Field Of Grains Research Articles

Collective Grain Interactions II. Non‐linear Collective Drag Force

AbstractIt is found that the collective effects operating at large distances from the grain surface can produce substantial scattering of the ion flux and create an additional collective drag force dominant for large grain densities. The consideration is restricted to large grain charges β = Zde 2a /TiλDi ≫ 1 and Ti /Te ≪ 1 (–eZd being the grain charge in units of electron charge, a being the grain size, λDi being the ion Debye radius and Te,i being electron and ion temperatures, respectively). For present dusty plasma experiments β ≈ 10–50, the large charges of grains are screened non‐linearly and the ion scattering creates non‐linear drag force. The present investigation considers effects of scattering by collective grain fields at large distances from the grains. It is found that the physical reason of the importance of collective drag force, calculated in this paper, is related to presence of weakly screened collective field of grains outside the non‐linear screening distance depending on grain densities. The amplitude of this collective fields of the grains is determined by non‐linear screening at non‐linear screening radius. It is shown that for dust densities of present experiments the collective drag force related to this scattering can be of the order of the non‐linear drag force caused by scattering inside the non‐linear screening radius or even larger. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pounded in Scotland

Pounded in Scotland

Ｃｏ−ＣｏＯ斜め蒸着テープの結晶粒内異方性磁界に関する考察

We investigated the relationship between the shape of clusters of Co crystallites, obtained by transmission electron microscopy (TEM), and the magnetic anisotropy field of grains (Hkgrain) of Co-CoO obliquely evaporated magnetic tape. Comparison of the former with the latter revealed that the Hkgrain of Co-CoO obliquely evaporated magnetic tape closely correlates with the aspect ratio (N) of clusters of Co crystallites. The magnitude of Hkgrain is defined as that of a magnetic field where the rotational hysteresis loss of the torque curve is zero. We investigated the relationship between the Hkgrain and saturation magnetization Ms, using samples prepared under various conditions. The results showed that Hkgrain becomes asymptotic to the value HkCo + Ms/2, determined by the maximum value of the shape anisotropy Ms/2 in the region of low Ms. The value of Hkgrain calculated by using a value of N obtained from TEM images was found to be close to the value of Hkgrain obtained by magnetic torque measurement, and this tendency was confirmed qualitatively.

Cleavage crack-growth-resistance of grain boundaries in polycrystalline Fe–2%Si alloy: experiments and modeling

Experiments were carried out on the modes of advance of cleavage cracking through a field of randomly oriented grains at −30 °C in the pure cleavage range (lower shelf) in very large grain size Fe–2%Si alloy, permitting detailed observations of the chronology of percolation of the cleavage crack front across a field of grains. Utilizing previously developed model-based information on the peak resistance to fracture of high angle grain boundaries, with a given tilt and twist misorientation across them, and detailed measurements of orientations of individual grains in the samples, it was possible to develop percolation maps of cleavage through the field of grains that for the initial phases of fracture through grains predicted the actual path with about 70% accuracy when compared with experimental observations. On the basis of these experiments and modeling a simple expression is projected for the effect of grain boundaries on the overall cleavage fracture resistance of low-carbon steel in the lower shelf region. The model gives a nearly 3-fold rise of cleavage resistance of the polycrystal over the single crystal, substantially independent of grain size. Additional experiments on decarburized 1010 steel of 50 μm grain size with an unusually low brittle-to-ductile transition temperature gave very similar results on the mode of percolation of cleavage cracking through grains even though in this case the cleavage propagation was under upper shelf conditions.

Straining of a polycrystal of Fe–Pd with martensite structure by uniaxial loading

This paper proposes a method to calculate the uniaxial stress–strain relationship of polycrystalline Fe–Pd, which has a martensite structure produced by cooling. Strain is caused by changes in the fractions of the three Bain corresponding variants which form the martensite structure within each grain. Internal stress and elastic energy are accumulated as straining proceeds and are caused by differences of eigenstrain (transformation strain) between differently oriented grains. The stress and elastic energy are evaluated on the basis of micromechanics. The stress acting on a grain due to surrounding grains is calculated using a mean field method. In this procedure, an averaging method is introduced, by which the mean field of grains having the same tensile direction, but having random lateral directions, are written in a closed form. The averaging method facilitates computations. It is shown that tension and compression differ in the stress–strain relationship.

Anisotropy field distribution measurements for high-density recording media

The magnetic anisotropy fields of grains measured using different methods (rotational hysteresis, Flanders & Shtrikman method, SPD and H C ( t)) for advanced high density recording media, are discussed. The magnetic anisotropy field found using the SPD (stiff) method is more reliable as compared to other methods.

Thermal stability in perpendicular recording media

Magnetic and structural properties of CoCrPt based perpendicular recording media required to achieve both high thermal stability and low-noise performance are discussed, based on experimental results obtained using a pulse magnetometer. Experimental and theoretical analysis reveal that for CoCrPt based media of 20 nm thickness having the thermal stability factor K u V/( kT)≈80, the maximum achievable value of the ratio of remanence coercivity to anisotropy field of grains, H r/ H k is ∼0.35. Furthermore, in order to realize a large squareness M r/ M s of nearly 1.0, which is required to resist the thermal agitation effect at low recording density, we find that the value of perpendicular magnetocrystalline anisotropy, K u needed to be more than three times larger than 2 πM s 2. X-ray diffraction patterns suggested that the suppression of structural imperfections in the HCP structure, particularly in the initial growth layer, is the key issue to induce larger K u value in the CoCrPt based media.

Temperature dependence of penetration field and irreversibility field of grain aligned Hg-1223 with Re doping

Magnetization loops have been measured on a grain aligned HgBa 2Ca 2Cu 3Re 0.2O y sample at temperatures of 5–110 K and under magnetic fields up to ±14 T for both field directions of H∥ c-axis and H⊥ c-axis. The magnetization loop is very asymmetric with respect to M=0 of the loop even at T=5 K, which comes from a strong surface barrier effect. The temperature dependence of the penetration field H p and the irreversibility field H irr and also the dimensionality of the vortex have been discussed in the frame work of the vortex penetration through the surface barrier.

Thermal agitation of magnetization in CoCrPt perpendicular recording media

Magnetic and structural properties of CoCrPt based perpendicular recording media required to achieve both high thermal stability and low-noise performance are discussed, based on experimental results obtained using a pulse magnetometer. Experimental and theoretical analysis reveal that for CoCrPt based media of 20 nm thickness having the thermal stability factor K/sub u/V/(kT)/spl ap/80, the maximum achievable value of the ratio of remanence coercivity to anisotropy field of grains, H/sub r//H/sub k/, is /spl sim/0.35. Furthermore, if the strictest condition of demagnetizing factor, 4/spl pi/, for perpendicular media is assumed, in order to realize a large squareness M/sub r//M/sub s/ of nearly 1.0, which is required to resist the thermal agitation effect, we find that the value of perpendicular magnetocrystalline anisotropy, K/sub u/, has to be more than 3 times larger than 2/spl pi/M/sub s//sup 2/.

Computer Identification of Structural Weaknesses in Locally Anisotropic Polycrystalline Materials

Computer experiments were performed to investigate behavior of mesoscopic stress responses in a simulated polycrystalline material sample containing a fairly large number of constituent grains for a number of polycrystalline materials. Kro¨ner-Kneer structure-based model was adopted and refined to provide an efficacious numerical approach to local mesoscopic stresses. The approach is developed on a concept of average fields of grains for arbitrarily polygon-shaped grains. Three criteria were proposed for classifying speculated material structure weaknesses in all simulated material samples. It is found that material structure weaknesses can be well correlated by defined “Orientation-Geometry Factor” and “Relevance Parameter.” Not only grain-orientation but also grain geometry exerts strong influences on mesoscopic stress distribution, hence the distribution of material structure weaknesses in simulated polycrystalline material samples. Computer experiments lead to correlated relationships that links material structure weaknesses with local microstructure, and a database for discrimination of material structure weaknesses in the material samples. The homogenization of materials with locally anisotropic microstructure is also discussed.

Domain boundary pinning and nucleation of ferroelectric (Pb1−xSrx)TiO3 ceramics

Domain boundary pinning and nucleation characteristics of ferroelectric (Pb1−xSrx)TiO3 ceramics under polarization switching have been studied in the present work. Transmission electron microscopy was employed to investigate domain boundary nucleation and interactions as the polarization switching proceeds. Results imply that domain boundary density increases as the electric field reverses repeatedly. Internal stress field in grains accumulates during domain intersecting with one another, and some straight 90° domain boundaries dissociate into zigzag-shaped boundaries which may grow and extend to intersect with an adjacent boundary. Complicated microstructural arrangements as well as defects were produced gradually after repeated domain nucleation and intersection. Therefore domain boundary mobility was retarded and the specimens showed fatigue phenomenon. Domain boundary interactions as well as the induced domain formation behavior are classified and discussed.

Effect of Pt on the Intergranular Exchange Coupling in CoCrTaPt Thin Film Media Prepared under Ultra Clean Sputtering Process

The effect of Pt addition on magnetic properties for CoCrTaPt media with extremely thin Cr underlayer of 2.5 nm fabricated under ultra clean sputtering process are connected with the magnetic anisotropy field of grains, the microsturucture, and intergranular magnetic coupling. As a results, (1)Pt addition more than 8 at% decreases the magnetrocrystalline anisotropy field of the grains. (2)Pt addition more than 8 at% disturbs a formation of Cr segregated rain boundary structure, inducing intergranular exchange coupling. (3) The upper limit of Pt addition is proposed to be 5-6 at%.

Intragranular critical current density in Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3 Ox tapes

Transport critical current density JJc as a function of external magnetic field perpendicular to a tape surface is investigated for Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox monofilamentary tapes at 77 K. The history effect of JJc on the magnetic field between increasing and decreasing fields is observed in a zero-field-cooled sample and is interpreted by the influence of trapped magnetic fields in grains on the intergranular (transport) current. The experimental results reveal the full flux penetration field Bp for the grains. Using a critical state model and the value of Bp, the intragranular critical current density JGc is estimated to be ~5000 A mm-2, which is nearly the same value among all samples and much larger than the value of JJc. The present study shows that the current carrying capacity for the tape is limited by the weak links at grain boundaries, and strong pinning in grains does not lead to a high transport critical current density unless further improvements in grain connectivity are made.

High coercive force and low intergranular coupling in CoCrTa thin-film recording media fabricated under ultra clean sputtering process

Ultra clean sputtering process (UC-process) was newly introduced in the fabrication of Co 85.5Cr 10.5Ta 4 thin-film media to establish the control of microstructure of films. By applying the UC-process, the remarkable high- H c value of about 2.8 kOe (15 nm/50 nm in CoCrTa/Cr thicknesses) was realized with no decrement in saturation magnetization. High- H c of about 2 kOe (40 nm in magnetic layer thickness) remains even for extremely thin Cr thickness of about 5 nm. Magnetic torque analysis has revealed that magnetic anisotropy field of grains H grain k in the UC-processed media shows large values of about 6–7.5 kOe, which are about 20–50% larger than those of media fabricated under the normal sputtering process commonly used. The result of temperature dependence of saturation magnetization suggests that the appearance of large H grain k is mainly attributable to the decrease in Cr content in grains. On the other hand, the enhancement of the formation of homogeneously segregated grain boundary structure, which corresponds to the decrease of intergranular exchange coupling, was also found. The ultra clean sputtering process enables the control of the fine structure and induces the excellent features for magnetic properties even in commonly used CoCrTa material.

SEASONAL VARIATION IN BACKSCATTERING FROM AGRICULTURAL LANDS ON RADAR IMAGERY

The results of the study of the effective scattering area (ESA) of agricultural lands on drained peat bogs in the Ryazan' section of the Meshchera Lowland during different observation periods are presented. An experimental radar survey was made during spring-summer 1986-1987 by means of a side-looking airborne radar (SLAR) at a wavelength of 2.5 cm. In the processing of radar survey materials, the water surface of inland water bodies was used as a reference level. Invariant calibration curves and comparable quantitative estimates of the relative effective scattering areas for different agricultural features (open soils, pastures, sown fields of grain, and cultivated crops) were obtained for five survey times.

High coercive force and low intergranular coupling in thin film media fabricated under the ultra clean sputtering process

The ultra clean sputtering process (UC process) was newly introduced in the fabrication of Co 62.5Ni 30Cr 7.5 and Co 85.5Cr 10.5Ta 4 thin film media to establish the new concept of controlling the microstructure. By applying the UC process, extremely high H c values of up to 3 kOe were realized without decreasing the saturation magnetization in both kinds of media. A high H c value was derived even in the media with very thin Cr underlayers. Magnetic torque analysis has revealed that the magnetocrystalline anisotropy field of grains, H k grain, is about 20–50% larger than that in the normally processed media, which is one of the dominant factors in realizing high H c values. The realization of large H k grain values was found to be mainly caused by the decrease in the Cr content of the grains. The enhancement of the homogeneously segregated grain boundary structure, which corresponds to a decrease in the intergranular exchange coupling, was also found to induce high H c values in these media. This desired segregated structure was observed even in the media with an extremely thin Cr underlayer of 2.5 nm. The low intergranular coupling realized induced very low media noise even in the CoNiCr media. The UC process is concluded to be an essential technology for fabrication of high density thin film recording media.

Method of measuring anisotropy field of a recording medium investigated by computer simulation

Three methods of measuring anisotropy field of a recording medium with the easy directions in the film plane were examined by computer simulation: (1) applying magnetic fields in a direction slightly tilted from the hard direction of the medium, (2) using rotational hysteresis loss, (3) applying perpendicular field to the film plane. The anisotropy fields obtained with these methods were smaller than the intrinsic anisotropy fields of grains (or particles). A method is proposed for obtaining the intrinsic anisotropy field of grains. In this method, magnetic torques are measured out-of-plane of a film. A method, which is an extension of the above method (1), for measuring the distribution of easy direction, is also examined and a revised method is proposed.

薄膜テープの結晶粒内異方性磁界とノイズ

The relation between the magnetic anisotropy field of grains and media noise in vacuum-evaporated Co-O films with oblique anisotropy is studied. The ratio of the coercive force to the magnetic anisotropy field of grains, which indicates the degree of magnetic isolation of the grains. With increasing thickness of the films, the ratio decreases and the media noise increases. The increase of the noise is caused by both the enlarged volume of the films and the weakened magnetic isolation of the grains. Though the degree of the magnetic isolation increases with increasing oxygen content, the media noise increases. It is considered that the increase of the noise is caused by the deterioration of the crystallographic orientation. The degree of magnetic isolation of the grains depends on the grain length. By making the films multilayered, the grain length is kept short and the degree of magnetic isolation of the grains is high.

Magnetic properties and microstructure of CoCrW films for longitudinal recording media

The magnetic properties and microstructure of CoCrW films were studied. A series of media with an identical structure but with a different CoCrM alloy film, where element M was varied, as the recording layer were deposited, and the capability of these alloy films as recording films was tested in view of the intergranular magnetic coupling in films. It is suggested that the intergranular magnetic coupling in CoCrW is reduced as much as that of CoCrTa. Anisotropy fields of grains in CoCrW films are almost the same as those in CoCrTa films. However, in-plane orientation of C-axes of Co alloy grains are higher than that of CoCrTa due to the better lattice matching to (100) textured Cr underlayer, and as a result they yield higher H/sub c/, S and S*.

Magnetic Anisotropy and Intergranular Magnetic Coupling in (Co86Cr12Ta2)100-xPtx Thin Film Media

Experimental characterization of the magnetic anisotropy field of grains H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grain</sup> , and the intergranular magnetic coupling in (Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">86</sub> Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100-x</sub> Pt <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thin film media was carried out by analyses of the magnetic torque and ΔM plot. The magnitude of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grain</sup> gradually increases from about 4 kOe to around 8 kOe with increasing Pt content up to 8 at%, mainly because of the increase in magnetocrystalline anisotropy. On the other hand, the ratio of the coercive force H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> to H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grain</sup> changes from about 0.2 to about 0.3, depending on the Pt content and substrate temperature at the time of the film's fabrication. It was clarified that the difference of the ratio of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> to H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grain</sup> in the examined media depends mainly on the difference in the strength of the intergranular exchange coupling. On the other hand, it is suggested that the knowledge of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grain</sup> should be taken into account, in order to clarify how much the intergranular magnetic coupling influences the coercive force through analysis of ΔM plot.