Reiko Sato Turtelli

Magnetoelectric Properties of CoFe

The CoFe₂O₄-BaTiO₃ core-shell structure composite has been successfully synthesized by wet chemical method. X-ray characterization showed that the composite consisted of two single phases: CoFe₂O₄ and BaTiO₃. The saturation magnetization of the CoFe₂O₄ component in composite was found to be similar to those of the bulk sample. It was observed that the longitudinal and transverse magnetoelectric coefficients of the core-shell structure are 18 times higher than those of the mixed structure

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The relaxation of magnetic permeability has been followed over a wide temperature range on amorphous magnetic ribbons (composition Fe 81.5 B 14.5 Si 4 ), prepared with different quenching rates, and thus characterized by thickness varying between 21 and 61μm. The measurements were performed on as quenched and annealed (2 h at 330°C in vacuum) samples, by taking the Fourier transform of permeability pulse envelopes, having a repetition time τ = 25 s. The magnitude F(ω, T) at an appropriate frequency \omega_{o}=4\pi at each temperature T is proportional to the decay of permeability after a time τ. By plotting this quantity vs. temperature (between 4 and 600 K), significant differences are found in the relaxation behavior of the examined samples, the relaxation effects being much more pronounced on thinner specimens. The room temperature relaxation is actually found to be closely proportional to the inverse of thickness, that is to quenching rare. These differences are somewhat, but not completely, eliminated on the annealed samples. These results thus provide evidence of structural differences in amorphous alloys due to different preparation procedures. The presence of maxima at higher temperatures on the curve of the permeability relaxation vs. T also provides evidence of differences in structural relaxation due to annealing in the amorphous state of the various samples.

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In order to investigate the effect of boron additions in the magnetostrictive behavior of Fe72Ga28 alloy, this alloy was doped with 0.5, 1.0, 1.5 and 2.0 at.% of boron. The results have shown that boron additions of 0.5 and 1.0% improve significantly the magnetostriction, the values being about two-fold higher compared to the undoped alloy. Microstructural evaluations suggest that boron additions suppress partially the D03 phase ordering, retaining the α-phase. Furthermore, the increase of lattice parameters of the α and D03 phases indicates that boron enters interstitially in the cubic lattice of these phases. Both effects are considered to be responsible for the increased magnetostriction observed in the (Fe0.72Ga0.28)99.5B0.5 and Fe0.72Ga0.28)99.0B1.0 alloys.

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The temperature dependence of the hysteresis loop, saturation magnetization, and saturation magnetostriction of rapidly quenched ribbons of the compositions Fe/sub 100-x/Si/sub x/ (x=12.1, 13.8, 20.0) and of FINEMET-type material with compositions Fe/sub 73.6/Cu/sub 1/Nb/sub 3/Si/sub 13.5/B/sub 9/ Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 16.5/B/sub 6/ were measured between room temperature and 400/spl deg/C. This paper analyzes the temperature dependence of the saturation magnetostriction and the saturation magnetization of the FINEMET ribbons after various heat treatments, assuming a superposition of the properties of the nanocrystalline Fe-Si and of the remaining amorphous phase. The behavior of the coercivity of Fe-Si prepared by rapid solidification follows roughly that of the (absolute value) of the magnetostriction. The Curie temperature of the remaining amorphous phase is enhanced.

-BaTiO

We report the effect of a short-time vacuum annealing (1073 K during 10 minutes) on structural phase transition temperatures and magneto-structural properties of as-quenched ribbons of the Heusler alloy Ni50.6Mn34.5In14.9. This alloy crystallizes in a single phase cubic B2-type austenite with a Curie point of TCA=284 K that with the lowering in temperature transforms into a martensite with TCM185 K. The direct and reverse martensitic phase transition temperatures were MS=257 K, Mf = 221 K, AS = 239 K, and Af = 266 K. After annealing austenite shows the highly ordered L21-type structure while the average chemical composition as well as the structural and magnetic transition temperatures were shifted to Ni50.2Mn34.3In15.5 and MS = 253 K, Mf = 238 K, AS = 257 K, Af = 265 K, ΔT = 13 K, TCA = 299 K and TCM207 K. In the annealed samples the magnetization changes associated to the magnetic and structural transitions are more abrupt and magnetization isotherms in both the austenitic and martensitic existence region show higher initial magnetic susceptibility and faster approach to saturation. Field-cooled hysteresis loops at 10 K were shifted along the negative H-axis for both samples, but a significant anomaly was evident on the left side of the hysteresis loop for as-quenched ribbons.

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In order to investigate the effect of the quenching rate on the magnetostriction of Fe_82.5Ga_17.5 alloy, a set of ribbons was prepared by varying the wheel velocities from 7.5 up to 25 m/s. Additionally Fe₈₅Al₁₅, Fe₈₁Al₁₉ and Fe₇₅Al₂₅ ribbons were prepared under similar conditions. The microstructure shows generally columnar particles and (100) texture perpendicular to the surface of the ribbons. For Fe-Al ribbons, the lattice constant and magnetostriction (which shows volume magnetostriction effect for the higher Al concentrations) increase and saturation magnetization decreases with Al content. However, the average grains size and the lattice constants of Fe_82.5Ga_17.5 and Fe-Al alloys, show large fluctuations with the wheel velocity. For Fe_82.5Ga_17.5, ribbons the magnetostriction values and its character with applied field depend strongly on the quenching rate. The dependence of the longitudinal magnetostriction with wheel speed exhibits a maximum of about 116 ppm at 15 m/s, which is related to the higher degree of (100) texture.

Core-Shell Structure Composite

A system for magnetic characterization of soft magnetic materials in low field is described. Fields of 0.02 A/m can be applied and a good sensibility is achieved by means of a Lock-In amplifier. Signals of 1 nV can be measured, and magnetic characterization with external stresses can be made. The measurements can be done from room temperature up to 973 K. The system allows to perform permeability, pinning field, Curie temperature and disaccommodation measurements automatically. >

Magnetic relaxation in amorphous ribbons prepared with different quenching rates

The behavior of the initial magnetic permeability (/spl musub 1/) of a set of Co/sub 75-x/Fe/sub x/Si/sub 15/B/sub 10/ amorphous ribbons (2/spl les/x/spl les/40) submitted to uniaxial stresses (/spl sigma/) and/or to a dc longitudinal magnetic field is studied in detail. A maximum in the /spl musub 1/ vs. /spl sigma/ curves is observed independently of the sign and absolute value of the magnetostriction constant, and is related to changes in the magnetic domain structure. >

Improved magnetostriction of Fe72Ga28 boron doped alloys

Measurements of the aftereffect of the magnetic permeability of a Fe-Ni-Mo-B amorphous alloy have been performed as a function of absolute temperature between 20 K and 300 K. The effect of a variation in the alloys rate of quenching from the melt is investigated. Measurements of saturation magnetization, saturation magnetostriction, and permeability aftereffect, have been performed at T=77 K on various Fe-based amorphous alloys. All results are discussed in terms of the current theories for the permeability aftereffect in amorphous ferromagnets.

Contribution of the crystalline phase Fe/sub 100-x/Si/sub x/ to the temperature dependence of magnetic properties of FINEMET-type alloys

The structure of annealed Sm(Co0.6Cu0.4)5 compounds, prepared with different Sm excess content, has been investigated by means of high resolution x-ray diffraction and scanning electron microscopy. The samples were also magnetically characterized by thermomagnetic analysis and M versus H curves at room temperature. Increasing Sm excess improves the compositional order of the 1?:?5 phase. The coercivity (HC) and the Curie temperature (TC) are both changed as a function of Sm excess content. The decrease in the structural defects density, resulting from the compositional order, is responsible for the observed magnetic behaviour.