The physical limits of high density recording in metallic magnetic thin film media

The fundamental physical limits of high density, saturation recording in high hysteresis loop squareness magnetic isotropic metallic media have been investigated based on observations of micromagnetic structure of recorded Co-Re thin films. Study based on the configuration of the "feather-like" ripple structures of the Lorentz TEM image at the transition region and electron deflection shadow image of the surface field configuration of the recorded media indicate that the physical limit of high density recording in the metallic media is the size of magnetic clusters existing in the film. For a Co-10 at % Re film with Hcbetween 200 to 500 oe., thickness of about 50 nm and S* > 90%, the maximum linear packing density the film can provide, as determined from the cluster size, would be less than 25,000 FRPI.     

The effect of the electron bombardment of NaCl substrates on the epitaxial growth of gold and silver films

The effect of the electron bombardment of NaCl substrates on the epitaxial growth of gold and silver films was investigated by transmission electron microscopy and Auger electron spectroscopy. The results indicate that anion surface vacancies are created during low energy electron bombardment; these vacancies, in turn, promote nucleation and subsequent epitaxial growth of vapor-deposited films. Nucleation is interpreted in terms of the attraction of metal atoms to the surface vacancies and the subsequent formation of a rigid cluster in an epitaxial orientation. These clusters then grow by the accretion of deposited atoms to form epitaxial islands which eventually coalesce with near-perfect register to form a continuous film characterized by a high degree of epitaxy and low defect densities.     

Observation of recorded magnetization patterns by electron holography

Magnetization patterns recorded on Co thin films prepared by oblique incidence vacuum deposition were directly observed by electron holography. Complicated ellipse-like interference fringes were observed along the sawtooth-like walls in the transition regions. Stray magnetic flux was observed in empty space beyond the edge of the film. The following results were obtained from analysis of these interference images. The smaller the product of the remanence and the film thickness, and the higher the coercivity, the narrower is the transition length that determines the limit of the recording density of a medium. It was confirmed that longitudinal magnetic recording of a density of up to 170 kBPI is attainable using Co film 30 nm thick and with coercivity of 112 kA/m. It was quantitatively proven that the intensity of the recorded magnetization in a medium is equal to its remanence when the magnetized area is distinguishable from the transition region.     

Ordering of free-standing Co nanoparticles

Colloidal Co particles of 11 nm diameter were deposited on Si substrate by spin coating and/or casting in magnetic field. A perpendicular magnetic field varying along the diagonal of the substrate was also applied. The samples were analyzed by transmission electron microscopy (TEM), field emission gun scanning electron microscopy (SEM-FEG), atomic and magnetic force microscopy (AFM/MFM). TEM micrographs show local order when a Co nanoparticle monolayer is deposited on Si. Drying the colloidal solution in a magnetic field leads to the formation of quite large clusters (0.3 μm) of Co nanoparticles. A stripe structure was then observed when the particles were deposited by casting in the varying magnetic field. AFM/MFM measurements show isolated Co clusters on the stripes. Magnetic features corresponding to the single Co cluster have been observed pointing out that all magnetic moments in the cluster are oriented along the field direction.     

Magnetic properties and first-order magnetic phase transition in single crystal FeRh thin film

To understand the mechanism of first-order magnetic phase transition in ordered FeRh thin films, the magnetic properties and first-order antiferromagnetic (AFM)–ferromagnetic (FM) phase transition behavior of single-crystal FeRh thin film are investigated in detail. The first-order magnetic phase transition is seen at a temperature of around 120 °C during heating and 145 °C cooling processes in perpendicular direction. The M–H loops measured isothermally amidst the AFM–FM transition regime show an opening at high magnetic field, which indicate a reversible AFM–FM transition induced by magnetic field. The clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition and the mechanism of nucleation and growth kinetics of the first-order magnetic phase transition in ordered FeRh thin film is quite similar to that of the crystallization of solids described by the Avrami model.     

Perpendicular magnetization structure of Co-Cr films

In a perpendicular recording system, a Co-Cr film as a medium is capable of storing very high density signals. Lorentz microscopy of 1000 kV TEM was used to observe the structure of recorded magnetizations in Co-Cr films having perpendicular anisotropy. A composite medium of a Co-Cr film with a soft magnetic back layer was shown by Lorentz microscopy to have a horseshoe magnetization structure. The stable antiparallel magnetization of transition in the Co-Cr layer determined the head-on magnetization structure of the soft magnetic back layer, which consists of a new straw-rope domain structure. The perpendicular magnetization structure of the Co-Cr film was found to consist of small domains magnetized through the film thickness which correspond to the columnar microstructure of the film. Since the intrinsic hysteresis loop of a Co-Cr film was shown to essentially have an ideal rectangular shape, it can be concluded that the Co-Cr layer of a composite film can be recorded by an ideal magnetizing process with negligible demagnetizing field at the transition.     

Torsional Oscillator Study of Thin He Films Under High Speed Rotation

Superfluidity is often compared with superconductivity and rotation is contrasted with magnetic field. In the case of superconductors, it is possible to destroy the superconducting state by application of a sufficiently high magnetic field. Agthough it is not yet technically feasible to destroy the superfluid state through rotation, significant modification of the superfluid can be achieved through rotation. In the experiments reported here we are studying the influence of rotation on the dissipation in superfluid films adsorbed in a porous medium. A new peak in the film dissipation is observed while under rotation at a temperature somewhat below the transition temperature. The dissipative feature we observe is related to the resugt for 2D films under rotation previously reported by Adams and Glaberson, however our observations for films in a porous medium under rotation differ considerably in detail. A theoretical discussion of these resugts has recently been given by Fukuda, et al. (to be published).     

FePt纳米薄膜研究进展

由于FePt在超高密度磁存储材料方面的广阔应用前景及其局限性,研究人员对FePT薄膜进行了大量的研究及改性.根据国外近期在此领域的研究现状,综述了单相、复相及掺杂FePt薄膜的制备以及对结构和性质的影响.复相或掺杂主要是通过结构的改变来降低L10晶相转变温度和FePt颗粒的大小,通过其耦合作用来影响FePt薄膜的磁学性能,使其成为超高密度存储器材料.     

Morphological and local magnetic properties of cobalt clusters electrodeposited onto indium tin oxide substrates

Magnetic force microscopy was employed in order to investigate the local magnetic distribution of individual cobalt clusters electrodeposited onto indium tin oxide surfaces. In addition, the size and shape of the clusters were analyzed by using scanning electron microscopy and atomic force microscopy at various electrodeposition potential values or growth stages. From these results, the cluster magnetic state at different cluster dimensions was identified and the critical cluster size for the magnetic transition from the single to the multi domain state was obtained. By analyzing the local magnetic states of the aggregates in terms of a theoretical single-domain ferromagnetic model, various magnetic parameters were inferred. Finally, in order to validate the experimental results, a micromagnetic simulation study was employed which correctly confirmed the experimental magnetic transition diagram, and also, provided some insights of other intrinsic magnetic properties of the aggregates.     

Magnetic properties of free ferromagnetic clusters in a Stern-Gerlach magnet

The magnetic properties of free magnetic metal clusters in a Stern-Gerlach magnetic field are studied. In particular we investigate magnetic resonance effects resulting from lattice anisotropy and cluster rotation. The anisotropy field fixed to the rotating atomic lattice of the cluster acts on the cluster magnetization like an RF field in NMR experiments. In our calculation we have used the Bloch equations and assumed different anisotropy field symmetries. A minimum in the magnetization as a function of the Stern-Gerlach field is obtained, if the distribution of the rotation frequency ω_rot is relatively narrow. For other conditions an almost superparamagnetic behavior emerges. In addition, the strength of the anisotropy field and the relaxation time are important quantities which determine the magnetic behavior of the clusters.     

化学沉积CoNiP薄膜的磁性及其在传感器中的应用研究

采用化学湿法沉积制备了Co-Ni-P磁性薄膜,研究了工艺条件对其磁性的影响,并对其作为磁旋转编码器磁鼓记录介质的记录特性进行了分析。研究结果表明,薄膜矫顽力和矩形比随施镀时间增加而下降。当施镀时间为3min时,Co-Ni-P薄膜矫顽力可达42506.4A/m,矩形比为0.55。X射线衍射及扫描电镜结果表明,随施镀时间增加,薄膜矫顽力下降,晶粒尺寸变大,并发生明显的择优取向。将化学沉积Co-Ni-P薄膜应用于磁旋转编码器的磁鼓记录介质,记录特性测试结果表明,输出信号良好,脉冲计数完整,波形良好,可完整写入2048对磁极。化学沉积Co-Ni-P磁性薄膜适合于高精度、高分辨率的磁旋转编码器的磁鼓记录材料。     

High resolution magneto-optical studies of the intermediate state in thin film superconductors

The structure of the intermediate state in superconducting lead films has been investigated as a function of magnetic field and film thickness. The detection system utilized the high specific Faraday rotation in thin films of a mixture of EuS and EuF₂ in combination with a polarizing microscope, yielding a resolution of about 1 μm. The thickness of the Pb films ranged between 0.7 and 9 μm, thus including the critical film thickness at which the transition from the intermediate state to the vortex state occurs. At low fields a liquid-like mixed state of multi-quanta flux tubes was observed which appeared to be stable up to increasing magnetic fields with decreasing film thickness. The diameter of these flux tubes varied approximately with the square root of the film thickness. At intermediate fields the intermediate state pattern was found to persist down to a film thickness of 0.7 μm, the smallest thickness investigated. The periodicity length of the intermediate state structure was in reasonable agreement with the non-branching model of Landau. Just below H_c, small superconducting domains were observed in increasing field, whereas long threads of superconducting material were formed abruptly in decreasing field. These superconducting threads were absent in the specimentsthinner than 1–2μ, being replaced by a liquid-like mixed state of superconducting tubes. After the passage of a sufficiently high electrical current through the specimen, the flux structure was found to be rearranged into long domains oriented predominantly perpendicular to the current, leading to current hysteresis effects. Finally, some dynamic observations were made during current induced flux flow.     

The measurement of magnetic microfields

A new transmission electron microscope method of measuring fine scale magnetic flux distributions, and in particular the fields associated with magnetic recording heads, is described. An island structured metallic foil or an amorphous film acts as a diffuse scatterer of transmitted electrons. A magnetic field in the region below the scatterer deflects the electron trajectories and as a result distorts the image of the scatterer. The local shearing of the image is compared to a reference zero field image by a double exposure of a photographic plate. Optical Fraunhofer diffraction by transmission through the developed plate shows Young's fringes which reveal the direction and magnitude of the local image shift which is related to the magnetic field component perpendicular to the electron trajectories. In some applications the method is potentially several orders of magnitude more sensitive than other electron beam methods.     

Domain splitting in thin films with in-plane anisotropy

Splitting of magnetic ripple into domains due to an external magnetic field was observed using the defocused electron microscopy method. The tested film was magnetized perpendicularly or parallel to the easy axis of the uniaxial anisotropy, and both the magnetic field direction and the anisotropy axis lay in the film plane. Splitting of the ripple into 1 μm domains was mainly reversible until the value of ripple amplitude reached 0.5 rad. Movements of the Néel walls and Néel-Bloch-Néel transformation of the domain walls were the main irreversible phenomena which led to hysteresis. Domain splitting of the ripple is properly described only by a Riedel model which includes the interactions between cores and tails of the periodically arranged Néel walls.     

Recent developments in lorentz electron microscopy

Several different methods of obtaining contrast for electron microscopy of magnetic specimens are reviewed. For the investigation of magnetization ripple and the structure of domain walls in magnetic films, two questions must be considered: the correct wave optical interpretation of Lorentz micrographs and the experimental attainment of coherence of the electron beam. Both questions have been sufficiently explored so that experimental results on magnetic film microstructure may be expected in the near future.     

High frequency characteristics of Fe65Co35 alloy cluster-assembled films prepared by energetic cluster deposition

Size-monodispersed Fe65Co35 alloy clusters whose average sizes ranged from 7 to 12 nm were produced by a plasma-gas-condensation (PGC)-type cluster deposition apparatus. Fe65Co35 alloy cluster-assembled films were further prepared at room temperature by energetic cluster deposition method. Positively charged clusters in a cluster beam were accelerated electrically and deposited onto a negatively biased substrate together with neutral clusters from the same cluster source, leading to the formation of a high-density Fe65Co35 alloy cluster-assembled films with good soft magnetic properties. High frequency magnetic characteristics of these films were studied at room temperature using a high-frequency permeameter (RMF-3000, Ryowa). The real part (micro') of permeability for the Fe65Co35 alloy cluster-assembled films prepared at bias voltage V(a) = -20 kV has a large value of micro' = 135 at f = 1.5 GHz, and imaginary part (micro") of permeability has a maximum (micro" approximately 190) at about 2.5 GHz.     

Quantitative Imaging of the Magnetic Configuration of Modulated Nanostructures by Electron Holography

By means of off‐axis electron holography the local distribution of the magnetic induction within and around a poly‐crystalline Permalloy (Ni₈₁Fe₁₉) thin film is studied. In addition the stray field above the sample is measured by magnetic force microscopy on a larger area. The film is deposited on a periodically nanostructured (rippled) Si substrate, which was formed by Xe⁺ ion beam erosion. This introduces the periodical ripple shape to the Permalloy film. The created ripple morphology is expected to modify the magnetization distribution within the Permalloy and to induce dipolar stray fields. These stray fields play an important role in spinwave dynamics of periodic nanostructures like magnonic crystals. Micromagnetic simulations estimate those stray fields in the order of only 10 mT. Consequently, their experimental determination at nanometer spatial resolution is highly demanding and requires advanced acquisition and reconstruction techniques such as electron holography. The reconstructed magnetic phase images show the magnetized thin film, in which the magnetization direction follows mainly the given morphology. Furthermore, a closer look to the Permalloy/carbon interface reveals stray fields at the detection limit of the method in the order of 10 mT, which is in qualitative agreement with the micromagnetic simulations.     

Field emission enhancement of carbon nitride films by annealing with different durations

Magnetron sputtered carbon nitride films (CN_x) were annealed at 750 °C for periods from 30 to 120 min. Effects of annealing with different durations on the field emission of CN_x films were investigated and related to the variations of chemical bonding and surface morphology induced by annealing. The results show that annealing effectively enhances field emission ability of the CN_x films and that the threshold field was lowered from 13 to 5 V/μm. The measurements of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) indicated that annealing leads to a loss of N content and to formation of more graphite-like sp² C clusters in the films, and simultaneously the film surface becomes rougher after annealing, all of which is attributed to the increased film field emission. A large number of sp² C clusters with good conductivity enables tunneling in the film, making electron emission easier, and moreover, a rougher surface also improves the field enhancement factor of the films. However, continuing to increase annealing time eventually lowers the field emission of the films.     

Structural investigation of Ge-Sb-Sn thin films using transmission electron microscopy

Atomistic structures of as-deposited and laser-induced-crystallized Ge-Sb-Sn layers have been examined using high-resolution electron microscopy (HREM) and nanobeam electron diffraction (NBED). Cross-sectional observations were performed on Ge-Sb-Sn thin films embedded in a multi-layered structure. Crystalline clusters were frequently observed in the HREM images of the as-deposited amorphous Ge-Sb-Sn thin film. Autocorrelation function analysis of the HREM image indicated a similarity between the structures of the crystalline clusters and that of rhombohedral Sb. Atomic pair-distribution functions obtained from the halo NBED intensity of the as-deposited amorphous Ge-Sb-Sn films also showed development of local structure whose atomic configuration is similar to that of the rhombohedral Sb. NBED revealed that the structure of the crystallized Ge-Sb-Sn thin film is also close to that of rhombohedral Sb. The atomistic structures of Ge-Sb-Sn thin films were compared with those of Ge-Sb-Te thin films and the rapid crystallization mechanism of these materials was discussed.     

磁控溅射薄膜生长全过程的计算机模拟研究

本文通过建立多尺度模型,结合模拟了磁控溅射中溅射原子的产生、溅射原子的碰撞传输、以及最终成膜的全过程,研究了基板温度、溅射速率、磁场分布和靶材-基板间距对薄膜生长过程与薄膜性能的影响.模拟结果显示,提高基板温度或降低溅射速率都会增加初期生长阶段薄膜的相对密度;磁场对靶的利用率有显著的影响,而对薄膜最终形貌的影响不大;增大靶材-基板间距会降低薄膜的粗糙度.