Magnetic and structural properties of FeX/Tb15 Å multilayers

Magnetic domains in Fe/Tb sputter-deposited multilayer films are studied magneto-optically using a Kerr microscope. These observations together with other experimental measurements (vibrating sample- and torque magnetometry) showed the important role of the iron-layer thickness d_Fe on the magnetic properties of the films. At certain combinations of the layer thicknesses both perpendicular and in-plane anisotropies are present in the film. The structural investigations indicate that a gradual increase of the iron-layer thickness d_Fe leads to a phase transition of iron from amorphous to crystalline at certain thicknesses of the iron layer, which is also reflected in the magnetic behavior of the films. The differences between the magnetic properties of the investigated multilayered Fe/Tb films and those of amorphous FeTb films are discussed as well.     

The oscillatory galvanomagnetic size-effect in multilayered structures

Within the framework of the modified semi-classical Fuchs-Sondheimer model, we investigated theoretically the electrical resistivity of multilayered structures (MLS) consisting of alternating metallic layers (of different purity and different thicknesses) in a transverse magnetic field as functions of the ratio of the adjacent layer thicknesses and the magnetic field value. We have derived both a general formula (valid at arbitrary values of layer thicknesses) and asymptotic expressions that are valid when metallic layers are thick or thin compared with the electron mean free path. We found a non-monotonic behavior in the resistivity vs. the value of an applied magnetic field. As we demonstrated, this behavior is sensitive to the characteristics of the electron scattering in the interlayer interfaces in low magnetic fields. Moreover, the MLS resistivity oscillates in high magnetic fields with the field value (or with the layer thicknesses). The oscillation includes the harmonics that correspond both to the each layer thicknesses and the total thickness. The intensity of the oscillation is determined by the diffusive electron scattering in the interfaces, and the oscillation amplitude is proportional to the coefficient of the electron transmission through the interlayer interfaces. We have calculated numerically the resistivity in a wide range of fields and layer thicknesses at various values of the parameters of the interface and bulk electron scattering.      

Studies on the coupling of interlayers in multilayered FeSi/Si amorphous films by cems

Multilayered FeSi/Si amorphous films with fixed FeSi layer thickness and different Si layer thicknesses have been studied by conversion electron Mössbauer spectroscopy at room temperature. The results showed that with decreasing the Si layer thickness, the hyperfine field of samples increased and the thickness of interface dead layers arisen from the atomic interdiffusion effect decreased. These are due to the coupling effect between the magnetic layers.     

Application of ferromagnetic resonance method to study of multilayer nanostructures

The ferromagnetic resonance spectra of multilayer nanostructures synthesized from magnetic layers based on amorphous Co₄₅Fe₄₅Zr₁₀ and nonmagnetic layers of the amorphous semiconductor α-Si have been experimentally studied. It is shown that the character of the spectrum depends strongly on thicknesses of magnetic and nonmagnetic layers and the structure of the boundary layer. The resonant fields are calculated within the effective-medium approximation. In some cases, the calculation results describe well the experimental data.     

Interface reactions in [Fe/B]<Subscript>n</Subscript> multilayers: a way to tune from crystalline/amorphous layer sequences to homogeneous amorphous Fe<Subscript>x</Subscript>B<Subscript>100-x</Subscript> films

[Fe/B]_{n ≥2} multilayers were prepared by thermal evaporation, ion-beam sputtering and laser ablation. By applying in situ electron spectroscopies (UPS, XPS) and monitoring the electrical resistance during layer growth, evidence could be provided for the occurrence of interface reactions within the range of studied deposition temperatures (77 K ≤T ≤300 K). These reactions result in amorphous Fe_xB_100-x phases, which are spatially restricted to a width of less than 3 nm at the original interface. The amorphicity of the reacted interlayers was unequivocally proven by additional high-resolution electron microscopy (HRTEM) and their characteristically changed magnetic properties. Due to the well-defined width of the interface reaction, homogeneous amorphous Fe_xB_100-x films can be obtained by reducing the individual Fe and B layer thicknesses to below the above reaction depth, while for larger thicknesses layer sequences of the crystalline/amorphous/crystalline type will result. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-731/502-2963, E-mail: hans-gerd.boyen@physik.uni-ulm.de     

Simulation of the MFM contrast from small low-coercive ferromagnetic nanoparticles in an external field

Calculations of the magnetic force microscopy (MFM) contrast from low-coercive ferromagnetic and superparamagnetic particles are presented. It is shown that the external field stabilizes the magnetic moment of particles, allowing the separation of contributions of van der Waals and magnetic interactions to the MFM phase contrast. The values of stabilizing magnetic fields are estimated.     

Morphology and the magnetic and conducting properties of heterogeneous layered magnetic structures [(Co45Fe45Zr10)35(Al2O3)65/a-Si:H]36

The morphology and the magnetic and conducting properties of an amorphous multilayer nanosystem [(Co₄₅Fe₄₅Zr₁₀)₃₅(Al₂O₃)₆₅/a-Si:H]₃₆ consisting of (Co₄₅Fe₄₅Zr₁₀)₃₅(Al₂O₃)₆₅ magnetic layers and semiconducting hydrogenated amorphous silicon (a-Si:H) layers of various thicknesses have been studied. Using a combination of methods (including polarized neutron reflectometry and grazing incidence small-angle X-ray scattering), it is shown that the magnetic and electrical properties of these multilayer structures are determined by their morphology. It is established that the magnetization and electric resistance of a sample is a nonmonotonic function of the a-Si:H layer thickness. Both characteristics are at a minimum for a structure with a semiconductor layer thickness of 0.4 nm. Samples with silicon layer thicknesses below 0.4 nm represent a three-dimensional structure of Co₄₅Fe₄₅Zr₁₀ grains weakly ordered in space, while in samples with silicon layer thicknesses above 0.4 nm, these grains are packed in layers alternating in the vertical direction. The average lateral distance between nanoparticles in the layer plane has been determined, from which the dimensions of metal grains in each sample have been estimated.     

Thickness dependence of electrical properties of ITO film deposited on a plastic substrate by RF magnetron sputtering

Indium tin oxide (ITO) films with various thicknesses in range of 40-280 nm were prepared onto a plastic substrate (PMMA). Deposition was carried out with RF magnetron sputtering method and the substrate temperature was held at ∼70 °C, in lack of the thermal damage to the polymer substrate. Changes in microstructure and electrical properties of ITO films according to their thicknesses were investigated. It was found that amorphous layer with thickness of 80 nm was formed at the interface on the polymer substrate and polycrystalline ITO could be obtained above the thickness. Conductivity of ITO films was found to be strongly dependent on the crystallinity. Consequently, it is suggested that crystallinity of the deposited films should be enhanced at the initial stage of deposition and the thickness of amorphous region be reduced in order to prepare high quality ITO thin films on polymer substrates.     

Formation of amorphous Al1−x Au x films as a consequence of low-temperature Al/Au-interface reactions in a multilayered system

Al/Au multilayers (average composition Al₂Au, individual layer thicknesses 1 nm Al and 0.71 nm Au) are prepared at 90 K by ion beam sputtering. The electrical resistance of the growing films is monitored in situ. From the results obtained in this way it can be concluded that interface reactions occur transforming the ultrathin layers into an amorphous phase, which is stable up to 255 K.For larger individual layer thicknesses (2.1 nm Au and 3 nm Al), the interface reaction into the amorphous state is incomplete. Based on a simple parallel-resistor model, one finds that the interface reaction into the amorphous phase is restricted to a thickness of less than 3.5 nm. The temperature dependence of the resistance of such thicker multilayers indicates the onset of interdiffusion of the yet unreacted material at T=200 K resulting in the crystalline Al₂Au-phase.     

Ferromagnetic resonance, magnetic properties, and resistivity of (CoFeZr)x(Al2O3)1 ? x/Si multilayer nanostructures

Comparative investigations of static magnetic properties, magnetoresistance, and ferromagnetic resonance data of multilayer nanostructures consisting of CoFeZr-Al₂O₃ composite magnetic layers and amorphous silicon semiconductor spacers were performed in a layer thickness range below 5 nm. The influence of layer dimension parameters and chemical peculiarities of silicon on the inner structure and type of magnetic interactions in nanostructures with 35 and 46 at % magnetic phase composite layers is discussed.     

Crystallization of amorphous SiC and superhardness effect in TiN/SiC nanomultilayers

TiN/SiC nanomultilayers with various constituent layer thicknesses were prepared by magnetron sputtering using TiN and SiC ceramic targets. X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, high-resolution transmission electron microscope, atomic force microscope and nanoindenter were employed to study the growth, microstructure and mechanical properties of these films. Experimental results revealed that amorphous SiC, which is more favorable under normal sputtering conditions, was forced to crystallize and grew epitaxially with TiN layers at thicknesses of less than 0.8 nm. The resultant films were found to form strong columnar structures, accompanied with a remarkable hardness increment. Maximal nanoindentation hardness as high as 60.6 GPa was achieved when SiC thickness was ∼0.6 nm. A further increase of SiC thickness caused the formation of amorphous SiC, which blocked the epitaxial growth of the multilayers, resulting in the decline of film's hardness. Additionally, investigations on multilayers different in TiN layer thicknesses showed that they are insensitive in both microstructure and hardness to the fluctuation of TiN layer thickness. The formation of epitaxially grown structure between crystalline SiC and TiN layers was found to be responsible for the obtained superhardness in multilayers.     

Preparation of a Single Layer of Luminescent Nanocrystalline Si Structures by Laser Irradiation Method

KrF excimer laser annealing on ultrathin hydrogenated amorphous Si films with various initial Si thicknesses is carried out to obtain a single layer of nanocrystalline Si structures. It is found that Si nanograins can be obtained with the area density as high as 10^11 cm^-2 under the irradiation with suitable laser fluence. Raman and planar transmission electron microscopy are used to characterize the formation process of Si nanocrystals from amorphous phase. Moreover, a strong photoluminescence is observed at room temperature from well-relaxed nanocrystalline Si structures.     

TiN/Al2O3纳米多层膜的共格外延生长及超硬效应

采用多靶磁控溅射法制备了一系列具有不同Al₂O₃调制层厚度的TiN/Al₂O₃纳米多层膜. 利用X射线能量色散谱、X射线衍射、扫描电子显微镜、高分辨透射电子显微镜和微力学探针表征了多层膜的成分、微结构和力学性能. 研究结果表明，在TiN/Al₂O₃纳米多层膜中，单层膜时以非晶态存在的Al₂O₃层在厚度小于1.5 nm时因TiN晶体层的模板效应而晶化，并与TiN层形成共格外延生长，相应地，多层膜产生硬度明显升高的超硬效应，最高硬度可达37.9 GPa. 进一步增加多层膜中Al₂O₃调制层的层厚度，Al₂O₃层逐渐形成非晶结构并破坏了多层膜的共格外延生长，使得多层膜的硬度逐步降低. 关键词： 2O₃纳米多层膜')" href="#">TiN/Al₂O₃纳米多层膜 外延生长 非晶晶化 超硬效应     

Hysteretic properties of nanostructured terbium films

The hysteretic properties of terbium films subjected to layered structuring by introducing nonmagnetic Ti and Si layers of a fixed thickness (2 nm) are studied in the temperature range 2–230 K. It is found that the variation of the Tb layer thickness in the range 1.5–360 nm and the nonmagnetic layer material leads to substantial changes in the magnetic hysteresis of the films and its temperature behavior. These changes are related to a change in the structural composition of the films, which consists of nanocrystalline, granulated, and amorphous Tb phases.     

Magnetic properties of Fe/Nd artificial superstructure films

Multilayered films with artificial superstructures were prepared by alternately depositing Fe and Nd in ultrahigh vacuum. The magnetic properties are studied from⁵⁷Fe Mössbauer spectroscopy. The hyperfine field in Fe layers and the direction of Fe magnetic moments depend on the Fe and Nd layer thicknesses. For films with certain Fe and Nd layer thicknesses, the direction of Fe magnetic moments is in-plane at 300 K but changes to be perpendicular at low temperatures. The direction of Fe magnetic moments is discussed in relation with the magnetization of interface Nd atoms.     

New possibilities for tuning ultrathin cobalt film magnetic properties by a noble metal overlayer

Complementary multiscale magneto-optical studies based on the polar Kerr effect are carried out on an ultrathin cobalt wedge covered with a silver wedge and subsequently with the Au thick layer. A few monolayers of Ag are found to have a substantial effect on magnetic anisotropy, the coercivity field, and Kerr rotation. The silver overlayer thickness-driven magnetic reorientation from easy axis to easy plane generates a new type of 90 degrees magnetic wall for cobalt thicknesses between 1.3 and 1.8 nm. The tuning of the wall width in a wide range is possible. Tailoring of the overlayer structure can be used for ultrathin film magnetic patterning.     

Recovery of soft magnetic properties of FeNiSm films by Ta interlayer

The magnetic properties of FeNiSm thin films with different thicknesses, different Ta interlayer thicknesses and different numbers of Ta interlayers were investigated. The single layer FeNiSm shows in-plane uniaxial anisotropy at a thickness below critical value, but shows weak perpendicular anisotropy with a stripe domain structure at thickness above the critical value. Experiments indicate that one or more Ta interlayers inserted into thick FeNiSm films with weak perpendicular anisotropy were effective not only in canceling the perpendicular anisotropy, but also in recovering the in-plane uniaxial anisotropy. Blocking of the columnar growth of FeNi grains by the Ta interlayer is considered to be responsible for this spin reorientation phenomenon. Moreover, the magnetization reversal mechanism in FeNiSm films with uniaxial anisotropy can be ascribed to coherent rotation when the applied field is close to the hard axis and to domain-wall unpinning when the applied field is close to the easy axis. The dynamic magnetic properties of FeNiSm films with uniaxial anisotropy were investigated in the frequency range 0.1-5 GHz. The degradation of the soft magnetic properties of magnetic thin films due to the growth of columnar grains can be avoided by insertion of a Ta interlayer.     

Influence of V and Mo overlayer on magneto-optical Kerr effect in ultrathin Co films

A method for characterization of sub-nanometer thick Co/V and Co/Mo interfaces is proposed that uses magneto-optical ellipsometry. Both the polar Kerr rotation and ellipticity are fitted simultaneously to different models of interface layer. The magneto-optical data are measured for varying thicknesses of the cobalt layer and overlayer by scanning of a laser beam over the samples with two orthogonal wedges. Decrease of magneto-optical effect at both interfaces Co/V and Co/Mo were observed, which corresponds to interface layers of thicknesses ranging from one to two monoatomic layers. In the case of vanadium, the interface layer is sharper and can be explained either by reduced magnetic moment of cobalt, or by anti-parallel magnetic moment of vanadium near the Co/V interface.     

Infrared optical properties of Li- and Xe-irradiated KTiOPO<Subscript>4</Subscript>

KTP was irradiated at 100 K and 295 K with Li ions and at 295 K with Xe ions. The infrared spectra of the Li-irradiated samples are consistent with a system consisting of a layer stack of undamaged KTP/amorphous KTP/undamaged substrate. Annealing of the samples leads to a growth of the covering layer at the expense of the amorphous layer. The damage yield of the sample irradiated at 100 K is noticeably higher, resulting in a greater thickness of the amorphous layer. The spectra show interference effects, indicating homogenous layer thicknesses. Such interference effects are absent for the Xe-irradiated samples. Spectral simulations revealed that there is no buried amorphous layer present in these samples. Instead, the latter samples consist of amorphous inclusions in undamaged crystalline KTP with a volume fraction depending on the energy dose. The spectra of the sample irradiated at an ion fluence of 3×10¹² cm^-2 are very similar to the spectrum of glassy KTP, indicating a strong structural relationship between ion-damaged amorphous KTP and glassy KTP. The dielectric function of amorphous KTP was determined and used, together with the principal dielectric functions of single-crystal KTP, to successfully simulate the spectra of a sample irradiated at an ion fluence of 2×10¹¹ cm^-2 by either the effective-medium approximation (EMA) or average-refractive-index theory (ARIT). PACS 42.70.Mp; 61.82 Ms; 78.30.-j     

57Fe Mössbauer study of Fe/Bi multilayers

Fe/Bi artificially structured films (ASF's) have been prepared by alternate depositions of Fe and Bi in an ultrahigh vacuum. X-ray diffraction measurements in the small angle range confirmed the formation of periodic structures in all the samples prepared on glass substrates cooled down to about 125K. the CEMS at room temperature indicated that the samples are ferromagnetic except the one with 2Å-thick Fe layers. The CEM spectra also indicated that the structure of Fe layers is amorphous when the Fe layer thicknesses are less than 15Å. Magnetization measurement and CEM spectrum at 6K show that Fe monolayers in the Fe/Bi ASF are ferromagnetic.