Magnetic Semiconductors Research Articles

Two-dimensional Heisenberg model with material-dependent superexchange interactions

The two-dimensional (2D) van der Waals ferromagnetic semiconductors, such as CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, and the 2D ferromagnetic metals, such as Fe$_3$GeTe$_2$ and MnSe$_2$, have been obtained in recent experiments and attracted a lot of attentions. The superexchange interaction has been suggested to dominate the magnetic interactions in these 2D magnetic systems. In the usual theoretical studies, the expression of the 2D Heisenberg models were fixed by hand due to experiences. Here, we propose a method to determine the expression of the 2D Heisenberg models by counting the possible superexchange paths with the density functional theory (DFT) and Wannier function calculations. With this method, we obtain a 2D Heisenberg model with six different nearest-neighbor exchange coupling constants for the 2D ferromagnetic metal Cr$_3$Te$_6$, which is very different for the crystal structure of Cr atoms in Cr$_3$Te$_6$. The calculated Curie temperature Tc = 328 K is close to the Tc = 344 K of 2D Cr$_3$Te$_6$ reported in recent experiment. In addition, we predict two stable 2D ferromagnetic semiconductors Cr$_3$O$_6$ and Mn$_3$O$_6$ sharing the same crystal structure of Cr$_3$Te$_6$. The similar Heisenberg models are obtained for 2D Cr$_3$O$_6$ and Mn$_3$O$_6$, where the calculated Tc is 218 K and 208 K, respectively. Our method offers a general approach to determine the expression of Heisenberg models for these 2D magnetic semiconductors and metals, and builds up a solid basis for further studies.

Gate-Controlled Magnetotransport and Electrostatic Modulation of Magnetism in 2D Magnetic Semiconductor CrPS4.

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2Dmagnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS4 -a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1eV-allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. The magnetoconductance, whichdepends strongly on gate voltage, is determined. reaching values as high as 5000% near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS4 multilayers employed in the study. The results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallicconductor.

First principles investigation of 3d transition metal doped SnO monolayer based diluted magnetic semiconductors

The two-dimensional (2D) SnO monolayer attracted capacious recognition on account of its incomparable applications in nanoelectronics. Diluted magnetic semiconductors (DMS) are realized via doping of transition metals (TM) in semiconductors for usage in spintronic devices. This work is carried out using first-principles method to substitute 3d TM impurity atoms (Sc, Ti, V, Cr, Co, Ni, Cu and Zn) in place of Sn cation sites in the SnO monolayers. The analysis of modification resulted after the doping process is carried out to investigate the changes in the structural, electronic and magnetic properties of the materials. The calculated formation energy is utilized to examine the structural stability of the doped materials. The material doped with Sc, Ti, V, Cr, Co and Cu showed magnetic behavior while no magnetization is detected in case of doping with Ni and Zn. The results are discussed in the light of calculated crystal field splitting and exchange interactions between the two impurity-doped atoms. The magnetic ground state and Curie temperature of the DMS are also estimated. The Sc, V and Co doping has a ferromagnetic ground state while Ti, Cr and Cu doped structures got the AFM ground state. The mixing of TM-3d orbitals with the host s and p states is considered responsible for magnetism and the magnetic moment. The outcomes of the study point out that the TM doped SnO monolayer based DMS may be helpful to realize the spin-TFT devices in the near future.

Two-dimensional GdI2/GeC van der Waals heterostructure: Bipolar magnetic semiconductor, high critical temperature and large magnetic anisotropy

The band alignment, intrinsic ferromagnetism, Dzyaloshinskii-Moriya interaction, magnetic anisotropy and critical temperature (TC) of a GdI2/GeC van der Waals (vdW) heterostructure were systematically investigated by first-principles calculations and Monte Carlo simulations. The proposed GdI2/GeC vdW heterostructure is a bipolar magnetic semiconductor with Type-I band alignment, large in-plane magnetic anisotropy energy (MAE) of − 0.912 mJ/m2 and high critical temperature of 466 K. In addition, under biaxial strains, the heterostructure transformed from semiconductor into metal, while the easy magnetization axis maintained in-plane and TC increased. The results indicated that the GdI2/GeC vdW heterostructure presented large MAE and high TC, having potential applications in spintronic devices.

Ab-initio study of electronic, magnetic, and optical properties of Ni mono-doped and (I, Ni) co-doped CdS nanowires

Diluted magnetic semiconductors typically display induced ferromagnetism and spin-dependent interactions at high Curie temperatures, which is advantageous for application in magneto-electronic devices. The causes of ferromagnetism and optical emission dynamics, which have complex microstructural and compositional properties, are still not fully understood. Here, using first-principles calculations, the impact of Ni(II) ion substitution on the electronic, optical, and magnetic properties of CdS NW has been studied for the first time. The results show that when the size of the nanowire increases, the bandgap and binding energy decrease. The most favorable site for Ni doping was found to be at the surface of the nanowire. The most stable configuration with Ni ions on the surface of the nanowire has AFM as its ground state, which may be described using the super-exchange mechanism. The ground state configuration shifts from AFM to FM due to the addition of the extra electron provided by iodine (I) co-doping, and the estimated Curie temperature is expected to be higher than the ambient temperature. The fundamental bandgap of CdS nanowire is redshifted with Ni(II) doping, and the observed d-d intra-band transition in the absorption spectra at the low energy side of the optical bandgap is well consistent with the experimental results. Furthermore, the optical bandgap and d-d intra-band transitions in the optical absorption spectra are correlated with the magnetic coupling between Ni(II) ions, and we find a red-shift/blue-shift of d-d intra-band transition peaks and bandgap in the FM/AFM coupled ions system, supporting the experimental observations. The enhanced optoelectronic and magnetic properties of (Ni, I) co-doped CdS nanowires suggest their possible application in spintronic and optoelectronic devices, such as remote sensing and photovoltaics.

Structural and electronic properties of substitutionally doped SnS2/WSe2 hetero-bilayer

Atom doping can efficiently modify the electronic properties of two-dimensional materials by adjusting the crystal structure and electronic states. Based on the density functional theory, we systematically investigate the influence of different doping atoms and different doping types on the structural, electronic, and magnetic properties of SnS2/WSe2 heterostructures. The calculation results indicate that single-atom doping can modify the electronic structure of SnS2/WSe2 heterojunction. The magnetic moments were found in D-SnS2/WSe2 (D = B, N, and Sb) and F–SnS2/WSe2 (FV, Mn, and Ta) heterobilayers. In the case of co-doping, the B–Mn, In–Mn, B–Re, In–Re, N–Mn, and N–Re co-doped SnS2/WSe2 systems transformed from a direct semiconductor to an indirect bandgap semiconductor with the increased bandgap. A wide range of band gaps will provide a promising application of doped SnS2/WSe2-based optoelectronic and magnetic semiconductor devices.

First-principles studies of charged defect states in intrinsic ferromagnetic semiconductors: the case of monolayer CrI3

Defects play significant roles in spin-current-related physical processes in intrinsic ferromagnetic semiconductors (FMSs), which are great promise for spintronics applications. However, current defect calculation methods cannot be used to investigate charged defects in FMSs due to the spin polarization of both the charged defect states and ionized carriers, which is not well treated in current defect calculation methods. In order to solve this problem, we propose a spin-distinguishable charge correction (SDCC) method that uses spin-polarized band edge charge density instead of spin-unpolarized uniform background charge density as the compensating charge for charged defects. We apply our method to study the defect properties of CrI3 monolayer and find it can be doped n-type under the Cr-rich growth condition but difficult to be doped p-type. The SDCC method proposed here is generally suitable for all FMSs, which will be useful for the studies of defect properties of magnetic semiconductors.

Structural evolution and coexistence of ferroelectricity and antiferromagnetism in Fe, Nb co-doped BaTiO3 ceramics

Multiferroics are materials that exhibit two or more primary ferroic properties within the same phase and have potential applications in sensors, spintronics and memory devices. Here, the dielectric, ferroelectric and magnetic properties of novel multiferroics derived from BaTi1−x(Fe0.5Nb0.5)xO3 (BTFN, 0.01 ≤ x ≤ 0.10) ceramics are investigated. Multiferroism in these ceramics is manifested by the coexistence of ferroelectric long-range ordering and antiferromagnetism. With increasing x-value, there is a structural evolution from a tetragonal perovskite to a mixture of tetragonal and cubic phases, accompanied by a decrease in the temperature of maximum permittivity. At room temperature, ferroelectric behaviour is evidenced by the presence of current peaks corresponding to domain switching in the current-electric field loops, while the observation of non-linear narrow magnetic hysteresis loops suggests dilute magnetism. The results indicate that in the x = 0.07 composition the antiferromagnetic order is established through an indirect super-exchange interaction between adjacent Fe ions.

Magnetic and Optical Properties of Dilute Magnetic Semiconducting (In0.9Mn0.1)2O3 Nanoparticles

Magnetic and Optical Properties of Dilute Magnetic Semiconducting (In0.9Mn0.1)2O3 Nanoparticles

Tailoring the magneto-electronic and optical properties of cobalt doped strontium titanate by first-principles calculations

Ferromagnetic semiconductors or half metals offer strategic advantage in spintronic applications due to their robust stray fields and ultrafast magnetic dynamics. Controlling their magnetization and readout of their magnetic state are essential for these applications but remain challenging. Herein, we present a density functional theory (DFT) based full potential linearized augmented plane wave (FP-LAPW) analysis of the magneto-optical properties of novel and most widely studied diamagnetic SrTiO3 (STO) doped with cobalt. Pure STO is non-magnetic wide band gap (3.25 eV) semiconductor. Our study shows that versatile transport and magnetic properties can be obtained in CoxSr1-xTiO3, ranging from magnetic semiconductor to half metallic ferromagnetic nature depending upon the dopant's concentration. Spin resolved electronic properties and non-zero values of calculated magnetic moments for doped supercells reveal that Co0·125Sr0·875TiO3 and Co0·25Sr0·75TiO3 are good DMSs while Co0·5Sr0·5TiO3 and Co0·75Sr0·25TiO3 are half metallic ferromagnets. The analysis of band structures and density of states reveal that partially filled Co-3d and O-2p states show hybridization. Moreover, calculated optical conductivity lie in the visible region of the order of 103 (Ω cm)−1 while reflectivity in this region is minimum. Our findings demonstrate Co doping as a powerful approach to control the magneto-optoelectronic physics in STO, which will stimulate further experimental research with promise for tailored magnetic and optoelectronic properties in Co doped STO systems.

Adsorption of Metal Atoms on SiC Monolayer

The electronic, magnetic, and optical behaviors of metals (M = Ag, Al, Au, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mn, Na, Ni) adsorbed on the SiC monolayer have been calculated based on density functional theory (DFT). The binding energy results show that all the M-adsorbed SiC systems are stable. All the M-adsorbed SiC systems are magnetic with magnetic moments of 1.00 μB (Ag), 1.00 μB (Al), 1.00 μB (Au), 1.01 μB (Bi), 1.95 μB (Ca), 1.00 μB (Co), 4.26 μB (Cr), 1.00 μB (Cu), 2.00 μB (Fe), 1.00 μB (Ga), 0.99 μB (K), 1.00 μB (Li), 3.00 μB (Mn), and 1.00 μB (Na), respectively, except for the Ni-adsorbed SiC system. The Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na-adsorbed SiC systems become magnetic semiconductors, while Bi, Ca, Co, K, and Li-adsorbed SiC systems become semimetals. The Bader charge results show that there is a charge transfer between the metal atom and the SiC monolayer. The work function of the K-adsorbed SiC system is 2.43 eV, which is 47.9% lower than that of pristine SiC and can be used in electron-emitter devices. The Bi, Ca, Ga, and Mn-adsorbed SiC systems show new absorption peaks in the visible light range. These results indicate that M-adsorbed SiC systems have potential applications in the field of spintronic devices and solar energy conversion photovoltaic devices.

Ferromagnetic coupling mechanism and vacancy defect regulation strategy of V-doped LiMgAs

First-principles calculations were conducted to investigate the electronic structure and magnetic coupling mechanism of V-doped LiMgAs. The regulation strategy of Li vacancy defects on magnetism was also adopted. The results showed that all the designed defective configurations exhibited thermodynamic stability evaluated from the formation energy. The V nearest neighbor doping configuration occupied the lowest formation energy of −9.26eV and performed the strongest stability. V dopant had advantages in magnetic introduction and the maximum atomic magnetic moment in this study was 6.40 μB/V. The ground state of the magnetic coupling was determined by the energy difference of the doping configurations for magnetic ions antiferromagnetic and ferromagnetic arrangements. The magnetic dopants existed in Li(MgV)As system by ferromagnetic coupling and the ferromagnetic stability weakened with the increase of the V atom separation. The incorporation of Li vacancy defect promoted the enhancement of ferromagnetism. The optimal design configuration was V atoms nearest neighbor doping with one Li vacancy defect with the energy difference of 0.27eV, which performed the strongest ferromagnetic stability. The removal of Li ion introduced the additional itinerant hole carrier and elevated the non-localized characteristics of the carriers in p-d hybrid orbitals, which facilitated the electron exchange between magnetic ions. In this paper, a novel type of dilute magnetic semiconductor with controllable carriers was designed and the mechanism of ferromagnetic coupling was revealed, which provided a theoretical reference for the subsequent studies.

Emergence of a Non-Van der Waals Magnetic Phase in a Van der Waals Ferromagnet.

Manipulation of long-range order in 2D van der Waals (vdW) magnetic materials (e.g., CrI3 , CrSiTe3 ,etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets that, in turn, affects the nanoelectronic /spintronic device performance. Counterintuitively, the current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr2 Te3 (TC2 ≈160K) in the parent vdW magnetic semiconductor Cr2 Ge2 Te6 (TC1 ≈69K). The coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magneto-transport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to the rather common poor environmental stability of the vdW magnets, the results open possibilities of finding air-stable novel materials having multiple magnetic phases.

Two-Dimensional Robust Ferromagnetic Semiconductors via Assembly of Magnetic Superatoms [Fe6S8(CN)6]5.

Cluster-assembled materials are of considerable interest owing to their unique properties and extensive application prospects. Nevertheless, the majority of cluster-assembled materials developed to date are nonmagnetic, limiting their applications in spintronics. Thus, two-dimensional (2D) cluster-assembled sheets with intrinsic ferromagnetism are very desirable. Here, via first-principles calculations, utilizing the recently synthesized magnetic superatomic cluster [Fe6S8(CN)6]5- as a building block, we design a series of thermodynamically stable 2D nanosheets [NH4]3[Fe6S8(CN)6]TM (TM = Cr, Mn, Fe, Co) with robust ferromagnetic ordering (Curie temperatures (Tc) up to 130 K), medium band gaps (from 1.96 to 2.01 eV), and sizable magnetic anisotropy energy (up to 0.58 meV per unit cell). Among these nanosheets, the [NH4]3[Fe6S8(CN)6]Cr is a bipolar magnetic semiconductor, whereas the other three ([NH4]3[Fe6S8(CN)6]TM (TM = Mn, Fe, Co) are half semiconductors. Additionally, the electronic and magnetic properties of [NH4]3[Fe6S8(CN)6]TM (TM = Cr, Mn, Fe, Co) nanosheets can be easily modulated by electron and hole doping via simply controlling the number of ammonium counterions. Furthermore, the Curie temperatures of the 2D nanosheets can be improved to 225 and 327 K by choosing 4d/5d transition metals TM = Ru and Os, respectively.

Author Correction: NiTe Magnetic Semiconductor Nanorods for Optical Limiting and Hydrogen Peroxide Sensor

Author Correction: NiTe Magnetic Semiconductor Nanorods for Optical Limiting and Hydrogen Peroxide Sensor

The penta-hexa silicene: A promising candidate for intrinsic room temperature magnetic semiconductor

Performing ab initio calculations, we investigate electronic and magnetic properties of a silicon allotrope (PH-silicene) composed entirely by six silicon pentagons and two silicon hexagons. The dynamically and mechanically stable PH-silicene hosts two-dimensional honeycomb spin structures, which can be antiferromagnetic, ferromagnetic, or ferrimagnetic depending on the applied tensile strain and/or number of stacked layers. In particular, the transition temperature of an in-plane antiferromagnetic ground state and a strain-induced ferromagnetic state of monolayer PH-silicene is found to be around 533 and 80 K, respectively. This unusual metal-free magnetism can be explained by the d0 charge transfer mechanism. On the other hand, we show that the PH-silicene is an indirect semiconductor with the bandgap of 0.585 eV. When stacking up to 4-layers, they vary from the semiconductor, the semimetal to the normal metal. Our findings suggest PH-silicene as a promising candidate for the room temperature magnetic semiconductor and will pave a way for silicon based spintronic devices.

Metal atoms adsorbed Ga2O3 monolayer: As a potential application in optoelectronic devices

Density functional theory based on first principles has been used to analyze the electronic, magnetic, work function, and optical absorption characteristics of metal atoms adsorbed Ga2O3 monolayer structures. The outcomes demonstrate that the Pt–Ga2O3 and Zn–Ga2O3 systems remain nonmagnetic semiconductor characteristics, while the Ag–Ga2O3, K–Ga2O3, Li–Ga2O3 and Na–Ga2O3 systems exhibit nonmagnetic metal behaviors. Meanwhile, the Al–Ga2O3, Cr–Ga2O3, Fe–Ga2O3 and Ni–Ga2O3 systems reveal magnetic metal behaviors, and the Au–Ga2O3, Bi–Ga2O3, Ge–Ga2O3 and Sn–Ga2O3 systems show magnetic semiconductor characteristics. Whereas the Co–Ga2O3 system emerges magnetic semimetal behavior. Furthermore, the work function of Ga2O3 adsorbed by metals are reduced. Importantly, after the adsorption of metal atoms, the optical absorption of Ga2O3 systems has been improved to varying degrees in the visible and infrared regions, and many absorption peaks have appeared, indicating that the introduction of metal atoms can improve the optical absorption of the Ga2O3 systems in both visible and near-infrared regions. Moreover, spherical metal nanoparticles placed on top of Ga2O3 nanosheets can excite localized surface plasmon resonance, which will greatly enhance the absorption properties of the structure. Therefore, the above results suggest that the metal atoms adsorbed Ga2O3 systems will be useful for the fabrication of spintronics, field emission devices and solar photovoltaic devices.

Possible Room-Temperature Ferromagnetic Semiconductors

Magnetic semiconductors integrate the dual characteristics of magnets and semiconductors. It is difficult to manufacture magnetic semiconductors that function at room temperature. Here, we review a series of our recent theoretical predictions on room-temperature ferromagnetic semiconductors. Since the creation of two-dimensional (2D) magnetic semiconductors in 2017, there have been numerous developments in both experimental and theoretical investigations. By density functional theory calculations and model analysis, we recently predicted several 2D room-temperature magnetic semiconductors, including CrGeSe3 with strain, CrGeTe3/PtSe2 heterostructure, and technetium-based semiconductors (TcSiTe3, TcGeSe3, and TcGeTe3), as well as PdBr3 and PtBr3 with a potential room-temperature quantum anomalous Hall effect. Our findings demonstrated that the Curie temperature of these 2D ferromagnetic semiconductors can be dramatically enhanced by some external fields, such as strain, construction of heterostructure, and electric field. In addition, we proposed appropriate doping conditions for diluted magnetic semiconductors, and predicted the Cr doped GaSb and InSb as possible room-temperature magnetic semiconductors.

Emerging Enhancement and Regulation Strategies for Ferromagnetic 2D Transition Metal Dichalcogenides.

Two-dimensionaltransition metal dichalcogenides (2D TMDs) present promising applications in various fields such as electronics, optoelectronics, memory devices, batteries, superconductors, and hydrogen evolution reactions due to their regulable energy band structures and unique properties. For emerging spintronics applications, materials with excellent room-temperature ferromagnetism are required. Although most transition metal compounds do not possess room-temperature ferromagnetism on their own, they are widely modified by researchers using the emerging strategies to engineer or modulate their intrinsic properties. This paper reviews recent enhancement approaches to induce magnetism in 2D TMDs, mainly using doping, vacancy defects, composite of heterostructures, phase modulation, and adsorption, and also by electron irradiation induction, O plasma treatment, etc. On this basis, the produced effects of these methods for the introduction of magnetism into 2D TMDs are compressively summarized and constructively discussed. For perspective, research on magnetic doping techniques for 2D TMDs materials should be directed toward more reliable and efficient directions, such as exploring advanced design strategies to combine dilute magnetic semiconductors, antiferromagnetic semiconductors, and superconductors to develop new types of heterojunctions; and advancing experimentation strategies to fabricate the designed materials and enable their functionalities with simultaneously pursuing the upscalable growth methods for high-quality monolayers to multilayers.

Molecular beam epitaxy growth of Transition Metal Dichalcogenide (Mo,Mn)Se2 on 2D, 3D and polycrystalline substrates

Magnetic doping of 2D materials such as Transition Metal Dichalcogenides is promising for enhancement of magneto-optical properties, as it was previously observed for 3D diluted magnetic semiconductors. To maximize the effect of magnetic ions, they should be incorporated into the crystal lattice of 2D material rather than form separated precipitates. This work shows study on incorporating magnetic manganese ions into the MoSe2 monolayers using molecular beam epitaxy. We test growth on various substrates with very different properties: polycrystalline SiO2 on Si, exfoliated 2D hexagonal Boron Nitride flakes (placed on SiO2/Si), monocrystalline sapphire, and exfoliated graphite (on tantalum foil). Although atomic force microscopy images indicate presence of MnSe precipitates, but at the same time, various techniques reveal effects related to alloying MoSe2 with Mn: Raman scattering and photoluminescence measurements shows energy shift related to presence of Mn, scanning transmission microscopy shows Mn induced partial transformation of 1H to 1T’ phase. Above effects evidence partial incorporation of Mn into MoSe2 layer.