We present a review of theories of tunnel magnetoresistance (TMR) putting an emphasis on the role of electron scattering due to randomness. We adopt the linear response theory and generalize the conductance formula to calculate the electrical conductance in layered structures. The effect of randomness is treated in the coherent potential approximation and⁄or direct numerical simulation. Because the momentum conservation parallel to the junction planes does not hold in the presence of randomness, number of tunneling path through the barrier increases. Using a simple tight-binding model, we demonstrate first how the tunnel conductance and TMR are influenced by the randomness and by the shape of the Fermi surface of the metallic electrodes. The theory is applied to epitaxial tunnel junctions of Fe⁄MgO⁄Fe. It is shown that the basic concept of tunneling in the presence of randomness is also hold in realistic junctions. Magnetoresistance of tunnel junctions with semimetals and those with manganites is studied to clarify the effect of their electronic structures and interaction on TMR.
This report describes the distribution of magnetic leakage flux from a cell phone and demonstrates where and how it is present by means of our original visualization technique. As a result of the visualization, the leakage flux from some parts can be clearly observed even when the phone is switched off. It is roughly estimated that the quantity of magnetic leakage flux is about 0.01 mT. An understanding of the presence and quantity of the magnetic leakage flux from cell phones obtained by using our technique has significant implications for the areas of human health, environmental medicine, scientific education, and engineering.
We investigated the tunnel magnetoresistance(TMR) and bias voltage dependence of the TMR with oxidized Al-Hf Alloy at the range of Hf content (CHf) from 0 at.% to 70 at.%. The TMR ratio was greatly increased from 9.6 % to 21.4 % when the CHf increased from 0 at.% to 17 at.%, and the critical AlHf thickness that appeared TMR raito was thinner with CHf increased. These results are explained by the consideration that a uniformity of the oxidation in the AlHf layer composed with an amorphous or a nano-crystallized state was better than that in the Al layer composed with the large grain-boundaries such as fcc-Al. The bias voltage dependence of the TMR with oxidized Al-Hf Alloy at CHf of 17 at.% also improved and the yield that tunnel junction had high breakdown voltage was better than that with Al-oxide barrier although barrier thickness became thin in order to decrease RA. These results indicate that the defects like pinholes in oxidized Al-Hf Alloy barrier decreased because the crystal structure of Al was changed into amorphous from fcc(111) by Hf.
Multi-layer chip coils were fabricated by the green sheet technique using polycrystalline Li-Zn-Cu ferrite or Ni-Zn-Cu ferrite, and their complex impedance spectra were analyzed with the help of numerical calculation. When Ni-Zn-Cu ferrite is used, the complex impedance spectra of multi-layer chip coils are very sensitive to residual stress, and they deviate markedly from the values calculated by using complex permeability, permittivity, and the chip coil dimensions. On the other hand, it was found that the complex impedance of a multi-layer chip coil using Li-Zn-Cu ferrite may be determined from the complex permeability and permittivity of the polycrystalline Li-Zn-Cu ferrite as well as the chip coil dimensions. Consequently, the use of Li-Zn-Cu ferrite made it easy for us to design the complex impedance of a multi-layer chip coil. It is that the difference is related to the magnetostriction of polycrystalline ferrite. Hence, it was clarified that polycrystalline Li-Zn-Cu ferrite has a great advantage for multi-layer chip coil applications.
Normal fluid-carrying pumps are classified into three types: turbo-type, volume-type, and others. This study deals with a newly developed turbo-type axial flow mini-pump, whose impeller is driven directly by electro-magnets with U-shaped cores located outside of the pipe. At the first step of this study, a prototype pump was produced, where a commercially available flat circular disc-type ferrite or neodymium permanent magnet is bonded adhesively to the top of each of the blades of the impeller, and this circular disc-type magnet is driven directly by eight electro-magnets. In the case of ferrite magnet, the magnet and the cores of electro-magnets are overlapped 10 mm in radial direction, but in the case of neodymium magnet, both are separated 2 mm. At the second step of this study, the fundamental pumping characteristics of this turbo-type mini-pump, such as maximum rotation speed in air and in water, pump head and flow rate as a function of the rotation speed of the impeller, were measured and discussed.
This paper presents a method for calculating the iron loss characteristics of a switched reluctance (SR) motor based on reluctance network analysis (RNA). The RNA model of the SR motor consists of a multiple number of nonlinear reluctances and magnetomotive forces (MMFs) exerted by a winding current. In addition to these elements, magnetic inductances that express the magnetic hysteresis are incorporated into the model. The RNA model of the SR motor is combined with its drive circuit and motion calculation circuits in SPICE, which is a general-purpose circuit simulator. Using the electromagnetic and motion-coupled models, we can calculate dynamic characteristics, including iron loss, quickly and accurately. We evaluate the validity of the proposed method by comparing the result with measured values.