Kyeong-Sup Kim

Low Temperature Magnetizaiton In Nanocrystalline Fe/sub 88/ Zr/sub 7/ B/sub 4/ Cu/sub 1/ Alloy

The authors report on the effect of annealing temperatures on the spin wave excitations for the nanocrystalline Fe/sub 88/Zr/sub 7/B/sub 4/Cu/sub 1/ alloy, observed from the low-temperature magnetization curve and the spectroscopic splitting g factor. The thermomagnetization curve at low temperature 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 within the annealing condition are calculated. The effective permeability, coercive force, saturation magnetization (B/sub 10/), magnetostriction and magnetic anisotropy of the annealed alloys are studied systematically. >

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. >

Ultra-soft magnetic properties in nanocrystalline Fe81B11Nb7Cu1 alloy

The extremely soft magnetic behaviors in the nanocrystalline Fe_(81)B₁₁Nb_7Cu₁ alloy annealed at 450 ℃ and 550 ℃ for 1 hour in a vacuum were investigated by means of the magnetoimpedance (MI) effect and the incremental permeability. Because the MI effect can be obtained only in ultra-soft magnetic materials, the improvement of magnetic softness by proper thermal treatment was carefully monitored by the MI effect for all annealed samples. The changes of the incremental permeability as a function of an external field were also measured to verify the magnetic softness along with the MI measurement.

Influence of exchange energy and magnetic anisotropy on the nanocrystalline alloy

We have studied the influence of exchange energy and magnetic anisotropy on the magnetic properties of nanocrystalline materials. Amorphous ribbons of composition Fe88Zr7B4Cu1 have been annealed between about 500 and 700 °C. Temperature dependence of magnetization was carried out and nanocrystallized magnetic entities were investigated using the mean field theory and random anisotropy model obtained ferromagnetic correlation length and magneto-crystalline anisotropy constant. The best soft magnetic properties were observed annealed at 650 °C for the smallest magneto-crystalline anisotropy constant (K1) of about 1.40×104 erg/cm3. We obtained average magnetic anisotropy of 0.81 erg/cm3 which is almost negligible compared with K1 at an annealing temperature of 650 °C. The local magneto-crystalline anisotropy is randomly averaged out by strong exchange interaction so that this mechanism provides the basis for the good soft magnetic properties.

Different behaviors of half-metallic ferromagnetism of Cr-doped AlN and InN

The electronic structure and magnetism are studied for the zinc-blende and wurtzite (Al,Cr)N, (Ga,Cr)N and (In,Cr)N by using the full potential linear muffin-tin orbital method. The energy gap (quasi-gap) in Cr-doped wurtzite InN decreases exponentially with increasing the Cr concentration from 0.027 to 0.166. The half-metallicity is retained in the whole range of concentrations considered, whereas for (Al,Cr)N, the half-metallic character disappears at a concentration of 0.166. The Cr magnetic moment in AlN is about 2.12–2.40μB Cr atom−1 with changing the Cr concentration, and for Cr in InN, it is a nearly constant value of 3.0 μB Cr atom−1.

Correction to scaling critical exponents and amplitudes for amorphous Fe–Mn–Zr alloys

Abstract Asymptotic and leading correction to scaling critical exponents and amplitudes have been determined for quenched amorphous Fe 90− y Mn y Zr 10 ( y =0–8) ferromagnets through an elaborate analysis of temperature dependence of spontaneous magnetization, zero-field susceptibility and low-field AC susceptibility data obtained in the asymptotic critical region. From this analysis, it is found that the values of the critical exponents and amplitudes do not depend on the alloy composition and are in good agreement with the values predicted for three-dimensional Heisenberg ferromagnet system. The observed experimental results are consistent with the concept of scaling in that the exponent equalities β = γ ( δ −1) and α =2(1− β )− γ are obeyed to a high degree of accuracy. These results show that both amorphous and crystalline materials behave similarly in the critical region though amorphous alloys show a wide asymptotic critical region than the crystalline materials. The presence of disorder does not seem to have any influence on critical behavior of the system investigated in the present work.

Temperature dependence of the magnetoimpedance effect in nanocrystalline Fe84Zr7B6Cu1Al2 alloy

Abstract The nanocrystalline Fe 84 Zr 7 B 6 Cu 1 Al 2 alloy was annealed at 450 and 550°C for 1 h to achieve the ultra-soft magnetic properties such as large magnetoimpedance ratio (MIR), the incremental permeability ratio (PR), nearly zero coercivity, zero magnetostriction, etc. The MIR and PR of the sample were measured from 100 kHz to 10 MHz in a cryogenic chamber where the temperature was varied from 10 to 300 K. The increment of MIR value is proportional to increasing temperature. The maximum PR values measured above 1 MHz remain almost temperature independent. However, the maximum PR values measured below 1 MHz show drastic increment at above 150 K due to thermal activation of magnetic domains.

Thermomagnetic properties and spin-wave excitation of the nanocrystalline Fe68.5Co5Mo3Cu1Si13.5B9 alloy

Abstract The effect of annealing temperatures on the spin-wave excitations for the nanocrystalline Fe 68.5 Co 5 Mo 3 Cu 1 Si 13.5 B 9 alloy have been investigated by observation of their low temperature magnetization curves. The temperature dependence of magnetization was measured with a SQUID magnetometer and spectroscopic splitting values g were estimated from ferromagnetic resonance (FMR) experiments. The thermomagnetization curve is found to obey Blochs law. From these experimental results, spin-wave stiffness constants under annealing conditions were calculated. Moreover, coercive force, initial permeability, ac power loss and magnetostriction of the annealed alloys have been studied systematically.

X-ray diffraction studies of an amorphous Fe80−xCoxSi6B14 alloy system

The atomic structure of single roll-quenched amorphous Fe80−xCoxSi6B14 (0 ≤x≤80 at.%) alloys have been studied by X-ray diffraction. The interference functions, atomic distribution functions, and radial distribution functions were calculated from the X-ray diffraction intensity data. The nearest-neighbor distance decreases from 2.560 ± 0.008 A to x = 0 (Fe80Si6B14) to 2.505 ± 0.008 A for x = 80 (Co80Si6B14), while the average coordination number of 12.50 ± 0.15 is independent of x. These results are consistant with the measured densities of the samples, which are found to increase with increasing cobalt content.

Magnetization study of glassy and nanocrystalline Fe82(Zr, Hf, Nb)7B10Cu1 alloys

Abstract We report the salient features of the magnetic properties of as-quenched and annealed Fe82(Zr, Hf, Nb)7B10Cu1 alloys. Temperature dependence of magnetization was carried out and magnetic entities were investigated using the spin-wave theory and mean-field theory. Annealed alloys have been shown to exhibit good soft magnetic properties especially in high-frequency range. The presence of a rather high concentration of boron appears to have suppress the growth of BCC-Fe grains and also stabilize the amorphous matrix. Our result reveals that the reason for good soft magnetic properties is associated with the reduction of the magnetic anisotropy by rather strong exchange energy when annealing temperature is increased.