Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Magnetic properties and structures of Fe/MgF2 multilayered films

Multilayers composed of Fe and MgF₂ with layer thicknesses between 9 Å and 100 Å and of 30 Å, respectively, were prepared with an ultrahigh-vacuum deposition technique. Medium-angle X-ray data show that the Fe layers in the BCC phase have considerable (1 1 0) texture. Periodicity due to multilayered structures was confirmed by a small-angle X-ray diffraction study and cross-section transmission electron microscope for films with Fe layer thicknesses >45 Å. In an Fe/MgF₂(9 Å/30 Å) sample, an island structure for the Fe layers was suggested by the existence of superparamagnetism in a film. At 4.2 K, enhancements of both magnetization and hyperfine field were observed in films having Fe layers thinner than 40 Å. The maxima in the magnetization (233 emu/g of Fe) and in the average hyperfine field (390 kOe) at 4.2 K were found in an Fe/MgF₂(9 Å/30 Å) film and were approximately 105% and 115% that of the bulk α-Fe, respectively. The thickness dependence suggests a 12% enhancement in the magnetic moment of interface Fe atoms. No exchange bias was found in the films, implying that antiferromagnetic fluorides are not formed at the interface, which is different from the case of Fe/LiF and Fe/CaF₂ multilayers.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Perpendicular magnetization in Fe/Ni bilayers on GaAs(0 0 1)

Following the concept of spin-injection into a semiconductor-based device, a ferromagnetic element (like a GMR multilayer structure) can be used as a spin filter. A high spin-polarization of the electrons can be realized by the preparation of a monocrystalline multilayer structure consisting of ultrathin films of a high magnetic polarization. In the case of ultrathin films, the manipulation of the easy-axis of magnetization is possible, by changing the anisotropy terms contributing to the effective anisotropy of the structure. We report on the structural and magnetic properties of Ni/Fe and Fe/Ni bilayers epitaxially grown on GaAs(0 0 1). By a proper choice of Fe and Ni sequences (Fe/Ni/GaAs) and their thickness (up to 3 ML of Fe on the top of Ni), the rotation of magnetization from the in-plane to the out-of-plane direction was achieved.     

Structural and magnetic properties of BCC Fe/Co (0 0 1) superlattices

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K₁) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m³ at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.     

In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

Stabilization of high spin FCC Fe in (Fe/Pd)n multilayers

Polycrystalline (Fe/Pd)_n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t_Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d_Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d_Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd)_n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ_B, which is about 20% larger compared to μ=2.2 μ_B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.     

Optical properties of Cr/Fe(t)/MgO (0 0 1) thin films

In this work we report on the optical properties of single-crystalline iron thin films. For this, Cr-capped Fe films with thickness, t, in the range 30–300 Å were prepared on MgO (0 0 1) by DC magnetron sputtering, and then studied by optical absorption technique within the range from 1.0 to 3.6 eV. All measurements were carried out at room temperature using a fiber optics spectrophotometer. The intensity of the transmitted light decreases with increasing film thickness. The optical constants of the films are deduced from a model that considers the transmission of light by two absorbing films on an absorbing substrate. The absorption coefficient of the Fe films is also calculated from the transmission data. The absorption spectra show the following characteristics: (i) two large absorption peaks centered at about 1.20 and 2.65 eV; and (ii) a sharp step near 1.40 eV. These structures are associated with conventional interband transitions of the iron film.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

Irreversible nanoscale morphology transformation of an Fe film on Mo(1 1 0) induced by a magnetic STM tip

STM tip-induced surface roughening is reported for a 6 Å Fe film grown on vicinal Mo(1 1 0) at 760 ± 15 K. Using a STM tip of the antiferromagnetic alloy MnNi and tunneling parameters of 0.1 nA and 70 mV, the film morphology was completely transformed over a period of 1 h at room temperature. The results indicate that there is a strong promotion of surface diffusion and interlayer mass transport by the local electric field between the tip and sample and/or a magnetic interaction between the tip and the film. The strain state of the film plays a part in the propagation of this transformation across the scanned area.     

Magnetic phase transitions in the Nd(Mn0.9Me0.1)O3 (Me=Al, Fe, Cr, Zn) perovskites

It is shown that perovskite NdMnO₃ is a weak ferromagnet with an anomalous magnetization behavior due to Nd sublattice contribution. Ferromagnetic component drastically increases whereas T_N slightly decreases when a part of manganese ions is replaced with Cr, Al, Fe, Zn. It is suggested that the Mn³⁺–O–Mn³⁺ superexchange interaction changes a sign in the microdomains enriched with Me=Cr, Al, Fe, Zn ions due to removing static Jahn–Teller distortions. All these substituted perovskites show a sharp drop of the magnetization as temperature decreases. A large temperature hysteresis indicates first-order phase transition. Below this transition neodymium magnetic moments orient opposite to a moment of manganese magnetic sublattice. It is supposed that this phase transition results from a change of the ground state of Nd ions.     

Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction

We report on the identification of Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite) in iron oxide thin films grown on α-Al₂O₃(0 0 0 1) by evaporation of Fe in an O₂-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O₂-atmosphere of 5 × 10^?5 mbar only hematite was detectable in the Raman spectrum.     

Field cooling-induced magnetic anisotropy in exchange biased CoO/Fe bilayer studied by ferromagneticresonance

Exchange-biased CoO/Fe bilayer grown on MgO (0 0 1) substrate by sputtering, studied by variable angle and temperature ferromagnetic resonance. Room temperature in-plane measurements reveal that the Fe layer was epitaxially grown on MgO substrate with a fourfold cubic symmetry. The data also show that the easy axis of magnetization is in the film plane and makes an angle of 45° with the [1 0 0] crystallographic direction of MgO substrate. The low temperature data exhibit a sudden onset of a field cooling-induced and shifted cubic anisotropy below the Néel temperature of CoO. This results in a twofold uniaxial or fourfold cubic symmetry for in-plane magnetic anisotropy depending on a field cooling direction. Low temperature measurements also present a reduction in the resonance fields due to the antiferromagnetic/ferromagnetic coupling. The developed theoretical model perfectly simulates the experimental data of coupled CoO/Fe bilayer.     

Ellipsometric detection of GaAs(0 0 1) surface hydrogenation in H2 atmosphere

Optical properties of MBE-grown GaAs(0 0 1) surfaces have been studied by spectroscopic ellipsometry under dynamic conditions of ramp heating and cooling after desorption of passivating As-cap-layer with low pressure H₂ atmosphere (14 Torr) applied to the surface. The temperature dependence of GaAs pseudo-dielectric function with atomically smooth (0 0 1) surface carrying the fixed Ga-rich (4 × 2) reconstruction was obtained for the temperature range of 160–600 °C. It is shown ellipsometrically that GaAs(0 0 1) heating in the molecular hydrogen atmosphere results in the formation of hydrogenated layer on the surface.     

Ab initio study of structural and magnetic properties of cubic Fe4N(0 0 1) surface

First-principles calculations are employed to study the structural and magnetic properties of fully-relaxed cubic Fe₄N(0 0 1) surfaces with both Fe₂- and Fe₂N-termination. The results of surface stability calculations show that the (0 0 1) surface of Fe₄N is most possibly existing with Fe₂N-termination. Slab structures have more localized features in the density of states especially for the Fe₂N-terminated surface due to structure relaxation. The average magnetic moments of Fe atoms increase with increasing thickness of slabs. The calculated interlayer distances indicate that the decreases of d₁₂ and d₂₃ result in stronger hybridization and shorter bond distances between Fe2 atom in the second layer and other atoms in surface or the third layers, which lead to variation of magnetic moments with different slab thicknesses.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Theoretical and experimental study of Fe/Cr nanometric quasiperiodic multilayers

An experimental and theoretical study of magnetization curves of Fe/Cr nanometric magnetic films grown with the structure of the quasiperiodic Fibonacci sequence is presented. Fe ferromagnetic films with interfilm exchange coupling provided by intervening Cr non-ferromagnetic layers were grown on MgO (100) by dc magnetron sputtering at 300 ^°C. The magnetization curves were investigated using the magneto-optical Kerr effect with the external field applied along the easy axis. The theoretical approach for this system is based on a realistic phenomenological model that includes the following contributions to free magnetic energy: Zeeman, cubic magneto-crystalline anisotropy, as well as bilinear and biquadratic exchange energies. Our numerical results are in very good agreement with the experimental data.     

Heteroepitaxial growth of thin InAs layers on GaAs(1 0 0) misoriented substrates: A structural and morphological comparison

Thin InAs epilayers were grown on GaAs(1 0 0) substrates exactly oriented and misoriented toward [1 1 1]A direction by atmospheric pressure metalorganic vapor phase epitaxy. InAs growth was monitored by in situ spectral reflectivity. Structural quality of InAs layers were studied by using high-resolution X-ray diffraction. No crystallographic tilting of the layers with respect to any kind of these substrates was found for all thicknesses. This result is discussed in terms of In-rich growth environment. InAs layers grown on 2° misoriented substrate provide an improved crystalline quality. Surface roughness of InAs layers depend on layer thickness and substrate misorientation.     

Magnetic and structural properties of Fe/Cu multilayers

Fe/Cu multilayers with Fe and Cu layers of equal thicknesses were grown by high-vacuum evaporation on Si(1 1 1) substrates at room temperature. The crystal orientation, the thickness of the elemental layers and the interplanar distances were analysed by both low- and high-angle X-ray diffraction in the θ–2θ configuration. The magnetic properties of Fe/Cu multilayers were studied by both a static and a dynamic technique, namely surface magneto-optical Kerr effect (SMOKE) and Brillouin light scattering (BLS). Longitudinal SMOKE cycles permitted us to determine the orientation of the easy axis of the magnetization and to put in evidence an appreciable in-plane magnetic anisotropy in multilayers with low periodicity and highly coherent structure. Polar loops were then used to determine the out-of-plane anisotropy fields, showing that both first- and second-order contributions are to be considered in order to reproduce the hysteresis cycle. BLS was then exploited to detect thermaly excited spin waves through inelastic scattering of light. The out-of-plane anisotropy fields evaluated by this high-frequency dynamic technique compare fairly well with the first-order values obtained by analysis of polar SMOKE hysteresis cycles.