Fe Monolayer Research Articles

Tuning exchange interactions in antiferromagnetic Fe/W(001) by 4d transition-metal overlayers

We use first-principles calculations based on density functional theory to study how the magnetic properties of an Fe monolayer on a W(001) surface---exhibiting a $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic ground state---can be modified by an additional $4d$ transition-metal overlayer. To obtain an overview of how the $4d$-band filling influences the exchange interactions in the Fe layer we have calculated the energy dispersion of spin spirals for $4d/\mathrm{Fe}/2\mathrm{W}$ unsupported quadlayers, in which the W(001) substrate is represented by only two atomic layers. Hybridization with the overlayer leads to a reduced ferromagnetic nearest-neighbor exchange interaction and the next-nearest neighbor exchange gains in strength. Surprisingly, we find that the $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic state is unfavorable for all systems with a $4d$ overlayer. For $4d$ overlayers from the beginning (Nb) or end (Pd) of the series we find a ferromagnetic ground state. As one moves to the center of the series there is a transition via a spin spiral (Mo, Rh) to a $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic ground state (Tc, Ru). We have studied the Mo, Ru, and Pd overlayer on Fe/W(001) representing the surface by a sufficiently large number of W layers to obtain bulklike properties in its center. The energy dispersions of spin spirals show qualitatively the same results as those from the $4d/\mathrm{Fe}/2\mathrm{W}$ quadlayers. The Dzyaloshinskii-Moriya interaction calculated upon including spin-orbit coupling shows significant strength and considerable frustration effects. The calculated magnetocrystalline anisotropy energy is large as well. All $4d/\mathrm{Fe}/\mathrm{W}(001)$ films are potential candidates for complex noncollinear spin structures.

Prediction of two-dimensional M2As (M = Mn, Fe) with high Curie temperature and large perpendicular magnetic anisotropy

Recently, with the miniaturization of electronic devices, there is an urgent need to use two-dimensional (2D) ferromagnetic materials with outstanding properties (such as high conductivity, better stability, etc.) and high Curie temperature (TC) in spintronic devices. By first-principles calculations based on density functional theory, we predict two new ferromagnetic metal monolayers M2As (M = Mn, Fe) with hight dynamic and thermal stabilities. Caculation results show that M2As monolayers possess high spin polarization ratio (27.3% for Mn2As and 52.6% for Fe2As), obvious out-of-plane magnetic anisotropy and large magnetic perpendicular anisotropy (101 μeV/Mn for Mn2As and 84 μeV/Fe for Fe2As). In addition, we found that the Mn2As monolayer exhibit high temperature ferromagnetism (TC = 450 K); while for the Fe2As monolayer, a high Curie temperature of 251 K was predicted. Moreover, we applied biaxial strain to the material to examine the changes in its magnetic properties and found that under a certain biaxial strain, Mn2As monolayer will have a ferromagnetic-antiferromagnetic phase transition, while Fe2As monolayer may exhibit a higher TC. Our findings render M2As (M = Mn, Fe) monolayers potential candidates for future spintronics and pave the way for exploring novel 2D magnetism in arsenic-based magnetic materials.

High-T c superconductor Fe(Se,Te) monolayer: an intrinsic, scalable and electrically tunable Majorana platform.

Iron-based superconductors have been identified as a novel platform for realizing Majorana zero modes (MZMs) without heterostructures, due to their intrinsic topological properties and high-Tc superconductivity. In the two-dimensional limit, the FeTe1−xSex monolayer, a topological band inversion has recently been experimentally observed. Here, we propose to create MZMs by applying an in-plane magnetic field to the FeTe1−xSex monolayer and tuning the local chemical potential via electric gating. Owing to the anisotropic magnetic couplings on edges, an in-plane magnetic field drives the system into an intrinsic high-order topological superconductor phase with Majorana corner modes. Furthermore, MZMs can occur at the domain wall of chemical potentials at either one edge or certain type of tri-junction in the two-dimensional bulk. Our study not only reveals the FeTe1−xSex monolayer as a promising Majorana platform with scalability and electrical tunability and within reach of contemporary experimental capability, but also provides a general principle to search for realistic realization of high-order topological superconductivity.

Multiple electrochemically accessible colour states in surface-confined metal–organic monolayers: stepwise embedding of individual metal centres

Sequential embedding of metal complexes of 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine to a surface-enhanced supports pre-functionalized with a templating layer results in hetero-bimetallic (Os–Fe, Co–Fe) and hetero-trimetallic (Co–Os–Fe) monolayer materials.

Первопринципные расчеты магнитной анизотропии пленок Fe и Co, разделенных прослойкой немагнитных металлов

This paper presents the results of numerical calculations of the magnetic characteristics of Co and Fe monolayer films on Cu and Pt surfaces using the VASP software package. The values ​​of the difference between the total energies of the antiferromagnetic and ferromagnetic configurations are calculated depending on the convergence parameters and the thickness of the nonmagnetic material. The values ​​of the magnetic anisotropy and magnetic moment of atoms in the structures of Co / Cu / Co, Fe / Pt / Fe, Co / Pt / Co, Pt / Co / Cu / Co / Pt are determined depending on the orientation of the surface face. For the (110) and (111) faces, the phenomenon of reorientation in the Co / Cu / Co structure is confirmed, when the anisotropy of the cobalt films parallel to the surface plane is replaced by the perpendicular anisotropy due to the introduction of an ultrathin platinum film into the structure.

Ab initio calculations of magnetic anisotropy of Co and Fe films on Cu and Pt surfaces

In this work, the results of numerical first-principles calculations of the magnetic characteristics for Co and Fe monolayer films on Cu(100) and Pt(100) slabs by using the VASP software package are presented. The values of the difference in the total energy of antiferromagnetic and ferromagnetic configurations depending on the parameters of convergence and thickness of non-magnetic material are calculated. Favorable magnetic anisotropy energy and values of the magnetic moment of atoms in the Co/Cu/Co, Co/Pt/Co, and Pt/Co/Cu/Co/Pt structures are determined. The phenomenon of reorientation in the Co/Cu(100)/Co structure, when the in-plane anisotropy of the cobalt films changes to an out-of-plane anisotropy due to the incorporation of an ultrathin platinum film into the structure is confirmed.

Back Cover: Probing Proximity‐Tailored High Spin–Orbit Coupling in 2D Materials (Adv. Quantum Technol. 9/2020)

Simultaneous extraction of the magnetic and spin-orbit coupling (SOC) information of an interface using the weak measurement assisted spin-Hall effect of light (SHEL) is demonstrated as a novel probe to characterize materials' magnetic properties, see article number 2000042 by Karthik V. Raman, Tharangattu N. Narayanan, Nirmal K. Viswanathan, and co-workers. A heterostructure of Fe and MoS2 monolayers is shown for enhanced SOC due to the proximity effect, which is proven via SHEL method and then confirmed using magneto-optic Kerr effect (MOKE) studies.

Towards skyrmion-superconductor hybrid systems

Spin-polarized scanning tunneling microscopy is used to identify the magnetic state of different thin films on a Re(0001) substrate, which becomes superconducting below 1.7 K. All magnetic films contain an Fe/Ir interface, which is known to facilitate the emergence of non-collinear magnetic order. For Fe monolayers on ultra-thin Ir films of different thickness we find several different atomic-scale magnetic states. For Pd/Fe bilayers we find nano-scale spin spirals as magnetic ground states for Ir thicknesses of three and four atomic layers. In applied magnetic fields skyrmions emerge, and in remanence non-trivial magnetic textures survive. This demonstrates the possibility to prepare skryrmion-hosting magnetic films on superconducting substrates.

Origin of the large voltage-controlled magnetic anisotropy in a Cr/Fe/MgO junction with an ultrathin Fe layer: First-principles investigation

Voltage-controlled magnetic anisotropy (VCMA) has attracted broad interest due to its high efficiency in switching magnetization. Large VCMA was experimentally observed in Cr/Fe/MgO junction with ultrathin Fe layer [Nozaki et al., Phys. Rev. Appl. 5, 044006 (2016)], whose underlying mechanism was still not clear however. The Cr/Fe/MgO/Fe magnetic tunnel junction (MTJ) is also well known for its quantum-well (QW) states and as-induced spin-dependent resonant tunneling [Greullet et al., Phys. Rev. Lett. 99, 187202 (2007)]. Here, in order to uncover the relation between the large VCMA and the QW states, we developed a $k$-resolved VCMA calculation method combined with the second-order perturbation theory to investigate it. We find the VCMA coefficient reaches $\ensuremath{-}297$ fJ/V m matching well with the previous experiment with three monolayers (MLs) of Fe. The coefficient oscillates strongly and even changes its sign with increasing the number of Fe MLs. Comparing the $k$-resolved VCMA with the Fermi surface of the interfacial Fe atom, the screening charges theory for VCMA was verified. For $2--9$ MLs Fe, interestingly, the QW states of ${\mathrm{\ensuremath{\Delta}}}_{1}$ electron at the $\mathrm{\ensuremath{\Gamma}}$ point provide large (no) contribution to the VCMA with odd (even) MLs. Moreover, the change of the orbital-resolved Fermi surface at the interfacial Fe atom also plays an important role on VCMA oscillation, which as well as the QW states results in the largest VCMA for 3-ML Fe. Our results deepen the understanding of the large VCMA in the Cr/Fe/MgO junction, which would be helpful to design a practical MTJ with large VCMA.

Electronic structure and magnetism of MTe2 (M = Ti, V, Cr, Mn, Fe, Co and Ni) monolayers

We study the electronic structure and magnetism of monolayer 3d transition-metal ditellurides MTe2 (M = Ti, V, Cr, Mn, Fe, Co and Ni) in trigonal prismatic H- and/or octahedral T-phase by means of the first-principles calculations. The results show that H-VTe2, T-VTe2, H-FeTe2 and T-MnTe2 monolayers exhibit intrinsic ferromagnetism, and the others have no ferromagnetism. The exchange splitting of V, Mn and Fe 3d orbitals is responsible for ferromagnetism. The exchange constant and the Curie temperature are estimated by using 2D Ising model and mean field theory. Among the four ferromagnetic monolayers, the H-VTe2 monolayer has the largest exchange constant and the corresponding Curie temperature is near room temperature. Calculations also show that the T-VTe2, H-VTe2 and H-FeTe2 monolayers have in-plane easy magnetization direction, while the easy direction of the T-MnTe2 monolayer is perpendicular to the layer. Moreover, we analyze the relative exchange strength of the four ferromagnetic monolayers based on the competition between the through-bond ferromagnetic interaction and the through-space antiferromagnetic interaction according to the Goodenough–Kanamori rules. The magnetocrystalline anisotropy is explained qualitatively based on the second-order perturbation theory from the spin-orbit coupling between 3d orbitals of M atoms.

Plumbene on a Magnetic Substrate: A Combined Scanning Tunneling Microscopy and Density Functional Theory Study.

As a heavy analog of graphene, plumbene is a two-dimensional material with strong spin-orbit coupling effects. Using scanning tunneling microscopy, we observe that Pb forms a flat honeycomb lattice on an Fe monolayer on Ir(111). In contrast, without the Fe layer, a c(2×4) structure of Pb on Ir(111) is found. We use density-functional theory calculations to rationalize these findings and analyze the impact of the hybridization on the plumbene band structure. In the unoccupied states the splitting of the Dirac cone by spin-orbit interaction is clearly observed, while the occupied Pb states are strongly hybridized with the substrate. In a freestanding plumbene we find a band inversion below the Fermi level that leads to the formation of a topologically nontrivial gap. Exchange splitting as mediated by the strong hybridization with the Fe layer drives a quantum spin Hall to quantum anomalous Hall state transition.

Modulated and monotonic monolayer-magnetism in hetero-epitaxial Fe on Cu(001)

We have combined two complementary techniques, element sensitive ex situ X-ray magnetic circular dichroism (XMCD) and in situ polarized neutron reflectivity (i-PNR), to determine the values of evolving magnetic moments obtained from a low symmetry system of hetero-epitaxial Fe monolayers (MLs), as a function of thickness. The samples were grown by magnetron sputtering on face-centered-cubic (fcc) Cu(001)/Si(001). Within experimental errors, we found a corroboration of the modulated moments from the XMCD and of the magnetic anisotropies from magnetization measurements with those obtained earlier from layer-by-layer i-PNR measurements. The modulation was attributed to initial island-like growth morphology assisted monolayer-magnetism during sputtering. Furthermore, analyzing the depth sensitive i-PNR profile of a bulk-like film, we developed a model characterized by monotonic magnetism involving collinear spins. The model is further described by a higher magnetization for the deeper layers, which decreases gradually for the upper layers before attaining saturation at its bulk value within a few MLs. This model contradicts the earlier proposed non-collinear model involving antiferromagnetic units, thus clarifying the long standing ambiguity in the coverage regime. The results have been compared with those existing, following the theoretical parameterized tight-binding model with satisfactory agreement. This study distinguishes the variation of monolayer-magnetism owing to the growth morphology from the layer-by-layer investigation vis-à-vis depth-profiling of bulk-like film. At the same time, it also promises the general possibility of depth-profiling using i-PNR in other complex multilayered systems on high flux neutron sources.

Adsorption of [formula omitted] molecule on the [formula omitted] substrate: Energetics, electronic and magnetic properties

Using spin-polarized density-functional calculations with generalized gradient approximation (GGA) as the exchange-correlation functional, we report the energetics of adsorption, as well as the electronic and magnetic properties of homonuclear bromine molecule deposited on the Fe/W(110) substrate. The Fe/W(110) substrate consist in Fe monolayer on the topmost W(110) layer such that the Fe atoms are bonded on the bridge adsorption sites of the W(110) surface. Our calculations show that when the Br2 molecule is adsorbed in parallel, oblique or vertical orientation with respect to the surface, the molecule dissociates. However, when the Br2 molecular adsorption is perpendicular to the bridge or top sites, non-dissociative adsorption occurs. We also found that the observed Br2 adsorption properties depend on the interaction between the 4p orbitals of adsorbed bromine specie and the Fe-3d orbitals of Fe atoms of the Fe/W(110) substrate. Furthermore, it is shown that when the bromine atoms are attached on its energetically preferred adsorption sites, the magnetic moments of Fe atoms are reduced while magnetic moment is induced on the bromine atoms. These results are consistent with a previous work on the O2 adsorption.

The chiral biquadratic pair interaction

Magnetic interactions underpin a plethora of magnetic states of matter, hence playing a central role both in fundamental physics and for future spintronic and quantum computation devices. The Dzyaloshinskii–Moriya interaction, , being chiral and driven by relativistic effects, leads to the stabilization of highly-noncollinear spin textures such as skyrmions, which thanks to their topological nature are promising building blocks for magnetic data storage and processing elements. Here, we reveal and study a new chiral pair interaction, , which is the biquadratic equivalent of the DMI. First, we derive this interaction and its guiding principles from a microscopic model, and we connect the atomistic form to the micromagnetic one. Second, we study its properties in the simplest prototypical systems, magnetic 3d transition metal dimers deposited on the Pt(111), Pt(100), Ir(111), and Re(0001) surfaces, resorting to systematic first-principles calculations. Lastly, we discuss its importance and implications not only for magnetic dimers but also for extended systems, namely one-dimensional spin spirals and complex two-dimensional magnetic structures, such as a nanoskyrmion lattice found in an Fe monolayer on Ir(111).

Tuning noncollinear magnetic states by hydrogenation

Two different superstructures form when atomic H is incorporated in the Fe monolayer on Ir(111). Depending on the amount of H provided, either a highly ordered p(2x2) hexagonal superstructure or an irregular roughly square structure is created. We present here spin-polarized scanning tunneling microscopy (SP-STM) measurements which reveal that in both cases the magnetic nanoskyrmion lattice state of the pristine Fe monolayer is modified. Our measurements of the magnetic states in these hydrogenated films are in agreement with superpositions of cycloidal spin spirals which follow the pattern and the symmetry dictated by the H superstructures. We thus demonstrate here the possibility to vary the symmetry of a non-collinear magnetic state in an ultrathin film without changing its substrate.

Electric Field Induced Room Temperature Null to High Spin State Switching: A Computational Prediction

AbstractBaTiO3 is a well‐known ferroelectric material, with an electric field‐induced coupled electrical/structural transition from a cubic to a tetragonal lattice above room temperature. Via DFT calculations, the magnetic properties of an Fe monolayer sandwiched between bulk‐like BaTiO3 are explored, showing how the magnetic moment of each Fe atom is switched from a null spin to a high spin (2 μB per Fe) state when the BaTiO3 structure changes from cubic to tetragonal. This effect occurs in an optimized range of interatomic distances at the interface, allowing the design of an above‐room temperature binary, electric field write/magnetic read device.

First-principles investigation of the interface magnetic anisotropy of Fe/SrTiO3

Interface effects in magnetic nanostructures play a critical role in the magnetic properties. By using first-principles density functional theory calculations, we investigate the electronic and magnetic properties of Fe/SrTiO3 interfaces, in which both the nonpolar surface SrTiO3(0 0 1) and the polar surface SrTiO3(1 1 0) are considered. A particular emphasis is placed on the magnetic anisotropy energy (MAE). Comparing MAE of the Fe/SrTiO3 interfaces and the corresponding Fe monolayers, we find the Fe/SrTiO3(0 0 1) interface decreases MAE, while the Fe/SrTiO3(1 1 0) interface increases MAE. The interface orbital hybridization and orbital magnetic moments are analyzed in detail to understand the different interface magnetic anisotropy. Our investigation indicates that interface engineering can be an effective way to modulate the magnetic properties.

Stability and magnetic properties of Fe double layers on Ir (111)

We investigate the interplay between the structural reconstruction and the magnetic properties of Fe double layers on Ir (111) substrate using first-principles calculations based on density functional theory and mapping of the total energies on an atomistic spin model. We show that if a second Fe monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal-close-packed to bcc (110)-like accompanied by a reduction of symmetry from trigonal to centered rectangular. Although the bcc-like surface has a lower coordination, we find that this is the structural ground state. This reconstruction has a major impact on the magnetic structure. We investigate in detail the changes in the magnetic exchange interaction, the magnetocrystalline anisotropy, and the Dzyaloshinskii-Moriya interaction depending on the stacking sequence of the Fe double layer. Based on our findings, we suggest a technique to engineer Dzyaloshinskii-Moriya interactions in multilayer systems employing symmetry considerations. The resulting anisotropic Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or antiskyrmions.

Efficient technique for ab-initio calculation of magnetocrystalline anisotropy energy

Ab-initio calculation of magnetocrystalline anisotropy energy (MCAE) often requires a strict convergence criterion and a dense k-point mesh to sample the Brillouin zone, making its convergence problematic and time-consuming. The force theorem for MCAE states that MCAE can be calculated by the band energy difference between two magnetization directions at a fixed potential. The maximally localized Wannier function can be utilized to construct a compact Hilbert space of low-lying electron states and interpolate band eigenvalues with high precession. We combine the force theorem and the Wannier interpolation of eigenvalues together to improve the efficiency of MCAE calculations with no loss of accuracy. We use a Fe chain, a Fe monolayer and a FeNi alloy as examples and demonstrate that the Wannier interpolation method for MCAE is able to reduce the computational cost significantly and remain accurate simultaneously, compared with a direct ab-initio calculation on a very dense k-point mesh. This efficient Wannier interpolation approach makes it possible for large-scale and high-throughput MCAE calculations, which could benefit the design of spintronics devices.

Enhanced ferroelectricity and conductance in iron-doped polystyrene sulfonate

We report on the synthesis of poly(styrene sulphonate) (PSS) doped with iron ions (PSS:Fe) and its electrical properties in J-Field Effect Transistors (J-FET). PSS:Fe films exhibited enhanced ferroelectric and conductance in comparison with non-doped PSS films, with a conductivity of 4.47 10−6 S/m. In comparison with traditional ferroelectric polymers, such as poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], PSS:Fe films had coercive fields three orders of magnitude smaller and remnant polarization six orders of magnitude higher. When operated with a negative gate voltage (VG), the drain current (ID) increased significantly, while positive gate voltages reduced the conductor channel and decreased ID. The proposed in-plane geometry with PSS:Fe monolayer films exhibits suitable ferroeletricity characteristics.