Bipolar Magnetic Semiconductor Research Articles

NbS2 Monolayers as Bipolar Magnetic Semiconductors for Multifunctional Spin Diodes and 3 nm Cold-Source Spin Field-Effect Transistors

NbS₂ Monolayers as Bipolar Magnetic Semiconductors for Multifunctional Spin Diodes and 3 nm Cold-Source Spin Field-Effect Transistors

Computational study on ferroelectric control over spin polarization in the bipolar magnetic semiconductor

Multiferroic van der Waals (vdW) heterostructures offer an exciting route toward the nanoelectronics and spintronics device technology. How to realize the mutual regulation between ferroelectric and magnetic materials has attracted extensive research. In this work, based on the density functional theory, we simulate a vdW multiferroic heterostructure based on the bipolar magnetic semiconductor material graphone and ferroelectric monolayer In2Te3 and further investigate its electronic properties. We find that direct contact between In2Te3 and graphone induces a transition in graphone from a ferromagnetic state to a non-magnetic state. Fortunately, the magnetic properties of graphone are preserved by using graphene as an intercalation layer, and the graphone monolayer changes from its original semiconductor to a half-metal in the graphone/graphene/In2Te3 vdW heterostructure for P↓ state. Furthermore, by adjusting the layer spacing of the heterostructure, the spin polarization states of graphone at the Fermi level (EF) are regulated between spin-up (S↑) and spin-down (S↓) with the reversal of ferroelectric polarization states. Our results not only provide a promising way to realize the half-metallicity in 2D magnetic materials but also computationally predict the ferroelectric control of the spin polarization state, which has great application potential in the next-generation nonvolatile electrically controlled spintronic devices.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking.

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

First-principles prediction of two-dimensional NiI[formula omitted] monolayer with adsorbed metal atoms

Monolayer NiI3 is a new two-dimensional (2D) intrinsic ferromagnetic Dirac half-metal material, and has attracted widespread attention due to its high Curie temperature and large magnetic anisotropy. In this work, we use first-principles calculations to study the physical properties of 2D NiI3 monolayer with adatom Y (Y=Li, Al, Mg, Ga). Our calculations show that Mg transforms the NiI3 monolayer from a half-metal to a bipolar magnetic semiconductor with a direct bandgap. While, the Li-NiI3, Al-NiI3, and Ga-NiI3 systems exhibit the band properties of 2D half-metals. In addition, the compressive strain can significantly increase the Curie temperatures of monolayer Li-NiI3 and Ga-NiI3. Especially under 1.5% compressive strain, the Mg-NiI3 system transitions from the ferromagnetic state to the antiferromagnetic state. Our study shows that atomic adsorption can optimize the properties of monolayer NiI3, and provides a theoretical basis for the use of atom adsorbed NiI3 system in experimental and device applications.

Two-Dimensional Room-Temperature Magnetism in Janus Mn2I3S3 and Cr2I3Se3 Monolayers with Tunable Magnetic Properties by Strain Engineering.

Exploring room-temperature intrinsic magnetism in two-dimensional (2D) materials for nanoscale spintronic devices has garnered significant interest. Achieving a high Curie temperature and substantial spin polarization in 2D ferromagnetic materials remains challenging. Drawing inspiration from the substantial enhancement of the Curie temperature observed in ferromagnetic CrIS monolayers by manipulating the covalent nature of Cr-S bonds, our study systematically delves into the electronic structure and magnetic properties of Janus M2X3Y3 (M = V, Cr, Mn, Fe, and Co; X = Cl, Br, I; Y = S, Se, and Te) monolayers through first-principles calculations. Our findings reveal that 15 kinds of these monolayers exhibit dynamic and thermodynamic stability while displaying diverse electronic and ferromagnetic characteristics. Notably, Mn2I3S3 demonstrates half-metallicity and in-plane magnetic anisotropy, while Cr2I3Se3 exhibits a half-semiconductor and perpendicular magnetic anisotropy. Consequently, Mn2I3S3 transforms from in-plane to perpendicular magnetic anisotropy through strain manipulation. Cr2I3Se3, under strain, transforms from a half-semiconductor to a bipolar magnetic semiconductor. The strong coupling caused by the M-Y bonds makes them have a Curie temperature higher than room temperature. The unique magnetic properties exhibited by the 2D Janus Mn2I3S3 and Cr2I3Se3 magnets hold promise for applications in spintronics. Our study provides a foundational understanding for future experimental explorations in this exciting research area.

Bipolar magnetic semiconductor and doping controllable spin transport property in 2D CoI2/MnBr2 heterostructure

The integration of two-dimensional heterostructure materials remains a fundamental way for the manipulation of spintronics in practical applications. Here, we predicted the transform of stripy antiferromagnetic (AFM) CoI2 and MnBr2 monolayers to interlayer AFM CoI2/MnBr2 heterostructure with intralayer ferromagnetic orders by using density functional theory. Interestingly, the CoI2/MnBr2 heterostructure exhibits a typical bipolar magnetic semiconducting state with type-I band alignments. Moreover, the half-metal/semiconductor transition and spin-up/spin-down polarization switching in CoI2/MnBr2 heterostructure can be effectively triggered by electron/hole doping. Our study provides the potential of AFM spintronics for information storage and processing.

Inducing abundant magnetic phases and enhancing magnetic stability by edge modifications and physical regulations for NiI2 nanoribbons.

Recently, a magnetic semiconducting NiI2 monolayer was successfully fabricated. To obtain richer magneto-electronic properties and find new physics for NiI2, we studied the zigzag-type NiI2 nanoribbon (ZNiI2NR) with edges modified by different concentrations of H and/or O atoms. Results show that these ribbons hold a higher energy stability, thermal stability, and magnetic stability, and the Curie temperature can be increased to 143 from 15 K for the bare-edged ribbons. They feature a half-semiconductor, bipolar magnetic semiconductor, or half-metal, depending on the edge-terminated atomic species and concentrations, and are closely related to the ribbon edge states, impurity bands or hybridized bands. By applying strain or an electric field, ribbons can achieve a reversible multi-magnetic phase transition among a bipolar magnetic semiconductor, half-semiconductor, half-metal, and magnetic metal. This is because strain changes the Ni-I bond length, resulting in a variation of bond configurations (weight of ionic and covalent bonds) and the number of unpaired electrons. The compressive strain can increase the Curie temperature because it makes the edged Ni-I-Ni bond angle closer to 90°, leading to the FM d-p-d superexchange interaction being increased. The electric field varies the magnetic phase because it alters the electrostatic potential of the ribbon edges, and the Curie temperature is enhanced under the electric field because the ribbon is changed to a metallic state (half-metal or magnetic metal), in which the magnetic Ni atoms satisfy the Stoner criterion and hold a large magnetic exchange coefficient and electron state density at the Fermi surface.

Influence of electronic correlation on the valley and topological properties of VSiGeP4 monolayer.

Valley is used as a new degree of freedom for information encoding and storage. In this work, the valley and topological properties of the VSiGeP4 monolayer were studied by adjusting the U value based on first-principles calculations. The VSiGeP4 monolayer remains in a ferromagnetic ground state regardless of the change in the U value. The magnetic anisotropy of the VSiGeP4 monolayer is initially in-plane, and then turns out-of-plane with the increase in the U value. Moreover, a topological phase transition is observed in the present VSiGeP4 monolayer with the increase in U value from 0 to 3 eV, i.e., the VSiGeP4 monolayer behaves as a bipolar magnetic semiconductor, a ferrovalley semiconductor, a half-valley metal characteristic, and a quantum anomalous Hall state. The mechanism of the topological phase transition behavior of the VSiGeP4 monolayer was analyzed. It was found that the variation in U values would change the strength of the electronic correlation effect, resulting in the valley and topological properties. In addition, carrier doping was studied to design a valleytronic device using this VSiGeP4 monolayer. By doping 0.05 electrons per f.u., the VSiGeP4 monolayer with a U value of 3 eV exhibits 100% spin polarization. This study indicates that the VSiGeP4 monolayer has potential applications in spintronic, valleytronic, and topological electronic nanodevices.

Theoretical prediction of C- and O-doped Hittorf’s violet phosphorene as bipolar magnetic semiconductor material

Hittorf’s violet phosphorene is a novel two-dimensional material with stable structure and excellent optoelectronic properties. Studying the doping effect helps to understand its physical essence and is of great significance in further developing nanoelectronic devices. In this paper, the first-principles method based on density functional theory is used to study the electromagnetic properties of the non-metallic element B-, C-, N-, and O-doped single-layer violet phosphene. The results show that there is no magnetism after having doped boron and nitrogen, and the system still behaves as a nonmagnetic semiconductor, while carbon doping and oxygen doping cause spin splitting, and the violet phosphorene transforms from a nonmagnetic semiconductor to a bipolar magnetic semiconductor, and its spin density is mainly distributed in the P atom and gap region, rather than on the impurity. The direction of spin polarization of its carrier can be reversed by adjusting the electric field of O-doped violet phosphorene. When a certain size of forward or reverse electrostatic field is applied, the band dispersion becomes stronger, and the O-doped violet phosphorene transforms into a half-metallic magnet with 100% downward or upward spin polarization at the Fermi level. The field effect spin filter based on O-doped violet phosphorene can reverse the direction of spin-polarized current by changing the direction of the gate voltage. This study shows that O-doped violet phosphorene is expected to be an ideal candidate material for two-dimensional spin field-effect transistors, bipolar magnetic spintronic devices, dual channel field effect spin filters, and field-effect spin valves.

First-principles prediction of two-dimensional Rare-earth intrinsic ferrovalley materials: Non-Janus GdXY (X≠Y=Cl,Br,I) monolayers

Two-dimensional ferrovalley magnetic materials have attracted much attention due to the applications in valley-based nonvolatile random access memories and valley filters. In this work, using first-principles calculations, we predict a promising class of bipolar magnetic semiconductors, namely non-Janus GdXY(X≠Y=Cl,Br,I) monolayers, which exhibit excellent mechanical and thermal stability, large magnetic moment (8 μB/Gd), and high Curie temperature (above 450 K). When magnetized along the ±z direction, a spontaneous valley polarization can be observed in non-Janus GdXY. Due to the non-zero Berry curvature, the anomalous Hall effect will be able to be observed in non-Janus GdXY. In addition, the system transforms into a semi-semiconductor from a bipolar magnetic semiconductor with increasing biaxial tensile strain. Under the strain of -4%∼+4%, the ferrovalley characteristics can be well maintained. Our findings not only reveal that non-Janus GdXY is a novel room-temperature ferrovalley semiconductor material, but also provide a new platform for designing spintronics and valley electronics devices.

First-principles prediction of the two-dimensional intrinsic ferrovalley material CeX2 (X=F,Cl,Br)

Two-dimensional (2D) ferrovalley semiconductor materials with intrinsic spontaneous valley polarization offer new prospects for valley electronics applications. However, there are only a limited number of known promising candidate materials, which are in urgent need of expansion. In particular, the room-temperature 2D ferrovalley materials are still lacking. In this study, we predicted novel 2D ferromagnetic CeX2 (X=Fe,Cl,Br) monolayers by using first-principles calculations. The monolayer CeX2 is a bipolar magnetic semiconductor with robust dynamical and thermal stabilities, and easy magnetization direction is in the plane. Due to the simultaneous breaking of both inversion symmetry and time-reversal symmetry, the monolayer CeX2 is exhibiting a spontaneous intrinsic valley polarization when magnetized along the out-of-plane z direction. Interestingly, monolayer CeBr2 is a spontaneous intrinsic ferrovalley material with a room temperature of 334 K and an obvious valley splitting of 32 meV. Due to the non-zero valley-contrast Berry curvature, monolayer CeBr2 is a candidate materials for realizing the anomalous valley Hall effect under a suitable applied electric field. Our study provides a theoretical reference for the design of valley electronic devices with anomalous valley Hall effect based on hole-doped CeX2.

First-principles prediction of intrinsic piezoelectricity, spin-valley splitting and magneto-crystal anisotropy in 2H-VS2 magnetic semiconductor

The piezoelectricity, valley character and magnetic properties of intrinsic ferrovalley 2H-VS2 monolayer are studied using density functional theory. The results suggest that 2H-VS2 monolayer exhibits obvious bipolar magnetic semiconductor characters with Curie temperature (TC) of 278 K, large piezoelectric coefficient d11 of 3.925 pm/V, in-plane magneto-crystal anisotropy (MA) and large valley splitting (ΔKK′) of 75.1 meV. Applying in-plane strain (-6% to 6 %) can well adjust the ΔKK′ and TC from 72 meV to 76.7 meV and from 98 K to 397 K, respectively, resulting an obvious change of electronic structures from bipolar magnetic semiconductor to spin gapless semiconductor phase. The magnetic anisotropy energy (MAE) of 2H-VS2 monolayer firstly increases and then decreases as the strain increases from −6% to 6 %, which mainly originates from the contribution of V dx2-y2 and dxy orbitals. In addition, 2H-VS2 monolayer retains a large valley-contrasting Berry curvature and non-zero anomalous Hall conductivity, indicating that the transverse velocity of carriers is opposite to the effect of in-plane longitudinal electric field. These results suggest that the 2H-VS2 monolayer can become good candidates for the spintronic and valleytronic applications.

Two-dimensional transition metal halide PdX 2 (X = F, Cl, Br, I): A promising candidate of bipolar magnetic semiconductors

Two-dimensional (2D) nanomaterials with bipolar magnetism show great promise in spintronic applications. Manipulating carriers’ spin-polarized orientation in bipolar magnetic semiconductor (BMS) requires a gate voltage, but that is volatile. Recently, a new method has been proposed to solve the problem of volatility by introducing a ferroelectric gate with proper band alignment. In this paper, we predict that the PdX 2 (X = F, Cl, Br, I) monolayers are 2D ferromagnetic BMS with dynamic stability, thermal stability, and mechanical stability by first-principles calculations. The critical temperatures are higher than the boiling point of liquid nitrogen and the BMS characteristics are robust against external strains and electric fields for PdCl2 and PdBr2. Then, we manipulate the spin-polarization of PdCl2 and PdBr2 by introducing a ferroelectric gate to enable magnetic half-metal/semiconductor switching and spin-up/down polarization switching control. Two kinds of spin devices (multiferroic memory and spin filter) have been proposed to realize the spin-polarized directions of electrons. These results demonstrate that PdCl2 and PdBr2 with BMS characters can be widely used as a general material structure for spintronic devices.

Chemically Controlled Reversible Magnetic Phase Transition in Two-Dimensional Organometallic Lattices.

Developing an efficient method to reversibly control materials' spin order is urgently needed but challenging in spintronics. Though various physical field control methods have been advancing, the chemical control of spin is little exploited. Here, we propose a chemical means for such spin manipulation, i.e., utilizing the well-known lactim-lactam tautomerization to reversibly modulate the magnetic phase transition in two-dimensional (2D) organometallic lattices. The proposal is verified by theoretically designing several 2D organometallic frameworks with antiferromagnetic to ferrimagnetic spin order transformation modulated by lactim-lactam tautomerization on organic linkers. The transition originates from the change in spin states of organic linkers (from singlet to doublet) via tautomerization. Such a transition further switches materials' electronic structures from normal semiconductors with zero spin polarization to bipolar magnetic semiconductors with valence and conduction band edges 100% spin polarized in opposite spin channels. Moreover, the magnitude of magnetic anisotropy energy also enhances by 5- to 9-fold.

Control of band polarity in two-dimensional VX2 (X = S, Se, and Te)

Bipolar magnetic semiconductor (BMS) has special electronic structures; i.e., its conduction band minimum (CBM) and valence band maximum (VBM) are completely spin-polarized in opposite directions. In this work, the band structures of 2H-VX2 (X = S, Se, and Te) are examined through first-principles calculations, and the results show that both 2H-VS2 and 2H-VSe2 are BMSs, while 2H-VTe2 is a unipolar magnetic semiconductor (UMS); i.e., its CBM and VBM show the same spin direction. Most interestingly, we find that electronic orbitals near the Fermi level of 2H-VX2 are occupied by dz2 and dxy orbitals, which can be effectively modulated by the biaxial strain. With appropriate strain modulations, 2H-VX2 can be BMS, UMS, or half-metal (HM). Our investigation reveals strain effects on the band structure of 2H-VX2, which greatly enhances their significance in spintronics.

Exquisite control of electronic and spintronic properties on highly porous Covalent Organic Frameworks (COFs): transition metal intercalation in bilayers

Two-dimensional covalent organic frameworks (2D COFs) are crystalline organic porous materials stacked in a layered fashion. In general, these materials have excellent structural tunability, which can be achieved through the various tools of organic synthesis. Their layered and porous nature makes them attractive candidates for electronics, optoelectronics, and catalysis. However, their application is still limited due to relatively poor π-delocalization and practical applications require controlling and tuning their electronic structure. In this paper, using hybrid density functional theory, we computationally explore a novel 2D COF architecture, consisting of only two crystalline atomic layers made of benzene, boroxine, and triazine rings. We study the intercalation of first-row transition metals in the bilayer to enhance and fine-tune their electronic and magnetic behavior. This resulted in the development of one pristine bilayer, 63 intercalated bilayers, and one trilayer 2D COF. We found that the concentration and position of transition metals in the structure can drastically change the 2D COFs’ electronic, magnetic, and spintronic features. Based on their spin-polarized electronic properties, these transition metal-intercalated 2D COFs have potential applications as water splitting catalysts, direct semiconductors in the visible range, half metals, half semiconductors, and bipolar magnetic semiconductors.

Quantum Transport Simulations of a Proposed Logic-In-Memory Device Based on a Bipolar Magnetic Semiconductor

Quantum Transport Simulations of a Proposed Logic-In-Memory Device Based on a Bipolar Magnetic Semiconductor

Two-Dimensional Metal-Organic Frameworks Towards Spintronics.

The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two-dimensional metal-organic frameworks (2D MOFs) highly prospective candidates for next-generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF-based spintronics materials. Then, we highlight the spin-transport properties of 2D MOF-based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

Two‐Dimensional Metal‐Organic Frameworks Towards Spintronics

AbstractThe intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two‐dimensional metal–organic frameworks (2D MOFs) highly prospective candidates for next‐generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF‐based spintronics materials. Then, we highlight the spin‐transport properties of 2D MOF‐based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

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.