Fcc Fe Films Research Articles

Magnetic properties and structural phase transition in ultrathin fcc Fe(111) and bcc Fe(111) films: First-principles study

The aim of this work is to investigate the structural and magnetic characteristics of Fe thin films with a triangular (hexagonal) lattice surfaces (fcc (111) and bcc (111)). The properties of these structures have been calculated using density functional theory (DFT) implemented in the full-potential local-orbital (FPLO) code. The results indicate a structural phase transition from fcc to bcc structure when the film thickness exceeds 23 Fe atomic monolayers. The considered fcc films prefer the low-spin ferromagnetic state with an average magnetic moment of about 1.0 μB per atom. This moment decreases with increasing film thickness until the critical thickness, where, after the structural transition to the bcc phase, it reaches a value close to that of bulk bcc Fe. Moreover, the values of the magnetic anisotropy energy are positive (perpendicular magnetic anisotropy) for the entire thickness range of films with fcc structure (in ferromagnetic low-spin state) and systematically decrease with increasing film thickness. The presented computational results explain the experimentally observed structural transition and may help to select appropriate substrates with suitable lattice parameters for the deposition of ultrathin Fe(111) films.

Iron carbide formation on thin iron films grown on Cu(1 0 0): FCC iron stabilized by a stable surface carbide

Thin iron films evaporated onto Cu(100) were carburized using ethylene to produce iron carbide surfaces for use as model systems in experimental research. XPS and AES confirm that ethylene dissociation produces a pure iron carbide. A maximum of 0.5 ML carbon can be deposited for film thicknesses below 12 ML where Fe grows as γ-iron (FCC). For thick, BCC-Fe(110) films, post-treatment with ethylene leads to carbon coverages beyond 0.5 ML where some carbon diffuses into the bulk. The film remains α-iron (BCC) and a different surface carbide with a (4 × 3) unit cell is found. On the thin FCC-Fe(100) films, carbon reconstructs the surface into a p4g(2 × 2)-Fe2C layer which has a special stability and acts as a carbon trap that prevents carbon diffusion into the bulk. Fe2C is thermally stable up to 700 K above which Fe diffuses into the copper substrate while leaving graphitic carbon behind. Carbon segregates to the surface during evaporation of iron on top of an Fe2C-covered FCC-Fe film and causes the film to retain the FCC structure up to a thickness of at least 30 ML, far beyond 12 ML where BCC-Fe forms on Cu(100) in absence of surface carbon.

Non-monotonic magnetic anisotropy behavior as a function of adsorbate coverage in Fe ultrathin films near the spin reorientation transition

Carbon monoxide (CO) adsorption on ultrathin fcc Fe films is known to result in the rotation of magnetization from out-of-plane to in-plane.

A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)

The initial oxidation of Fe is a fundamental issue due to its importance for corrosion and catalysis. In this study, we investigate the initial oxidation of epitaxial fcc and bcc Fe thin films grown on a Cu(100) substrate at room temperature. Fe thin films deposited with two different thicknesses on Cu(100) were prepared by molecular beam epitaxy in order to obtain the fcc Fe(100) and bcc Fe(110) surfaces. The structures and the chemical composition of the Fe films were evaluated by LEED and photoelectron spectroscopy, using the facilities at the TEMPO beamline of the Synchrotron SOLEIL. The Fe 2p and O 1s high-resolution spectra were collected during oxidation at a constant O2 pressure in order to build the kinetic curves, which are discussed taking into account the Fromhold-Cook theory. Our result revealed the different initial oxidation kinetics of the films and showed that bcc Fe(110) surface has a much higher reactivity with O2 than the fcc Fe(100). The Fe 2p peak analysis suggests the formation of Fe1−xO and an Fe3+-rich oxide on the bcc Fe(110) surface and Fe1−xO on the fcc Fe(100) surface.

Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100)

Metastable face-centered cubic (fcc) Fe/Cu(100) thin films are good candidates for ion-beam magnetic patterning due to their magnetic transformation upon ion-beam irradiation. However, pure fcc Fe films undergo spontaneous transformation when their thickness exceeds 10 ML. This limit can be extended to approximately 22 ML by deposition of Fe at increased CO background pressures. We show that much thicker films can be grown by alloying with Ni for stabilizing the fcc γ phase. The amount of Ni necessary to stabilize nonmagnetic, transformable fcc Fe films in dependence on the residual background pressure during the deposition is determined and a phase diagram revealing the transformable region is presented.

Inhomogeneous temperature dependence of the magnetization in fcc-Fe on Cu(001)

Using soft-x-ray magnetic resonant reflectivity measurements, we have studied the layer-resolved temperature dependence of the magnetic structure in a six-monolayer-thick fcc-Fe film grown on Cu(001) by pulsed laser deposition. Temperature-dependent reflectivity curves were fitted on the basis of the Heisenberg Hamiltonian involving only a small number of temperature-independent interlayer and intralayer exchange interaction parameters. Our study supports the model of the formation of the blocks of layers with robust magnetic structure whereas the interblock interactions are relatively weak. The temperature-dependent magnetization profile derived on the basis of the exchange interaction parameters reveals a strong inhomogeneity within the film, which is in agreement with the recent theoretical prediction.

FeMn/Fe/Co/Cu(1,1,10) films studied using the magneto-optic Kerr effect and photoemission electron microscopy

FeMn/Fe/Co/Cu(1,1,10) films were grown epitaxially and investigated using the magneto-optic Kerr effect and photoemission electron microscopy. We found that FeMn/Fe/Co/Cu(1,1,10) exhibits the same properties as FeMn/Co/Cu(1,1,10) for the ferromagnetic phase of the face centered cubic (fcc) Fe film but a different property for the non-ferromagnetic phase of the fcc Fe film. This result indicates that the characteristic property reported in the literature for FeMn/Co/Cu(001) comes from the FeMn spin structure and is independent of the ferromagnetic layer.

In-situ magnetic nano-patterning of Fe films grown on Cu(100)

Metastable paramagnetic face-centered cubic (fcc) Fe films grown on a Cu(100) single crystal at room temperature can be transformed to the ferromagnetic body-centered cubic (bcc) structure by ion irradiation. We have employed this technique to write small ferromagnetic patches by Ar+ irradiation through a gold coated SiN mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer (ML) Fe films grown in ultrahigh vacuum as well as 22-ML films stabilized by dosing carbon monoxide during growth. The structural transformation of these nano-patterned films was investigated using scanning tunneling microscopy. In both 8 and 22-ML fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic transformation was confirmed by magnetic force microscopy.

Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)

We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) $\ensuremath{\gamma}$ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) $\ensuremath{\alpha}$ phase. A structural transformation of these films from fcc to bcc can be induced by ${\text{Ar}}^{+}$ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing ${\text{Ar}}^{+}$ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.

Spin structure in ultrathin fcc-Fe films on Cu(001)

Spin structure in ultrathin fcc-Fe films on Cu(001)

The role of structural variations in the magnetism of Fe/Cu(1 1 1): First-principles calculations

The effects of three crucial structural variations, i.e., sub-monolayer coverage, intermixing, and stacking fault, on the magnetism of Fe/Cu(1 1 1) were quantitatively investigated using density functional theory calculations. A single Fe atom (0.25 ML) on the fcc (2 × 2) Cu(1 1 1) surface has a higher magnetic moment (3.18 μ B) than bulk Fe (2.15 μ B). The calculated magnetic moments of 0.5, 0.75 and 1 ML coverage Fe on the Cu(1 1 1) surface, were 3.01 μ B, 2.87 μ B and 2.72 μ B, respectively. For intermixing between 1 ML of Fe and Cu(1 1 1), the magnetic moment of the Fe adatoms monotonically decreased with the intermixing amount. The electron density of states showed that the decreasing magnetic moment of Fe adatoms resulted from the filling of down-spin 3 d-electrons below Fermi level by the hybridization of 3 d-electron among Fe atoms. The influence of Cu(1 1 1) atoms on the electronic structure and magnetic moment was observed to be negligible. In the artificially prepared 2 and 3 ML thick Fe films on Cu(1 1 1), the magnetic moments of Fe atoms were not dependent on the stacking sequences of Fe layers. However, the demagnetization energy calculation revealed a more stable perpendicular magnetization for fcc Fe films compared to hcp Fe films on Cu(1 1 1). In brief, the structure dependence of magnetism of Fe films on Cu(1 1 1) mainly originates from the bonding among Fe atoms, while the effect of bonding with Cu atoms was negligible.

New Model for Magnetism in Ultrathin fcc Fe on Cu(001)

Using soft-x-ray resonant magnetic scattering in combination with first-principles calculations for noncollinear magnetic configurations we present a new model of the magnetism in ultrathin fcc Fe films on Cu(001). We find the presence of blocks with robust magnetic structure, while the relative directions of the moments of different blocks are sensitive to the detailed atomic structure and temperature. The magnetic noncollinearity is directly demonstrated, which has not been possible so far.

Growth and structural transition of Fe ultrathin films on Ni(111) investigated by LEED and STM

The growth of Fe ultrathin films on Ni(111) at room temperature has been investigated by low-energy electron diffraction and scanning tunneling microscopy. In the initial deposition, a pseudomorphic fcc-Fe(111) monolayer with triangular line defects grows on the Ni substrate. Triangular lines consist of atomic-size dark spots located in the Ni atom sites and are interpreted in terms of the misfit-induced atomic vacancies in the Ni substrate. With further deposition, the second monolayer Fe film, which consists of three elongated domains of a striped structure, grows on the first monolayer fcc-Fe film. The striped structure results from the growth of a distorted bcc-Fe(110) phase with one-dimensional Nishiyama-Wassermann orientation. Further deposition leads to the growth of thicker bcc (110) films on the distorted bcc(110) domains, forming three-dimensional ridgelike islands. The first and second monolayer Fe films grow in an incomplete layer-by-layer growth mode and the thicker Fe films grow in a three-dimensional island growth mode.

Fe–Ni Surface Alloy Formation on Ni(111) Investigated by Scanning Tunneling Microscopy

The structure of Fe films grown on Ni(111) with an Fe coverage of 3.2 monolayers and its conversions during heating in ultrahigh vacuum have been investigated by scanning tunneling microscopy (STM). The Fe films consist of elongated domains stacked with four to six monatomic layers of Fe, forming ridgelike structures. A phase transition from fcc(111) to bcc(110) with the Nishiyama–Wassermann orientation relationship occurs from at least the third monatomic layer. A triangular network structure and large accumulated bcc-Fe islands are formed on the surface after heating the Fe films at 550 K. The network is considered to be misfit dislocation loops caused by the lattice misfit between the fcc-Fe films and the Ni substrate. A surface structure consisting of two distinct randomly distributed spots and a well-ordered (5 ×5) superstructure are observed after further heating at 620 and 700 K, respectively, and they are caused by the formation of the Fe–Ni surface alloy on the Ni(111) surface.

Structural phase diagram, magnetism, and exchange-biased behavior of Fe films onFexMn1−x∕Cu(001)

The fcc-bcc structural phase transition and the interlayer exchange-bias coupling in a $\mathrm{Fe}∕{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}∕\mathrm{Cu}(001)$ system are studied in a wide $x$ range with different growth temperatures. For depositions of Fe at $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ on the ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$ layer with $xl0.35$, the transition occurs below 15 ML and the resulting bcc Fe films are ferromagnetic with exchange bias. The fcc Fe is stabilized up to 20 and 25 ML for the depositions on the ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$ layer with $xg0.35$ at 150 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. Both fcc Fe films show no magnetic signal. The relation comprising the structure and magnetic coupling of the Fe films might be attributed to the $x$-dependent interface roughness at $\mathrm{Fe}∕{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$.

Magnetism of Fe films grown on Co(100) studied by spin-resolved Fe3sphotoemission

The magnetic properties of fcc Fe films grown on fcc Co(100) have been studied by means of spin-resolved Fe $3s$ core-level photoemission and characteristics of the measured spin-resolved $3s$ spectra for fcc Fe films have been investigated. The spin-resolved $3s$ spectra measured on 3.9 and 6.6 monolayer (ML) fcc Fe films are similar in spectral shape. The $3s$ majority-spin spectra for these fcc Fe films show weaker intensity on the high-binding-energy side than the spectrum previously reported for bulk bcc Fe. The spin-resolved $3s$ spectra for the fcc Fe films are analyzed by cluster model calculation consisting of four Fe atoms. In the analysis by the cluster model calculation, effects of the interatomic configuration interaction on the spin-resolved $3s$ spectra for fcc Fe films are discussed. Itinerancy of $3d$ electrons is found to be an important factor in describing the spin-resolved Fe $3s$ spectra. The $z$ spin momentum estimated by the cluster calculation indicates that both the Fe films are in a high-spin ferromagnetic state near the surface. On the other hand, the spin polarization at the background in the spectrum for the 6.6 ML Fe film is much smaller than that for the 3.9 ML film. This variation of the background spin polarization indicates that the magnetic moment averaged up to deeper layers of Fe film is suppressed in the 6.6 ML film.

Magnetic structure of fcc Fe films on Co(1 1 1)

The collinear magnetic structure of n ( n = 1, …, 6) monolayers (ML) of Fe films deposited on Co(1 1 1) substrate is investigated by ab initio density functional theory within a generalized gradient approximation. We have found that the ferromagnetic ground state exists only for one Fe monolayer. For higher thickness, a sequence of antiferromagnetic coupling with low magnetic moments in the inner layers of films is identified as ground state.

Phonon Density of States of Pseudomorphic fcc-Fe Film

The phonon density of states of a pseudomorphic fcc-Fe film with ∼3 monolayers has been investigated by nuclear resonant inelastic scattering. The fcc-Fe layers are intercalated into an interface between monolayer hexagonal boron nitride and a Ni(111) substrate with a commensurate relation. The in-plane nearest-neighbor distance of the fcc-Fe film is shorter than that of uncompressed fcc-Fe precipitates in Cu by -1.8%. The compression increases the mean force constant of the fcc-Fe film, which is higher than that of uncompressed fcc-Fe precipitates by 5.8%.

Reconstruction of the clean and H covered “magnetic live surface layer” of Fe films grown on Cu(100)

The surface of 6–7 monolayer thick fcc Fe films grown at room temperature on a Cu(1 0 0) substrate is characterized by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The STM images show a p4g(2 × 2) structure at 5 and 80 K, but not at 300 K. LEED, however, indicates an expansion of the interlayer distance and lateral distortions of similar magnitude both at 150 K and at 300 K. No evidence for a significant change of the surface structure is detected by a LEED spot profile analysis between 150 and 300 K. We attribute the apparent absence of the reconstruction in the STM images at 300 K to surface dynamics caused by domain boundary motion. The particular surface structure with bond angles and distances similar to bcc Fe suggests a driving force of the reconstruction which is similar to that operative in the fcc-to-bcc transition of bulk Fe. Dosing less than 5 L H2 decorates the p4g(2 × 2) surface reconstruction, while higher hydrogen doses transform the surface reconstruction to p(2 × 1).

Checkerboard pattern of the interlayer coupling between two Co films acrossFe∕CuandCu∕Co∕Cuspacer layers grown onCu(100)

Quantum well (QW) states and oscillatory interlayer coupling in $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Fe}∕\mathrm{Co}∕\mathrm{Cu}(001)$ are investigated by angular resolved photoemission spectroscopy and x-ray magnetic linear dichroism. We find that the QW states in $\mathrm{Cu}∕\mathrm{Fe}∕\mathrm{Co}∕\mathrm{Cu}(001)$ depend very little on the magnetic state of the fcc Fe films. The interlayer coupling between the Co films across the $\mathrm{Cu}∕\mathrm{Fe}$ spacer layer displays a checkerboard pattern in $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Cu}$ thickness plane. The presence of the fcc Fe ferromagnetic live layer at the $\mathrm{Cu}∕\mathrm{Fe}$ interface is shown to be responsible for the checkerboard pattern, which was confirmed by experiments on $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}∕\mathrm{Cu}(100)$ system.