Bcc Fe Films Research Articles

Influence of substrate surface reconstruction on the growth and magnetic properties of Fe on GaAs(001)

Abstract : We have studied the magnetic and structural properties of epitaxial bcc Fe(001) films grown at 175 C on molecular-beam epitaxy-prepared GaAs(001)-2*4 and -c(4*4) reconstructed surfaces, with film thicknesses ranging up to ~30 ML (~43 ). We present measurements of the thickness-dependent evolution of the magnetic properties of the Fe films as determined by in situ magneto-optic Kerr effect. We find that the magnetic properties and growth mode are similar for both 234 and c(434) reconstructions, although the initial adsorption sites and island nucleation as measured by scanning tunneling microscopy are clearly dominated by the substrate surface reconstruction. The onset of room-temperature ferromagnetism occurs at 6 ML for growth on both GaAs surface reconstructions. At this coverage, the measured Curie temperature (100 C) is significantly reduced from that of bulk alpha-Fe (770 C). The anisotropy is dominated by a uniaxial component such that the two (110) axes are inequivalent for all coverages studied. Shape anisotropy does not appear to play a significant role.

Kerr effect study of the onset of magnetization in Fe films on GaAs(001)-2×4

We have studied the magnetic behavior of epitaxial bcc Fe(001) films grown on molecular beam epitaxy-prepared GaAs(001)-2×4 substrates, with film thicknesses ranging up to ∼16 monolayers (ML) (∼23 Å). We present measurements of the thickness-dependent evolution of the magnetic properties of the Fe films as determined by in situ magneto-optical Kerr effect. Particular attention is given to the substrate-induced in-plane uniaxial magnetic anisotropy. We find that the onset of room-temperature ferromagnetism occurs between 4 and 8 ML, and that the anisotropy is dominated by a uniaxial in-plane component such that the two 〈110〉 axes are inequivalent for all coverages studied. Although the initial nucleation and growth results in 2D islands elongated along [1̄10], the in plane easy axis is perpendicular to this direction along [110], indicating that shape anisotropy does not play a significant role.

Using ferromagnetic resonance to measure the magnetic moments of ultrathin films

A technique that uses ferromagnetic resonance (FMR) to measure the saturation magnetization of ultrathin ferromagnetic films is described. It has been used to measure the layer averaged magnetic moments of ultrathin Fe films located in Ag/Fe/Ag, Au/Fe/Ag, Cu/Fe/Ag, Pd/Fe/Ag, and Ni/Fe/Ag structures relative to that of an Au/5.7 ML Fe/Ag reference film. The ratios obtained using method have a total measurement error of a little over 1%. The measurements were carried out to investigate theoretical predictions that Fe atoms located at or near surfaces and interfaces should possess enhanced magnetic moments compared those of Fe atoms in the bulk. All of the bcc Fe(001) films used in our work were approximately 5 monatomic layers (ML) thick and were all grown on bulk fcc Ag(001) substrates. The covering layers of Cu, Pd, and Ag were all 7 ML thick while the covering layers of Ni were 2–3 ML thick and the covering layers of Au were 20 ML thick. All FMR measurements were carried out at a temperature of 77 K. An Ag/5.5 ML Fe/Ag specimen and an Ag/10.9 ML Fe/Ag specimen (the thick Fe specimen) were determined to have a moment of ratio of 1.06 ± 0.01. This compared well to a layer averaged, ground state, moment ratio of 1.05 calculated for the two films using results obtained from published first principles calculations. Ratios of 1.08 ± 0.01 and 1.10 ± 0.01 were obtained for Au/5.7 Fe/Ag and Cu/5.8 Fe/Ag structures, respectively, when compared to the thick Fe specimen. In a similar manner, ratios of 1.11 ± 0.01 and 1.12 ± 0.01, respectively, were determined for Pd/5.6 Fe/Ag and Pd/5.7 Fe/Ag sandwiches. Finally, the layer averaged moment of a 2 Ni/5.7 Fe/Ag structure was in a ratio of 1.15 ± 0.01 with the thick Fe specimen while two 3 Ni/5.7 Fe/Ag sandwiches were in ratios of 1.24 ± 0.01 and 1.20 ± 0.01, respectively. All measured values were in excellent agreement with published first-principles calculations and with the layer averaged moments obtained from polarized neutron reflection (PNR) studies carried out on many of our specimens.

Magnetic anisotropy of thin Fe films grown on Ni

The magnetic anisotropy of a series of thin Fe films, grown on Ni films with constant thickness 140 Å on Si substrates, has been studied at different temperatures by SQUID magnetometry. It is found that the magnetic anisotropy changes with the Fe layer thickness. A significant change of both the interface and volume anisotropies of Fe films has been observed in the thickness range between 40 and 60 Å which is correlated to a structural phase transition of the Fe films from fcc-like to bcc with increasing Fe layer thickness. Furthermore, the deduced volume anisotropy of bcc Fe films thicker than 60 Å is much larger than the magnetocrystalline anisotropy of bulk bcc Fe which may be attributed to strains in the films.

Interaction of CO with heteroepitaxial fcc- and bcc-Fe films on Cu(100)

Up to 10 monolayers iron is known to grow with an fcc structure on Cu(100), while thicker iron films exhibit the usual bcc structure. Using UPS and TDS measurements we find significant differences in the interaction of CO with both types of iron surfaces. On a CO covered fcc iron film the intensity ratio I(4σ) I(1π + 5σ) in the He II excited UP spectra is nearly two, and CO desorbs without dissociation at 360 K. On the other hand, on a 50 ML thick bcc-Fe film both CO-induced peaks have nearly the same intensity, and adsorbed CO molecules dissociate at temperatures just above 300 K as judged from the appearance of O(2p) emission at 5.7 eV. This interesting difference of the CO interaction with fcc- and bcc-Fe films is discussed in terms of different CO adsorption sites and adsorbate orientations on both surfaces, in that a tilted CO species as found on a bcc-Fe surface does not exist on the fcc-Fe(100) film.

Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio bandstructure theory

Using a spin-polarized full-potential linear muffin-tin orbital method we present calculations of the nonlinear magneto-optical Kerr effect for thin bcc Fe films within a slab geometry. Film layer dependent contributions to the Kerr spectrum are determined. Thus, we calculate the magneto-optical Kerr spectra for the linear and nonlinear case. Our results show clearly that the Kerr spectra of thin films are characteristicly different from those at surfaces of bulk materials. In the case of linear Kerr spectra of Au/Fe(bcc)/Au(001) films our theoretical results are in good agreement with observed frequency- and thickness-dependent spectra.

Magnetic anisotropy of glide-distorted fcc and of bcc ultrathin Fe/Cu(001) films.

The effective anisotropy fields, ${H}_{\mathrm{eff}}$, of Fe films of 2 to 15 monolayers thick grown on Cu(001) were measured at 100 K for two growth conditions; 100 K growth with a room temperature anneal and room-temperature growth. First-order anisotropy constants, ${K}_{v}$ and ${K}_{s}$, are derived for the thickness independent anisotropy energy term and the thickness dependent anisotropy energy term, respectively. For 100 K growth, ${K}_{v}$ for the glide-distorted fcc Fe film is two orders of magnitude larger than for the bcc Fe film and g30 times larger than for bulk bcc Fe. The fcc film has ${K}_{s}=0.26$ ergs/${\mathrm{cm}}^{2}$, compared to 0.94 ergs/${\mathrm{cm}}^{2}$ for the bcc Fe film. The perpendicular easy axis in the glide-distorted fcc Fe, for either growth temperature, is observed only because both ${K}_{v}$ and ${K}_{s}$ result in large perpendicular anisotropy energies. A conversion to an in-plane easy axis occurs as the thickness dependent anisotropy energy decreases with increasing Fe thickness in the bcc phase and is not directly a result of the phase transformation to bcc Fe. Room-temperature growth gives similar anisotropy constants.

Structural Transformations During Growth of Epitaxial Fe(001) Thin Films on Cu(001) and Pt(001)

AbstractWe have used grazing incidence x-ray diffraction to observe the structural evolution during growth of sputter-deposited epitaxial Fe films on Cu(001) and Pt(001). We find that on Cu(001), Fe is fcc up to a thickness of 10–12 monolayers, whereupon bcc Fe is observed in first the Pitsch and then the Bain orientations. The fcc Fe shows some relaxation of the misfit from the Cu, as do the Pitsch orientation bcc, which is in tension, and the Bain orientation bcc, which is in compression. All three Fe variants exist in a 40 monolayer thick film. On Pt(001) the Fe grows as bcc with the Bain orientation. However, a thin (20 å) bcc Fe film is transformed to fcc Fe with cube-on-cube orientation by subsequent deposition of Pt. This behavior is consistent with intermixing of Pt into the Fe layer, which lowers the mismatch and bulk chemical energies of the fcc phase relative to that of the bcc phase.

Enhanced magnetic moments in bcc Fe films.

We present evidence for enhanced magnetic moments in epitaxial Fe films in proximity with Ag, Au, Cu, and Pd overlayers prepared by molecular-beam epitaxy on singular Ag(001) substrates in a combined study using polarized-neutron reflection (PNR) and ferromagnetic resonance (FMR). Second versions of equivalent samples were investigated in order to test the repeatability of the results, and all PNR measurements were corrected for background and diffuse scattering. Within experimental error, measurements of the absolute value of the magnetic moment per atom made using PNR were found to agree with measurements of the relative magnetization using FMR, and measurements on repeated samples were also found to agree. An average magnetic moment per Fe atom of 2.58\ifmmode\pm\else\textpm\fi{}0.09${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ was determined for a Ag/5.5 ML Fe structure, significantly enhanced from the value of ${\mathrm{\ensuremath{\mu}}}_{\mathrm{Fe}}$=2.33\ifmmode\pm\else\textpm\fi{}0.05${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ obtained for a Ag/10.9 ML Fe reference sample. Corresponding values of 2.48\ifmmode\pm\else\textpm\fi{}0.08${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and 2.50\ifmmode\pm\else\textpm\fi{}0.10${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ were determined for Cu/5.8 ML Fe and Cu/5.7 ML Fe structures, respectively. Values of the layer averaged moment of 2.66\ifmmode\pm\else\textpm\fi{}0.05${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and 2.6\ifmmode\pm\else\textpm\fi{}0.2${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ were determined for Pd/5.6 ML Fe and Pd/5.7 ML Fe structures, respectively, assuming that no induced polarization occurs in the interface Pd layers.The degree of enhancement measured in equivalent samples is seen to be in agreement within experimental error. The magnitude of the enhancement (12--20)% we observe in the samples of thickness close to 6 ML is slightly higher than that predicted by recent band-structure calculations, but our results show conclusively that the enhancement can be attributed to the interface atoms. The temperature dependence of the magnetization has also been measured using PNR and the results compared with that predicted from spin-wave theory using FMR measurements of the effective spin-wave gap. Good agreement is found for the Ag/Fe and Cu/Fe systems for which spin-wave gap temperatures of 0.1 and 0.12 K, respectively, are determined by PNR, compared with values of 0.15 and 0.11 K deduced from FMR, where the agreement for Au/Fe is less good. Both FMR and PNR data are consistent with the largest gap occurring for the Pd/Fe interface system but the value of the spin-wave gap determined by FMR is in this case smaller than the value obtained from fitting the PNR data.

The Magneto-Optical Quantum Size Effect in bcc-Fe(001) and (110) Ultrathin Films

ABSTRACTRecently, the oscillation of the magneto-optical effect with respect to the ferromagnetic layer thickness was observed for bcc-Fe(001) ultrathin films. The oscillation periods have been explained by the theory concerning the formation of quantum well states in the ferromagnetic layer. In this paper, a physical picture how the electron confinement produces the quantum size effect of the magneto-optical responses is introduced, and experimental results on the magnetooptical effects of the bcc-Fe films of different orientations are presented. The (001) oriented films show clear oscillation. On the other hand, the (110) oriented films does not show any oscillation. The reason is discussed from the band structure and growth modes.

Magnetic and structural instabilities of ferromagnetic and antiferromagnetic Fe/Cu(100)

Fe wedges epitaxially grown on Cu(100) have been employed to investigate the interplay between magnetic and structural instabilities. 2–4 monolayer (ML) clean Fe films grown at room temperature are ferromagnetic with perpendicular easy axes. bcc Fe films≳11 ML thick are ferromagnetic with in-plane easy axes. Most importantly, 6–11 ML fcc Fe films are antiferromagnetic and have a ferromagnetic surface. Films grown below 200 K and annealed to room temperature do not exhibit the antiferromagnetic phase, but remain ferromagnetic and undergo a spin-reorientation transition from perpendicular to in plane at ∼6 ML. A new phase diagram for Fe/Cu(100) is proposed as a function of thickness and growth temperature. In addition, an impurity-stabilized layer-by-layer growth that persists to 30–40 ML Fe is also reported.

Magnetic response of ultrathin Fe on MgO: A polarized neutron reflectometry study

The magnetization of ultrathin bcc Fe films (two and three monolayers) on MgO was measured and compared with the behavior predicted for a two-dimensional ferromagnet. The experiment indicated that no hysteresis was present in the magnetization. Instead, the magnetization at low temperature was affected by a marked field cooling effect. These observations lead to the conclusion that films of Fe on MgO of such thickness exhibit superparamagnetic behavior as if they were not entirely continuous. In contrast, films thicker than five monolayers exhibit a magnetic response close to that of bulk iron.

Surface and step magnetic anisotropy.

Cu(11n) surfaces vicinal to Cu(001) are characterized by (001) terraces separated by monoatomic steps. Growth of thin epitaxial magnetic films on such terraced substrates, e.g., Co/Cu(1 1 13), leads to unexpected magnetic anisotropy associated with the surface steps. This anisotropy remains largely unexplained, and is of potential technological importance. We have studied the anisotropy of fcc Co/Cu (1 1 13) films using in-situ magneto-optical Kerr effect (MOKE), and find an in-plane uniaxial anisotropy favoring magnetization parallel to the steps. The strength of this anisotropy decreases with increasing film thickness. In this paper we focus on the interpretation of this anisotropy in terms of the N\'eel model. The N\'eel anisotropy energy for such a film has the form ${\mathit{E}}_{\mathrm{film}}$= ${\mathit{E}}_{\mathrm{bulk}}$-2${\mathit{E}}_{\mathrm{surface}}$/t-2${\mathit{E}}_{\mathrm{step}}$/(td), where t and d are film thickness and terrace width, respectively. The last term includes contributions from sites at both the upper edge and inner corner of the step and makes the dominant contribution to the step-induced anisotropy. This model properly accounts for the preferred direction of magnetization parallel to the steps in Co/Cu(1 1 13). bcc Fe/W vicinal to (001) shows a preference for in-plane magnetization perpendicular to the steps for t2.5 monolayers. The N\'eel model also predicts this anisotropy for a bcc Fe film provided the magnetization lies in the film plane, not in the (001) plane.

Magnetic anisotropy in ultrathin films grown on vicinal surfaces (abstract)

Growth of thin epitaxial magnetic films on terraced substrates, such as Co/Cu(1 1 13) vicinal to (001), leads to unexpected magnetic anisotropy associated with the surface steps. This anisotropy remains largely unexplained and is of potential technological importance. We have studied the anisotropy of fcc Co/Cu(1 1 13) films of 3–20 ML thicknesses using in situ MOKE. The presence of monoatomic steps on the Cu(1 1 13) surface induces a uniaxial anisotropy favoring magnetization parallel to the steps. The strength of this anisotropy decreases with increasing film thickness. In this paper we focus on the interpretation of this anisotropy in terms of the Néel model.1,2 Vicinal surfaces are characterized by (001) terraces separated by monoatomic steps. The Néel anisotropy energy for such a surface has the form EtotalN= Ebulk atoms+(Esurface atoms /t) + Estep-edge atoms /td), where t and d are film thickness and terrace width, respectively. The last term includes contributions from sites at both the top and bottom of the step and is responsible for the step-induced anisotropy. For 10 ML of Co on Cu(1 1 13), the magnitude of this term is approximately 0.3 erg/cm2, favoring M parallel to the steps, as we have observed. Fe/W vicinal to (001) films show a preference for in-plane magnetization perpendicular to the steps.3 The Néel model predicts this anisotropy for a bcc Fe film provided the magnetization lies in the film plane, not in the (001) plane.

Perpendicular magnetic anisotropies in ultrathin BCC iron films and surfaces

The perpendicular magnetic anisotropy in one- to six-monolayer BCC Fe films is studied by the Green-function method based upon the Heisenberg model (S>or=1) with out-of-plane spin orientation of the atoms at the surfaces. The various anisotropy constants resulting from surface single-ion anisotropy have been derived for the first time. According to the anisotropy constants as functions of the thickness of ultrathin films and temperature, we explain that there is a perpendicular remanence for an intermediate range of thickness of ultrathin films and temperature. The Curie temperature and the spontaneous magnetization of each atomic plane are calculated by taking into account the exchange interaction of the first- and second-neighbour couplings. It has been shown that in a one-monolayer film the second-neighbour exchange coupling was enhanced markedly owing to loss of the first-neighbour atoms and great enhancement of the overlap of the electron cloud of the second-neighbour atoms. It is proved that the behaviours of ferromagnetic transition and perpendicular remanence of ultrathin films depend strongly on the requirement that the spin be perpendicularly pinned to some degree at the surfaces.

Hyperfine interactions in bcc (110) iron coated with ruthenium

Using FLAPW method we have calculated the electronic structure of the Ru/Fesystem simulating it by a three- layer slab of an ideally constructed bcc Fe(110) film covered on each side with Ru(110) monolayer. For comparison, results for five layer iron slab as well as the Ag 3Fe Ag slab in the same geometry are presented. I have calculated hyperfine magnetic field and electric field gradient (EFG). Valence charge distribution and moments are also calculated. The major feature is the strong change in the valence electrons contribution to hyperfine field in the outermost Fe layers in both Ag/3Fe/Ag and Ru/3Fe/Ru systems, as compared to five-layer Fe slab. The influence of Ag and Ru does not differ much if moments and valence charged in muffin-tin spheres of Fe are concerned. Electric field gradients change for different coating which is the evidenceof strong modifications in distribution of the local electron densities.

Diffuse polarised neutron reflection studies of ultrathin bcc Fe(001) epitaxial films

Polarised neutron reflection (PNR) has recently been developed as a highly sensitive magnetometric probe of ultrathin epitaxial ferromagnetic films. In this contribution we demonstrate the importance of accounting for the effect of diffuse scattering from the sample substrate and overlayers in using PNR to accurately estimate magnetic moments and illustrate these results by analysing the beam profile obtained from sandwiched ultrathin bcc Fe films prepared by MBE.

Magnetization of ultrathin bcc Fe films on MgO.

The magnetization of body-centered-cubic iron was measured by means of polarized neutron reflection on a set of films of different thickness (4, 6, 8, and 16 \AA{}). The films were epitaxially deposited onto polished (100) MgO substrates and covered by protective layers of gold. The results show that at 40 K all films are ferromagnetic, with an average magnetic moment per iron equal to 2.2\ifmmode\pm\else\textpm\fi{}0.2${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ regardless of film thickness. However, the thinnest films exhibit a Curie temperature lower than that of the bulk and a magnetization axis tilted out of the film surface. These results are discussed in the light of the detailed morphology of the films, and compared with the theoretical predictions.

New magneto-optical transition in ultrathin Fe(100) films.

Magneto-optical polar Kerr effects in ultrathin bcc Fe(100) films sandwiched by Au layers were investigated in the 1.6-5.3 eV photon energy range at room temperature. A new peak appeared in the off-diagonal conductivity spectrum at around 3.5-4.5eV for the Fe films thinner than 10 A. The peak energy is lower for thinner Fe films. The absolute value of e xy at 3.7eV in the 3 A thick Fe film is 2 times as large as that in the bulk Fe

Spin-polarized electron gun for electron spectroscopies

A spin-polarized electron gun for electron spectroscopies is described in detail. The gun consists of an electron source based on a negative electron affinity GaAs photocathode coupled to an appropriate transport electron optics. The gun has been designed with the aid of ray-tracing analysis and then accurately tested. It produces a transversely polarized (P0∼27%) electron beam at variable energy with a small spot size and angular spread (less than 2 mm and 5°, respectively). Such performances are attained up to sample currents as high as 10 μA for the whole beam energy range (8–50 eV). As an application we present data on spin-dependent absorbed current spectroscopy from bcc Fe films epitaxially grown on Ag(100).