Chong-Oh Kim

Magnetotransport of p-type GaMnN assisted by highly conductive precipitates

GaMnN growth on GaAs (100) using a GaN single precursor via molecular beam epitaxy was undertaken. The grown layers revealed p-type conduction. It is confirmed that p-GaMnN reveals room temperature ferromagnetism with hysteresis loop having a coercivity of ∼100 Oe. The segregated phase showing a transition temperature of ∼200 K is assigned to Mn3GaN, and which enhances the conductivity of the surrounding GaMnN region. As a consequence, the GaMnN layer with segregation revealed an anomalous Hall effect at room temperature proving magnetotransport in GaMnN phase. The enhanced conductivity of GaMnN by the highly conductive second phase also revealed the importance of the role of the free carriers in the carrier-mediated ferromagnetism.

Enhanced carrier-mediated ferromagnetism in GaMnN by codoping of Mg

The effects of Mg codoping on the structural, electronic transport, and magnetic properties of GaMnN films are investigated. Mg was shown to compete with Mn for incorporation into the growing films at impingement, and to replace Mn. The partial replacement of Mn by Mg in GaMnN has resulted in an increase of conductivity of the region. The enhanced transport property due to the highly efficient dopant Mg resulted in a remarkable increase of the saturation magnetization, indicating an interaction between Mn and Mg for the carrier-mediated ferromagnetism. The increased carrier population by Mg codoping enabled a full mediation among the Mn atoms, and consequently, suppressed the paramagnetic rise in the temperature-dependent magnetization measurement.

Soft chemical synthesis and characterization of Ni0.65Zn0.35Fe2O4 nanoparticles

Mixed ferrite nanoparticles of Ni0.65Zn0.35Fe2O4 using metal nitrates were prepared by three different low temperature methods of sol gel in polyvinyl alcohol matrix, coprecipitation with NaOH as coprecipitating base, and sol gel in citric acid matrix, separately to estimate the influence of synthesis on the structural and magnetic parameters of this high saturation magnetization NiZn ferrite composition. In the final stage of the synthesis, the coprecipitated sample was washed and filtered several times before drying and finally heat treated at 350°C for 1h to get the ultrafine powder. Whereas the other two samples, which were transformed into dried gel after mixing and drying, were heat treated at 350°C for 1h, respectively, to find out the dried gel burnt out in a self-propagating combustion manner to form a fluffy powder. X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer measurements are performed on all three samples. Though the composition is the same for all thr...

Control of iron nanoparticles size and shape by thermal decomposition method

Iron nanoparticles were prepared by thermal decomposition under 180/spl deg/C with 0.256M iron pentacaronyl and 0 M-0.085M oleylamine as a surfactant. Sizes of the synthesized iron nanoparticles are from 4.8nm to 10.9nm depending on the decomposition times in the range from 10 to 60min. The surfactant concentration is a key factor for controlling the size and shape of iron nanoparticles. The synthesized iron particles without surfactant are flocculated to be clusters of 20-50nm. In the case of using 0.0425M surfactant, the obtained particles are almost sphere. While oleylamine concentration was added up to 0.085M, several iron nuclei are agglomerated together regularly. It is confirmed that the synthesized iron particles are amorphous, but the surface of the particles is easily oxidized and to be spinel structured iron oxide by HRTEM.

A model for asymmetric giant magnetoimpedance in field-annealed amorphous ribbons

A phenomenological model for the asymmetric giant magnetoimpedance (GMI) in field-annealed amorphous ribbons is developed. The effect of a surface crystalline layer on the GMI response is described in terms of an effective bias field appearing due to a coupling between the crystalline layer and amorphous phase. It is shown that the presence of the bias field changes drastically the GMI profile. At low frequencies, the domain-walls motion leads to a steplike change in the GMI response. At high frequencies, the domain-walls motion is damped, and the GMI profile exhibits asymmetric two-peak behavior. The calculated dependences are shown to be in a qualitative agreement with results of experimental studies of the asymmetric GMI in field-annealed Co-based amorphous ribbons.

The role of exchange coupling on the giant magnetoimpedance of annealed amorphous materials

The characteristics of giant magnetoimpedance (GMI) profiles were measured in annealed amorphous Co66Fe4B15Si15 ribbons in the open air as functions of annealing time, temperature and field. The GMI profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called ‘‘GMI-valve’’ in samples produced with the optimum annealing parameters: time taX8 h, temperature TaE3801C and in the weak-field range, 0: 5O epHap 3O e: The GMI-valve is related to an exchange coupling of a bias-field with the magnetization of the soft amorphous phase, where the bias-field is caused by HCP-Co, FCC-Co and/or Co2Si crystalline phases in a B and Si depleted layer. r 2002 Elsevier Science B.V. All rights reserved.

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.

Separation of reversible domain-wall motion and magnetization rotation components in susceptibility spectra of amorphous magnetic materials

The reversible susceptibility spectra are measured for rectangular Co66Fe4NiB14Si15 samples with various easy-axis angles, α, relative to the sample axis. A phenomenological method is proposed for the reversible spectra to separate the relaxation processes of domain-wall motion and magnetization rotation. The separation provides a method for measuring the static susceptibilities and the relaxation frequencies for the two reversible magnetization processes. The α and the longitudinal stress dependence show that the separated spectra with relaxation frequencies near 360 kHz and 1.6 MHz correspond to relaxations of domain-wall motion and to magnetization rotation, respectively.

Structural and Magnetic Characterizations of Coprecipitated Ni–Zn and Mn–Zn Ferrite Nanoparticles

Two mixed ferrite systems, namely Ni0.65Zn0.35 Fe2O4 (Ni-Zn) and Mn0.75 Zn0.18 Fe2.07 O4 (Mn-Zn) have been prepared by coprecipitation method, and then the resulting ultrafine powders were heat treated at different temperatures from 200 to 800degC for improved crystallinity and magnetic properties. The samples were characterized by X-ray diffraction, vibrating sample magnetometry, and ferromagnetic resonance spectrometry. As a result of the heat treatment, the average particle size has been found to increase from 9.9 to 15.7 nm for Ni-Zn ferrites and from 2.4 to 10.2 nm for Mn-Zn ferrites, and the corresponding magnetization values have increased from 9.1 to 23 emu/g for Ni-Zn ferrites and from 7.9 to 11.7 emu/g for Mn-Zn ferrites, respectively. The results are discussed in the light of changes in particle size and inversion degree parameter for cationic distribution at nanoscales

Effect of surfactants on the size and shape of cobalt nanoparticles synthesized by thermal decomposition

Cobalt nanoparticles with various morphologies were synthesized by thermal decomposition of cobalt acetate in the presence of various surfactants at 260°C. A combination of surfactants consisting of sufficient amount of oleic acid together with polyvinylpyrrolidone and oleylamine resulted in well-dispersed cubic cobalt nanoparticles of ∼25nm in average size. When 1,2-dodecanediol was added as a reducing agent to the surfactant mixture, triangular-prism-shaped nanoparticles of ∼50nm in average size were synthesized. Furthermore, an injection of trioctylphosphine into the reactor as an additional surfactant decreased the particle size to ∼10nm. The XRD pattern of the prism-like particles corresponded to hexagonal close-packed crystalline phase of cobalt.