexchange interactions can be strong enough to survive at room temperature because of the weakening of antiferromagnetic direct-exchange interactions. On the basis of the explored mechanism, a zinc-blende binary transition metal compound, chromium carbide, is predicted to be an intrinsic ferromagnetic tetrahedral semiconductor with a Curie temperature that is as high as ∼1900 K. These findings not only expand the understandings of magnetism in semiconductors but also are of great interest for room-temperature spintronic applications.

Sacrificial Templating: A Route to Europium-II Oxide (EuO) Particles with Arbitrary Shape Prepared Indirectly by Hostile Takeover

Shape–property correlations of nanocrystals have recently moved into focus in materials science research. Magnetic properties, for instance, depend strongly on shape. Because crystal morphology is determined by a certain set of lattice planes (hkl) representing the surfaces, the achievable shapes are set by crystallographic symmetry. For instance, for a cubic crystal (90° angles) system it is very hard to realize hexagonal crystal shapes (120° angles). Breaking this paradigm is, thus, highly challenging. Here, we present a synthesis concept suitable for the synthesis of atypical Europium(II) oxide (EuO) particles. EuO is interesting as it is one of few materials known that belong to the class of intrinsic ferromagnetic semiconductors. It is shown that ZnO nanoparticles (rod-like, hexagonal platelets, spherical, and dumbbells) act as a sacrificial template and can be converted to the corresponding EuO particles by preservation of structural features, when treated with Eu-vapor. The transformation proceeds ...

Theoretical prediction of two-dimensional CrOF sheet as a ferromagnetic semiconductor or a half-metal

Two-dimensional chromium oxide fluoride CrOF sheet was studied based on density functional theory. The investigation indicates that the CrOF sheet is an intrinsic ferromagnetic semiconductor. The corresponding Curie temperature is 150 K. In particular, the Curie temperature increases up to 410 K under hole doping and the CrOF sheet becomes a half-metal. The versatile electronic and magnetic properties indicate that the two-dimensional CrOF sheet can be a promising candidate for next-generation spintronic devices.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6

Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.

Theoretical prediction of a two-dimensional intrinsic double-metal ferromagnetic semiconductor MnCoO4

A two-dimensional double-metal oxide MnCoO4 was predicted to be an intrinsic ferromagnetic semiconductor by using density functional theory. The low cleavage energy 0.36 J·m−2, which is similar to that of graphene, indicates that it can be easily exfoliated. The bulk structure has an antiferromagnetic ground state while the ferromagnetic configuration is the ground state against two antiferromagnetic and three ferrimagnetic configurations in the two-dimensional structure. The spin flip gaps for valence and conduction bands are 0.41 and 0.10 eV calculated with the HSE06 density functional, which are much larger than the thermal energy at room temperature. The Curie temperature obtained from the Monte Carlo simulation is 40 K. Under 9% tensile strain, the spin flip gaps increase largely so that the spin flip can be suppressed. The direct antiferromagnetic coupling between the Mn and Co atoms reduces largely while the indirect ferromagnetic couplings between two Mn or two Co atoms mediated by the O atoms do not decrease much in the stretched structure. The Curie temperature increases to 230 K, higher than the dry ice temperature. Moreover, phonon dispersion indicates that the MnCoO4 is also stable under the tensile stain. Therefore, two-dimensional MnCoO4 could be a good candidate for low-dimensional spintronics.

Electron transport in heavily doped GdN

We report measurements of electron transport phenomena in the intrinsic ferromagnetic semiconductor GdN doped with $1.3\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ electrons. The conductivity, carrier concentration, and thermoelectric power are compared with expectations based on an LSDA+$U$ band structure. In the ferromagnetic state the carriers fill the majority-spin conduction band pockets to the bottom of the minority-spin band. The resistance implies an electron mobility of 18 ${\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at zero temperature, and in turn a mean-free path of 10--30 nm. Spin disorder scattering rapidly reduces the mobility near the 70 K Curie temperature (${T}_{\text{C}}$). The thermoelectric power is negative in the paramagnetic phase, as expected for a $n$-type conductor, with a magnitude that is in agreement with the Fermi energy implied by the band structure. The thermopower reverses sign to be positive in the ferromagnetic phase, which correlates with a strongly temperature-dependent electron diffusion from spin-disorder scattering that increases rapidly as the temperature rises toward ${T}_{\text{C}}$.

Novel two-dimensional ferromagnetic semiconductors: Ga-based transition-metal trichalcogenide monolayers.

Ga-based two-dimensional transition-metal trichalcogenides with tunable ferromagnetic semiconducting properties are explored by first-principles calculations. It is found that the CrGaTe3 monolayer is an intrinsic ferromagnetic semiconductor with an indirect bandgap of 0.3 eV and exhibits strong dynamical, thermal and mechanical stabilities in the free-standing form. Its Curie temperature of 71 K estimated by Monte Carlo simulations can be significantly enhanced up to near room-temperature (263 K) by 5% biaxial tensile strain at which its structure still remains dynamically stable, while compressive strain can turn the monolayer from a ferromagnetic semiconductor to an antiferromagnetic semiconductor and finally to a ferromagnetic metal with a Curie temperature of 473 K at -5% strain. It is revealed that the pronounced strain tunability originates from the competition between a variety of magnetic exchange interactions. The CrGaSe3 monolayer is found to exhibit similar intrinsic ferromagnetic semiconducting properties and strain tunability. The exceptional properties render Ga-based transition-metal trichalcogenides promising candidates for spintronic applications.