Seong Cho Yu

Magnetic properties of nanocrystalline Cu-Fe-Co alloys processed by mechanical alloying

The effects of Co addition on structural and magnetic property changes during mechanical alloying of Fe and Cu have been studied by using X-ray diffractometry (XRD) and magnetometry. Supersaturated fcc solid solutions with crystallite size of /spl sim/7 nm have been prepared by mechanical alloying Cu/sub 0.5/Fe/sub 0.5-x/ (x=0.0, 0.1, 0.2, 0.4) powders. The variation of saturation magnetization is similar to the Slater-Pauling curve, even though the magnetization is smaller than that for the Fe-Co binary alloy. The Curie temperature of the solid solution increases with Co content.

Microstructure and magnetic properties of Cu0.8(Fe1−xCox)0.2 alloy powders manufactured by a mechanical alloying process

Changes in structural and magnetic properties during mechanical alloying and heat treatment were studied in Cu0.8(Fe1−xCox)0.2 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) alloys using x‐ray diffractometry, transmission electron microscopy, and magnetometry. Supersaturated solid solutions with a fcc crystal structure were obtained in all alloys by mechanical alloying. The grain sizes of the solid solutions were about 20 nm. Magnetization of the supersaturated solid solutions could be explained by a mixture of two types of particles showing paramagnetism and ferromagnetism. The variation in magnetization with Co content in the solutions was similar to the Slater–Pauling curve. Fe–Co with a bcc structure precipitates during annealing of the Cu0.8Fe0.1Co0.1 alloy. With increasing annealing temperature ferromagnetic behavior becomes dominant due to a coarsening of the precipitates.

Frequency characteristics of permeability in weak-field annealed Co-based amorphous ribbon

The Co-based amorphous ribbons were annealed under the weak-field of a few hundred A/m at a temperature of 380C for 8 h in an air atmosphere. The annealing field Ha was applied along the sample axis during the annealing to develop the bias field in crystalline layer on the sample surface. Two relaxation due to wall motion and rotational magnetization are revealed in low and high frequency region, respectively, for as-quenched and annealed samples in vacuum under zero field. For the annealed samples in air, the low frequency relaxation due to domain wall motion drastically decreases, while the high frequency relaxation due to rotational magnetization remains irrespective of Ha These results indicate that the wall of longitudinal domains are strongly coupled with the bias field, while the rotational magnetization is enhanced in the annealed samples under the field.

Temperature dependence of magnetization in nanocrystalline CuFeCo alloys

Abstract The temperature dependence of the magnetization in nanocrystalline Cu 0.5 Fe 0.5- x Co x ( x =0.0, 0.1, 0.2, 0.4) powders, manufactured by mechanical alloying in a high-energy ball mill, has been studied by means of magnetometry and ferromagnetic resonance experiments. The Curie temperatures of the solid solutions increase with Co content. The spin wave stiffness constant and the range of the exchange interaction increase with increasing Co content, indicating the hardening of the exchange interaction by Co substitution.

Annealing temperature dependence of permeability spectra and asymmetrical giant magnetoimpedance in Co-based amorphous ribbon

The permeability spectra and giant magnetoimpedance (GMI) profiles have been measured as a function of annealing temperature, Ta in field-annealed Co-based amorphous ribbon. The permeability spectra show the superposition of two dispersions of Debye type for Ta⩽200 °C. As Ta increases over 250 °C, the low frequency dispersion begins to decrease, and eventually disappears for Ta⩾350 °C due to the exchange coupling with the bias field formed in surface crystalline layer. However, the high frequency dispersion increases with Ta up to 350 °C and then becomes nearly constant. The GMI profile measured at 0.1 MHz shows a one peak in the samples of Ta⩽300 °C. As Ta increases over 350 °C, the GMI-valve is revealed due to the influence of bias field on wall motion. As the Ta increases over 450 °C, the asymmetric two peaks of GMI profile are shown instead of the GMI valve due to the damping of wall motion after the crystallization.

Local Structure and Magnetic Properties of Fe-Mn Nanocrystalline Alloys Fabricated by Mechanical Alloying Technique as a Function of Milling Time

Structural and magnetic properties of Fe??Mn?? nanocrystalline alloys prepared by the mechanical alloying technique (using commercial Fe and Mn powders as the precursors) are studied as a function of milling time, 1 hr to 48 hrs. The nano-crystallite size and shape are examined by using scanning electron microscopy (SEM). The effect of milling time on structural characterization was investigated using X-ray diffractometer (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). Both XRD and EXAFS studies showed that the alloying process should be completed after 36 hrs milling. Concerning the magnetic behavior, the data obtained from superconducting quantum interference devices (SQUID) exhibited both magnetic saturation (Ms) and coercivity (H c ) depend strongly on the milling time, which are related to the changes in the crystallite size and magnetic dilution.

A class of micromachined magnetic resonator for high-frequency magnetic sensor applications

A class of LC resonators for micromagnetic sensor devices is reported, which is fabricated by means of the microelectromechanical system (MEMS) technique. The micro-LC resonator consists of a solenoidal microinductor with a bundle of soft magnetic microwire cores and a capacitor connected in parallel to the microinductor. The core magnetic material is a tiny glass-coated Co83.2B3.3Si5.9Mn7.6 microwire fabricated by a glass-coated melt-spinning technique. The solenoidal microinductors fabricated by the MEMS technique were 500–1000μm in length with 10–20 turns. The changes of inductance as a function of external magnetic field in microinductors with appropriately annealed microwire cores were varied as much as 370%. Because the permeability of ultrasoft magnetic microwire is changing rapidly as a function of external magnetic field, the inductance ratio as well as magnetoimpedance ratio (MIR) in a LC resonator is varied accordingly as a function of external magnetic field. The MIR curves can be tuned very p...

Critical behavior and magnetocaloric effect of LaFe10−xBxSi3 alloy ribbons

We have studied the magnetic properties and magnetocaloric effect of LaFe10−xBxSi3 (x = 1, 2, and 3) alloy ribbons prepared by a rapidly quenching method. The partial replacement of Fe for B in LaFe10−xBxSi3 leads to a rapid decrease in the Curie temperature (TC) from 425 K for x = 1, through 310 K for x = 2, to 190 K for x = 3. Among LaFe10−xBxSi3 ribbons, only LaFe7B3Si3 (i.e., x = 3) has a dominancy of amorphous phase. The M2 versus H/M plots prove this sample exhibiting a second-order magnetic phase transition. The detailed analyses of M(H) data around TC based on the modified Arrott plot introduced critical values of TC ≈ 192 K, β = 0.354 ± 0.013, γ = 1.355 ± 0.032, and δ = 4.8 ± 0.1. These values are close to those expected for the 3D-Heisenberg model (β = 0.365 and γ = 1.336), indicating an existence of ferromagnetic short-range interactions. Concerning the magnetic entropy change (ΔSm), we have found its maximum ΔSm achieved just around TC, which are 1.04 and 1.42 J/kg K for x = 2 and 3, respectiv...

Influence of fabrication conditions on giant magnetocaloric effect of Ni–Mn–Sn ribbons

The magnetocaloric effect of Ni50Mn50?xSnx ribbons (x?=?11?15) prepared by using melt-spinning and subsequent annealing has been investigated. The x-ray diffraction data of specimens show that all the samples are partially crystallized with Ni2MnSn phase. The magnetic transitions of these ribbons strongly depend on Sn-concentration and annealing process. Particularly, the antiferromagnetic?ferromagnetic transition is just observed at a narrow range of the Sn-concentration (x?=?12?14). The positive magnetic entropy changes occurring at the transition temperature of the antiferromagnetic phase are quite large, |?Sm|max?=?5.7?J?kg?1?K?1 (for x?=?13) with external magnetic field change ?H?=?12?kOe. Besides that, the negative magnetic entropy changes take place near Curie temperature and their magnitude is also large, |?Sm|max?=?1.9?J?kg?1?K?1 (for x?=?13). The obtained results indicate that Ni50Mn50?xSnx ribbons are good candidates for magnetic refrigeration application at room temperature.

The magnetization study of B containing TbFe thin films

Abstract We report the magnetic behaviour of TbFeB thin films fabricated by RF magnetron sputtering technique. In order to obtain the intrinsic magnetic property, the temperature dependence of magnetization was measured using a SQUID magnetometer and VSM during heating from 5 to 500 K, with an applied field of either 10 or 50 kOe. Abnormal behavior in the temperature dependence of magnetization measured at 10 kOe is observed in all the specimens. Magnetization is very small at low temperatures and it initially increases with increasing temperature, reaches a maximum, and then decreases with the further increase of temperature. This result can be explained in terms of the existence of the conical spin structure which is caused by the noncollinear magnetic couplings between Fe atoms, and between Tb atoms due to the structural disorder.