Nano-sized magnetic instabilities in Fe/NiO/Fe(001) epitaxial thin films

We report on a magnetic imaging study of the Fe/NiO/Fe(001) trilayer structure, by means of X-ray photoemission electron microscopy (XPEEM) and spin-polarised low-energy electron microscopy (SPLEEM). Two different magnetic couplings between the Fe layers are observed depending on the NiO thickness being greater or smaller than a critical value. Very small magnetic domains and domain walls are observed in the top Fe layer. They are dramatically smaller than those observed in the Fe substrate, and have a convoluted topology. Furthermore they seem to be unstable with respect to an applied magnetic field for any NiO thickness except that corresponding to the transition between the different coupling regimes. The phenomenology of such magnetic nano-structures and the dependence of the magnetic behaviour of the layered structure on the NiO spacer thickness are discussed on the basis of the experimental results and of state-of-the-art theoretical models.     

Magnetic anisotropy and morphology of Fe epitaxial layers grown on MgO/InAs heterostructures

In-plane magnetic anisotropy and the corresponding morphology of Fe epitaxial layers have been investigated with respect to underlying MgO growth temperature when epitaxial Fe/MgO layers are grown on InAs (001) substrates. Coexistence of three-dimensional Fe islands with strong in-plane textures along <110> and (100) is observed on 4 nm thick MgO layers grown on 200 degrees C, leading to the absence of magnetic anisotropy. Meanwhile, the partially relaxed MgO layers grown above 300 degrees C give rise to two-dimensional Fe layers with cubic magnetic anisotropy. The higher MgO growth temperature accelerates the two-dimensional layer formation of the subsequent Fe as well as the advent of cubic anisotropy by reducing underlying strain within the MgO layer.     

Layer-by-layer growth of thin epitaxial Fe3Si films on GaAs (001)

Molecular beam epitaxy of Fe₃Si films on GaAs (001) is studied in situ by grazing incidence X-ray diffraction. Fe₃Si grows layer-by-layer. During deposition the growth front roughens as indicated by the damping of the X-ray oscillations and corresponding atomic force micrographs. The X-ray oscillations are modified during growth at substrate temperatures of 180 °C and below.     

Diode Structures Based on (In,Fe)Sb/GaAs Magnetic Heterojunctions

Technical Physics Letters - The current–voltage characteristics of light-emitting diodes based on InGaAs/GaAs heterostructures with an injector made of (In, Fe)Sb diluted magnetic...     

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH_x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH₃) pressure of 1 Torr.     

Magnetic epitaxial films: Applications and potential

Magnetic films in amorphous, polycrystalline and single-crystal form have shown promise for many years as a method for miniaturizing magnetic components in the design of electronic devices. With the exception of a few specialized applications, e.g. thin film Permalloy memory elements and magnetic tape heads, widespread use of these films has not yet been realized. Recent advances in the preparation of high quality non-metallic single-crystal garnet films have brought a number of important components much closer to reality.The most immediate widespread use of the single-crystal magnetic film is bubble memory devices. These devices exploit recent advances in the magnetic garnet technology and achieve an extremely high data storage density. They are expected to supplant mechanical tape and disc recorders in some applications soon.Another class of single-crystal magnetic film devices with great potential but not as well developed as the bubble devices are the planar yttrium iron garnet microwave devices. Included in this category are limiters, filters and magnetically tunable oscillators, which are compatible with conventional microwave integrated circuits. Magnetostatic surface wave devices represent another class of devices which rely upon high quality single-crystal magnetic films. These devices are capable of carrying out signal processing functions such as fixed, variable and tapped delay, convolution and pulse compression directly at microwave frequencies.This paper concentrates on the material parameters which are most important in the bubble, filter and magnetostatic surface wave applications, on how the parameters can be optimized and on the fundamental limits they place upon device performance.     

Magnetic effect in multilayer Fe/Cr films

Ferromagnetic resonance measurements on different Fe/Cr multilayer samples with the same thickness of the spacing Cr layers suggest that these multilayers have a different magnetic behavior depending on the thickness of the active Fe layers. In this work we show that the change of magnetic behavior from bulk mode to surface mode is observed only when the thickness of the Fe layers is less than 4 nm. This is attributed to the Fe layer thickness and to the relative thickness of the magnetic and non-magnetic layers. ©1999Kluwer Academic Publishers     

Synthesis and properties of epitaxial SnO2 films deposited on MgO (100) by MOCVD

Epitaxial tin oxide (SnO₂) thin films have been prepared on MgO (100) substrates at 500-600 °C by metalorganic chemical vapor deposition method. Structural and optical properties of the films have been investigated in detail. The obtained films were pure SnO₂ with the tetragonal rutile structure. An in-plane orientation relationship of SnO₂ (110) [010]//MgO (200) [110] between the film and substrate was determined. Two variant structure of SnO₂ were analyzed. The structure of the film deposited at 600 °C was investigated by high-resolution transmission electron microscopy, and an epitaxial structure was observed. The absolute average transmittance of the SnO₂ film at 600 °C in the visible range exceeded 90%. The optical band gap of the film was about 3.93 eV.     

The magnetic domain structure in low anisotropy epitaxial garnet films

Single-crystal films of (YCa)₃(FeGe)₅O₁₂ garnets were prepared by the liquid phase epitaxial method on the (111) plane of Gd₃Ga₅O₁₂ substrates. These magnetic bubble domain films have comparable uniaxial anisotropy energy, cubic anisotropy energy and demagnetizing energy. A photographic technique was developed to measure the angles of inclination of the domain magnetization vectors with respect to the normal to the sample surface. Without an applied magnetic field these inclination angles were 58° and 122°. A simple stripe domain model was used to provide an explanation of the experimental results. This model can easily be extended for other types of mixed anisotropy cases.     

Magnetic properties and microstructures of perpendicular anisotropy (SmCo7/Cu)n films

Pseudobinary SmCo_7 − xCu_x intermetallic compound films with a TbCu₇-type structure were prepared by sputtered (SmCo₇/Cu)_n multilayer. After annealing, the spacer layer Cu (0.5-1.5 nm) diffused into the SmCo₇ matrix and stabilized the SmCo₇ meta-stable phase. The resulting microstructure was investigated by transmission electron microscopy. Perpendicular anisotropy was obtained by introducing a Cu/Ti dual underlayer during sputtering, resulting in prefer-orientated SmCo₇ (00L) X-ray diffraction peaks. This study observes a maze-like domain pattern in perpendicular anisotropy films and finds single- and multi-domain particles in correlated AFM and MFM images.     

Fe/GaAs(100)界面构成和电子态的同步辐射研究

利用同步辐射光电子能谱技术详细研究了Fe/GaAs(100)的界面反应和电子结构。在界面处,Fe与As形成稳定的化学键,而Ga则溶解到Fe薄膜中形成合金,但反应只能在界面处发生,形成窄的反应层。Fe沉积后改变了GaAs表面的电子结构,重新钉扎了费米能级位置,引起费米能级向价带顶移动0.27eV。另外,Fe的生长模式在沉积过程中发生改变,而且存在As和Ga的扩散现象,同步辐射价带谱也证实了这一点。     

Magnetic anisotropy of the ultrathin Fe layers in Fe/NiFe/Fe/Cu multilayers and their effect to the magnetoresistance

The effects of the deposition of ultrathin ⁵⁷Fe layers on both sides of the NiFe layers in NiFe/Cu multilayers were investigated by focusing on their structural, magnetic and magnetoresistance properties. Conversion electron Mössbauer spectroscopy measurements showed an out-of-plane magnetic anisotropy of the Fe layers. The magnetoresistance curves showed an unusual shape, where up to three peaks were observed. Eight variables computer simulations, based on a phenomenological model that considers bilinear and biquadratic couplings between layers with cubic and in-plane uniaxial anisotropies, were used in order to calculate the best-fitting magnetization curves for the NiFe/Cu and Fe/NiFe/Fe/Cu multilayers. Both model and Mössbauer spectroscopy results showed that it is the rotation of the Fe magnetic moment from out-of-plane to in-plane orientation that provokes the unusual magnetoresistance curve shape. The observed reduction of the magnetoresistance amplitude with the addition of one monolayer of Fe in the NiFe/Cu multilayer was attributed to a less-effective spin-dependent scattering that occurs at Fe/Cu and Fe/NiFe interfaces than at the NiFe/Cu interfaces.     

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100)_B3, (110)_B3, and (111)_B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100)_B3 substrates, Fe(100)_bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110)_bcc and Fe(111)_bcc single-crystal films are respectively obtained on GaAs(110)_B3 and GaAs(111)_B3 substrates at RT-300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0-9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc-Fe crystal.     

Stress-induced anisotropy in LPE grown Ni(Fe,Al)2O4 films

Preliminary results are reported about the growth of single crystal Ni(Fe,Al)₂O₄ films, grown by means of liquid phase epitaxy on (111)MgO and on (111)ZnGa₂O₄ substrates using a PbO-B₂O₃-Fe₂O₃ solvent. While films grown upon MgO show stress relief at the growth temperature, films grown upon ZnGa₂O₄ possess a tensile strain due to elastic deformation. Since λ₁₁₁ for NiFe₂O₄ is strongly negative a stress-induced uniaxial anisotropy is present in the films. Stripe domains can be observed with the Bitter technique and when a magnetic field is applied perpendicular to the plane of the film, magnetic bubbles with a diameter of ～2 μm appear. A bubble stability factor q exceeding unity is obtained. For the first time magnetic bubbles are found in LPE grown spinel ferrites.     

Orientation control of epitaxial Ge thin films growth on SrTiO3 (100) by ultrahigh vacuum sputtering

We report the orientation control of crystalline Ge (111) and Ge (001) growth on SrTiO₃ (100) substrate by adjusting the temperature of substrate. It is found that the substrate temperature plays an important role for the formation of crystalline Ge with different surface orientations and interfacial chemical configuration during the sputtering process. At 500 °C, Ge (111) with good crystalline quality is formed, while Ge (001) is preferably grown on SrTiO₃ substrate at 650 °C. Our results show the possibility of manipulating the surface orientations during Ge growth on SrTiO₃ by controlling the substrate temperatures.     

Magnetic properties of epitaxial thin films and bulk of Eu(Ni,Mn)O3 perovskites

Magnetic properties of EuNi_xMn_1 − xO₃ were studied in bulk form and, for a specific composition (x = 1/3), as epitaxial thin film. Paramagnetic properties are interpreted as a VanVleck contribution of the ⁷F Eu³⁺ configuration, showing linear decrease of the effective moment and VanVleck susceptibility as x(Ni) increases. The ordered state was studied for three particular compositions which describe different regimes: canted-antiferromagnetism for x(Ni) = 1/4 and ferromagnetism for x(Ni) = 1/2, the composition x(Ni) = 1/3 being at the frontier of both regimes. Nominal composition x(Ni) = 1/3 was chosen for epitaxial films, since it corresponds to a critical concentration for which a maximum number of pairs Mn³⁺-Mn⁴⁺ and optimal double-exchange interactions are found. The coercive field is enhanced, and the field dependence of T_max is reduced as if the coherent ferromagnetic phase is favoured at the boundary of the magnetic domains.     

Stress variation in epitaxial GaAs films on silicon under thermal treatment

High quality epitaxial GaAs films of 1.8 and 6.3 μm thickness on silicon substrates were examined for lattice distortion, misalignment and curvature by X-ray diffraction (Bond method) at 20–400 °C. These films were deposited by the metal-organic chemical vapour deposition method on the (001) plane of silicon using a buffer layer produced at T_b = 370 or 460 °C. A top layer was then grown at T_t = 560 or 650 °C. The GaAs films contract more strongly on cooling than the substrate, which causes a curvature and a tetragonal distortion below a critical temperature T_c. This temperature varies on thermal treatment at 200–400 °C and approaches T_b, the growth temperature of the buffer layer. The tetragonal distortion can be stabilized, so that T_c approximates T_b, if the GaAs films are annealed for several days at 400 °C.