Robust Intrinsic Ferromagnetism Research Articles

Sub-THz High Spin Precession Frequency in van der Waals Ferromagnet Fe3GaTe2.

The 2D magnet Fe3GaTe2 has received considerable attention for its high Curie temperature (TC), robust intrinsic ferromagnetism, and significant perpendicular magnetic anisotropy (PMA). In this study, the dynamic magnetic properties of Fe3GaTe2 are systematically investigated using an all-optical pump-probe technique. We find that the spin precession frequency (f) is as high as 351.2 GHz at T = 10 K under a field of H = 70 kOe. However, it decreases to 242.8 GHz at 300 K, mainly due to the reduced effective PMA field (Hkeff). The Gilbert damping factor (α) is modest, which increases from 0.039 (10 K) to 0.075 (300 K) owing to the enhanced scattering rate. Interestingly, when Fe3GaTe2 is coupled with 2 nm of Co, the Hkeff, f, and α just decrease slightly, highlighting the dominant influence of Fe3GaTe2. These findings substantially deepen our understanding of Fe3GaTe2, promoting the development of spintronic devices based on advanced 2D magnetic materials.

Above-room-temperature intrinsic ferromagnetism in ultrathin van der Waals crystal Fe3+xGaTe2

Two-dimensional (2D) van der Waals (vdW) magnets are crucial for ultra-compact spintronics. However, so far, no vdW crystal has exhibited tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin regime. Here, we report the tunable above-room-temperature intrinsic ferromagnetism in ultrathin vdW crystal Fe3+xGaTe2 (x = 0 and 0.3). By increasing the Fe content, the Curie temperature (TC) and room-temperature saturation magnetization of bulk Fe3+xGaTe2 crystals are enhanced from 354 to 376 K and 43.9 to 50.4 emu·g−1, respectively. Remarkably, the robust anomalous Hall effect in 3-nm Fe3.3GaTe2 indicates a record-high TC of 340 K and a large room-temperature perpendicular magnetic anisotropy energy of 6.6 × 105 J m−3, superior to other ultrathin vdW ferromagnets. First-principles calculations reveal the asymmetric density of states and an additional large spin exchange interaction in ultrathin Fe3+xGaTe2 responsible for robust intrinsic ferromagnetism and higher TC. This work opens a window for above-room-temperature ultrathin 2D magnets in vdW-integrated spintronics.

Layer dependent magnetism and topology in monolayer and bilayers ReX 3 (X = Br, I)

The realization of robust intrinsic ferromagnetism in two-dimensional materials with the possibility to support topologically non-trivial states has provided the fertile ground for novel physics and next-generation spintronics and quantum computing applications. In this contribution, we investigated the formation of topological states and magnetism in monolayer and bilayer systems of ReX 3 (X = Br, I), with PBE, ACBN0 (self-consistent Hubbard-U), excluding/including van der Waals (vdW) corrections and/or spin–orbit coupling. Bulk ReX 3 (X = Br, I) is predicted to crystallize in space group (#148), similar to CrI3, with monolayer exfoliation energies that are comparable or less than that of graphite. The topological character of the monolayer and bilayer systems of ReX 3 (X = Br, I) is derived from anomalous Hall conductivity computations. Topologically non-trivial states in ReX 3 (X = Br, I) are absent in the Hubbard-U computations if vdW interactions are included, a prediction that is attributed to the large Hubbard-U difference between the chemical constituents, ΔU ∼ 1.5–1.6 eV, and a significant ∼2.0%–3.6% compressive in-plane strain introduced by vdW interactions. In contrast to the fragile and likely absent topological states in ReX 3 (X = Br, I), magnetic properties are robust and independent of the level of theory: ferromagnetic monolayers are coupled antiferromagnetically to bilayers, with an energy separation between ferromagnetic and antiferromagnetic bilayer spin configurations that could be as low as 0.02 meV/Re (f = 4.8 GHz), well within the microwave range. This suggests that layer dependent magnetism in ReX 3 (X = Br, I) may support a microwave controllable magnetic qubit, consisting of a superposition of antiferromagnetic and ferromagnetic bilayer states.

The Versatile Electronic, Magnetic and Photo-Electro Catalytic Activity of a New 2D MA2 Z4 Family*.

The recent successful growth of MoSi2 N4 and WSi2 N4 monolayers led to the discovery of a new class of the two-dimensional (2D) MA2 Z4 materials with no known 3D layered allotropes, which renders great possibilities to integrate diverse properties by proper design of sandwiched "MZ2 " building blocks and "A-Z" passivation layers. In this work, the dynamic stability, electronic properties, and surface reactivity of the new MA2 Z4 family, in which M is Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, A refers to Si or Ge, and Z is N, P or As, is theoretically probed. Among the proposed 54 possible combinations, about 42 compositions are dynamically stable, which vary from non-magnetic, anti-ferromagnetic, to ferromagnetic semiconductors, metals and half-metals. In particular, the VB (V, Nb, Ta) MA2 Z4 possesses robust intrinsic ferromagnetism that is essential for spintronics applications. In regard to surface activity, most MA2 Z4 , particularly N- or P-terminated IVB and VB MA2 Z4 , have high catalytic potential for hydrogen evolution, and the ▵GH of non-magnetic MA2 Z4 is highly correlated to the highest occupied p electronic states of the surface Z atoms. The photocatalytic activity is also evaluated. MoSi2 N4 and WSi2 N4 within 4 % tensile strain are capable of photocatalytic overall water splitting. The findings indicate the new 2D MA2 Z4 family has fascinating properties and possesses strong potential for applications but not limited to electronics, spintronics and catalysts, which will stimulate the interests of experimental synthesis.

High-temperature ferromagnetism in monolayers MnGaX3 (X = Te, Se)

Two-dimensional (2D) ferromagnetic monolayers are highly desirable in the spintronic field owing to their atomic-thickness and controllable spin degree of freedom. We investigated the 2D transition-metal trichalcogenides MnGaX3 (X = Te, Se, S) as the promising candidate monolayer ferromagnets via first-principles calculations. Our calculations show that monolayer MnGaTe3 and MnGaSe3 are ferromagnetic (FM) metal and half-metal with large magnetic moments and sizeable magneto-crystal anisotropy energy. Both the monolayers are mechanical and dynamical stable, and can be exfoliated from the corresponding layered crystals. More importantly, Monte Carlo simulations predict high Curie temperature for MnGaTe3 (720 K) and MnGaSe3 (910 K). Plus, their ferromagnetic configurations become more stable under the increasing biaxial tensile strain from 0 to 5%. Metallic MnGaTe3 converts into half-metal under biaxial tensile strain, and the band gap of semiconducting spin-channel of half-metallic MnGaSe3 increases under the increasing strain. The distinct half-metallic and robust intrinsic ferromagnetism at high temperature render the two monolayers attractive in spintronics.

Intrinsic ferromagnetism and topological properties in two-dimensional rhenium halides.

The realization of robust intrinsic ferromagnetism in two-dimensional (2D) materials in conjunction with the intriguing quantum anomalous Hall (QAH) effect has provided a fertile ground for novel physics and for the next-generation spintronic and topological devices. On the basis of density functional theory (DFT), we predict that layered 5d transition-metal heavier halides (TMHs), such as ReX3 (X = Br, I), show intrinsic ferromagnetism with high spin polarization and high Curie temperatures. The outstanding dynamic and thermodynamic stability ensures their experimental feasibility. The strong spin-orbit coupling (SOC) of Re makes the electronic structure of the ReI3 monolayer topologically nontrivial with a large Chern number (C = -4). DFT+U calculations reveal that the 2D system undergoes a nontrivial to trivial transition with increasing on-site Hubbard Coulomb interaction U through the emergence of a Dirac cone. This transition is corroborated by the emergence of chiral edge states and the anomalous Hall conductivity. These findings not only demonstrate room-temperature ferromagnetism in atomically thin 5d TMHs, but also pave the way for the potential realization of the QAH effect with high Chern numbers in pristine 2D layers.

Two-Dimensional Co2S2 monolayer with robust ferromagnetism

Design and synthesis of two-dimensional (2D) materials with robust intrinsic ferromagnetism is highly desirable due to their potential applications in spintronics devices. In this work, we identify a new 2D cobalt sulfide (Co2S2) material by using first-principles calculations and particle swarm optimization (PSO) global structure search. We show that the 2D Co2S2 is most stable in the litharge type tetragonal structure with space group of P4/nmm. The elastic constants, phonon spectrum, and molecular dynamics simulation confirm its mechanical, dynamical and thermal stability, respectively. It is also found that Co2S2 monolayer is a ferromagnetic metal with a Curie temperature up to 404 K. In addition, we propose a feasible procedure to synthesize the Co2S2 monolayer by chemically exfoliating from bulk TlCo2S2 phase.

Microstructure and Magnetic Properties of Sn1 − x Ni x O2 Thin Films Prepared by Flash Evaporation Technique

An effort was made to develop semiconductor oxide-based room temperature dilute magnetic semiconductor (DMS) thin films based on wide band gap and transparent host lattice with transition metal substitution. The Sn\(_{\mathrm {1}-x}\)Ni\(_{x}\textit {O}_{\mathrm {2}}\) (\(x\,= \mathrm {0.00, 0.03, 0.05, 0.07, 0.10, and \,0.15}\)) thin film samples were prepared on glass substrates by flash evaporation technique. All the samples were shown single phase crystalline rutile structure of host SnO\(_{\mathrm {2}}\) with dominant (110) orientation. The Ni substitution promotes reduction of average crystallite size in SnO\(_{\mathrm {2}}\) as evidenced from the reduction of crystallite size from 40 (SnO\(_{\mathrm {2}}\)) to 20 nm (Sn\(_{\mathrm {0.85}}\)Ni\(_{\mathrm {0.15}}\textit {O}_{\mathrm {2}}\)). In the energy dispersive spectra as well as X-ray photoelectron spectra of all the samples show, the chemical compositions are close to stoichiometric with noticeable oxygen deficiency. The crystalline films were formed by coalescence of oval-shaped polycrystalline particles of 100 nm size as evidenced from the electron micrographs. The energy band gap of DMS films decreases from 4 (SnO\(_{\mathrm {2}}\)) to 3.8 eV (x \(=\) 0.05) with increase of Ni content. The magnetic hysteresis loops of all the samples at room temperature show soft ferromagnetic nature except for SnO\(_{\mathrm {2}}\) film. The SnO\(_{\mathrm {2}}\) films show diamagnetic nature and it converts into ferromagnetic upon substitution of 3 % Sn\(^{\mathrm {4+}}\) by Ni\(^{\mathrm {2+}}\). The robust intrinsic ferromagnetism (saturation magnetization, 21 emu/cm\(^{\mathrm {3}}\)). Further increase of Ni content weakens ferromagnetic strength due to Ni-O antiferromagnetic interactions among the nearest neighbour Ni ions via O\(^{\mathrm {2-}}\) ions. The observed magnetic properties were best described by bound magnetic polarons model.

Robust intrinsic ferromagnetism and half semiconductivity in stable two-dimensional single-layer chromium trihalides

Single-layer chromium trihalides constitute a series of stable 2D intrinsic FM half semiconductors with large magnetic anisotropy energies.

Half-Unit-Cell α-Fe2O3 Semiconductor Nanosheets with Intrinsic and Robust Ferromagnetism

The synthesis of atomically thin transition-metal oxide nanosheets as a conceptually new class of materials is significant for the development of next-generation electronic and magnetic nanodevices but remains a fundamental chemical and physical challenge. Here, based on a "template-assisted oriented growth" strategy, we successfully synthesized half-unit-cell nanosheets of a typical transition-metal oxide α-Fe2O3 that show robust intrinsic ferromagnetism of 0.6 μB/atom at 100 K and remain ferromagnetic at room temperature. A unique surface structure distortion, as revealed by X-ray absorption spectroscopy, produces nonidentical Fe ion environments and induces distance fluctuation of Fe ion chains. First-principles calculations reveal that the efficient breaking of the quantum degeneracy of Fe 3d energy states activates ferromagnetic exchange interaction in these Fe(5-co)-O-Fe(6-co) ion chains. These results provide a solid design principle for tailoring the spin-exchange interactions and offer promise for future semiconductor spintronics.

A first principles study of thiol-capped Au nanoparticles: Structural, electronic, and magnetic properties as a function of thiol coverage

We have studied the stability of thiolated Au38 nanoparticles (NPs) via density functional theory based calculations varying the coverage from 0 up to 32 molecules. Three different initial core arrangements were considered for the cluster, spherical, tubular, and bi-icosahedral, while thiol groups were attached to the cluster via the sulfur atom either as single molecules or forming more complex staple motifs. After molecular dynamics runs several metastable configurations are found at each coverage thus allowing to analyze the properties of the NPs in the form of ensemble averages. In particular, we address the structural and electronic properties as a function of the number of thiols. The study emphasizes the strong influence of the core structure on the stability of the NPs, and its interplay with the thiol coverage and adsorption geometries. The magnetic properties of the NPs have also been explored via spin-polarized calculations including spin-orbit coupling. No evidence for the existence of a robust intrinsic ferromagnetism is found in any of the structures.