Ultrathin Fe Films Research Articles

Spin-wave propagation steered by electric field modulated exchange interaction.

Combined ab initio and micromagnetic simulations are carried out to demonstrate the feasibility on the electrical manipulation of spin-wave propagation in ultrathin Fe films. It is discovered that the exchange interaction can be substantially weakened under the influence of electric field applied perpendicular to the magnetic film surface. Furthermore, we demonstrate that the electric field modified exchange constant could effectively control the propagation of spin waves. To be specific, an external applied electric field of 5 V/nm can effectively weaken exchange interaction by 80% and is sufficient to induce nearly twofold change of the wavenumber. This discovery may open a door to energy-efficient local manipulation of the spin wave propagation utilizing electric fields, which is crucial for both fundamental research and spin wave based logic applications.

Ab-initio study of the magneto-optical properties of the ultrathin films of Fen/Au(001)

With the aim of understand the microscopic origin of the magneto-optical response in the Fe ultrathin films, we used the first principle full-relativistic Spin-Polarized Relativistic Linear Muffin-Tin Orbitals with Atomic Sphere Approximation. We performed an ab-initio study of the structural, magnetic and magneto-optical properties of Fe deposited on semi-infinite Au(001). The structure and growth of the film leads to a pseudomorphic body centered tetragonal structure with tetragonality ratio c/a=1.62, and the pseudomorphic growth is found to be larger than 3 monolayers. The magnetic study revealed a ferromagnetic phase with a large magnetic moment compared to the bulk one. The magneto-optical response is calculated via the polar magneto-optical Kerr effect over a photon energy range up to 10eV. The most important features of the Kerr rotation spectra are interpreted trough the interband transitions between localized states.

Predicting epitaxial orientations and lattice structure in ultrathin magnetic thin films

Metastable phases, such as bcc Co or Ni and hcp Fe or Ni, reportedly possess extraordinary magnetic properties for epitaxial ultra-thin films. To understand phase stability of these epitaxy-oriented phases upon substrate lattices, we calculated novel phase diagrams of Co, Fe, and Ni ultrathin films by considering the chemical free energy, elastic strain energy, and surface energy. Verified by experimental data in the literatures, the stable epitaxy-oriented phases are readily identified from the phase diagrams. The stabilization of these metastable phases is determined by the interplay between orientation-dependent elastic strain energy and surface energy.

Structure and stability of ultrathin Fe films on W(110)

The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono- and bilayers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bilayer on a tungsten substrate has been addressed. To complete the physical picture, an iron trilayer has also been analyzed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the $[1\overline{1}0]$ direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.

Study of high-temperature oxidation of ultrathin fe films on Pt(100) by using X-ray photoelectron spectroscopy

High-temperature oxidation of iron thin films deposited on Pt(100) surfaces was studied by using X-ray photoelectron spectroscopy (XPS). Upon an oxygen exposure of 300 Langmuir onto a 7.5- monolayer (ML) Fe film at 830 K, about 2 monolayers of the Fe film were oxidized as Fe3O4 while the remaining Fe atoms diffused into the substrate. For 1.25-, 2.5-, and 3.75-monolayer Fe films, only about a monolayer of the Fe film was oxidized as FeO, regardless of the number of Fe atoms. The oxide layers on the 7.5-monolayer Fe film were observed to be stable upon post-annealing at 1030 K.

Self-Terminated Electrodeposition of Ni, Co, and Fe Ultrathin Films

Self-terminated Fe-group metal (Ni, Co, Fe) electrodeposition occurs at potentials negative of the onset of water reduction where OH– generation leads to the formation of a blocking hydroxide monolayer. Quenching of metal deposition is accompanied by an increase in dissipative energy loss in microbalance experiments attributed to increased hydrogen bonding to the adjacent double layer. Pulse deposition at −1.5 VSSCE in 5 mmol/L (NiCl2, CoCl2, FeSO4) – 0.1 mol/L NaCl pH 3.0 electrolytes yields fully coalesced ultrathin films of Ni, Co, Fe, or alloys thereof, on Au. The film thickness is controlled by the nucleation, growth, and termination dynamics constrained by the electrochemical cell time constant. Precipitation of bulk Ni(OH)2 and related phases is minimized by using short deposition times and dilute metal cation concentrations to limit supersaturation. The rapid deposition of smooth, compact ultrathin Fe, Co, and Ni films should facilitate mechanistic and durability studies of Fe-group metal catalysi...

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO (001) film by magneto-optic Kerr effect**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274033, 11474015, and 61227902), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20131102130005), and the Beijing Key Discipline Foundation of Condensed Matter Physics.

The magnetization reversal process of Fe/MgO (001) thin film is investigated by combining transverse and longitudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy, the typical magnetization reversal process of Fe ultrathin film can take place via either an “l-jump” process near the easy axis, or a “2-jump” process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.

Computational investigation of stable formation condition for FexNi1−x alloy films on paramagnetic substrate

The adsorption process of ultrathin ferromagnetic Fe and Ni alloy film on a nonmagnetic metal substrate is studied by the variational spin-density functional method. The conditions for the stable formation of ferromagnetic films with respect to island adsorption are discovered with a change of values of the coverage parameter and alloy component concentration. The equilibrium values of vacuum gap and film thickness are determined from the minimum of the total interfacial energy. The energy characteristics of films FexNi1-x on close-packed surfaces W, Cu and Ag with effects of magnetic ordering are calculated for different temperatures.

Controlling the growth of epitaxial graphene on metalized diamond (111) surface

The 2-dimensional transformation of the diamond (111) surface to graphene has been demonstrated using ultrathin Fe films that catalytically reduce the reaction temperature needed for the conversion of sp3 to sp2 carbon. An epitaxial system is formed, which involves the re-crystallization of carbon at the Fe/vacuum interface and that enables the controlled growth of monolayer and multilayer graphene films. In order to study the initial stages of single and multilayer graphene growth, real time monitoring of the system was preformed within a photoemission and low energy electron microscope. It was found that the initial graphene growth occurred at temperatures as low as 500 °C, whilst increasing the temperature to 560 °C was required to produce multi-layer graphene of high structural quality. Angle resolved photoelectron spectroscopy was used to study the electronic properties of the grown material, where a graphene-like energy momentum dispersion was observed. The Dirac point for the first layer is located at 2.5 eV below the Fermi level, indicating an n-type doping of the graphene due to substrate interactions, while that of the second graphene layer lies close to the Fermi level.

Exchange bias ofTbPc2molecular magnets on antiferromagnetic FeMn and ferromagnetic Fe films

Improving the magnetic stability of single-molecule magnets is a key challenge facing molecular spintronics. We use x-ray magnetic circular dichroism to explore the possibility of magnetically stabilizing ${\mathrm{TbPc}}_{2}$ molecules by attaching them to ultrathin Fe and FeMn films. We show that ${\mathrm{TbPc}}_{2}$ deposited on antiferromagnetic FeMn films exhibits magnetic hysteresis and exchange bias as a consequence of coupling to the uncompensated interfacial Fe spins. The FeMn-thickness dependence of the coercive field and exchange bias of ${\mathrm{TbPc}}_{2}$ is similar to that of inorganic ferromagnetic/antiferromagnetic systems. The magnetic remanence is comparable with the fraction of molecules attached to pinned interfacial Fe spins. The Tb magnetic moments are antiferromagnetically coupled to the Fe thin films as well as to the uncompensated Fe spins at the FeMn interface. The sign of the coupling changes from antiferromagnetic to ferromagnetic after doping the interface with electron-donor Li atoms.

Nanopatterning of magnetic domains: Fe coverage of self-assembled alumina nanostructure

Nanosized ultrathin magnetic films were prepared by controlling the deposition of Fe onto an oxidized NiAl(001) surface with an alumina nanostructure on it. Because the ultrathin ferromagnetic Fe films on the bare NiAl(001) surface are separated by paramagnetic Fe nanoparticles on the alumina stripes, as determined by scanning electron microscopy with spin analysis, they form rectangular domains with sizes ranging from tens of nanometer to larger than a micrometer. Magnetic domain patterning can thus be achieved by controlling the Fe coverage and nanostructured template.

Adsorption and Magnetic Properties of Ultrathin Fe and Ni Alloy Films with an Inhomogeneous Magnetization Distribution

Theoretical description of ultrathin ferromagnetic Fe and Ni alloy film adsorption on a nonmagnetic metal substrate is considered by the spin-density functional method. The conditions for the formation of ultrathin ferromagnetic films stable with respect to island adsorption with a change in the coverage parameter θ and Fe content concentration x are revealed. The equilibrium vacuum gap Dmin and the equilibrium film thickness hmin are determined from the minimum of the total interfacial energy. The energy and magnetic characteristics of FexNi1-x films on close-packed W and Cu surfaces are calculated in dependence on concentration x for different temperatures.

Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, ml/ms. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states.

Noncollinearity of the canted spins across ultrathin Fe films on vicinal Ag surfaces

We investigate the depth dependence of the canted magnetization in a bcc Fe ultrathin film grown on a vicinal Ag substrate. This study is performed for different Fe thicknesses, in the vicinity of the spin reorientation transition, by soft x-ray resonant magnetic reflectivity (SXRMR). Above 5.5 monolayers, the high spatial resolution of SXRMR allows us to describe a gradient of the canted magnetization. The tilt angle is larger near the Ag interface and decreases towards the surface. This noncollinearity is ascribed to a different balance of various anisotropies across the layer thickness. Below that value, a large increase of the tilt angle is observed and ascribed to the spin reorientation transition. Unlike the previous situation, the tilt angle is found to be homogeneous throughout the film. This can be related either to the out-of-plane spin reorientation associated to a reduced number of monolayers or to the $q$-space limitation of the SXRMR technique at the Fe ${L}_{3}$ edge.

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.

Separation of linear and quadratic magneto-optic Kerr effects in ultra-thin Fe films using a rotating field method

A rotating field method was applied to separate linear and quadratic magneto-optic Kerr effects (MOKE) of Fe/GaAs(001) and Fe/MgO/GaAs(001) epitaxial films, and two quadratic MOKE terms could be quantitatively separated. The linear MOKE was independent of crystal orientation, but the quadratic MOKE showed a sinusoidal relationship with a 90° period. The experimental results are fully consistent with theory based on the transfer matrix method.

Oxidation Kinetics of Thin and Ultrathin Fe Films

We have studied oxidation kinetics of Fe thin lms under atmospheric conditions using the fact that metallic iron is a ferromagnet but ultrathin natural iron oxides are practically nonmagnetic at room temperature. As a consequence, oxidation is associated with a loss in ferromagnetism. Fe thin lms were deposited onto 1.5 nm V thick bu er layer using UHV magnetron sputtering. As a substrate we have used Si(100) wafers with an oxidised surface. Results show that all samples with an initial Fe thickness greater than 6 nm oxidize practically instantaneously, whereby a constant amount of 2.5 nm of metal is transformed into oxides. For iron thickness lower than 6 nm the time constant for oxidation increases considerably reaching a value of 30 days for the initial Fe thickness equal to 4 nm.

Freestanding single-crystalline magnetic structures fabricated by ion bombardment

Starting from an ultrathin Fe film grown epitaxially on top of a GaAs(001) substrate, we show that freestanding structures can be created by ion-beam treatment. These structures are single-crystalline blisters and only a few nanometers thick. Anisotropic stress in the rim of a blister induces magnetic domain states magnetized in the direction normal to the blister edge. Experimental evidence is provided that the lateral size can be confined by starting from a nanostructured template.

Direct evidence of antiferromagnetic exchange interaction in Fe(001) films: Strong magnon softening at the high-symmetryM¯point

We report on the direct observation of a large unusual antiferromagnetic exchange interaction in Fe(001) films. By measuring the magnon dispersion relation over the entire Brillouin zone of an ultrathin Fe(001) film on Rh(001), we demonstrate that the signature of this unusual antiferromagnetic exchange interaction can be observed at the high-symmetry $\overline{\mathrm{M}}$ point. The exchange parameters are quantified by comparing the measured magnon dispersion relation with the results of our first-principles calculations. We suggest a way of examining the existence of an antiferromagnetic exchange interaction in layered magnetic structures and also a way of quantifying its strength.

Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.