Fcc Fe Films Research Articles

Theoretical analysis of ultrathin FCC Fe films on Cu(1 1 1) substrates

We report ab-initio investigations of the magnetic structure and reconstruction of 1 and 4 monolayer thick films of γ-Fe on Cu (1 1 1) substrates. We find that the bilayer antiferromagnetic structures oriented along [0 0 1] direction representing the magnetic ground state of Fe/Cu(0 0 1) films determine also the magnetic structure of Fe/Cu(1 1 1) even in the monolayer limit.

Theoretical analysis of ultrathin FCC Fe films on (111) substrates

We report ab-initio investigations of the magnetic structure and reconstruction of 1 and 4 monolayer thick films of γ-Fe on Cu (1 1 1) substrates. We find that the bilayer antiferromagnetic structures oriented along [0 0 1] direction representing the magnetic ground state of Fe/Cu(0 0 1) films determine also the magnetic structure of Fe/Cu(1 1 1) even in the monolayer limit.

Magnetic profile and interlayer exchange coupling in fccFen/Nim(001) superlattices

We have performed ab initio calculations to study the electronic and magnetic structures of fcc ${\mathrm{Fe}}_{n}/{\mathrm{Ni}}_{m}(001)$ superlattices. A ferromagnetic coupling between Fe and Ni atoms is always obtained at the interfaces for any value of $m$ or $n.$ The interlayer exchange coupling between adjacent ${\mathrm{Ni}}_{m}$ slabs through ${\mathrm{Fe}}_{n}$ spacer displays an oscillatory behavior which is strongly modified when mixed Fe-Ni interfaces are considered. The iron atoms close to the interface form an induced ferromagnetic arrangement whereas the atoms inside the fcc Fe film give rise to an antiferromagnetic order. The compositional ordering at the Fe/Ni interface introduces a $c(2\ifmmode\times\else\texttimes\fi{}2)$ type of antiferromagnetism in the Fe layers in the case of ferromagnetic interlayer exchange coupling.

Carbon monoxide adsorption on 4 monolayer thick fcc-Fe/Cu(1 0 0): infrared reflection absorption and low energy electron diffraction studies

Adsorption behavior of carbon monoxide (CO) on 4 monolayer (ML) thick fcc-Fe films grown on Cu(1 0 0) has been investigated with IR reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). The as-grown film gives a 5×1 LEED pattern which remains unchanged during CO exposure at 90 K. Simultaneous IRRAS measurements show that the band due to CO adsorbed on the on-top site dominates the spectra at saturation coverage. In contrast CO exposure at room temperature leads to change in the LEED pattern from 5×1 to diffuse c(2×2) accompanied by preferential adsorption of CO on the bridge site of the film surface. These results suggest that room temperature CO exposure gives rise to a change in the reconstructed surface of the as-grown film and c(2×2) overlayer of CO adsorbed on the bridge site, while CO adsorption takes place on the on-top site of the 5×1 reconstructed surface with no long-range ordered overlayer at 90 K.

Morphology and magnetic structure of Fe/Cu(100)

The structure and magnetism of epitaxially grown Fe films on Cu(100) at 300 K have been investigated in situ using reflection high-energy electron diffraction, low-energy electron diffraction, scanning tunnelling microscopy and magneto-optical Kerr effect. Based on a careful thickness calibration, the three well-known regions with distinct magnetic properties are determined accurately for Fe films on Cu(100). We claim that the traditional region II should be further divided into two parts: II(a) and II(b), where the former remains region II in the traditional sense but the latter is a transition region between regions II and III. A theoretically predicted temperature-driven martensitic phase transition from face-centred-cubic (fcc) Fe to body-centred-cubic Fe is indeed realized in the transition region II(b). In addition, the total magnetic moments as a function of film thickness and its temperature dependence have also been investigated. The results exclude the collinear type-1 antiferromagnetic configuration as the magnetic structure for fcc Fe films on Cu(100). It is proposed that a spin-density-wave state is responsible for the magnetic structure.

Onset of magnetic order in fcc-Fe films on Cu(100).

On the basis of an ab initio theory of metallic magnetism in layered materials, we investigate the onset of magnetic order in thin (2-8 layers) fcc-Fe films on and embedded in Cu(100) substrates. In particular, we find an oscillatory dependence of the Curie temperatures on embedding depth, in excellent agreement with experimental data. The thermally induced spin fluctuations are treated within a mean-field disordered local moment picture and give rise to layer-dependent "local exchange splittings" in the electronic structure even in the paramagnetic phase. These features determine the magnetic intralayer and interlayer interactions which are strongly influenced by the presence and extent of the Cu cap.

Oscillatory interlayer coupling mediated by fcc-Fe/Co(1 0 0) films

The magnetism of epitaxial fcc-Fe films deposited on Co(1 0 0) and sandwiched between two Co(1 0 0) films was investigated by X-ray magnetic circular dichroism at various temperatures. The dependence of the Fe magnetism on the film thickness is complex and qualitatively similar on Co(1 0 0) and in fcc-Co/Fe/Co(1 0 0) trilayers. The fcc-Fe film magnetization presents a pronounced oscillation suggesting a partial antiferromagnetic ordering in the 5–10 monolayers (ML) thickness range. The decrease in temperature down to 100 K does not exhibit any change in the magnetic properties of the Fe-film. The fcc-Fe films mediate an oscillatory, indirect coupling in Co/Fe/Co(1 0 0) structures that alternates in correspondence with the changes of the Fe magnetization.

Magnetic structure of fcc Fe on Cu(111)

We report ab initio calculations on the magnetic structure of fcc Fe films on Cu(111) for a coverage up to 6 ML considering all possible collinear spin orders in the $p(1\ifmmode\times\else\texttimes\fi{}1)$ surface cell. The total magnetic moment of the film undergoes a high-to-low spin transition around 3 ML coverage, in agreement with recent experiments. The spin transition is due to the competition between surface/interface effects with enhanced moments and preferentially ferromagnetic coupling between adjacent monolayers on the one hand and bulklike spin-density wave correlations with a wavelength of about 3 ML on the other hand.

Probing the Magnetic Forces in fcc-Fe(001) Films by Means of Surface Phonon Spectroscopy

Helium atom scattering experiments on Fe films of a few monolayers grown on a Cu(001) substrate demonstrate that (a) in a magnetic film the interatomic magnetic forces can be directly probed from the temperature dependence of the Rayleigh wave frequency across the Curie temperature, and (b) the Rayleigh wave velocity and the topmost interlayer spacing, measured as a function of the film thickness, show a simultaneous $10%$ reduction at 3 to 4 monolayers---an indication of large magnetostrictive effects.

Structural and magnetic properties of ultrathin fcc Fe films on Cu(001): Full-potential LAPW studies

Structural and magnetic properties of ultrathin fcc Fe films on Cu(001): Full-potential LAPW studies

Magnetism and interlayer coupling in fcc Fe/Co films

The magnetism of epitaxial fcc Fe films deposited on Co(100) and sandwiched between two Co(100) films was investigated by x-ray magnetic circular dichroism. The dependence of the Fe magnetism on the film thickness is complex and qualitatively similar on Co(100) and in fcc Co/Fe/Co(100) trilayers. The fcc Fe film magnetization presents a pronounced oscillation, suggesting a partial antiferromagnetic ordering in the 5--10 monolayer thickness range. The fcc Fe films mediate an oscillatory, indirect coupling in Co/Fe/Co(100) structures that alternates in correspondence with the changes of the Fe magnetization.

Infrared reflection absorption spectroscopic study for CO adsorption on molecular beam epitaxially grown Fe films on Cu(1 1 1)

Adsorption of CO at 90 K on molecular beam epitaxially grown Fe films on Cu(1 1 1) has been investigated using IR reflection absorption spectroscopy (IRRAS). Reflection high energy electron diffraction images reveal Fe films up to 4 ML (monolayer) thickness have an fcc(1 1 1) structure and a mixture of fcc(1 1 1) and bcc(1 1 0) structures for 4–8 ML thick films. A bcc(1 1 0) structure dominates the films beyond 8 ML in thickness. The C–O stretching frequency at saturation coverages observed by IRRAS represents these structural differences clearly. For 1–4 ML thick fcc Fe films C–O stretch is located at 2000 cm −1 and the 6–8 ML thick films yield two C–O stretch bands at 2000 and 2032 cm −1, the latter of which is dominant for films more than 10 ML thick.

Brillouin light scattering study of ferromagnetically coupled Cu/Fe(110)/Cu/Fe(110)/Cu/Si(111) heterostructures: Bilinear exchange magnetic coupling

Epitaxial $\mathrm{Cu}(60 \AA{})/\mathrm{Fe}(20\AA{})/\mathrm{Cu}{(d}_{\mathrm{Cu}})/\mathrm{Fe}(60\AA{})/\mathrm{Cu}(60\AA{})$ heterostructures with the Cu spacer thickness ${d}_{\mathrm{Cu}}$ ranging between $0$ and 33 \AA{} have been grown on $7\ifmmode\times\else\texttimes\fi{}7$ reconstructed surface of Si(111) substrates. Fcc(111) Fe films grow epitaxially on the Cu(111) buffer layer up to a thickness of 6--8 \AA{}, while, for larger thicknesses, one observes the appearance of three-dimensional bcc Fe(110) domains in the Kurdjumov-Sachs orientation. Brillouin light scattering (BLS) from thermally excited spin waves has been exploited in order to study the interlayer exchange coupling between the two Fe films at room temperature. The experimental spin-wave frequency dependence on the applied magnetic field is simulated using a simple model which includes first-order intrinsic volume and interface anisotropies and takes the bilinear exchange interaction between the two ferromagnetic layers into account. We have found that for thicknesses of the Cu spacer larger than $6 \AA{},$ the Brillouin spectrum consists of two spin-wave modes due to the Fe double layered structure which depends on the Cu-layer thickness. The coupling is found to be ferromagnetic for the whole range of Cu spacer thicknesses investigated. Using the magnetic parameters determined by the above analysis, we have carried out a detailed calculation of the BLS cross section assuming the dynamic magnetization to be constant across each magnetic film. A very good agreement between the calculated and the measured cross sections has been obtained.

Structural analysis of Fe/Ni(001) films by photoelectron diffraction

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0--14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fcc to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ in-plane orientation. Interlayer spacings of $2.05\ifmmode\pm\else\textpm\fi{}0.06\AA{},$ $2.01\ifmmode\pm\else\textpm\fi{}0.03\AA{},$ and $1.85\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ were found in the bcc region, between bcc and fcc layers and in the fcc region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was $2.49\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of $2.04\ifmmode\pm\else\textpm\fi{}0.04\AA{}$ and in plane n-n distance of $2.47\ifmmode\pm\else\textpm\fi{}0.01\AA{}.$ At 14 ML the PD pattern collected over a 94\ifmmode^\circ\else\textdegree\fi{} azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ alignment.

Dependence of the Curie temperature on the Cu cover layer inx−Cu/Fe/Cu(001)sandwiches

A strong reduction of the Curie temperature ${T}_{C}$ has been observed for room-temperature-grown fcc Fe films on Cu(001) when covered with 1 monolayer (ML) Cu for all Fe thicknesses up to the fcc-bcc transition of the Fe film at \ensuremath{\approx}11 ML. At 2 ML Cu coverage this decrease of ${T}_{C}$ partially recovers and approaches a constant lower value on further increasing Cu coverage. The correlation of this observed magnetic behavior with electronic and possible structural changes of the Fe film upon Cu coverage is discussed.

Non-pseudomorphous epitaxial growth of fcc Fe on Cu(100) observed by reflection high-energy electron diffraction (RHEED)

A high-precision reflection high-energy electron diffraction (RHEED) analysis of the in-plane lattice spacing of the surface during epitaxial growth of fcc Fe films on a Cu(100) single crystal substrate is presented. As a function of Fe coverage, initially the in-plane fcc Fe lattice spacing is found to decrease drastically until a minimum is reached, while at higher coverages the smaller equilibrium value for bulk fcc Fe (3.59Å), rather than the value of the Cu substrate (3.615Å), is approached. This proves that fcc Fe films on Cu(100) grow strain free and are non-pseudomorphous, contrary to earlier assumptions.

Structure and magnetism of FCC-Fe films on Cu(1 0 0)

The atomic and magnetic structure of FCC-Fe layers on a Cu(1 0 0) substrate is investigated by ab initio calculations as a function of the thickness of the film. Fe films with a thickness varying between 1 and 3 ML stabilize a ferromagnetic (FM) ground state. For thicker films with an even number of Fe layers (4, 6 and 8 ML) a bilayer AF magnetic structure develops, while for an odd number of Fe layers (5, 7 and 9 ML) various spin structures coexist. The magnetic properties of Fe thin layers are compared to those of bulk FCC-Fe biaxially strained to the lattice constant of the Cu substrate and the dependence of magnetic energies in the form of the exchange-correlation functional is discussed.

Structural and magnetic studies of fcc Fe films with self-organized lateral modulation on striped Cu(110)–O(2×1) substrates

Face-centered cubic (fcc) Fe wedges of 0–12 ML were grown by means of molecular beam epitaxy onto a novel substrate: flat Cu(110) with an oxygen-induced, long-range ordered striped phase, and studied in situ with medium energy electron diffraction (MEED) and the surface magneto-optical Kerr effect (SMOKE). In contrast to Fe growth on either clean or oxygen-saturated Cu(110), the films on the striped substrates retain a layer-by-layer growth mode up to 6–7 ML and are fcc at least up to 12 ML. In addition, satellite peaks were observed on both sides of the MEED (0, 0) streak, indicating a long-range-ordered lateral modulation of the Fe surface. We postulate that the Fe films grow conformally onto the original striped substrate. SMOKE studies show that these fcc Fe wedges are ferromagnetic with an easy axis along the original stripes for Fe thickness >4 ML and a remanant magnetization that increases linearly with thickness beyond 4 ML.

Photoelectron diffraction in magnetic dichroism: Surface live magnetic layers in fcc Fe/Co(001)

We combine photoelectron diffraction and magnetic dichroism in photoemission to obtain magnetic and structural information regarding ultrathin films on an element-resolved basis. Crystallinity-induced effects in the magnetic dichroism of $2p$ core level photoelectrons present valuable structural information regarding magnetically inhomogeneous films. The location of ferromagnetic layers in structurally relaxed fcc Fe films (eight monolayers) grown at room temperature on Co/Cu(001) is found to be at the surface, with nonferromagnetic layers underneath. Tetragonally distorted fcc Fe films (one and three monolayers) are found to be entirely ferromagnetic.

Magnetism of atomically thin fcc Fe overlayers on an expanded fcc lattice:Cu84Al16(100)

We present experimental data on the magnetic properties of atomically thin fcc (\ensuremath{\gamma}-phase) Fe films (1--6 atomic layer nominal thickness) epitaxially grown on ${\mathrm{Cu}}_{84}{\mathrm{Al}}_{16}(100)$ obtained by linear magnetic dichroism in the angular distribution of Fe $3p$ core photoelectrons excited by linearly polarized synchrotron radiation. The sign and magnitude of the Fe $3p$ photoemission magnetic asymmetry indicates the onset of in-plane ferromagnetism at 2.5(2) monolayer (ML) thickness of \ensuremath{\gamma}-Fe. The Curie temperature is 288(2) K for 4 ML thickness. The magnetic splitting of the Fe $3p$ $\mathrm{mj}$ core hole sublevels is 1.10(2) eV, i.e., the same value as measured for a bcc-Fe(100) surface where large surface and near-surface enhanced moments contribute. These results characterize the epitaxial \ensuremath{\gamma}-Fe on ${\mathrm{Cu}}_{84}{\mathrm{Al}}_{16}(100)$ as a high-spin ferromagnet for thickness up to 4 ML, with an average magnetic moment per iron atom of 2.5(1)${\ensuremath{\mu}}_{B}.$ A phase transition occurs between 4 and 5 ML thickness: the magnetic order of the pseudomorphic \ensuremath{\gamma}-Fe film decreases consistently with the breaking into two phases with the deeper layers in a low-spin and/or antiferromagnetic phase and surface restricted ferromagnetism, similar to the case of \ensuremath{\gamma}-Fe/Cu(100).