Fe Monolayer Research Articles

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.

Intrinsic noncollinear magnetization in Fe/Cr superlattices

Magnetic moments distribution in Fe3Crn superlattice series with fixed middle Fe monolayer and number of Cr monolayers (MLs) n from 1 to 45 is computed in the framework of collinear and noncollinear Periodic Anderson model. The superlattices are composed of layers in (001) and (110) plane with ideal interface. The total energy shows that noncollinear orientation of the magnetic moments remains the ground state for all superlattices with Cr thickness above 5 MLs. Distribution of the magnetic moments for Fe/Cr(001) superlattices depends on parity of the Cr MLs. For odd numbers Cr magnetic moments are canted and symmetrically distributed between the neighboring Fe slabs. The values of Cr moments are enhanced at the interface and weakened to the bulk in the middle. For even numbers of Cr MLs quasi-helicoidal magnetic moments distribution consisting of two interleaved spirals is found. The moments are screwing sequentially from Fe/Cr interface to perpendicular orientation, keeping the angles and moments for some successive MLs, and then continue screwing towards the next interface. In Fe/Cr(110) superlattices the magnetic moments of two nonequivalent atoms in the monolayer are canted to each other near Fe/Cr interface and then swing the direction on perpendicular to the fixed Fe moments.

Magnetic excitations in ultrathin magnetic films: Temperature effects

The idea of investigating large wave-vector magnetic excitations in ultrathin films by spin-polarized electron spectroscopy is briefly reviewed. The historical background of the paper is based on the personal experience of the authors who collaborated and discussed with Douglas Mills regarding this subject. Douglas Mills' impact on the understanding of fundamental mechanisms involved in the excitation process and the development of the theory of magnetic excitations is outlined.In addition, the temperature effects on the large wave-vector magnetic excitations in ultrathin Fe films are addressed. The experimental results of magnon excitations in the pseudomorphic Fe monolayer on W(110) are presented. The temperature dependence of the magnon dispersion relation is discussed.

Calculations of magnetic states and minimum energy paths of transitions using a noncollinear extension of the Alexander-Anderson model and a magnetic force theorem

Calculations of stable and metastable magnetic states as well as minimum energy paths for transitions between states are carried out using a noncollinear extension of the multiple-impurity Alexander-Anderson model and a magnetic force theorem which is derived and used to evaluate the total energy gradient with respect to orientation of magnetic moments -- an important tool for efficient navigation on the energy surface. By using this force theorem, the search for stable and metastable magnetic states as well as minimum energy paths revealing the mechanism and activation energy of transitions can be carried out efficiently. For Fe monolayer on W(110) surface, the model gives magnetic moment as well as exchange coupling between nearest and next-nearest neighbors that are in good agreement with previous density functional theory calculations. When applied to nanoscale Fe islands on this surface, the magnetic moment is predicted to be 10\% larger for atoms at the island rim, explaining in part an experimentally observed trend in the energy barrier for magnetization reversal in small islands. Surprisingly, the magnetic moment of the atoms does not change much along the minimum energy path for the transitions, which for islands containing more than 15 atom rows along either $[001]$ or $[1\bar{1}0]$ directions involves the formation of a thin, temporary domain wall. A noncollinear magnetic state is identified in a $7\times 7$ atomic row Fe island where the magnetic moments are arranged in an antivortex configuration with the central ones pointing out of the $(110)$ plane. This illustrates how the model can describe complicated exchange interactions even though it contains only a few parameters. The minimum energy path between this antivortex state and the collinear ground state is also calculated and the thermal stability of the antivortex state estimated.

Complex trend of magnetic order in Fe clusters on4dtransition-metal surfaces. I. Experimental evidence and Monte Carlo simulations

We demonstrate the occurrence of compensated spin configurations in Fe clusters and monolayers on Ru(0001) and Rh(111) by a combination of x-ray magnetic circular dichroism experiments, first-principles calculations, and Monte Carlo simulations. Our results reveal complex intracluster exchange interactions which depend strongly on the substrate $4d$-band filling, the cluster geometry, as well as lateral and vertical structural relaxations. The importance of substrate $4d$-band filling manifests itself also in small nearest-neighbor exchange interactions in Fe dimers and in a nearly inverted trend of the Ruderman-Kittel-Kasuya-Yosida coupling constants for Fe adatoms on the Ru and Rh surface.

Spin-correlations and magnetic structure in an Fe monolayer on 5d transition metal surfaces

We present a detailed first principles study on the magnetic structure of an Fe monolayer on different surfaces of 5d transition metals. We use the spin-cluster expansion technique to obtain parameters of a spin model, and predict the possible magnetic ground state of the studied systems by employing the mean field approach and, in certain cases, by spin dynamics calculations. We point out that the number of shells considered for the isotropic exchange interactions plays a crucial role in the determination of the magnetic ground state. In the case of Ta substrate we demonstrate that the out-of-plane relaxation of the Fe monolayer causes a transition from ferromagnetic to antiferromagnetic ground state. We examine the relative magnitude of nearest neighbour Dzyaloshinskii–Moriya (D) and isotropic (J) exchange interactions in order to get insight into the nature of magnetic pattern formations. For the Fe/Os(0 0 0 1) system we calculate a very large D/J ratio, correspondingly, a spin spiral ground state. We find that, mainly through the leading isotropic exchange and Dzyaloshinskii–Moriya interactions, the inward layer relaxation substantially influences the magnetic ordering of the Fe monolayer. For the Fe/Re(0 0 0 1) system characterized by large antiferromagnetic interactions we also determine the chirality of the 120° Néel-type ground state.

Spin Tuning of Electron-Doped Metal–Phthalocyanine Layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state.

Graphene-protected Fe layers atop Ni(111): Evidence for strong Fe-graphene interaction and structural bistability

The intercalation of different metals underneath the graphene sheet has opened the possibility of preparing new low-dimensional structures that could present quantum-size effects. Knowing the surface crystallograpy of these low-dimensional systems is mandatory for a complete understanding of their physical and chemical properties. In this work, we present a combined low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy, and first-principles calculations study of the structural properties of graphene-protected Fe films on Ni(111). The results indicate that graphene interacts strongly with the topmost Fe atoms, similar to graphene on Ni(111). For one and two Fe monolayer films, the Fe is isostructurally deposited on Ni (fcc on fcc), and graphene is deposited commensurably with the underlying Fe surface atoms. For one Fe monolayer case, the LEED data indicate the coexistence of two types of crystalline domains, named top-fcc and bridge-top structures. Our first-principles calculations show that for the graphene/Fe/Ni system, the total energies of the the top-fcc and bridge-top structures are nearly degenerate, consistent with the observed bistability.

Magneto-transport properties of Fe thin films in an external electric field

Magneto-transport properties in Fe thin films in an external electric field (E-field) were investigated by means of the first-principles full-potential linearized augmented plane-wave method. Results for an Fe monolayer predict a reduction (enhancement) in the in-plane dc electric (intrinsic Hall) conductivity due to a change in the band structure around the Fermi energy when the E-field is introduced. In addition, a magnetization reorientation from the out-of-plane direction to the in-plane direction leads to an abrupt change in the conductivity. Modification of the ac electric conductivity by an E-field is also presented.

Influence of oxygen and hydrogen adsorption on the magnetic structure of an ultrathin iron film on an Ir(001) surface

We present a detailed ab initio study of the electronic structure and magnetic order of an Fe monolayer on the Ir(001) surface covered by adsorbed oxygen and hydrogen. The results are compared to the clean Fe/Ir(001) system, where recent intensive studies indicated a strong tendency towards an antiferromagnetic order and complex magnetic structures. The adsorption of an oxygen overlayer significantly increases interlayer distance between the Fe layer and the Ir substrate, while the effect of hydrogen is much weaker. We show that the adsorption of oxygen (and also of hydrogen) leads to a $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic order of the Fe moments, which is also supported by an investigation based on a disordered local moment state. Simulated scanning tunneling images using the simple Tersoff-Hamann model hint that the proposed $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic order could be detected even by nonmagnetic tips.

Oxygen on an Fe monolayer on W(110): From chemisorption to oxidation

The adsorption of oxygen on a pseudomorphic iron monolayer deposited on a W(110) surface was studied experimentally and theoretically. Standard surface characterization methods, such as Auger electron spectroscopy and low energy electron diffraction, and specific nuclear methods, such as conversion electron Mössbauer spectroscopy (CEMS) and nuclear resonant scattering of synchrotron radiation, combined with theoretical calculations based on the density functional theory allowed us to determine the structure of the oxygen adsorbate and the electronic properties of iron atoms with different oxygen coordinations. The oxygen-(3×2) structure on the iron monolayer was recognized and was interpreted to be a state with oxygen chemisorbed on the non-reconstructed surface with modest electron transfer from iron to oxygen. A transition from chemisorbed oxygen to the onset of Fe-oxidation is revealed by distinct changes in the CEMS spectra.

Manipulating the magnetic anisotropy of 3d transition-metal films on Cu(001) and their alloys on Rh(001) by electric field

The mechanism of electric field (EF) effects on the magnetocrystalline anisotropy (MCA) in metallic films is investigated by first-principles calculations. Start with a simple system of Fe, Co and Ni monolayer on Cu(001) substrate, we show that the key factor for a large EF-induced MCA modification is that the energy bands cross of d3z2−r2 and dyz (or dxz and dxy) is close to the Fermi level. In order to enhance the MCA modification by EF, 4d metal substrates (Rh, Pd) are also discussed. In particular, we find that the magnetization direction can be switched from out-of-plane to in-plane by a small EF for Fe1–xCox alloy films on Rh(001) substrate with x=0.5.

Quantifying perpendicular magnetic anisotropy at the Fe-MgO(001) interface

We show that Fe-MgO interfaces possess strong perpendicular magnetic anisotropy of 1.0 ± 0.1 erg/cm2 in fully epitaxial MgO/V/Fe/MgO(001) and MgO/Cr/Fe/MgO(001) heterostructures. The sign and amplitude of the total anisotropy are quantified as a function of Fe thickness using magnetometry and ferromagnetic resonance. There is a transition from out-of-plane to in-plane anisotropy for 6 Fe monolayers in V/Fe/MgO and only 4 monolayers in Cr/Fe/MgO. A detailed study of the Fe magnetization and effective anisotropy in both systems explains this difference and quantifies the Fe-MgO interface anisotropy.

Magnetic Properties of bcc-Fe(001)/C60 Interfaces for Organic Spintronics

The magnetic structure of the interfaces between organic semiconductors and ferromagnetic contacts plays a key role in the spin injection and extraction processes in organic spintronic devices. We present a combined computational (density functional theory) and experimental (X-ray magnetic circular dichroism) study on the magnetic properties of interfaces between bcc-Fe(001) and C(60) molecules. C(60) is an interesting candidate for application in organic spintronics due to the absence of hydrogen atoms and the associated hyperfine fields. Adsorption of C(60) on Fe(001) reduces the magnetic moments on the top Fe layers by ∼6%, while inducing an antiparrallel magnetic moment of ∼-0.2 μ(B) on C(60). Adsorption of C(60) on a model ferromagnetic substrate consisting of three Fe monolayers on W(001) leads to a different structure but to very similar interface magnetic properties.

Effect of Fe–O distance on magnetocrystalline anisotropy energy at the Fe/MgO(001) interface

We report first-principles calculations on the magnetocrystalline anisotropy energy (MAE) of an Fe monolayer sandwiched by MgO. We found that by increasing the interlayer distance between Fe and O by about 8% from its equilibrium value, the perpendicular interfacial magnetic anisotropy can be enhanced as high as 2.75 erg/cm2, which is three times larger than that at the equilibrium distance. The analysis of MAE based on the second-order interactions of the spin-orbit coupling shows that the energy position of the majority-spin dz2 orbital is of central importance in determining MAE. Our results suggest that increasing the Fe–O distance in the Fe/MgO system is an important material-design direction for high-performance magnetic memories.

Search for stable ferromagnets amongAIV/Fe digital alloys (AIV=Si, Ge) using first-principles calculations

Using first-principles electronic structure calculations we investigate the existence of stable ferromagnets among the $A$${}^{\mathrm{IV}}$/Fe digital alloys ($A$${}^{\mathrm{IV}}=\mathrm{Si},\mathrm{Ge}$), modeled as periodic sequence of Fe monolayers in the $A$${}^{\mathrm{IV}}$ host. Total-energy calculations and the magnetic force theorem are exploited for accurate determination of the magnetic ordering. To estimate the critical temperatures, Monte Carlo simulations are employed, while the renormalization-group analytical expressions are applied to assess the impact of the interlayer exchange on the critical temperature values. According to our results, among the systems under consideration only the Ge-based alloys feature a stable ferromagnetic ordering at nonzero temperature. The critical temperatures of these systems were found to be strongly dependent on the underlying crystal structure.

Trends in the magnetic properties of Fe, Co, and Ni clusters and monolayers on Ir(111), Pt(111), and Au(111)

We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and Au(111). For this a fully relativistic {\em ab-initio} scheme based on density functional theory has been used. We analyse the element, size and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions as well as the magnetic anisotropy energy in these systems. Our results show that the atomic spin magnetic moments in the Fe and Co clusters decrease almost linearly with coordination on all three substrates, while the corresponding orbital magnetic moments appear to be much more sensitive to the local atomic environment. The isotropic exchange interaction among the cluster atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is in general one or two orders of magnitude smaller when compared to the isotropic one. For the magnetic properties of Ni clusters the magnetic properties can show quite a different behaviour and we find in this case a strong tendency towards noncollinear magnetism.

First-Principles Study of Magnetic Properties of Co/Pt(111) Film in Electric Field

Technology of electric field (EF) control of magnetic properties is attractive for spintronic devices. In particular, it has been shown that the EF can change the magnetization and the magnetic anisotropy. According to an experimental study, an EF less than 0.1V/nm causes a large change of 40% in the magnetic anisotropy energy (MAE) of the Fe/Au(001) film. Also, another experimental study using the Co/Pt(111) film in the EF has shown that the Curie temperature of the film is changed up to 6K by applying an EF of about +0.2V/nm while down to 6K by applying an EF of about 0:2V/nm. A first-principles study of the MAE in the EF for the Fe/ Pt(001) film was carried out; it has been found that the change in the MAE is in proportion to the EF with the slope rate of 0.03meV/Fe per V/nm for Fe/Pt(001). Another firstprinciples study of the MAE in the EF for the Fe monolayer was carried out; it has been found that the MAE is affected considerably by applying the EF. On the other hand, to our knowledge, there are no first-principles studies of the Co/ Pt(111) film in the EF although the Co films are as important as the Fe films. In this study, we investigate the magnetic properties of the Co/Pt(111) film in the EF carrying out the first-principles calculations. We take account of the EF by introducing electrode far above Co/Pt(111) and changing the total number of electrons in the film. We calculate the spin moments and the numbers of electrons of the constituent atoms as well as the MAE of the film self consistently. We employ the fully relativistic full-potential linearcombination-of-atomic-orbitals method based on the density-functional theory within the local spin density approximation. The exchange-correlation energy functional used in this study is the Perdew–Wang parameterization of the Ceperley–Alder results. As shown in Fig. 1, the Pt(111) surface is modeled by a three-layer slab. We denote the Pt atoms in the top, middle, and bottom Pt layers by Pt(I), Pt(II), and Pt(III), respectively. We assume hcp stacking of the Co adlayer on the Pt(111) surface because the Co adatom prefers to occupy an hcphollow site on the Pt(111) surface. We use the lattice constant of the triangular lattice, 2.78 A, which is calculated from the lattice constant of fcc Pt, 3.92 A. All the interlayer distances between adjacent atomic layers of Co/Pt(111) are optimized calculating the forces acting on the Co and Pt atoms in the absence of the EF. The optimized interlayer distances are shown in Fig. 1. The EF is applied as follows. We change the total number of electrons in the film; we denote this change per Co atom by N. We introduce the following external potential originated in the electrode shown in Fig. 1:

Fe nanoparticles on ZnSe: Reversible temperature dependence of the surface barrier potential

The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of $\ensuremath{\sim}$1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.

Ab initiostudy of spin-spiral noncollinear magnetism in a free-standing Fe(110) monolayer under in-plane strain

We investigate the magnetic phase transition from collinear ferromagnetic (FM) ordering to noncollinear spin-spiral (SS) ordering in an Fe(110) monolayer under in-plane strain by performing fully unconstrained first-principles spin-density-functional calculations. The FM Fe(110) monolayer undergoes a FM-SS phase transition on the application of in-plane compression, whereas the application of tension keeps the system FM. The stability and wavelength of the excited SS state are further increased by compressive strains, especially along [$\overline{1}$10]. The FM-SS transition in the isotropically strained monolayer is dominated by competing exchange interactions between the ferromagnetically coupled first neighbor and the antiferromagnetically coupled second neighbor; the third neighbor also contributes to the transition under anisotropic strain. In addition, we demonstrate the stabilization mechanism of SS noncollinear magnetism from the electronic band structures: The noncollinear SS state is stabilized by a remarkable interband repulsion between the majority and minority spins, which occurs under in-plane compression.