T.K. Kim

Temperature dependence of magnetoimpedance effect in amorphous Co66Fe4NiB14Si15 ribbon

The temperature dependence of the magnetoimpedance (MI) effect is important both for scientific study and for thermal stability of MI sensors. We have performed the measurement of MI effect in amorphous Co66 Fe4 NiB14 Si15 (Metglas 2714A) ribbon from a cryogenic chamber where the temperature of the sample can vary from 10 to 300 K. The ac current was fixed at 10 mA for all measured frequencies ranging from 100 kHz to 10 MHz. The magnetoimpedance ratio (MIR) was revealed the drastic increment as a function of MIR(T)=MIR(0)exp(cT2), where c is a constant. The measured MIR values at room temperature are usually 2–3 times larger than the data measured at 10 K for all measured frequencies. However, the shapes of the MIR curves are remained. This result shows the potential application of the MI effect for a temperature sensor.

Magnetoimpedance effect in the nanocrystalline Fe–Zr–Cu–B–Al alloy system

The magnetoimpedance (MI) of Fe92−x−yZr7BxCu1Aly (x=2,4,6,8,y=0,0.5,1,1.5) alloys has been measured to investigate the influence of the structural changes in the crystallization process as well as the changes of the soft magnetic properties such as permeability, magnetic anisotropy, etc. after thermal treatment. The MI measurements were carried out along the ribbon axis with longitudinal magnetic field. The nanocrystalline alloy system was prepared by the rapid quenching technique in an Ar atmosphere. Ultrasoft magnetic behavior has been observed in all samples annealed at 550 °C. The x-ray diffraction and transmission electron microscopy results show the α-Fe phase appeared in the annealed alloy system at 550 °C. The maximum magnetoimpedance ratio value in Fe84Zr7B3Cu1 alloy annealed at 550 °C was reached as much as 1100%. The field annealing does not contribute to improve the magnetoimpedance effect in these nanocrystalline alloy system.

MAGNETOIMPEDANCE EFFECT IN NANOCRYSTALLINE FE90-XBXZR7CU1AL2 (X=2,4,6,8) ALLOYS

The magnetoimpedance of Fe90−xBxZr7Cu1Al2 (x=2,4,6,8) alloys has been measured to investigate the influence of the structural changes in the crystallization process as well as the changes of magnetic properties such as permeability, coercivity, magnetic anisotropy, etc. after thermal treatment. The frequency of magnetoimpedance measurement ranged from 100 kHz to 10 MHz, and the current was fixed at 10 mA for all measurements. Annealing was performed at temperatures of 350, 450, and 550 °C for 1 h in a vacuum. Ultrasoft magnetic behavior has been observed in the samples annealed at 450 °C. The magnetoimpedance ratio coincides with the softness of magnetic properties of thermally treated samples. A giant change of the longitudinal permeability as a function of external field is also observed.

Phases of melt-spun Sm1−xFe7+x alloys and magnetic properties of their nitrides

Sm1-xFe7+x (0 less than or equal to x less than or equal to 0.9) alloys were induction-melted and rapidly quenched at various wheel velocities of 20-50 m/s. The results of X-ray diffraction show that for the composition of 0 less than or equal to x less than or equal to 0.1 the rhombohedral Th2Zn17-type structure is the main phase, and for the composition of 0.2 less than or equal to x less than or equal to 0.9 the hexagonal TbCu7-type structure is formed. AC initial susceptibility experiments indicate that at annealing temperatures below 580 degrees C the amorphous phase is crystallized to the TbCu7-type phase, while annealing up to 800 degrees C the TbCu7-type phase is transformed to the Th2Zn17 phase. After nitrogenation, the optimal condition for the best magnetic properties is obtained in Sm12.5Fe87.5Ny, quenched at 50 m/s and annealed at 700 degrees C for 30 min, and consequently nitrogenated at 460 degrees C for 4 h. The best result for the coercivity H-i(c) of the Sm12.5Fe87.5Ny achieved is 18.9 kA/cm (23.7 kOe).

Coercivity enhancement by Zn addition in hot deformed NdFeB magnets

Anisotropic NdFeB magnets were fabricated by the single-stage hot deformation method from commercial isotropic MQPA powder. The effect of Zn addition on the magnetic properties was investigated and the considerable increase of coercivity was observed. The increment of coercivity was about 57% in the case of 0.4 wt% Zn addition compared with that for additive-free magnets.

Temperature dependence of the magnetization of nanocrystalline Fe/sub 68.5/Co/sub 5/Nb/sub 3/CuSi/sub 13.5/B/sub 9/ alloy

The authors report on the effect of annealing temperatures on the spin wave excitations for the nanocrystalline Fe/sub 68.5/Co/sub 5/Nb/sub 3/CuSi/sub 13.5/B/sub 9/ alloy, observed from the low-temperature magnetization curves and the spectroscopic splitting g-factors. The temperature dependence of magnetization was measured with a vibrating sample magnetometer and spectroscopic splitting g-values were estimated from ferromagnetic resonance experiments. Coercive force, initial permeability, and AC power loss of the annealed alloys have been studied systematically. The thermomagnetization curve is found to obey the Bloch law, M/sub s/(T)=M/sub s/(0) (1-BT/sup 3/2/-CT/sup 5/2/). From these experimental results, spin wave stiffness constants and the range of the exchange interaction within the annealing condition were calculated. The source of the excellent soft magnetic properties in the alloy with ultrafine grains may be the softening of the spin wave, in addition to the grain size effect. >

Magnetic properties of new Nd-Fe-B nanocrystalline with /spl alpha/-Fe as main phase

The /spl alpha/-Fe based Nd-Fe-B magnet with low Nd content has been investigated to develop a new type of rare earth magnet. The amorphous Nd/sub 4/Fe/sub 85.5/B/sub 10.5/ alloy prepared by a rapid quenching technique was crystallized to form only two phases: /spl alpha/-Fe as the main phase, and Nd/sub 2/Fe/sub 14/B as the secondary phase, The coercivity and remanence of the studied alloy were 95.5 kA/m and 1.2 T, respectively. The additions of Mo and Cu to Nd/sub 4/Fe/sub 85.5/B/sub 10.5/ alloy reduced the mean grain size and increased the coercivity up to 207 kA/m. Furthermore, the substitution of 8 at% Co for Fe in Nd/sub 4/Fe/sub 82/B/sub 10/Mo/sub 3/Cu/sub 1/ improved the coercivity, remanence, and Curie point. The coercivity, remanence, and energy product of the Nd/sub 4/Fe/sub 74/Co/sub 8/B/sub 10/Mo/sub 3/Cu/sub 1/ alloy were 219 kA/m, 1.4 T, and 103.5 kJ/m/sup 3/, respectively.

Nb‐added high‐coercivity Nd‐Fe‐B melt‐spun ribbon with high remanence

The effects of composition and additives on the microstructures and magnetic properties of Nd‐Fe‐B melt‐spun ribbons were studied. Experimental results have revealed that homogeneous grain structure with fine grain size was obtained on composition close to 2‐14‐1 stoichiometry with additives. It was also found that Nb was very effective in increasing both remanence and coercivity in a Nd‐Fe‐B melt‐spun ribbon. A high‐energy product of 151.2 kJ/m3 (19.0 MGOe) was obtained from an isotropic (Nd0.5Pr0.5)12Fe72Co8B6Nb2 melt‐spun ribbon with a remanence of 0.926 T and a coercivity exceeding 1200 kA/m (∼15 kOe).

Enhancement of permeability of Co-based amorphous alloy by two-step cooling method

Abstract The soft magnetic properties of Co-based amorphous alloy have been investigated as a function of cooling condition followed by high-temperature annealing. Furnace cooling to the Curie temperature and subsequent water quenching (F&W method) considerably increased the initial permeability by 60% and 100% higher than those of the furnace-cooled and water-quenched samples, respectively. The F&W sample also showed superior high-frequency characteristics. This phenomenon was analyzed in terms of internal thermal strees and the magnetic ordering. We confirmed the alloy annealed by F&W shows better shielding effectiveness

Measurement accuracy of a pulsed field magnetometer designed for rare earth based permanent magnets

A pulsed field magnetometer with maximum field of 120 kOe has been constructed. The pulsed field generated by a 18 kJ capacitor bank system combined with a pulsed coil was oscillation damped with a period of 10.12 ms. The specimen space was designed to heat up to 200/spl deg/C by flowing hot N/sub 2/ gas. Magnetic hysteresis curve was measured by induction method during 10.12 ms of a pulsed duration from the first positive to the second positive peak. The accuracy of PFM is 2% within the space of /spl plusmn/2 mm/spl times/8 mm at the center of the J-coil.