Electron doping as a handle to increase the Curie temperature in ferrimagnetic Mn3Si2X6 (X = Se, Te).

By analysing the results of ab initio simulations performed for Mn3Si2X6 (X = Se, Te), we first discuss the analogies and the differences in electronic and magnetic properties arising from the anion substitution, in terms of size, electronegativity, band widths of p electrons and spin-orbit coupling strengths. For example, through mean-field theory and simulations based on density functional theory, we demonstrate that magnetic frustration, known to be present in Mn3Si2Te6, also exists in Mn3Si2Se6 and leading to a ferrimagnetic ground state. Building on these results, we propose a strategy, electronic doping, to reduce the frustration and thus to increase the Curie temperature (TC). To this end, we first study the effect of electronic doping on the electronic structure and magnetic properties and discuss the differences in the two compounds, along with their causes. Secondly, we perform Monte-Carlo simulations, considering from the first to the fifth nearest-neighbor magnetic interactions and single-ion anisotropy, and show that electron doping efficiently raises the TC.

Moiré magnetic exchange interactions in twisted magnets.

In addition to moiré superlattices, twisting can also generate moiré magnetic exchange interactions (MMEIs) in van der Waals magnets. However, owing to the extreme complexity and twist-angle-dependent sensitivity, all existing models fail to fully capture MMEIs and thus cannot provide an understanding of MMEI-induced physics. Here, we develop a microscopic moiré spin Hamiltonian that enables the effective description of MMEIs via a sliding-mapping approach in twisted magnets, as demonstrated in twisted bilayer CrI3. We show that the emergence of MMEIs can create a magnetic skyrmion bubble with non-conserved helicity, a 'moiré-type skyrmion bubble'. This represents a unique spin texture solely generated by MMEIs and ready to be detected under the current experimental conditions. Importantly, the size and population of skyrmion bubbles can be finely controlled by twist angle, a key step for skyrmion-based information storage. Furthermore, we reveal that MMEIs can be effectively manipulated by substrate-induced interfacial Dzyaloshinskii-Moriya interactions, modulating the twist-angle-dependent magnetic phase diagram, which solves outstanding disagreements between theories and experiments.

Interplay between quantum anomalous Hall effect and magnetic skyrmions.

SignificanceQuantum anomalous Hall effect (QAHE) and magnetic skyrmion (SK), as two typical topological states in momentum (K) and real (R) spaces, attract much interest in condensed matter physics. However, the interplay between these two states remains to be explored. We propose that the interplay between QAHE and SK may generate an RK joint topological skyrmion (RK-SK), characterized by the SK surrounded by nontrivial chiral boundary states (CBSs). Furthermore, the emerging external field-tunable CBS in RK-SK could create additional degrees of freedom for SK manipulations, beyond the traditional SK. Meanwhile, external field can realize a rare topological phase transition between K and R spaces. Our work opens avenues for exploring unconventional quantum states and topological phase transitions in different spaces.

Ferromagnetic barrier induced large enhancement of tunneling magnetoresistance in van der Waals perpendicular magnetic tunnel junctions.

van der Waals (vdW) intrinsic magnets are promising for miniaturization of devices beyond Moore's law for future energy efficient nanoelectronic devices and have been successfully used for constructing high performance vdW magnetic tunnel junctions (vdW MTJs). Here, using first principles calculations, we investigate the magnetic anisotropy, spin-dependent transport and tunneling magnetoresistance (TMR) effect of vdW MTJs formed by sandwiching a ferromagnetic (FM) monolayer CrI3 or non-magnetic monolayer ScI3 barrier between two vdW FM Fe3GeTe2 electrodes, respectively. It is found that two vdW MTJs possess strong perpendicular magnetic anisotropy. Moreover, due to no barrier for majority-spin transmission within half-metallic CrI3 barrier and the difference between majority- and minority-spin conduction channels of the Fe3GeTe2 electrode, a high TMR ratio of about 3100% is achieved in vdW MTJs based on the Fe3GeTe2/CrI3/Fe3GeTe2 vdW heterostructure. In contrast, a smaller TMR ratio of about 1200% is produced in vdW MTJs based on the Fe3GeTe2/ScI3/Fe3GeTe2 vdW heterostructure due to the strong suppression of ScI3 for majority-spin transmission in the case of the parallel state of magnetization of two FM electrodes. Our results provide a promising route for the design of vdW perpendicular MTJs with a high TMR ratio.

Perpendicular Magnetic Anisotropy and Dzyaloshinskii-Moriya Interaction at an Oxide/Ferromagnetic Metal Interface.

We report on the study of both perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) at an oxide/ferromagnetic metal (FM) interface, i.e., BaTiO_{3} (BTO)/CoFeB. Thanks to the functional properties of the BTO film and the capability to precisely control its growth, we are able to distinguish the dominant role of the oxide termination (TiO_{2} vs BaO) from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at an oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is 2 times larger than that of the TiO_{2}-BTO/CoFeB, while the DMI of the TiO_{2}-BTO/CoFeB interface is larger. We explain the observed phenomena by first principles calculations, which ascribe them to the different electronic states around the Fermi level at oxide/ferromagnetic metal interfaces and the different spin-flip process. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.

Interface-induced perpendicular magnetic anisotropy in Co2FeAl/NiFe2O4 superlattice: first-principles study.

A light element magnetic tunnel junction with perpendicular magnetic anisotropy (PMA) is crucial for the realization of high thermal stability and low critical switching current in next-generation high-density nonvolatile memory. Using first-principles calculations, we investigate the structure and magnetic anisotropy of a Co2FeAl/NiFe2O4 superlattice. It is found that the most energetically favorable configurations for Co2FeAl(001)/NiFe2O4(001) interfaces are when the interface O atoms in NiFe2O4 are on top of the interface metal atoms in Co2FeAl due to the bonding between interface O atoms in NiFe2O4 and interface metal atoms in Co2FeAl. Interestingly, a large PMA of up to 1.07 mJ m-2 can be obtained at the interface between Co-terminated Co2FeAl and NiO-terminated NiFe2O4 and the interface Co atoms play an important role in establishing the large PMA at the Co2FeAl/NiFe2O4 interface. The d-orbital-resolved magnetic anisotropy energy of interface and surface Co atoms reveals that compared to surface Co, the matrix element differences between dz2 and dyz as well as dx2-y2 and dxy orbitals of the interface Co provide large contributions to the PMA of interface Co, which originates mainly from the different electron occupations of dz2, dyz, dx2-y2 and dxy orbitals between the interface Co and surface Co due to the bonding between interface Co atoms in Co2FeAl and interface O atoms in NiFe2O4. Our results indicate that the Co2FeAl/NiFe2O4 heterostructures are promising candidates for achieving large interfacial PMA in light element heterostructures.

Strain-induced N-N bonding and magnetic changes in monolayer intrinsic ferromagnetic TmN2 (Tm = Tc and Nb).

The modulation of magnetic property in two-dimensional (2D) intrinsic ferromagnets is important for their future application in spintronic devices at the nanoscale. In this work, using first-principles calculation, we investigate the effects of strain on the structures, electronic structures and magnetic properties of monolayer 1H-NbN2 and 1H-TcN2. The results show that both the unstrained monolayer 1H-NbN2 and 1H-TcN2 are 2D intrinsically ferromagnetic (FM) metal, in which the magnetic moment of the 1H-NbN2 and 1H-TcN2 comes mainly from the d orbitals of Nb atom and the p orbitals of N atom, respectively. Remarkably, two neighboring N atoms in the unstrained 1H-NbN2 form N-N bond, while those in the 1H-TcN2 do not. When lattice constant a increases to 3.17 Å, monolayer 1H-TcN2 undergoes N-N nonbonding-bonding transition at which the distance between the N atoms d N-N suddenly drops by almost 25%. In particular, due to the bonding between two neighboring N atoms, the magnetic moment of N atoms in 1H-TcN2 are quenched and the ground state transfers to non-magnetic. In contrast, when a decreases to 3.18 Å, monolayer 1H-NbN2 undergoes N - N bonding-nonbonding transition at which the d N-N suddenly increases from 1.79 Å to 1.97 Å. The N-N bonding-nonbonding transition induces the magnetic moments to transfer from the d orbitals of Nb atom to the p orbitals of N atom, while ground state of monolayer 1H-NbN2 remains FM metal.

Giant Enhancements of Perpendicular Magnetic Anisotropy and Spin-Orbit Torque by a MoS2 Layer.

2D transition metal dichalcogenides have attracted much attention in the field of spintronics due to their rich spin-dependent properties. The promise of highly compact and low-energy-consumption spin-orbit torque (SOT) devices motivates the search for structures and materials that can satisfy the requirements of giant perpendicular magnetic anisotropy (PMA) and large SOT simultaneously in SOT-based magnetic memory. Here, it is demonstrated that PMA and SOT in a heavy metal/transition metal ferromagnet structure, Pt/[Co/Ni]2 , can be greatly enhanced by introducing a molybdenum disulfide (MoS2 ) underlayer. According to first-principles calculation and X-ray absorption spectroscopy (XAS), the enhancement of the PMA is ascribed to the modification of the orbital hybridization at the interface of Pt/Co due to MoS2 . The enhancement of SOT by the role played by MoS2 is explained, which is strongly supported by the identical behavior of SOT and PMA as a function of Pt thickness. This work provides new possibilities to integrate 2D materials into promising spintronics devices.

Evidence of a strong perpendicular magnetic anisotropy in Au/Co/MgO/GaN heterostructures.

We report a strong perpendicular magnetic anisotropy (PMA) in Au/Co/MgO/GaN heterostructures from both experiments and first-principles calculations. The Au/Co/MgO heterostructures have been grown by molecular beam epitaxy (MBE) on GaN/sapphire substrates. By carefully optimizing the growth conditions, we obtained a fully epitaxial structure with a crystalline orientation relationship Au(111)[1̄10]//Co(0001)[112̄0]//MgO(111)[101̄]//GaN(0002)[112̄0]. More interestingly, we demonstrate that a 4.6 nm thick Co film grown on MgO/GaN still exhibits a large perpendicular magnetic anisotropy. First-principles calculations performed on the Co (4ML)/MgO(111) structure showed that the MgO(111) surface can strongly enhance the magnetic anisotropy energy by 40% compared to a reference 4ML thick Co hcp film. Our layer-resolved and orbital-hybridization resolved anisotropy analyses helped to clarify that the origin of the PMA enhancement is due to the interfacial hybridization of O 2p and Co 3d orbitals at the Co/MgO interface. The perpendicularly magnetized Au/Co/MgO/GaN heterostructures are promising for efficient spin injection and detection in GaN based opto-electronics without any external magnetic field.

Large magnetic anisotropy and strain induced enhancement of magnetic anisotropy in monolayer TaTe2.

Monolayer TaTe2 holds great potential for the realization of large magnetocrystalline anisotropy due to strong spin-orbit coupling (SOC) interactions of Ta. Here, we systematically investigate the electronic structure, magnetism and magnetocrystalline anisotropy of monolayer TaTe2 under different strains by means of first-principles calculations. The results show that monolayer TaTe2 is a ferromagnetic metal and exhibits a large in-plane magnetic anisotropy energy (MAE) of -11.38 meV per TaTe2. It is worth noting that the magnetic moment, magnetic coupling and magnetic anisotropy of monolayer TaTe2 are significantly enhanced by strain. In particular, when tensile strain increases from 0% to 8%, the MAE of monolayer TaTe2 greatly increases from -11.38 to -15.14 meV per TaTe2. By analyzing the density of states and the contribution to magnetocrystalline anisotropy (MCA) from the SOC interaction between two d orbitals of Ta atoms based on second-order perturbation theory, it is concluded that a large MAE of monolayer TaTe2 is mainly contributed by the SOC interaction between opposite spin dxy and dx2-y2 orbitals of Ta atoms and the significant increase of the negative contribution to MCA from the SOC interaction between opposite spin dxy and dx2-y2 orbitals under strain is the reason why the MAE of monolayer TaTe2 is significantly enhanced by strain. Our results indicate that monolayer TaTe2 is a promising candidate suitable for applications in magnetic storage devices.

Curvature-enhanced Spin-orbit Coupling and Spinterface Effect in Fullerene-based Spin Valves.

We investigated curvature-enhanced spin-orbit coupling (SOC) and spinterface effect in carbon-based organic spin valves (OSVs) using buckyball C60 and C70 molecules. Since the naturally abundant (12)C has spinless nuclear, the materials have negligible hyperfine interaction (HFI) and the same intrinsic SOC, but different curvature SOC due to their distinct curvatures. We fitted the thickness dependence of magnetoresistance (MR) in OSVs at various temperatures using the modified Jullière equation. We found that the spin diffusion length in the C70 film is above 120 nm, clearly longer than that in C60 film at all temperatures. The effective SOC ratio of the C70 film to the C60 film was estimated to be about 0.8. This was confirmed by the magneto-electroluminescence (MEL) measurement in fullerene-based light emitting diodes (LED). Next, the effective spin polarization in C70-based OSVs is smaller than that in C60-based OSVs implying that they have different spinterface effect. First principle calculation study shows that the spin polarization of the dz(2) orbital electrons of Co atoms contacted with C60 is larger causing better effective spin polarization at the interface.

A first-principles study on the magnetic properties of nonmetal atom doped phosphorene monolayers.

In order to induce magnetism in two-dimensional semiconductors for their applications in spintronic devices and novel chemical and electronic properties of semiconducting phosphorene, the geometrical structure, electronic and magnetic properties of doped phosphorene monolayers with a series of nonmetal atoms, including H, F, Cl, Br, I, B, C, Si, N, As, O, S and Se, were systematically investigated using first-principles calculations. The results show that although the substitutional doping of H, F, Cl, Br, I, B, N, O, S or Se results in large structural deformation at the doping sites of phosphorene monolayers, all neutral nonmetal atom doped systems are stable. The calculated formation energies reveal that the substitutional doping of numerous nonmetal atoms in phosphorene monolayer are possible under appropriate experimental conditions, and the charged dopants C(-), Si(-), S(+) and Se(+) are stable. Moreover, the substitutional doping of H, F, Cl, Br, I, B, N, As, C(-), Si(-), S(+) or Se(+) cannot induce magnetism in phosphorene monolayer due to the saturation or pairing of valence electrons of dopant and its neighboring P atoms, whereas ground states of neutral C, Si, O, S or Se doped systems are magnetic due to the appearance of an unpaired valence electron of C and Si or the formation of a nonbonding 3p electron of a neighboring P atom around O, S and Se. Furthermore, the magnetic coupling between the moments induced by two Si, O, S or Se are long-range anti-ferromagnetic and the coupling can be attributed to the hybridization interaction involving polarized electrons, whereas the coupling between the moments induced by two C is weak.