Observation of above-room-temperature ferromagnetism in chemically stable layered semiconductor RhI3

Two-dimensional ferromagnetic semiconductors of rare-earth monolayer GdX2 (X = Cl, Br, I) with large perpendicular magnetic anisotropy and high Curie temperature

Two-dimensional (2D) ferromagnetic semiconductors (FMSs) hold exciting and promising potential for application in spintronic devices at the nanoscale. Currently, most 2D FMSs are based on 3d electrons; 4f electrons can provide nontrivial magnetism but have been much less studied to date. This paper presents a theoretical study, via first-principles calculations, of EuSn2X2 (X = P, As) monolayers based on rare-earth cations with f-electrons. The results show that EuSn2X2 monolayers possess a large magnetization (7 μB/Eu), a controllable magnetic anisotropy energy, and a unique d-electron-mediated f–f exchange mechanism. Both types of EuSn2X2 (X = P, As) monolayers are FMSs with indirect bandgaps of 1.00 and 0.99 eV, respectively, based on the Heyd–Scuseria–Ernzerhof (HSE06) method, which can be transform to direct bandgap semiconductors under biaxial strain. Interestingly, under the latter, spin–orbit coupling interaction gradually replaces the dipole–dipole interaction in the dominant position of magnetic anisotropy, resulting in the magnetic easy axis changing from in-plane to the more desirable out-of-plane. Considering their excellent dynamic, thermal, and mechanical stabilities and small cleavage energy, these EuSn2X2 monolayers can be exfoliated from their synthesized bulk. Our study not only helps to understand the properties of 2D 4f rare-earth magnets but also signposts a route toward improving the performance of EuSn2X2 monolayers in nano-electronic devices.

Hole-mediated ferromagnetic coupling in two-dimensional CrI3/VSe2 van der Waals heterostructures

In recent years, 2D ferromagnetic semiconductors have attracted much attention because of its potential application in spintronic devices. Using first‐principles calculations, the magnetic and optical properties of intrinsic and chalcogen‐doped VCl3 monolayers are investigated. In contrast to previous work, VCl3 monolayer is proved to be an antiferromagnetic semiconductor rather than a Dirac half‐metal after considering the electronic correlation effect. At a low S concentration x between and , S‐doped VCl3 monolayer forms a ferromagnetic semiconductor with a large bandgap and a strong exchange splitting in both valence and conduction bands. As the doping content x increases above , S‐doped VCl3 monolayer will change to be an anti‐ferromagnetic semiconductor and a non‐magnetic metal successively. Moreover, Se‐ and Te‐doped VCl3 monolayers can also form robust ferromagnetic semiconductors at low doping concentration. In particular, the Curie temperature of Se‐doped VCl3 monolayer can reach 170 K, higher than that of S‐ and Te‐doped VCl3 monolayers. At last, chalcogen‐doped VCl3 monolayers have enhanced optical absorption in the visible regions compared to intrinsic VCl3 monolayer. The results show that chalcogen‐doped VCl3 monolayers have promising potential applications in future spintronic and optoelectronic devices.

Two-dimensional (2D) ferromagnetic (FM) semiconductors with high Curie temperature (T c ) and large perpendicular magnetic anisotropy (PMA) are promising for developing next-generation magnetic storage devices. In this work, we investigated the structural, electronic, and magnetic properties of MoF3 and Janus Mo2F3 X 3 (X = Cl, Br, I) monolayers by first-principles methods. These materials are 2D FM semiconductors with large PMA and half-semiconducting character as both VBM and CBM belonging to the spin-up channel. Biaxial strain can modulate band gap, reverse easy magnetization axis, and induce magnetic phase transitions in MoF3 monolayer and its Janus structures. Compared to MoF3 monolayers, Janus Mo2F3 X 3 monolayers can preserve the structural ability and the FM ground state over a wider range of strain. The magnetic anisotropy energies (MAEs) of these 2D materials can be enhanced to greater than 1 meV/Mo by tensile strains. Intrinsic T c of MoF3 monolayer and its Janus structures are less than 110 K and are insensitive to strain. However, hole doping with a feasible concentration can achieve a room-temperature half-metallicity in these 2D materials. The required hole concentration is lower in Janus Mo2F3 X 3 monolayers than MoF3 monolayer. Our results indicate that MoF3 and Janus Mo2F3 X 3 (X = Cl, Br, I) monolayers are promising candidates for 2D spintronic applications and will stimulate experimental and theoretical broad studies.

Two-dimensional (2D) ferromagnetic semiconductors hold a great potential for nano-electronic and spintronic devices. Nevertheless, their experimental realization remains a big challenge. Through first-principles calculations, we here demonstrate the possibility of realizing 2D ferromagnetic semiconductors simply by exfoliating layered crystals of CrXTe3 (X = Si, Ge). The exfoliation of CrXTe3 is feasible due to its small cleavage energy, and CrXTe3 nanosheets can form free-standing membranes. Interestingly, upon exfoliation, the ferromagnetism and semiconducting character are well preserved from bulk to the nanosheet form. Long-range ferromagnetic order with a magnetization of 3 μB per Cr atom is confirmed in 2D CrXTe3 from classical Heisenberg model Monte Carlo simulations. Both bulk and 2D CrXTe3 are indirect-gap semiconductors with their valence and conduction bands fully spin-polarized in the same direction, which is promising for spin-polarized carrier injection and detection. We further demonstrate the tunability and enrichment of the properties of CrXTe3 nanosheets via external operations. Under moderate tensile strain, the 2D ferromagnetism can be largely enhanced. By pure electron doping or adsorbing nucleophilic molecules, CrXTe3 nanosheets become 2D half metals.

The discovery of two-dimensional (2D) ferromagnetic semiconductors holds significant promise for advancing Moore's law and spintronics in-memory computing, sparking tremendous interest. However, the Curie temperature of explored 2D ferromagnetic semiconductors is much lower than room temperature. Although plenty of 2D room-temperature ferromagnetic semiconductors have been theoretically predicted, there have been formidable challenges in preparing such metastable materials with ordered structures and high stability. Here, utilizing a novel template-assisted chemical vapor deposition strategy, we synthesized layered MnS2 microstructures within a ReS2 template. The high-resolution atomic structure representation revealed that monolayer MnS2 microstructures well crystallize into a distorted T-phase. Room-temperature ferromagnetism was confirmed through vibrating sample magnetometer measurements, microzone magnetism imaging techniques, and transport characterization. Theoretical calculations indicated that the room-temperature ferromagnetism originates from the Mn-Mn short-range interaction. Our observation not only offered the experimental confirmation of the intrinsic room-temperature ferromagnetism in layered MnS2, but also provided an innovative strategy for the growth of 2D metastable functional materials.

CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising behavior with mean field type interactions where the critical exponents β, γ, and δ are 0.323 ± 0.006, 0.835 ± 0.005, and 3.585 ± 0.006, respectively, at the Curie temperature of 64 K. We propose that the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor.

CrGeTe3 recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike CrSiTe3 whose magnetism can be understood using the 2D-Ising model, CrGeTe3 exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior CrGeTe3 around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis, which show that the magnetism of CrGeTe3 follows the tricritical mean-field model with the critical exponents \b{eta}, {\gamma}, and {\delta} of 0.240, 1.000, and 5.070, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for CrGeTe3 should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.

The 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

The emergence of atomically thin valleytronic semiconductors and 2D ferromagnetic materials is opening up new technological avenues for future information storage and processing. A key fundamental challenge is to identify physical knobs that may effectively manipulate the spin‐valley polarization, preferably in the device context. Here, a novel spin functional device that exhibits both electrical and magnetic tunability is fabricated, by contacting a monolayer MoSe2 with a 2D ferromagnetic semiconductor Cr2Ge2Te6. Remarkably, the valley‐polarization of MoSe2 is found to be controlled by a back‐gate voltage with an appreciably enlarged valley splitting rate. At fixed gate voltages, the valley‐polarization exhibits magnetic‐field and temperature dependence that corroborates well with the intrinsic magnetic properties of Cr2Ge2Te6, pointing to the impact of magnetic exchange interactions. Due to the interfacial arrangement, the charge‐carrying trion photoemission predominates in the devices, which may be exploited to enable drift‐based spin‐optoelectronic devices. These results provide new insights into valley‐polarization manipulation in transition metal dichalcogenides by means of ferromagnetic semiconductor proximitizing and represent an important step forward in devising field‐controlled 2D magneto‐optoelectronic devices.

2D ferromagnetic semiconductors are recognized as the cornerstone of next-generation spintronics devices. However, their practical applications are severely hindered by the low Curie temperature, which originates from the weak d-p-d ferromagnetic superexchange interaction. H- anion with short ionic radius can effectively shorten the distance between magnetic centers and simultaneously induce a perfect 180° superexchange angle to strengthen the magnetic coupling, thus achieving high-temperature magnetic ordering. Here, by first-principles calculations, such a case in 2D Ruddlesden-Popper phase hydride double perovskite A4NiVH8 (A = Na, K, Rb) is demonstrated. These hydride monolayers possess quite good thermodynamic stability and can retain their structures under normal pressure at least at 500 K. Magnetic and electronic properties calculations reveal that they are all ferromagnetic semiconductors with high Curie temperatures (up to 429 K) and superior electron mobilities (up to 5522 cm2 V-1 s-1, based on the deformation potential theory). In addition, monolayer Na4NiVH8 exhibits the characteristics of a bipolar magnetic semiconductor with gate-tunable spin polarization.

Spintronic devices, using the spins of electrons as information processing, have generated world-wide interest. Just as graphene, transition-metal dichalcogenides, and black phosphorus revolutionized condensed matter and materials engineering, the discovery of two-dimensional (2D) van der Waals (vdW) magnetic materials is expected to open a new horizon in material science and enable the potential development of spintronics. In fact, 2D magnetism has been investigated for decades while the experimental validation was unable to achieve till recently. The recent exciting 2D ferromagnetic breakthroughs, such as monolayers CrX3 (X = Cl, Br), monolayer Fe3GeTe2, and bilayer CrGeTe3 from their vdW bulk down to atomically thin, have also pushed forward researches on novel magnetic properties and creative concepts. In contrast to the traditional magnetic thin films, 2D vdW ferromagnetic materials (FM) largely decouple from the substrates, allow electrical control and are open to chemical functionalization. Without clear targets or guidelines, traditional trial-and-error experiments face the fundamental challenges of long time and high costs. Computational simulations, which serving as an important first step in exploring possible applications of new materials, can not only predict novel 2D materials but also suggest their possible synthesis routes. Many interesting cases, such as the growth of 2D borophene (B) and tellurene (Te), thermoelectricity in tin selenide (SnSe), ferroelectricity in tin telluride (SnTe), and high carrier mobilities in black phosphorene, have been confirmed by experiments, showing the accuracy of computational methods and their ability in facilitating experimental exploration in 2D space. Compared to other computational methods, the first-principles method, which has been the most widely used tools in designing new materials, only require a few basic physical constants and the atomic position coordinates. It is valuable mainly in two important aspects: (1) It can be used to predict and design new materials with novel properties, and (2) it provides understanding of the physics underlying the properties of new materials to replace the expensive and time-consuming physical test. Therefore, first-principles method based on density functional theory is effective for investigating new materials. In fact, the rapid development of 2D magnetic materials benefits from theoretical simulation. For example, the recent star ferromagnetic bilayer CrGeTe3, monolayer CrI3, and monolayer Fe3GeTe2 were also first predicted theoretically, and they have recently been experimentally made, which shows the strong power of first-principles calculations in designing these spintronics materials. In this review, we highlight the overall picture of recent progress, current challenges, and future prospects on theoretical design of FM materials. We hope this review provides basic understanding the importance of first-principles calculations in facilitating new discoveries and the accurate characterization of 2D FM materials. To achieve this, we first give the reason why ferromagnetic order exists in 2D space at finite temperature theoretically. Then, we summarize the discovery processes and magnetic properties of recent landscape of several 2D ferromagnetic semiconductors, metals, and half-metals, using 2D CrI3, CrGeTe3, Fe3GeTe2, and FeCl2 as the examples, respectively. Finally, we highlight the existing problems of designed 2D FM materials and propose possible directions in computational simulations for further development. Of course, this review cannot cover all 2D FM materials, and readers can also refer to other recent reviews and references therein for more low-dimensional FM materials.

The combination of semiconductivity and tunable ferromagnetism is pivotal for electrical control of ferromagnetism and next‐generation low‐power spintronic devices. However, Curie temperatures (TC) for most traditional intrinsic ferromagnetic semiconductors (≤200 K) and recently discovered two‐dimensional (2D) ones (<70 K) are far below room temperature. 2D van der Waals (vdW) semiconductors with intrinsic room‐temperature ferromagnetism remain elusive considering the unfavored 2D long‐range ferromagnetic order indicated by Mermin–Wagner theorem. Here, vdW semiconductor CrxGa1−xTe crystals exhibiting highly tunable above‐room‐temperature ferromagnetism with bandgap 1.62–1.66 eV are reported. The saturation magnetic moment (Msat) of CrxGa1−xTe crystals can be effectively regulated up to ≈5.4 times by tuning Cr content and ≈75.9 times by changing the thickness. vdW CrxGa1−xTe ultrathin semiconductor crystals show robust room‐temperature ferromagnetism with the 2D quantum confinement effect, enabling TC 314.9–329 K for nanosheets, record‐high for intrinsic vdW 2D ferromagnetic semiconductors. This work opens an avenue to room‐temperature 2D vdW ferromagnetic semiconductor for 2D electronic and spintronic devices.

Two-dimensional (2D) semiconducting transition metal dichalcogenides can be used to make high-performance electronic, spintronic, and optoelectronic devices. Recently, room-temperature ferromagnetism and semiconduction in 2D VSe2 nanoflakes were attributed to the stable 2H-phase of VSe2 in the 2D limit. Here, our first-principles investigation shows that a metastable semiconducting H′ phase can be formed from the H VSe2 monolayer through uniaxial stress or uniaxial strain. The calculated phonon spectra indicate the dynamical stability of the metastable H′ VSe2 and the path of phase switching between the H and H′ VSe2 phases is calculated. For the uniaxial stress (or strain) scheme, the H′ phase can become lower in total energy than the H phase at a transition point. The H′ phase has stronger ferromagnetism and its Curier temperature can be enhanced by applying uniaxial stress or strain. Applying uniaxial stress or strain can substantially change spin-resolved electronic structures, energy band edges, and effective carrier masses for both of the H and H′ phases, and can cause some flat bands near the band edges in the strained H′ phase. Further analysis indicates that one of the Se–Se bonds in the H′ phase can be shortened by 19% and the related Se–V–Se bond angles are reduced by 23% with respect to those of the H phase, which is believed to increase the Se–Se covalence feature and reduce the valence of the nearby V atoms. Therefore, structural and bond reconstruction can be realized by applying uniaxial stress in such 2D ferromagnetic semiconductors for potential spintronic and optoelectronic applications.