Antiferromagnetic Semiconductor Research Articles

Geometrically frustrated Gd2Ti2O7 oxide: A comprehensive exploration of structural, magnetic, and magnetocaloric properties for cryogenic magnetic cooling applications

The magnetocaloric effect (MCE) has been extensively studied in various magnetic solids, aiming to both facilitate practical applications in magnetic cooling and deepen our understanding these materials’ intrinsic properties. This study presents a systematic investigation of Gd2Ti2O7 oxide, characterized by a geometrically frustrated magnetic structure, focusing on its structural and magnetic characteristics through experimental analysis and theoretical calculations. Emphasis is placed on its magnetic phase transition (MPT) and magnetocaloric properties, scrutinizing temperatures as low as 0.4 K and magnetic fields up to 16 T. The results reveal that Gd2Ti2O7 oxide possesses a crystalline cubic pyrochlore-type structure with an antiferromagnetic (AFM) semiconductor ground state. The material undergoes a first-order MPT in low field and low-temperature regimes, transitioning to a second-order MPT above 2 K up to 16 T. Evaluation of magnetocaloric parameters, including maximum magnetic entropy change, temperature-averaged entropy change, relative cooling power, and refrigerant capacity, indicates moderate values under low magnetic field changes, while significantly large values are achievable under high magnetic field changes. This finding suggests that potential cryogenic magnetocaloric materials (MCMs) with optimal performance should exhibit low MPT temperatures induced by magnetic frustration and possess a ground state with a sufficiently large magnetic moment induced by low magnetic fields, thus avoiding excessively strong AFM coupling. This study lays the groundwork for future cryogenic MCM design and provides valuable insights into the intrinsic properties of geometrically frustrated magnetic materials.

Electronic and optical properties of semiconducting BeN4 nanoribbons

Recently, a novel two-dimensional (2D) Dirac semimetal BeN4 have been successfully synthesized (Phys. Rev. Lett. 126, 175,501, 2021). However, the zero bandgap of monolayer BeN4 prevents it from performing as a semiconductor in next-generation optoelectronics and electronics. Here we employ first-principles calculations to show that cutting BeN4 monolayer along zigzag direction can open moderate band gaps. The zigzag BeN4 nanoribbons with Be termination are nonmagnetic (NM) semiconductors with a moderate direct band gap and good optical absorption. The nanoribbons with edges occupied by Be and N atoms are nonmagnetic semiconductors or ferromagnetic (FM)/NM metals, and the electronic properties are closely related to the type of boundary N atoms. The configuration with both edges occupied by the pentagonal N atoms is an antiferromagnetic (AFM) semiconductor, while the system with both edges situated by hexagonal N atoms is a NM direct band gap semiconductor. If one edge is terminated by hexagonal N atoms and the other one is pentagonal N atoms, the nanoribbon will behave as a FM semiconductor. We also explore the electronic properties of zigzag BeN4 nanoribbons with hydrogen passivation. These excellent properties endow zigzag BeN4 nanoribbons with enormous potential for applications in optoelectronic and spintronic devices.

Acoustic and optical phonons in quasi-two-dimensional MPS3 antiferromagnetic semiconductors

We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ−A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications.

Electronic Noise Spectroscopy of Quasi-Two-Dimensional Antiferromagnetic Semiconductors.

We investigated low-frequency current fluctuations, i.e., electronic noise, in FePS3 van der Waals layered antiferromagnetic semiconductor. The noise measurements have been used as noise spectroscopy for advanced materials characterization of the charge carrier dynamics affected by spin ordering and trapping states. Owing to the high resistivity of the material, we conducted measurements on vertical device configuration. The measured noise spectra reveal pronounced Lorentzian peaks of two different origins. One peak is observed only near the Néel temperature, and it is attributed to the corresponding magnetic phase transition. The second Lorentzian peak, visible in the entire measured temperature range, has characteristics of the trap-assisted generation-recombination processes similar to those in conventional semiconductors but shows a clear effect of the spin order reconfiguration near the Néel temperature. The obtained results contribute to understanding the electron and spin dynamics in this type of antiferromagnetic semiconductors and demonstrate the potential of electronic noise spectroscopy for advanced materials characterization.

Nonvolatile Electric Control of Antiferromagnet CrSBr.

van der Waals magnets are emerging as a promising material platform for electric field control of magnetism, offering a pathway toward the elimination of external magnetic fields from spintronic devices. A further step is the integration of such magnets with electrical gating components that would enable nonvolatile control of magnetic states. However, this approach remains unexplored for antiferromagnets, despite their growing significance in spintronics. Here, we demonstrate nonvolatile electric field control of magnetoelectric characteristics in van der Waals antiferromagnet CrSBr. We integrate a CrSBr channel in a flash-memory architecture featuring charge trapping graphene multilayers. The electrical gate operation triggers a nonvolatile 200% change in the antiferromagnetic state of CrSBr resistance by manipulating electron accumulation/depletion. Moreover, the nonvolatile gate modulates the metamagnetic transition field of CrSBr and the magnitude of magnetoresistance. Our findings highlight the potential of manipulating magnetic properties of antiferromagnetic semiconductors in a nonvolatile way.

Competition between Electron–Phonon and Spin–Phonon Interaction on the Band Gap and Phonon Spectrum in Magnetic Semiconductors

Using the microscopic s-f model and Green’s function theory, we study the temperature dependence of the band gap energy Eg and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of Eg and the phonon spectrum, there is a kink at the phase transition temperature TC or TN due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I>0,R>0, Eg decreases with increasing SPI and EPI, whereas for I<0,R>0, there is a competition; Eg increases with raising the EPI and decreases for enhanced SPI. For R<0, in both cases, the SPI and EPI reduce Eg. The magnetic field dependence of Eg for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO (I>0, R<0), whereas ω = 151 cm−1 in EuSe (I>0, R>0) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data.

Realizing ferromagnetic semiconductors in SrRu1−xZrxO3 alloys

Using first principle calculations, we study the structural, electric and magnetic properties of SrRu1−xZrxO3 (0 ≤ x ≤ 1). The spin-polarization calculations present that SrRu1−xZrxO3 is a ferromagnetic metal at x = 0, a ferromagnetic semiconductor at x = 0.125, 0.25, an antiferromagnetic semiconductor at x = 0.5 and a nonmagnetic insulator at x = 1, which is in agreement with available experiments. As increasing Zr contents, the lattice parameters and band gaps of SrRu1−xZrxO3 increase while the energy difference between antiferromagnetic and ferromagnetic states decreases. Through Ru-O and Zr-O bonds, hybridization between Ru 4d and Zr 4d states near the Fermi level becomes strong. As a result, Ru 4d states split and then metal-insulator transition occurs at x = 0.125 due to Zr. Ferromagnetic semiconductors are first predicted in SrRu1−xZrxO3 alloys, which may have potential applications in spintronic devices.

Electronic, magnetic and transport properties of anti-ferromagnetic semiconductor BaGd2X4(X S, Se)

Electronic, magnetic and transport properties of anti-ferromagnetic semiconductor BaGd2X4(X S, Se)

Structure, electronic, and magnetic properties of 3d transition metal intercalated borophene/BP heterostructures

AbstractThe intercalation of various atoms or molecules has become one promising way to manipulate the electronic and magnetic properties of layered materials. Using density functional calculations, we explored the 3d transition metal (TM) intercalated α‐borophene/black phosphorus (α‐B/BP) heterostructure, TM@(α‐B/BP) (TM = Sc‐Ni), on their structure, electronic and magnetic properties. Our results demonstrate that TM@(α‐B/BP)s can be ferromagnetic (FM), antiferromagnetic (AFM) and nonmagnetic depending on the choice of TM atoms, and most systems have large magnetic anisotropic energy. Particularly, Ti@(α‐B/BP) is AFM semiconductor with Néel temperature of 470 K, which is much higher than room temperature. Moreover, the electronic and magnetic properties of TM@(α‐B/BP)s can be further altered by the TM intercalation concentration. Our results provide a feasible way to design promising candidates for applications in electronic and information storage devices.

Two-Dimensional Transition Metal Boride TMB12 (TM = V, Cr, Mn, and Fe) Monolayers: Robust Antiferromagnetic Semiconductors with Large Magnetic Anisotropy.

Currently, two-dimensional (2D) materials with intrinsic antiferromagnetism have stimulated research interest due to their insensitivity to external magnetic fields and absence of stray fields. Here, we predict a family of stable transition metal (TM) borides, TMB12 (TM = V, Cr, Mn, Fe) monolayers, by combining TM atoms and B12 icosahedra based on first-principles calculations. Our results show that the four TMB12 monolayers have stable antiferromagnetic (AFM) ground states with large magnetic anisotropic energy. Among them, three TMB12 (TM=V, Cr, Mn) monolayers display an in-plane easy magnetization axis, while the FeB12 monolayer has an out-of-plane easy magnetization axis. Among them, the CrB12 and the FeB12 monolayers are AFM semiconductors with band gaps of 0.13 eV and 0.35 eV, respectively. In particular, the AFM FeB12 monolayer is a spin-polarized AFM material with a Néel temperature of 125 K. Moreover, the electronic and magnetic properties of the CrB12 and the FeB12 monolayers can be modulated by imposing external biaxial strains. Our findings show that the TMB12 monolayers are candidates for designing 2D AFM materials, with potential applications in electronic devices.

Exciton Polaritons in Emergent Two-Dimensional Semiconductors.

The "marriage" of light (i.e., photon) and matter (i.e., exciton) in semiconductors leads to the formation of hybrid quasiparticles called exciton polaritons with fascinating quantum phenomena such as Bose-Einstein condensation (BEC) and photon blockade. The research of exciton polaritons has been evolving into an era with emergent two-dimensional (2D) semiconductors and photonic structures for their tremendous potential to break the current limitations of quantum fundamental study and photonic applications. In this Perspective, the basic concepts of 2D excitons, optical resonators, and the strong coupling regime are introduced. The research progress of exciton polaritons is reviewed, and important discoveries (especially the recent ones of 2D exciton polaritons) are highlighted. Subsequently, the emergent 2D exciton polaritons are discussed in detail, ranging from the realization of the strong coupling regime in various photonic systems to the discoveries of attractive phenomena with interesting physics and extensive applications. Moreover, emerging 2D semiconductors, such as 2D perovskites (2DPK) and 2D antiferromagnetic (AFM) semiconductors, are surveyed for the manipulation of exciton polaritons with distinct control degrees of freedom (DOFs). Finally, the outlook on the 2D exciton polaritons and their nonlinear interactions is presented with our initial numerical simulations. This Perspective not only aims to provide an in-depth overview of the latest fundamental findings in 2D exciton polaritons but also attempts to serve as a valuable resource to prospect explorations of quantum optics and topological photonic applications.

First principles study of two-dimensional high-temperature antiferromagnetic semiconductor MoICl2 under strain effect

Two-dimensional antiferromagnetic materials with room-temperature magnetism are ideal materials for antiferromagnetic spintronic devices. In this paper, antiferromagnetic monolayer MoICl2 based on first-principles calculations is reported. We predict that monolayer MoICl2 is an antiferromagnetic semiconductor with an indirect bandgap. In the study of strain-induced effects on the system, the electronic structure, bandgap, and magnetic anisotropy energy of monolayer MoICl2 undergo significant changes. However, its magnetic ground state remains unaffected and continues to exhibit antiferromagnetic behavior. Particularly, we predict that the Néel temperature (TN) of monolayer MoICl2 is 53 K and that TN can be further increased to 380 K at a biaxial compressive strain of 6%. The results indicate that monolayer MoICl2 can serve as a potential candidate material for spintronics applications.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr.

van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

Layer-dependent electronic and magnetic properties of two-dimensional graphitic molybdenum carbide

Intrinsic magnetic two-dimensional (2D) materials with high critical temperature are highly desired in advanced spintronics applications. Via first-principles calculations, we firstly predict that two-dimensional molybdenum carbide (with a chemical formula of Mo2C12) monolayer is a highly stable antiferromagnetic (AFM) semiconductor with a band gap around 1 eV and a high Néel temperature of 420 K. We then show that the multilayer (Mo2C12)n, where n is the number of layers, exhibits interesting electronic and magnetic properties that are sensitively dependent on the number of layers. The stability of the AFM configuration and the energy gap rapidly decrease with the number of layers. When n≤5, (Mo2C12)n remains AFM, while magnetic moments are mainly located on surface Mo atoms, and Mo atoms on top and bottom surfaces have opposite spin polarizations. When n>5, the AFM phase is unstable and the material becomes metallic. These layer-tunable properties make (Mo2C12)n potentially useful for various electronics and spintronics applications. As one example, an intriguing (Mo2C12)5 based magnetic metal–semiconductor–metal heterojunction is proposed in this work.

Double transition metal MBenes: A family of 2D MXenes with tunable magnetic properties

The emergence of two-dimensional (2D) magnetic materials is critical to the development of nanoscale spintronic devices. However, the lack of intrinsic magnetism limits their applications. In this work, we propose a universal strategy to develop experimentally feasible 2D MBenes with intrinsic magnetism. Using high-throughput first-principles calculations, 546 kinds of ordered double transition metal MBenes with chemical formula (M′2/3M″1/3)2B2 (M′ and M″ = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co) were developed and studied in detail. Among them, we have identified 21 (M′2/3M″1/3)2B2 with robust magnetic ordering and good structural stability, including 15 ferromagnetic metals, 1 ferromagnetic half-metal, 3 antiferromagnetic metals, and 2 antiferromagnetic semiconductors. Remarkably, (Cr2/3Zr1/3)2B2 is a ferromagnetic half-metal with a high Curie temperature of 610 K and a strong out-plane magnetic anisotropy energy of 305 μeV per Cr atom. (Mn2/3Sc1/3)2B2 and (Fe2/3Nb1/3)2B2 are antiferromagnetic semiconductors with Néel temperatures of 257 and 462 K, respectively. These excellent magnetic properties make the double transition metal MBenes desirable for 2D spintronics.

Zeeman splitting in doped antiferromagnetic semiconductors

Zeeman splitting of conduction electrons in doped antiferromagnetic materials in the presence of a strong spin–orbit interaction is considered. Using symmetry arguments the Hamiltonian and the spectrum of conduction electrons near some time-reversal invariant momenta were constructed. It was shown, that in the vicinity of some time-reversal invariant momenta the external magnetic field perpendicular to the Neel axis does not lead to the Zeeman splitting. The Kramers degeneracy is preserved in that case. The absence of the Zeeman splitting in antiferromagnets leads to the absence of spin zeros in quantum oscillation experiments. Recent experimental confirmation of the predicted effects in organic antiferromagnet and High-Tc Superconductors is discussed.

Magnetic bias field driven ferro- and antiferromagnetic semiconductors as double negative media for microwave and terahertz ranges

In this work, a theoretical study of the backward electromagnetic waves (BEMWs) existing in the bigyrotropic left-handed media controlled by a magnetic bias field is presented. The bigyrotropic media are both longitudinally and transversely magnetized ferromagnetic (FM) or antiferromagnetic (AFM) semiconductors (SCs) with electric and magnetic loss. It is shown that the BEMWs are observed both in the microwave and terahertz frequency ranges, in which the effective material parameters of the FM or AFM SCs are double negative. We demonstrate the control of the BEMW dispersion characteristics not only by changing the magnetic bias field direction and strength, but also by variation a material magnetization and thickness, an electron concentration in a solid-state plasma, as well as the electric and magnetic loss.

Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime

Over the past few decades, exciton-polaritons have attracted substantial research interest due to their half-light-half-matter bosonic nature. Coupling exciton-polaritons with magnetic orders grants access to rich many-body phenomena, but has been limited by the availability of material systems that exhibit simultaneous exciton resonances and magnetic ordering. Here we report magnetically-dressed microcavity exciton-polaritons in the van der Waals antiferromagnetic (AFM) semiconductor CrSBr coupled to a Tamm plasmon microcavity. Using angle-resolved spectroscopy, we reveal an exceptionally high exciton-photon coupling strength, up to 169 meV, demonstrating ultrastrong coupling that persists up to room temperature. By performing temperature-dependent spectroscopy, we show the magnetic nature of the exciton-polaritons in CrSBr microcavity as the magnetic order changes from AFM to paramagnetic. By applying an out-of-plane magnetic field, we achieve effective tuning of the polariton energy while maintaining the ultrastrong exciton-photon coupling strength. We attribute this to the spin canting process that modulates the interlayer exciton interaction.

Bipolar ferromagnetic semiconductors and dipole-modulated magnetism in two-dimensional Janus transition metal dihalides

Two-dimensional (2D) transition metal dihalides (TMDHs) have attracted great interest owing to their unique magnetic and semiconductor properties. Compared with the mirror/inversion symmetric materials, 2D Janus materials possess vertical intrinsic dipole moment, which offer a versatile platform for the fundamental physics and future spintronic devices. Here, we systematically explore the magnetic and electronic properties of the 2D Janus transition metal dihalides MXY (M = Co and Ni; X ≠ Y = Cl, Br, and I) monolayers and bilayers by using density functional theory. The monolayer CoClBr, NiClBr, and NiBrI are bipolar ferromagnetic semiconductors that possess the valence and conduction band edges of different spin channels. The magnetism of the bilayer CoClBr, NiClBr, and NiBrI is highly dependent on the accumulated dipole moments of the two adjacent layers. When the dipole moments in both layers are aligned in the same direction and the accumulated dipole moments are nonzero, the systems are antiferromagnetic half semiconductors. However, when the dipole moments in the two layers are opposite and the accumulated dipole moments are zero, the systems are A-type antiferromagnetic semiconductors. Our findings are helpful to understand the magnetism of Janus TMDHs and guide experiments in exploring their potential application in spintronic devices.

DFT study of Ag-X (X = Pd, Pt, Ag) decorated (8,0) single wall carbon nanotube for spintronic applications

The main purpose of the present article is an investigation of the electronic and magnetic properties of (8,0) single-wall carbon nanotube (CNT80) decorated with a diatomic configuration of Ag and X (X = Pd, Pt, and Ag). Density functional theory and the Grimme-D2 approach in spin polarization mode are employed. It is concluded that Ag and X are adsorbed on CNT80 in the dimer form rather than singly atomic adsorption. The spin-distinct electronic band structure and the densities of states are also calculated. The calculations show that Ag-Pd/CNT80 is a bipolar semiconductor with an energy gap of 0.27 eV for spin-up, 0.47 eV for spin-down, and 0.15 eV for spin-flip electrons. Ag-Pt/CNT80 is an anti-ferromagnetic p-type indirect semiconductor with a gap of 0.187 eV for temperatures below 188 K. After 188 K up to around 330 K, this structure converts to an indirect bipolar semiconductor, with an energy gap of 0.31 eV (0.37 eV) for the spin-up (down) channel. Adsorption of Ag dimer onto CNT80 leads to the p-type nonmagnetic semiconductor. As an application of the proposed materials, the absorption of CO as a polar molecule on the Ag-Pd/CNT80 structure is also investigated, and a proposed structure as a spintronic CO sensor is introduced.