Teiko Okazaki

Microstructures and magnetic properties of rapidly solidified CoNiGa ferromagnetic shape memory alloys

The transformation characteristics of CoNiGa ribbons resemble those of bulk alloys. However, the transformation temperatures in the ribbons are higher than those of the bulk suggesting stress-induced martensites. TEM observations reveal arch typical twins and precursor tweed structures. The twins can be wavy suggesting a small boundary energy.

Effect of rapid solidification on giant magnetostriction in ferromagnetic shape memory iron-based alloys

Abstract Ferromagnetic shape memory Fe–29.6 at.% Pd alloy ribbons prepared by the rapid solidification, melt-spinning method, showed a giant magnetostriction of 830 microstrain when an external magnetic field of 7 kOe was applied nearly normal to the ribbon surface at room temperature. This ribbon’s magnetostriction was several times as large as conventional polycrystalline bulk’s one before rapid solidification. The magnetostriction in the rolling direction depended strongly on a direction of applied magnetic field. We considered that this phenomenon is caused by a rearrangement of activated martensite twin variants just below the austenite phase transformation temperature. We investigated their basic material properties, i.e. the dependencies of magnetostriction on temperature as well as on magnetic angular orientation to the surface, magnetic properties, crystal structure, surface texture morphology and shape memory effect of Fe–29.6 at.% Pd ribbon samples by comparing with conventional bulk sample. It can be concluded that the remarkable anisotropy of giant magnetostriction of ribbon sample is caused by the unique uniaxial-oriented fine grain structure formed by the melt-spinning method. In addition, we confirmed the possibility of rapidly solidified Fe–Pt ribbon as a new kind of iron-based ferromagnetic shape memory alloys for magnetostrictive material.

Fabrication of multiferroic composite actuator material by combining superelastic TiNi filler and a magnetostrictive Ni matrix

This research aims to design and verify a new magnetically driven multiferroic composite actuator material which is characterized by large strain and a rapid response speed. The composite actuator material is designed to consist of a magnetostrictive Ni matrix and superelastic TiNi alloy fiber fillers whose volume fraction was changed. The spark plasma sintering (SPS) method which is characterized by short time and low temperature processing was used to join these two different material elements. Cross-sectional observation by optical micrograph, mechanical compression tests, and magnetostriction measurements were carried out to verify the perfection of the adhesion state of the TiNi filler and nickel matrix as well as the amplification effect of magnetostriction. As the experimental result, the amplification effect of about a maximum of 1.5 times magnetostriction was verified in the fabricated composite actuator.

Magnetoelectric Coupling in Multi-Ferro Fe-Pd/PZT/Fe-Pd Laminate Composites

Magnetoelectric(ME) coupling in multi-ferroic composite designed by using two material elements of Pb(Zr, Ti)O3 (PZT: 260μm) and magnetostrictive alloys(Galfenol based Fe-Ga-X(X=Al, Co), ferromagnetic shape memory Fe-Pd) was investigated. Output ME Voltage under the driving a.c.magnetic field Hac at f = 1 Hz in Fe81Ga19/PZT/ Fe81Ga19, Fe80Ga15Al5 /PZT/ Fe80Ga15Al5, Fe76Ga17Co7/PZT/ Fe76Ga17Co7 composites, in which the magnetostrictive foils(50μm thickness) were prepared by rapid solidification, increased in proportion to the actuated force than the magnetostriction.Moreover，output ME Voltage in Fe70Pd30 / PZT/ Fe70Pd30 laminates, in which the Fe70Pd30 films (10μm thickness) on PZT were deposited by sputtering method, exhibited 8 V at Hac of 175Oe, which is larger than 7.16 V for Fe76Ga17Co7 (50μm)/PZT/ Fe76Ga17Co7 (50μm) composites

GIANT MAGNETORESISTANCE IN GRANULAR FERROMAGNETIC SYSTEMS OF NI3MN ALLOY

Giant magnetoresistance (GMR) phenomena were investigated in the inhomogeneous phase of Ni3Mn alloy, where magnetically ordered single-domain clusters are distributed in a disordered nonmagnetic matrix. The observed GMR varies with the long-range-order parameter S and has values ranging from -0.5% to -1.8% for 0.1 and the volume fraction e of the ferromagnetic clusters. The GMR is maximized at ~33 nm3 and e~0.25, these values being obtained from analysis using a superferromagnetic model.

Magnetic Properties and Microstructures of Rapidly Solidified FePd Alloy Ribbons

It is well known that FePd alloys are effective as a magneto-thermoelastic actuator material, because they have large magnetostriction and shape memory effect. In order to use the alloys for a micro-actuator, magnetic properties and microstructures have been examined as for rapidly solidified Fe-29.6 at% Pd alloy ribbons. The ribbons exhibit a large magnetostriction at room temperature and good shape memory effect. Magnetostriction and coercive force of the ribbons markedly depend on the direction of the applied magnetic field. Maximum values of magnetostriction and coercive force are obtained at θ = 85 degree (θ is the angle between the magnetic field and the ribbon plane). Relief effects corresponding to the formation of FCT martensite variants are observed on the grains. X-ray diffraction profile at room temperature shows that FCT martensitic phase and FCC parent phase coexist in the ribbon. Dense striations are observed in the TEM bright field images of FCT martensite plates. Selected area electron diffraction patterns revealed the striations to be thin twins.

Magnetoelectric Effect of Fe70Pd30 Ferromagnetic Shape Memory Alloy Film: Lead Zirconate Titanate Trilayer Composites at Low and High Magnetic Field Frequencies

Magnetoelectric (ME) coupling in multiferroic composites of Fe70Pd30/lead zirconate titanate (PZT)/Fe70Pd30 trilayers was investigated. An Fe70Pd30 ribbon and Fe70Pd30 films were prepared by rapid solidification and sputtering techniques, respectively. The saturated value of magnetostriction of the ribbon 30 µm in thickness was 95 ppm at a steady magnetic field of 200 Oe. Those of the films were 22 to 67 ppm depending on the film thickness. The output ME voltage VME of the trilayer composites, which was measured at a driving ac magnetic field Hac at 1 Hz, increased with increasing magnetostriction. The VME of the ribbon/PZT/ribbon composite exhibited a very large peak at a resonant frequency of 64 kHz and responded to an Hac of 0.0025 Oe. However, for the film/PZT/film composite, VME linearly increased with increasing Hac in a wide frequency range of 1 to 104 Hz.

Magnetic X-ray absorption fine structure for Ni-Mn alloys.

Magnetic X-ray absorption fine-structure (XAFS) spectra have been measured for Ni-Mn alloys. The magnetic XAFS in the near-edge region (X-ray absorption near-edge structure, XANES) and X-ray magnetic circular dichroism (XMCD) of the Mn and Ni K-edge for Ni(1-x)Mn(x) (x = 0.25, 0.24 and 0.20) show that (i) the local magnetic structure around the Mn atom is quite different from that around the Ni atom, and (ii) the peak intensity in the magnetic XANES of the Mn K-edge depends on the magnetization of the sample in contrast to the Ni K-edge. The Mn K-edge magnetic EXAFS (extended XAFS) for Ni(0.76)Mn(0.24) is also measured. The second and fourth peaks in the Fourier transform are observed to be enhanced in comparison with the non-magnetic EXAFS, indicating that the second- and fourth-shell Ni atoms are replaced by Mn atoms due to heat treatment (atomic ordering). Semi-relativistic theoretical calculation explains the observed magnetic EXAFS.

Determination of the local structure of the first and second shells in ordered and disordered Ni-Mn alloys

The coordination numbers and the interatomic distances for 50, 75 and 80 at.% Ni-Mn alloys in ordered and disordered states are presented. A new method for determining the first and second nearest neighbor coordination numbers in a binary alloy is applied. It is shown that magnetic properties of these alloys depend on short range order in atomic arrangement.

Magnetostrictive Galfenol Torque Sensor Devices for Smart by-Wire Steering System in Automobile Technology

Polycrystalline Galfenol (Fe-Ga-X, X=Al, C, Zr etc.) alloys were fabricated as a bulk sample from rapid-solidified powders or ark-melted and annealing process method for enhancing various engineering applicabilities of this magnetostrictive alloy. Especially, (Fe-Ga0.15-Al0.05)99.0-Zr0.5-C0.5 [at.%] sample showed a maximum magnetostriction of λmax=90ppm to 150ppm as well as a tensile stress over σ=800MPa. This large magnetostriction is mainly caused by non-precipitating of the ordered A2 phases without the excessive precipitation of ordered phases such as fcc ordered L12, bcc ordered D03 phases and the remained [100] oriented strong textures by a heat treatment. Based on the improvements of these properties in the developed bulk Galfenol alloys, secondarily, we will introduce an application as a smart torque sensor by utilizing Galfenol-ring around the shaft for steering-by-wire system of automobile. A torque sensing system by using the magnetostrictive ring of Galfenol alloy was developed and magnetic flux leakage from the ring attached on the rotating shaft was experimentally measured by using differential Hall probe sensor. The sensitivity of this type of torque-sensor shows a strong dependency of metallurgical microstructure and the residual stress (i.e.hoop-stress) in the ring due to sensor shows a strong dependency of the residual stress (i.e.hoop-stress) in the ring due to the fitting level. A promising result on ring-type and single-structured inverse magnetostrictive torque sensor will be presented.