Fe Film Thickness Research Articles

Magnetization of uncovered and V-covered ultrathin Fe(100) films on V(100)

We used polarized neutron reflectometry (PNR) to determine the absolute magnetic moment of uncovered and V-covered Fe films in the thickness range from $0.3\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The films were prepared by molecular beam epitaxy on a V(100) buffer layer grown on a MgO(100) crystal. The magnetic moment shows a linear dependence on the Fe film thickness with a reduction (compared to the Fe bulk value) of the magnetic moment equivalent to $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe for the V-covered films and a reduction equivalent to $0.03\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe for the uncovered Fe films. For the case of the $\mathrm{V}∕\mathrm{Fe}∕\mathrm{V}$ samples we observe a much smaller reduction of the magnetic moment than reported for experiments on $\mathrm{Fe}∕\mathrm{V}$ multilayers. As theoretical calculations show a strong decrease of the magnetic moment for an interface alloy we conclude that the larger reduction of the magnetization in $\mathrm{Fe}∕\mathrm{V}$ multilayers is due to an increase in interface roughness with increasing film thickness. For the uncovered Fe(100) films we find a much smaller reduction of the magnetic moment than in earlier in situ PNR experiments on $\mathrm{V}(110)∕\mathrm{Fe}(110)$ where we observed a reduction equivalent to $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe.

Magnetooptic properties of Fe∕Pd and Co∕Pd bilayers under hydrogen absorption

The magnetooptical (MO) properties of hydrogenated Fe∕Pd and Co∕Pd bilayers were studied as a function of the H2 pressure. For samples with a Pd overlayer thickness 5.3nm and Fe film thickness in the range of 4.0–11.0nm, the fractional change in MO response under hydrogen loading, ΔθK∕θK0, increases approximately 12%, independent of the Fe film thickness. For H2 pressures less than PH2=25Torr, the enhancement obeys Sievert’s law (ΔθK∕θK0∝PH21∕2). Thicker Pd overlayers increase the MO enhancement, with remarkably large enhancement of 50% for a Fe(4.1nm)∕Pd(10.0nm) sample. Because ΔθK∕θK0 is independent of the Fe thickness, this effect results from a change in the optical properties of the Pd overlayer. This was confirmed by vibrating sample magnetometry. In contrast, no effect is observed in the Co∕Pd bilayers, presumably due to a small amount Co interdiffusion that prevents the Pd from absorbing H2.

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).

Interaction of CO and NO with Fe thin films grown on Rh(1 0 0) surface

Chemisorption of CO and NO on Fe thin films on Rh(1 0 0) has been studied by LEED, AES, HREELS and TDS methods. The analysis of the LEED (0 0) beam I( V) curves shows that a compressed interlayer spacing in the pseudomorphic fct Fe films is expanded at 4 ML thickness. HREELS for CO adsorbed 1 ML thick Fe film displays two C–O stretch bands at 2060 and 1950 cm −1 due to CO bonded at atop and bridge sites, respectively. These two CO bands merge into a broad band centered at 2000 cm −1 with increasing film thickness. In addition, a weak C–O stretch band stays at 1630 cm −1 in the film thickness range of 1–5 ML, which can be assigned to CO bound to fourfold hollow sites. TPD of CO saturated Fe films shows three molecular desorption states and one recombinative desorption as similarly as bulk bcc Fe(1 0 0) surface. The two desorption states at low temperature sequentially occur from atop sites initially and then from bridge sites. In contrast, NO adsorption displays an intense N–O stretch mode at 1825 cm −1 with a shoulder band at 1620 cm −1 in the entire thickness range of 1–5 ML. These bands are assigned as NO bound to atop and bridge sites. On heating, NO bonded at atop sites produces the desorption peak at 350 K and the dissociation of NO is completed at 450 K. In the adsorption of CO and NO, there is no appreciable change in the stretching frequencies accompanied with the increase in interlayer spacing within the resolution of HREELS.

Out-of-plane magnetized Fe-based multilayers on GaAs(0 0 1)

We report on the out-of-plane magnetization in Fe/Ni and Fe/Au bilayers grown on GaAs(0 0 1). For Ni thicknesses of several monolayers, the magnetization of the Fe/Ni bilayer increases linearly with the Fe film thickness, which is typically observed when Fe grows in the BCC-structure. For thicknesses up to 8 ML, Ni is nonmagnetic such as in the Fe/Ni/GaAs(0 0 1) structure. This could be due diffusion of Ga. Up to thickness of 2 ML the Fe overlayer is magnetized out-of-plane, however at low temperatures only. The nature of the out-of-plane spin orientation is discussed, also in view of the crystallographic structure of the Fe/Ni bilayer and by comparison with the Fe/Au bilayer grown on GaAs(0 0 1).

Magneto-optical properties of Fe/Cr/Fe/MgO/Fe structures epitaxially grown on GaAs(001)

Fe/Cr/Fe trilayers were epitaxially grown on atomically flat GaAs(001). For the thickness of Cr spacer layer corresponding to antiferromagnetic coupling, “reversed” minor hysteresis loops were measured with longitudinal magneto-optical Kerr effect (MOKE), i.e., a negative “magnetization” signal was detected when the thicker bottom Fe layer was saturated along the applied field. This behavior is interpreted by depth variations of the MOKE sensitivity. Magnetization reversal shows that both antiferromagnetic switching and spin–flop transition fields depend on the ratio of both Fe film thicknesses. The shape of the MOKE loops becomes more complex with further deposition of MgO and Fe layers on the top of the Fe/Cr/F/GaAs(001) stack. Superconducting quantum interference device measurements confirm the interpretation of the MOKE loops and demonstrate homogeneity and sharpness of the interfaces in the structures.

Effect of the interlayer coupling on the Ni spin reorientation in Ni/Fe/Co/Cu(100)

Ni/Fe/Co/Cu(100) films were epitaxially grown and investigated by photoemission electron microscopy. The magnetic correlation of the Ni and Co films was investigated by element-specific domain images. We found that the Ni magnetization exhibits a continuous rotation in the spin reorientation transition (SRT) region and that the Ni SRT thickness oscillates with the Fe film thickness.

Spin motion of photoelectrons

Ab initio and model calculations demonstrate that the spin motion of electrons transmitted above the vacuum energy through ferromagnetic films can be investigated by means of angle- and spin-resolved core-level photoelectron spectroscopy. The motion of the photoelectron spin polarization can be regarded as a combination of a precession around and a relaxation towards the magnetization direction. For ultrathin Fe films on Pd(001), its dependence on the Fe film thickness and on the Fe electronic structure is studied systematically. In addition to elastic and inelastic scattering, the effect of band gaps on the spin motion is addressed in particular.

Linear and quadratic magneto-optical Kerr effects in continuous and granular ultrathin monocrystalline Fe films

The effects of nanostructuration on the magneto-optic properties of ultrathin monocrystalline iron films grown on MgO (001) are investigated through the magneto-optic coefficients, which have a linear and quadratic dependence on the magnetization. The photon energy dependence of such magneto-optic coefficients is determined by measuring the relative variations of the reflectivity (ΔR p p /R p p ) for p-polarized incident and reflected light (p-p polarized) when the magnetization rotates in the plane of the sample. Thick Fe films present a magneto-optical anisotropy, which has a quadratic dependence on the magnetization. Such anisotropy is strongly reduced in nanostructured iron thin films formed by nanometric iron islands. The modifications induced by the nanostructuration are stronger for the magneto-optical coefficients, which have a quadratic dependence on the magnetization in contrast to the linear terms. A self-consistent effective-medium formalism is presented that explains the modifications induced in the magneto-optical coefficients by nanostructuration.

Exchange bias in the Fe/KCoF3 system: A comprehensive magnetometry study

Exchange bias was studied in the Fe/KCoF3 ferromagnet/antiferromagnet system. KCoF3 can be deposited onto single crystal of Fe, either in the polycrystalline or single crystal form, depending on growth conditions. The samples were grown by molecular beam epitaxy on Ga-terminated GaAs (100) wafers. We study effects of the crystal state of the fluoride, thickness of the Fe film, crystallographic orientation of the Fe, and temperature on exchange bias. The structures with single crystal KCoF3 show that the exchange bias is well correlated with the coercivity at low temperatures and vanishes at a temperature close to the Néel temperature. Both the magnitude of the exchange bias and the blocking temperature of the samples with the polycrystalline fluoride were significantly reduced compared to the single crystal structures. As the Fe film thickness was increased, the exchange bias decreased for all samples. In contrast, the blocking temperature remained unchanged for the samples with the single-crystal fluoride. The exchange bias measured along the easy anisotropy axis of the Fe was slightly larger than that measured along the hard axis. In addition, all samples exhibited a weak training effect.

In situ oxidation of epitaxial Fe(1 1 0) films grown on Mo(1 1 0)/Al 2O 3(1 1 −2 0) substrates

The surface morphology of in situ oxidized 200 Å thick Fe(1 1 0) films grown on Al 2O 3(1 1 −2 0) substrates using a Mo(1 1 0) buffer layer was investigated by low energy electron diffraction and scanning tunneling microscopy (STM). The epitaxially grown Fe(1 1 0) films were oxidized to magnetite, Fe 3O 4(1 1 1), in an oxygen atmosphere. For a low oxygen exposure of 6 L at room temperature a c(2 × 2)-O-reconstruction and for an intermediate oxygen exposure of 100 L followed by annealing at 600 K the formation of FeO(1 1 1) was observed. After high oxygen exposures (⩾1500 L) and annealing at 800–1000 K the formation of Fe 3O 4(1 1 1) films of high crystalline quality has been achieved. Atomically resolved STM images of the Fe 3O 4(1 1 1) surface show a hexagonal symmetry with a 6 Å periodicity. The system was identified as a well-ordered epitaxial Fe 3O 4(1 1 1) layer on the Fe(1 1 0)/Mo(1 1 0)/Al 2O 3(1 1 −2 0) system.

Spin reorientation induced by Ni atoms in Fe/Cu(001)

We determined the spin reorientation transition (SRT) in face-centered-tetragonal (fct) Fe/Cu(001) from out of plane to in plane, as induced by Ni-capping atoms, by observing the magneto-optical Kerr effect. The SRT takes place when the Ni-capping layer thickness exceeds a critical value dcNi, at which point the perpendicular anisotropy of the fct-Fe film becomes equal to the in-plane anisotropy of the Ni-capping layer. We found that dcNi decreases linearly with increasing Fe film thickness. By extrapolating to zero Ni thickness, the “true” critical thickness for the fct-Fe films is estimated to be 4.3 monolayers. This result is very close to the value obtained using Co-capping layers as reported in literature.

Determination of spin distributions in ion-beam magnetic textured iron films by magnetic orientation Mössbauer spectroscopy

A promising approach by which to gain information about the spin distributions in different sample layers using depth-sensitive angle-dependent conversion electron Mössbauer spectroscopy is presented in combination with magneto-optical Kerr effect, Rutherford backscattering spectrometry and x-ray diffraction investigations. Three 77 nm thick Fe films containing a 15 nm Fe57 marker layer at different depths were deposited on Si(100) substrates and irradiated under well defined mechanical stress with 200 keV Xe ions. The distribution of the spins in the sample plane was analyzed by Mössbauer spectroscopy and by tilting the sample toward the incident γ beam and observing the variation in angle-dependent relative intensity of the sextet lines. For the sample with the sensitive marker layer in the middle of the ferromagnetic film, all spins were found to be oriented exactly along the mechanical stress axis. This agrees with the angular properties of the macroscopic hysteresis curves measured by the magneto-optic Kerr effect.

Thickness‐dependent interface magnetism of Fe/Cr/Fe trilayers

AbstractElectronic structure and magnetic properties of FeN/Cr3/FeN bcc(100) trilayers (N is the number of atomic layers) are studied on the basis of ab initio calculations performed within the framework of the density functional theory in the local spin density approximation, using the full‐potential LAPW method in slab geometry. According to our calculations, the magnetic moment of a surface Fe layer increases monotonically from 2.45 μB for N = 1 to 2.92 μB for N = 3 and practically does not change at further increase of the Fe film thickness. However, the magnetic moment in the interface Fe layer changes nonmonotonically with the Fe film thickness increase and is close to the surface value at N = 5 and 7. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Role of spin current in magnetic relaxations of metallic multilayer films

In our recent studies the relaxation processes were investigated by ferromagnetic resonance (FMR) using magnetic single, Au/Fe/GaAs(0 0 1), and double layer, Au/Fe/Au/Fe/GaAs(0 0 1), structures prepared by molecular beam epitaxy (MBE). Single and double layer structures were grown on (4×6) reconstructed GaAs(0 0 1) templates. The samples have high crystalline quality with the magnetic properties corresponding to bulk Fe, only modified by interface anisotropies. The magnetic relaxation in these structures is well described by Gilbert damping with no extrinsic contributions to the FMR line width. These structures provide an excellent opportunity to investigate non-local damping which is caused by spin transport across a non-magnetic spacer. In the double layer structures thin Fe layers were separated from a second thick Fe layer of 40 monolayer (ML) thickness by a 40 ML thick Au(0 0 1) spacer. The main result of these studies is that the ultrathin Fe films in magnetic double layers acquire an additional Gilbert damping. The additional FMR line width followed an inverse dependence on the Fe film thickness which is expected for interface damping. The spin pump model will be discussed and it will be shown that it is closely related to the dynamic part of interlayer exchange coupling.

Effects of Fe Film Thickness and Pretreatments on the Growth Behaviours of Carbon Nanotubes on Fe-doped (001) Si Substrates

Transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to investigate the effects of Fe film thickness and pretreatment conditions on the growth behaviours of carbon nanotubes on the Fe-deposited Si (001) substrates. The Fe films are deposited for 20 s–20 min by pulse-laser deposition. SEM results show that the growth characteristics of carbon nanotubes are strongly dependent on the Fe film thickness and pretreatment conditions. It is shown that the pretreatments at 800°C (and also chemical vapour deposition growth) cause the continuous Fe films to be broken up into nanoparticles (8–50 nm across) and discontinuous islands (100 nm–1.1 µm in size). It is also shown that the Fe nanoparticles are essentially required for the formation of aligned carbon nanotubes. Furthermore, for the Ar-pretreated sample, a carbonaceous layer is formed near the surface region. TEM results show direct evidence that a base growth mode is responsible for the growth of carbon nanotubes in the present work. Based on TEM and SEM results, the pretreatment atmosphere dependence of the growth behaviours of carbon nanotubes is discussed.

Magnetism in exposed and coated nanoclusters studied by dichroism in X-ray absorption and photoemission

We have studied the evolution of the magnetic behaviour of size-selected Fe nanoclusters from isolated particles adsorbed on a surface exposed in UHV to dense interacting assemblies. The clusters were produced by a UHV-compatible gas aggregation source and were studied in situ by synchrotron radiation techniques based on dichroism. Isolated and exposed Fe clusters deposited on highly oriented pyrolitic graphite substrates exhibit orbital and spin magnetic moments that are, respectively, up to 250% and 10% higher than the values measured in thick Fe films. The moments decay towards the bulk value with increasing cluster size or with increasing particle density on the surface. The total moment in the supported clusters is not as high as observed in free clusters of the same size. The individual particles appear to be magnetically isotropic but a strong in-plane coherent anisotropy develops as the coverage is increased to a complete cluster monolayer. Coating the deposited Fe clusters with a Co film substantially increases the spin moment on the Fe.

Magnetic uniaxial anisotropy of Fe films grown on vicinal Ag(001)

Step-induced magnetic anisotropy was investigated in Fe films grown on a vicinal Ag(001) substrate using the surface magneto-optic Kerr effect technique. We found that the step-induced magnetic anisotropy is independent of the Fe film thickness up to 61 ML. Step decoration with Ag or Pd atoms also shows no effect on the step-induced magnetic anisotropy. These results indicate that the step-induced magnetic anisotropy in this system does not localize at the step edges. We suggest that strain inside the Fe film due to the large lattice mismatch between fcc Ag and bcc Fe in the normal direction of the film is the dominant contribution to the step-induced magnetic anisotropy in this system.

Strain determination in ultrathin bcc Fe layers on Si(001) by x-ray diffraction

Good quality body-centered cubic iron layers can be grown on Si(001) at room temperature using thin ${\mathrm{FeSi}}_{2}$ or ${\mathrm{CoSi}}_{2}$ silicide buffer layers. The in-plane and out-of-plane strains are measured by x-ray diffraction (XRD). Due to the difference in Fe and Si parameters of +5.6%, the thin Fe layers undergo a strong tetragonal distortion. For the samples deposited on ${\mathrm{FeSi}}_{2},$ the measured biaxial in-plane compressive strain ranges from values as large as -3.8% in the thinnest films (25 ML) to -1% in the thicker layers (>40 ML). In the samples deposited on ${\mathrm{CoSi}}_{2},$ a part of the iron layer clearly observable up to 40 ML, apparently grows in pseudomorphy with the silicon, whereas another dominant part above 15 ML partly relaxes and behaves as on ${\mathrm{FeSi}}_{2}$ templates but with systematically larger strains. The evolution of the in-plane cubic effective magnetic anisotropy constant versus Fe film thickness observed in previous work, can be explained by means of fourth-order (in spin) magnetoelastic coupling in the iron lattice with the strain components determined by XRD. Finally, the observed ratio between the perpendicular and in-plane strain components differs substantially from linear elasticity theory predictions and indicates the importance of anharmonic effects and/or defects in the description of the elastic properties of such films.

Upper bound for the magnetic proximity effect extracted from Brillouin light scattering

The effective magnetic thickness of Fe films and of Fe/Al and Fe/Pd bilayers is determined using Brillouin light scattering. The magnetic thickness is extracted by fitting the field dependence of the frequencies of two magnon modes. Within experimental errors of about 1 \AA{}, no change in the effective magnetic thickness of the Fe layers was detected. If a net magnetization does exist in Pd when in contact with Fe, it is sufficiently different in nature so as to not modify the Fe spin waves.