Fe Monolayer Research Articles

Effect of strain on the magnetic states of transition-metal atoms doped monolayer WS2

The effects of transition-metal atoms doping and elastic strain on the magnetic properties of monolayer WS2 are investigated by the first-principles calculation. Firstly, we investigate the magnetic properties of a series of 3d transition-metal atoms (from Sc to Zn) doped monolayer WS2. Our calculations show that doping of transition-metal atoms from Sc to Cr results in nonmagnetic states, while Mn, Fe, Co, Ni, Cu and Zn doping can induce magnetism in WS2 monolayer. The localized nonbonding 3d electrons of TM atoms lead to the formation of magnetic moments. Meanwhile, our calculations indicate that the elastic strain can be used to manipulate the spin polarization of TM-3d orbital. Specifically, the magnetic moment of Fe and Co-doped monolayer WS2 increase with the increase of tensile strain, while the magnetic moment of Mn, Ni, Cu and Zn-doped WS2 monolayer initially increase and then decrease with the increase of tensile strain. On the other hand, the magnetic moment of Fe, Co, Mn, Ni, Cu and Zn-doped WS2 monolayers decrease with the increase of compressive strain. While the magnetic moment of Sc, Ti, V and Cr-doped WS2 monolayers remain 0μB independent of tensile strain and compressive strain.

Tuning the electronic structures and magnetism of two-dimensional porous C2N via transition metal embedding.

Based on first-principles calculations, the electronic structures and magnetism are investigated in 3d transition metal (TM)-embedded porous two-dimensional (2D) C2N monolayers. Numerical results indicate that except Mn and Co atoms, other TM atoms can be embedded stably in the 2D C2N monolayer. Moreover, the magnetic moments of the TM-embedded C2N monolayer depend highly on the atomic number of the TM atoms. The Sc, Ti, V, Cr, Mn, Fe, Co and Ni atom-embedded C2N monolayers possess a ferromagnetic ground state, while embedding Cu can induce paramagnetic characteristics in the 2D C2N monolayer. Meanwhile, the Zn-embedded C2N monolayer exhibits a nonmagnetic ground state. These results indicate that the magnetism of 2D C2N monolayers can be tuned via embedding TM atoms.

Multiferroic heterostructures for spin filter applications: Anab initiostudy

Novel imaging spin-filter techniques, which are based on low energy electron diffraction, are currently of high scientific interest. To improve the spin-detection efficiency a variety of new materials have been introduced in recent years. A new class of promising spin-filter materials are represented by multiferroic systems, as both magnetic and electric ordering exist in these materials. We have investigated Fe/BaTiO3(001), which defines a prominent candidate due to its moderate spontaneous polarization, for spin filter applications calculating diffraction patterns for spin polarized electrons incident on the Fe surface. Motivated by the fact that spin polarized low energy electron diffraction is a powerful method for the determination of the properties of surfaces we investigated the influence of switching the BaTiO3 polarization on the exchange and spin orbit scattering as well as on reflectivity and figure of merit. This system obviously offers the possibility to realize a multiferroic spin filter and manipulating the spin-orbit and exchange scattering by an external electric field. The calculations have been done for a large range of kinetic energies and polar angles of the diffracted electrons considering different numbers of Fe monolayers.

Tuning magnetic anistropy by charge injection and strain in Fe/MoS2 bilayer heterostructures

Ferromagnetic/MoS2 junctions are significant building blocks for spintronics. Our first-principle calculations show that magnetic anisotropy energy (MAE) and the direction of magnetization in the Fe/MoS2 system can be tailored by injecting charge or strain engineering. We demonstrate that these two approaches have a significant influence on the magnitude of the MAE. The hybridization between Fe 3 d and Mo 4 d electrons changes the occupation of d orbitals around the Fermi level, which leads to the strong dependence of MAE on the external charge injection or strain. MAE increases up to ten times higher than that of the neutral system when injecting charge. More intriguing is that the biaxial strain can affect the easy-axis switching in Fe/MoS2, while the process does not occur in the single Fe monolayer.

Two-dimensional iron oxide bi-and trilayer structures on Pd(100)

The growth morphology and structure of iron oxide films, prepared by postoxidation of Fe monolayers on a Pd(100) surface, have been investigated in a multitechnique study, using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), high-resolution x-ray photoelectron spectroscopy (HR-XPS) and x-ray absorption spectroscopy (XAS), both using synchrotron radiation, and comprehensive density functional theory (DFT) analysis. A two-dimensional (2-D) hexagonal O–Fe–O trilayer phase has been generated at submonolayer Fe coverages, which converges into two different 2-D hexagonal Fe–O bilayer structures at one monolayer. One phase exhibits a c(8×2) coincidence structure and is associated with a stoichiometric FeO(111) bilayer. The second phase displays a superstructure of triangular loops, which is understood from DFT modeling as excess O ad-atoms in the terminating oxygen layer, thus corresponding to a FeO bilayer with a formal FeO1.125 stoichiometry. Annealing the latter in ultrahigh vacuum to 770K results in the pure c(8×2) wetting layer. The thermodynamic stability of the O–Fe–O trilayer and FeO bilayer phases is analyzed in the DFT framework and is found to be in good agreement with the experiment. The absence of a c(4×2)-Fe3O4 phase in the experimental phase diagram, which is found to be stable by DFT and is experimentally encountered for other transition metal oxide films, such as Ni-, Co-, and Mn-oxide on Pd(100), is ascribed to kinetic reasons.

Dirac fermions in a Fe ultrathin film

We show the existence of massive Dirac fermions in electronic band structures of a few Fe atomic layers with perpendicular magnetization. Based on a tight binding model fitted to ab-initio band structure, we observe four distinct massive Dirac fermions near the Fermi level, which result from atomic spin-orbit coupling of Fe and a band inversion between Fe $4s$-$3d_{x^2-y^2}$ hybrid orbital band and $3d_{xy}$ orbital band. These lead to a valence band with finite Chern integer (+2) and chiral edge modes near the Fermi level. When the chemical potential is set inside the Dirac gap by carrier doping, the Hall conductivity exhibits a plateau-like structure with quantized value $2\frac{e^2}{h}$, and orbital magnetization shows a prominent increase, latter of which is mostly due to chiral orbital motion of electrons along the edge modes. We discuss the stability of the Dirac fermions in Fe(001) monolayer on MgO(001) substrate and Fe(001) bilayer case.

Arsenic-bridged magnetic interactions in an emerging two-dimensional FeAs nanostructure on MnAs

The extreme case of an Fe monolayer deposited onto a manganese arsenide (MnAs) substrate is analyzed using density functional theory. We find that an FeAs quasi-two-dimensional antiferromagnetic surface nanostructure emerges. This nanostructure, which is magnetically nearly decoupled from the substrate, is due to bonding effects arising from the arsenic atoms bridging the Fe magnetic interactions. These interactions are studied and modeled using a Heisenberg-type Hamiltonian. They display an angular dependence which is characteristic of superexchange-like interactions, which are of the same order of magnitude as those appearing in Fe-based pnictides.

Topological orbital magnetization and emergent Hall effect of an atomic-scale spin lattice at a surface

We predict the occurrence of a novel type of atomic-scale spin lattice in an Fe monolayer on the Ir(001) surface. Based on density functional theory calculations we parametrize a spin Hamiltonian and solve it numerically using Monte Carlo simulations. We find the stabilization of a three-dimensional spin structure arranged on a $(3\ifmmode\times\else\texttimes\fi{}3)$ lattice. Despite an almost vanishing total spin magnetization we predict the emergence of orbital magnetization and large anomalous Hall effect, to which there is a significant topological contribution purely due to the real space spin texture at the surface.

Growth and magnetic domain structure of ultrathin Fe films on Rh(001)

The growth and magnetic domain structure of ultrathin Fe films epitaxially grown on face-centered cubic (fcc) Rh(001) is investigated by spin-polarized scanning tunneling microscopy (SP-STM) at low temperatures $(T=5.5\text{K})$. Our results indicate that the cleaning procedure applied to the Rh(001) substrate plays an important role for the formation of a clean and well-ordered Fe film. The Fe monolayer exhibits an out-of-plane antiferromagnetic $c(2\ifmmode\times\else\texttimes\fi{}2)$ spin structure. Islands of the second Fe layer are found to be out-of-plane ferromagnetic. Coalescence of the double-layer islands results in the formation of larger domains, which extend over several hundred nanometers for a closed two layer film. Further increasing the Fe coverage leads to a reduction of the domain size and the formation of patterns that are reminiscent of stripe domains driven by the competition of surface/interface and shape anisotropy.

Magnetic Nano-skyrmion Lattice Observed in a Si-Wafer-Based Multilayer System.

Growth, electronic properties, and magnetic properties of an Fe monolayer (ML) on an Ir/YSZ/Si(111) multilayer system have been studied using spin-polarized scanning tunneling microscopy. Our experiments reveal a magnetic nano-skyrmion lattice, which is fully equivalent to the magnetic ground state that has previously been observed for the Fe ML on Ir(111) bulk single crystals. In addition, the experiments indicate that the interface-stabilized skyrmion lattice is robust against local atomic lattice distortions induced by multilayer preparation.

Magnetic phase diagram of an Fe monolayer on W(110) and Ta(110) surfaces based onab initiocalculations

We present detailed investigations of the magnetic properties of an Fe monolayer on W and Ta (110) surfaces based on the ab initio screened Korringa-Kohn-Rostoker method. By calculating tensorial exchange coupling coefficients, the ground states of the systems are determined using atomistic spin dynamics simulations. Different types of ground states are found in the systems as a function of relaxation of the Fe layer. In case of W(110) substrate this is reflected in a reorientation of the easy axis from in-plane to out-of-plane. For Ta(110) a switching appears from the ferromagnetic state to a cycloidal spin spiral state, then to another spin spiral state with a larger wave vector and, for large relaxations, a rotation of the normal vector of the spin spiral is found. Classical Monte Carlo simulations indicate temperature-induced transitions between the different magnetic phases observed in the Fe/Ta(110) system. These phase transitions are analyzed both quantitatively and qualitatively by finite-temperature spin wave theory.

Influence of the local atom configuration on a hexagonal skyrmion lattice.

Spin-resolved scanning tunneling microscopy is used to reveal a commensurate hexagonal nanoskyrmion lattice in the hcp stacked Fe monolayer on Ir(111). The exact nature of the spin configuration is due to magnetic interactions between the Fe atoms and the Ir substrate, either originating from polarization effects, or due to a three-site hopping mechanism of the Dzyaloshinsky-Moriya interaction leading to a canting of the Dzyaloshinsky-Moriya vector with respect to the interface.

Stability and magnetization of free-standing and graphene-embedded iron membranes

Inspired by recent experimental realizations of monolayer Fe membranes in graphene perforations, we perform ab initio calculations of Fe monolayers and membranes embedded in graphene in order to assess their structural stability and magnetization. We demonstrate that monolayer Fe has a larger spin magnetization per atom than bulk Fe and that Fe membranes embedded in graphene exhibit spin magnetization comparable to monolayer Fe. We find that free-standing monolayer Fe is structurally more stable in a triangular lattice compared to both square and honeycomb lattices. This is contradictory to the experimental observation that the embedded Fe membranes form a square lattice. However, we find that embedded Fe membranes in graphene perforations can be more stable in the square lattice configuration compared to the triangular. In addition, we find that the square lattice has a lower edge formation energy, which means that the square Fe lattice may be favored during formation of the membrane.

Magnetic correlations beyond the Heisenberg model in an Fe monolayer on Rh(0 0 1)

Motivated by a recent experimental observation of a complex magnetic structure (Takada et al 2013 J. Magn. Magn. Mater. 329 95) we present a theoretical study of the magnetic structure of an Fe monolayer deposited on Rh(0 0 1). We use a classical spin Hamiltonian with parameters obtained from ab initio calculations and go beyond the usual anisotropic Heisenberg model by including isotropic biquadratic interactions. Zero-temperature Landau–Lifshitz–Gilbert spin dynamics simulations lead to a complex collinear spin configuration that, however, contradicts experimental findings. We thus conclude that higher order multi-spin interactions are likely needed to account for the magnetic ordering of the system.

Electric-field-induced modification of the magnon energy, exchange interaction, and curie temperature of transition-metal thin films.

The electric-field-induced modification in the Curie temperature of prototypical transition-metal thin films with the perpendicular magnetic easy axis, a freestanding Fe(001) monolayer and a Co monolayer on Pt(111), is investigated by first-principles calculations of spin-spiral structures in an external electric field (E field). An applied E field is found to modify the magnon (spin-spiral formation) energy; the change arises from the E-field-induced screening charge density in the spin-spiral states due to p-d hybridizations. The Heisenberg exchange parameters obtained from the magnon energy suggest an E-field-induced modification of the Curie temperature, which is demonstrated via MonteCarlo simulations that take the magnetocrystalline anisotropy into account.

Magnetism and magnetocrystalline anisotropy of 3dtransition metal monolayers on Pt(001): a density-functional study.

We systematically investigate the effects of having Pt as a substrate and/or capping layer on the magnetism and magnetocrystalline anisotropy (MCA) of 3d transition metal (TMs; Cr, Mn, Fe, and Co) monolayers (MLs) by using a first-principles calculationl method. We found that Fe and Co MLs are ferromagnetic (FM) on a Pt(001) surface, but Mn and Cr MLs are antiferromagnetic (AFM). The magnetic moments are quite robust with additional Pt-capping. Furthermore, Pt-capping enhances the small perpendicular MCA (meV) of Fe/Pt(001) significantly to 4.44 meV. Our electronic structure analyses indicate that strong hybridization between Pt-5d and TM-3d orbitals plays a crucial role in determining magnetic ordering and MCA. For comparison we also calculated magnetism and MCA of 3d TM MLs on Ta(001) with and without Ta-capping.

First-principles analysis of a homochiral cycloidal magnetic structure in a monolayer Cr on W(110)

The magnetic structure of a Cr monolayer on a W(110) substrate is investigated by means of first-principles calculations based on noncollinear spin density functional theory (DFT). As magnetic ground state we find a long-period homochiral left-rotating spin spiral on top of an atomic-scale antiferromagnetic order of nearest-neighbor atoms. The rotation angle of the magnetic moment changes inhomogeneously from atom to atom across the spiral. We predict a propagation direction along the crystallographic $[001]$ direction with a period length of $|\ensuremath{\lambda}|=14.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, which is in excellent agreement with a modulation of the local antiferromagnetic contrast observed in spin-polarized scanning tunneling microscope experiments by Santos et al. [New J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction as the origin of the homochiral magnetic structure, competing with the Heisenberg-type exchange interaction and magnetocrystalline anisotropy energy. From DFT calculations we extract parameters for a micromagnetic model and thereby determine a considerable inhomogeneity of the spin spiral, increasing the period length by 6% compared to homogeneous spin spirals. The results are compared to the behavior of a Mn and Fe monolayer and Fe double layer on a W(110) substrate.

Local tunnel magnetoresistance of an iron intercalated graphene-based heterostructure

The lateral variation of the tunnel magnetoresistance (TMR) of a graphene-based vertical heterostructure is studied by spin-polarized scanning tunneling microscopy (SP–STM) using an Fe-coated probe tip. The well-defined heterostructure is obtained by the intercalation of a magnetic Fe monolayer at the graphene/Ir(1 1 1) interface. Its structure is characterized by a moiré pattern with a high corrugation. In contrast to the Fe / Ir(1 1 1) surface, graphene/Fe / Ir(1 1 1) exhibits ferromagnetic order with an out-of-plane easy magnetization axis. At the nanometer scale, our experiments reveal that the moiré pattern induces a lateral variation of the TMR, which reaches 80%. The measured TMR at valleys of the moiré pattern is higher than at hills. We interpret this modulation in terms of a different hybridization between graphene and Fe at valleys and hills due to a different graphene–Fe distance at these sites, which leads to a different transmission of spin-polarized states.

Surface strain improves molecular adsorption but hampers dissociation for N2 on the Fe/W(110) surface.

We compare the adsorption dynamics of N(2) on the unstrained Fe(110) and on a 10% expanded Fe monolayer grown on W(110) by performing classical molecular dynamics simulations that use potential energy surfaces calculated with density functional theory. Our results allow us to understand why, experimentally, the molecular adsorption of N(2) is observed on the strained layer but not on Fe(110). Surprisingly, we also find that while surface strain favors the molecular adsorption of N(2) it seems, on the contrary, to impede the dissociative adsorption. This result contrasts with previous examples for which strain is found to modify equally the energetics of chemisorption and dissociation.

Tunable magnetic properties of monoatomic metal-oxide Fe/MgO multilayers

Metal-oxide ${\mathrm{[Fe}}_{K}/(\text{MgO}){}_{L}{\mathrm{]}}_{N}$ multilayers were grown on MgO(001) substrates for various integer numbers $(K,L)$ of the Fe(001) and MgO(001) monolayers, respectively, and the number of repetitions $N$ varied from 6 to 30. Room-temperature conversion electron M\ossbauer spectroscopy (CEMS) measurements proved that the magnetic properties of these monoatomic multilayers were extremely sensitive to both the Fe and MgO sublayer thicknesses. A stable ferromagnetic state and a strong perpendicular magnetization component at room temperature were obtained by changing the sublayer thickness and the number of multilayer repetitions. The analysis of the CEMS spectra in correlation with the magneto-optic Kerr effect measurements indicated a complicated domain structure in this special type of metal-insulator material. The vortexlike domain structure was confirmed by micromagnetic simulations.