Magnetic and Microstructure of Triple C/Co73Cr22Tas/Ti for Perpendicular Media

The magnetic properties and microstructure of triple C/CoCrTa/Ti perpendicular recording films have been studied. Magnetic measurements show that the optimal thickness of Ti underlayer is 4Ohm and that of CoCrTa is 35 nm. The optimal value of substrate temperature is found to be 400℃. A suitable Ti underlayer causes a magnetic layer to have a near-perfect hcp texture, with Co grains in the （002） preferred orientation. The film with needle-like grains is more suitable for perpendicular recording films.     

Magneto-transport properties of dilute granular ferromagnets

We present magnetoresistance (MR) measurements performed on quench-condensed granular Ni thin films which are on the verge of electric continuity. In these systems, the electric conductivity is believed to be governed by the resistance between a very small number of grains. The films exhibit sharp resistance jumps as a function of magnetic field. We interpret these findings as being the result of magneto-mechanical distortions that occur in single grains which act as bottlenecks in the dilute percolation network. The observed features provide a unique measure of magnetostriction effects in nano-grain structures as well as being able to shed light on some of the properties of regular granular magnetic films.     

Variations from Zn1-xCoxO Magnetic Semiconductor to Co-ZnCoO Granular Composite

We investigate the variations from as-deposited Zn1-x： Cox O magnetic semiconductors to the post-annealed Co- ZnCoO granular composite. The as-deposited Zn1-x Cox 0 magnetic semiconductor deposited under thermal nonequilibrium conditions is composed of Zn1-x. Cox O nanograins of high Co concentration. The room-temperature ferromagnetism with high magnetization and large negative magnetoresistance are found in the as-deposited samples. By annealing, the samples become of granular composite consisting of the Co metal grains and the remanent Zn1-x CoxO matrix. Although the magnetization is enhanced after annealing, the spin-dependent negative magnetoresistance disappears at room temperature. The magnetoresistance observed in the annealed samples in the high field region has no relation with the ferromagnetism, which in turn indicates that the roomtemperature ferromagnetism and large negative magnetoresistance observed in the as-deposited are the intrinsic properties of the Zn1-x Cox O magnetic semiconductor.     

Evolution of structural and magnetic properties of FePt/C granular films with thermal annealing

We report the structural and magnetic properties of as-deposited and thermally annealed FePt/C granular multilayer films. The as-deposited system exhibits a disordered fcc FePt phase with an average grain size of 3 nm. Thermal annealing at 650 °C results in partial L1₀ ordering and an associated grain growth to 7 nm. Mössbauer measurements show that there is no non-magnetic component present, suggesting that carbon resides only in the grain boundary region. The ferromagnetic grains are magnetically decoupled.     

Magnetic transport properties in iron/iron-oxide films

Iron/iron-oxide granular films were fabricated using reactive dc magnetron sputtering. Their structural, magnetic and transport properties were systematically studied. XPS and TEM confirmed the coexistence of Fe, FeO and Fe₂O₃. A metal–insulator transition was observed with the increasing of the oxygen component in the film. The temperature dependencies of longitudinal resistivity ρ_xx and anomalous Hall resistivity ρ_xy were discussed. We found the enhancement of ρ_xy and investigated the scaling law between anomalous Hall coefficient R_s and ρ_xx. In all the samples, R_s was found to be proportional to ρ_xx when ρ_xx is small, which indicated the skew scattering is dominant.     

Study of interactions in Co–SiO2 granular films by means of MFM and magnetization measurements

A study of Co[SiO₂] granular films using magnetic force microscopy (MFM), transmission electron microscopy and DC magnetization is presented. MFM shows large strip magnetic domains in the percolated system and allows a more direct investigation of the magnetic interaction among grains in the film with the lowest metal volume fraction.     

Magnetic hysteresis of the magnetoresistance and the critical current density in polycrystalline YBa2Cu3O7−δ–Ag superconductors

A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa₂Cu₃O_7−δ–Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density J_c(B_a) and magnetoresistance R(B_a). The R(B_a) curves show a minimum in their decreasing branch occurring at B=B_min which was found to be both the excitation current I_ex and the maximum applied magnetic field B_am dependent. In addition, for a certain value of B_am>5 mT, we have observed that B_min increases with increasing I_ex and reaches a saturation value. The J_c(B_a) curves show a maximum in decreasing applied magnetic fields occurring at B=B_max. We have also found that B_max increases with increasing B_am and reaches a saturation value. The minimum in the R(B_a) and the maximum in J_c(B_a) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high-T_c materials.     

A critical path approach for elucidating the temperature dependence of granular hopping conduction

We revisit the classical problem of granular hopping conduction’s σ∝exp[–(T₀/T)] temperature dependence, where σ denotes conductivity, T is temperature, and T₀ is a sample-dependent constant. By using the hopping conduction formulation in conjunction with the incorporation of the random potential that has been shown to exist in insulator-conductor composites, it is demonstrated that the widely observed temperature dependence of granular hopping conduction emerges very naturally through the immediate-neighbor critical-path argument. Here, immediate-neighbor pairs are defined to be those where a line connecting two grains does not cross or by-pass other grains, and the critical-path argument denotes the derivation of sample conductance based on the geometric percolation condition that is marked by the critical conduction path in a random granular composite. Simulations based on the exact electrical network evaluation of finite-sample conductance show that the configurationaveraged results agree well with those obtained using the immediate-neighbor critical-path method. Furthermore, the results obtained using both these methods show good agreement with experimental data on hopping conduction in a sputtered metal-insulator composite Ag_x(SnO₂)_1–x, where x denotes the metal volume fraction. The present approach offers a relatively straightforward and simple explanation for the temperature behavior that has been widely observed over diverse material systems, but which has remained a puzzle in spite of the various efforts made to explain this phenomenon.     

Arch generated shear bands in granular systems

We propose an arch based model, on cubic and square lattices, to simulate the internal mobility of grains, in a dense granular system under shear. In this model, the role of the arches in granular transport presents a non-linear dependence on the local values of the stress components that can be modeled geometrically. This non-linearity is very important since a linear dependence on the stress will make the models behave similarly to viscous fluids, which will not reproduce highly interesting properties of the sheared systems such as shear bands. In particular, we study a modified Couette flow and find the appearance of shear bands in accordance with the literature.     

Physical and chemical properties of nanostructured CoO-AlOx granular films

A series of nano CoO-AlO_x granular films were prepared by Ar-O₂ reactive rf sputtering and their resistive and magnetoresistive properties were measured for studying the spin-dependent properties. Transmission electron microscopy and X-ray photoelectron spectroscopy are also utilized to investigate the structures and chemical states of these samples. Based on our results, the particles of CoO with radius around 2 nm could be magnetic at room temperature and provide a large tunneling magnetoreststance of −6% through the AlO_x barrier.     

Magnetic properties of Mn-substituted GdCoxMn1−xO3 and LaCoxMn1−xO3

Partial substitution of manganese by cobalt in rare-earth perovskites REMnO₃ leads to unusual magnetic phenomena because of the simultaneous presence of Mn³⁺, Mn⁴⁺, Co²⁺ and Co³⁺ species. The magnetic nature of the RE cation plays a fundamental role in the magnetic properties. We present herein two specific families: for RE=La the magnetic behavior of the |Co+Mn| network is observed, while for Gd its strong magnetic moment interacts with the transition metals, leading to a spin reversal state. Magnetic interactions are maximized at x=0.50, as if two regimes exist: for x<0.5 Co substitutes Mn in the REMnO₃ manganite, and for x>0.5 Mn substitutes Co in the RECoO₃ cobaltite.     

Origin of enhanced coercivity in (Fe65Co35)x(Al2O3)1-x granular thin films

The magnetic behaviors of granular (Fe₆₅Co₃₅)_x(Al₂O₃)_1−x films have been examined. The results show that the coercivity at 5 K first increases sharply from 80 Oe for x = 0.1 to 700 Oe for x = 0.3 and then drops with increasing x. This behavior differs from the system of Fe-SiO₂. Considering the interfacial area, we can theoretically explain the difference between these systems. It is concluded that the peak value of coercivity becomes smaller and occurs at a lower value of x for a more rapidly increasing granular size with volume fraction.     

Improvement of magnetic intergranular isolation and evaluation of read/write characteristics on SmCo5 perpendicular magnetic thin films

SmCo₅ alloy is a promising candidate for ultra-high-density perpendicular magnetic recording (PMR) media because of its high uniaxial magnetocrystalline anisotropy K_u of more than 1.1×10⁸ erg/cm³. Previously, we successfully achieved high K_u in a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, in order to apply the SmCo₅ films to practical PMR media, it is necessary to decrease medium noise. A granulated magnetic film comprising of small and magnetically decoupled grains is effective in reducing the medium noise. In this paper, we have proposed a new granular film that is fabricated by partial thermodiffusion of Cu between the Sm-Co continuous layer and the Cu underlayer, which is granulated using compositional segregation caused by the addition of Ta₂O₅. We have analyzed the magnetic properties, magnetic domain size, and magnetization reversal process of the proposed SmCo₅ film. The magnetic domain size decreased and the magnetization reversal process changed from the magnetic-wall-motion mode to a coherent rotation mode to some extent on isolation of magnetic grains. The read/write characteristics of granulated SmCo₅ double-layered media were also evaluated. The medium noise decreased and the signal-to-noise ratio increased for the granulated double-layered (PMR) medium.     

A model for the packing of irregularly shaped grains

In this paper we will attempt to address the problem of the packing properties of granular materials composed of irregularly shaped grains (using configurational statistical mechanics). In particular, we will develop a model for a system of irregular grains based upon perturbing a packing of mono- or poly-disperse spheres. In the mono-disperse case we will show that the system packs less densely than a packing of perfect spheres, except when local correlations between configurations of grains are taken into account. The opposite is found to be true for a perturbation expansion based upon poly-disperse spheres. Finally we will show that for a bi-disperse packing of spheres phase segregation occurs for any size ratio and discuss whether this is to be expected.     

The effects of Cu doping on the magnetoresistive behavior of perovskites La0.7Ca0.3MnO3

The electronic and magnetic properties of Cu-doped perovskite La_0.7Ca_0.3Mn_1−xCu_xO₃ obtained by doping Cu on its Mn sites have been studied. The perovskite structure was found to remain intact up to the highest doping level of x=0.20. At low Cu concentration (x=0.05) the temperature-dependence of resistivity of the material exhibited up to two peaks corresponding to the magnetic transitions from the PM to the FM phase, and from the FM to the AFM phase. In general, the doping level was found to suppress the ferromagnetic ordering of the material, increase its resistivity, and produce large values of magnetoresistance near the resistivity peak. These results were explained as due to the formation of the antiferromagnetic phase.     

Numerical modelings of superconducting wires for AC loss calculations

Superconducting properties of superconducting wires as well as the influence of their composite structure and twisting should be taken into account for their numerical modeling for AC loss calculations. Furthermore, complicated electromagnetic conditions in electrical apparatuses under which superconducting wires are used influence their AC loss properties; superconducting wires carry their transport current and are exposed to the external magnetic field whose direction and magnitude vary spatially. A series of numerical models of superconducting tapes based on the finite element method has been developed. In each model, some of the above-mentioned factors that could influence the AC loss properties are taken into account. The models are formulated with the current vector potential and the scalar magnetic potential (T–Ω method). Superconducting property is given by the E–J characteristic represented by a power law. The current distributions in non-twisted and twisted superconducting tapes carrying their transport current and/or exposed to the external magnetic field are calculated with these models to estimate their AC loss. The current distribution in a short piece of superconducting tape exposed to AC magnetic field is also calculated.     

The novel YMn2D6 deuteride synthesized under high pressure of gaseous deuterium

The novel intermetallic deuteride YMn₂D₆ was synthesized under high deuterium pressure. In order to identify the structure and characterize the magnetic properties of this deuteride the powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Mn X-ray absorption near edge structure (XANES) and magnetization measurement (SQUID) were carried out. The crystal structure, the chemical state of Mn and the magnetic properties of this novel deuteride were examined and discussed. It should be noted that the structure of YMn₂D₆ (F-43m) differs dramatically from C15 symmetry of the parent material. Such a great rearrangement of the metal lattice due to deuterium absorption is rather exceptional for C15 Laves phase.     

Lattices of nonspherical ferromagnetic grains with magnetic dipole interaction: Theory and experimental examples

The magnetic properties (ground state, magnetic phase diagram, and phase transitions in a magnetic field) of two-and three-dimensional lattices of ferromagnetic grains with the intergrain dipole interaction are studied. The main attention is paid to the lattices formed by nonspherical grains (prolate and oblate ellipsoids of revolution) and their extreme forms (rodlike and disc-shaped grains). An analysis shows that the conclusions of the theory are in good agreement with the results of experiments.     

Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La_1−xCd_xMnO₃ (0x0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (T_p1), which is close to paramagnetic to ferromagnetic transition (T_c) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (T_P2) below T_p1. This transition becomes more prominent and the transition temperature (T_p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.