Spin-dependent quantum interference from epitaxial MgO thin films on Fe(001)

Spin-dependent electron reflection from MgO thin films grown on Fe(001) was measured using spin-polarized low energy electron microscopy. The electron reflectivity exhibits quantum interference from which two MgO energy bands with Delta1 symmetry were determined in experiment. We found that a bulklike MgO energy gap is fully established for MgO film thicker than 3 atomic monolayers and that the electron reflectivity from the MgO/Fe interface exhibits a spin-dependent amplitude and a spin-independent phase change.     

Solid-state synthesis in Ni/Fe/MgO(001) epitaxial thin films

Solid-state synthesis in Ni/Fe/MgO(001) bilayer epitaxial thin films has been studied experimentally. The phase sequence Fe/Ni→(～350°C)Ni₃Fe→(～400°C)NiFe→(～ 550°C)γ_par is formed as the annealing temperature increases. The crystal structure in the invar region consists of epitaxially intergrown single-crystal blocks consisting of the paramagnetic γ_par and ferromagnetic NiFe phases, which satisfy the orientation relationship [100](001)NiFe ∥ [100](001) γ_par. It has been shown that the nucleation temperatures of the Ni₃Fe, NiFe, and γ_par phases coincide with the temperatures of solid-state transformations in the Ni-Fe system.     

The effect of GaAs(001) substrate roughness on the magnetic properties of epitaxial Fe films

The effect of GaAs(001) surface roughness on the magnetic properties of MBE-grown Fe films having a thickness t in the interval from 12 to 140 Å is investigated by the ferromagnetic resonance method. The films were deposited at room temperature with rates of 9 and 3 Å/min. For films grown on substrates with the rms deviation of the roughness σ≈10 and 30 Å, the spectrum is essentially dependent on the relationship between t and σ. At t≤σ and t≥3σ, a single absorption line is observed, whereas at σ≤t≤3σ, two absorption lines are present. These features of the spectra are related to the island growth of the films and the influence of roughness on island coalescence.     

Structural and magnetic properties of ultrathin epitaxial Fe films on GaAs(001) and related semiconductor substrates

This article presents a review of the structural and magnetic properties of ultrathin epitaxial Fe films on GaAs(001) and related semiconductor substrates. Interest in these systems and Fe/GaAs(001) in particular has increased significantly over the last two decades, largely due to the emergence of the field of magnetoelectronics. Since then numerous studies of molecular beam epitaxy of Fe on GaAs(001) have been carried out, making it by far the best researched Fe/semiconductor(001) system. Issues such as magnetic anisotropy in the ultrathin regime, however, remain controversial with contradictory reports in the literature giving rise to considerable controversy within the field. By carefully scrutinizing the enormous amount of literature published on Fe/GaAs(001) so far and analysing these results within the wider context of Fe/semiconductor(001) systems, this article tries to settle some of these controversial issues, hence providing a long overdue ‘common denominator' for research in this area.     

Interplay between anisotropic strain relaxation and uniaxial interface magnetic anisotropy in epitaxial Fe films on (001) GaAs

Grazing incidence x-ray diffraction study of Fe epitaxial ultrathin films (1.5-13 nm) on GaAs (001) reveals an anisotropy of both domain shape and strain, with [110] and [1-10] as the principal directions. It is shown that the observed thickness-dependent strain anisotropy, together with a uniaxial interface term, can provide an unambiguous explanation to the usual in-plane magnetic anisotropy and its thickness dependence observed in this magnetic thin-film system.     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Strain-induced polarization rotation in epitaxial (001) BiFeO3 thin films

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO3 thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO3 films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO3 with biaxial strain while the spontaneous polarization itself remains almost constant.     

First-principles study of ferromagnetism in epitaxial Si-Mn thin films on Si(001)

Density-functional theory calculations are employed to investigate both the epitaxial growth and the magnetic properties of thin Mn and MnSi films on Si(001). For single Mn adatoms, we find a preference for the second-layer interstitial site. While a monolayer Mn film is energetically unfavorable, a capping-Si layer significantly enhances the thermodynamic stability and induces a change from antiferromagnetic to ferromagnetic order. For higher Mn coverage, a sandwiched Si-Mn thin film (with CsCl-like crystal structure) is found to be the most stable epitaxial structure. We attribute the strong ferromagnetic intralayer coupling in these films to Mn 3d-Si 3s3p exchange.     

Raman study of ultrathin Fe3O4 films on GaAs(001) substrate: stoichiometry,epitaxial orientation and strain

The growth and characterization of high‐quality ultrathin Fe₃O₄ films on semiconductor substrates is a key step for spintronic devices. A stable, single‐crystalline ultrathin Fe₃O₄ film on GaAs(001) substrate is obtained by post‐growth annealing of epitaxial Fe film with thicknesses of 5 and 12 nm in air. Raman spectroscopy shows a high ability to convincingly characterize the stoichiometry, epitaxial orientation and strain of such ultrathin Fe₃O₄ films. Polarized Raman spectroscopy confirms the unit cell of Fe₃O₄ films is rotated by 45° to match that of the Fe (001) layer on GaAs, which results in a built‐in strain of − 3.5% in Fe₃O₄ films. The phonon strain‐shift coefficient(−126 cm⁻¹) of the A_1g mode is proposed to probe strain effect and strain relaxation of thin Fe₃O₄ films on substrates. It can be used to identify whether the Fe layer is fully oxidized to Fe₃O₄ or not. Copyright © 2011 John Wiley & Sons, Ltd.     

Magnon excitation by spin injection in thin Fe/Cr/Fe films

The technique of Brillouin light scattering is used to observe strong excitation of magnons in antiferromagnetically coupled trilayers of Fe/Cr/Fe at room temperature. The magnons are driven out of equilibrium by a microwave current applied in the trilayer through point contacts. The magnitude of the scattering intensity is investigated as a function of the magnon wave number and applied magnetic field. Confirming recent theoretical predictions, the observations provide strong evidence of electronic spin injection in the rf driving field.     

Phase‐controlled epitaxial growth of iron oxide thin films on MgO(001) and LaAlO3(001) substrates

We have deposited epitaxial iron oxide thin films on MgO(001) and LaAlO₃(LAO)(001) substrates, resulting in different phase stabilities. Atomic force microscopy images revealed a smooth surface. Detailed X‐ray diffraction (XRD) measurements were performed to confirm the epitaxial growth and to analyze the atomic growth configuration. We found that (00l) oriented γ‐Fe₂O₃ was the stable phase on MgO(001) substrates, whereas $ (1\bar 102) $ oriented α‐Fe₂O₃ was stable on LAO(001). Magnetic hysteresis loop measurements revealed typical ferrimagnetic behavior for γ‐Fe₂O₃ on MgO, whereas the magnetization of α‐Fe₂O₃ on LAO was relatively small and consistent with an antiferromagnetic order. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

FMR and SMFMR investigation of epitaxial Fe/GaAs(0 0 1) thin films with Si and Ge overlayer

The magnetic anisotropy and magnetoelastic properties of epitaxial iron films prepared by DC magnetron sputtering on single crystal GaAs(0 0 1) substrate and covered with a protective Si or Ge layer have been investigated by means of the ferromagnetic resonance and strain-modulated ferromagnetic resonance. It has been shown that the uniaxial and cubic anisotropy constants as well as two magnetoelastic constants strongly depend on the thickness of the film. The surface components of the cubic anisotropy and magnetoelastic constants have been determined.     

X-ray pole-figure study of the epitaxial growth of C60 thin films on mica (001)

The growth of epitaxial C₆₀ thin films on mica(001) by thermal evaporation has been studied in detail by X-ray pole-figure measurements. The influence of the deposition rate, the substrate temperature and the film thickness on the in-plane epitaxial arrangements and the formation of twins has been investigated. It has been demonstrated that the C₆₀ growth is determined by two independent and equivalent C₆₀-crystal grain alignments (type-A and type-B). The nearly six-fold symmetry of the mica(001)-substrate surface offers the three-fold fcc-(111)-oriented C₆₀-crystal grains two equivalent crystal alignments. A high deposition rate of 0.5 Å/s is responsible for the formation of twins at a substrate temperature of 150°C, which diminishes by a higher substrate temperature of 200°C. By a decrease of the deposition rate down to 0.08 Å/s the twins vanish at a film thickness of 200 nm and at the substrate temperature of 150°C. Under the same sublimation conditions, in addition to the type-A and type-B crystal orientations, the growth of the thin C₆₀ films starts with a slight fibre texture which does not appear at a larger film thickness.