Yukiko Yasukawa

Switching of magnetic easy-axis using crystal orientation for large perpendicular coercivity in CoFe2O4 thin film

Perpendicular magnetization and precise control over the magnetic easy axis in magnetic thin film is necessary for a variety of applications, particularly in magnetic recording media. A strong (111) orientation is successfully achieved in the CoFe2O4 (CFO) thin film at relatively low substrate temperature of 100 °C, whereas the (311)-preferred randomly oriented CFO is prepared at room temperature by the DC magnetron sputtering technique. The oxygen-deficient porous CFO film after post-annealing gives rise to compressive strain perpendicular to the film surface, which induces large perpendicular coercivity. We observe the coercivity of 11.3 kOe in the 40-nm CFO thin film, which is the highest perpendicular coercivity ever achieved on an amorphous SiO2/Si substrate. The present approach can guide the systematic tuning of the magnetic easy axis and coercivity in the desired direction with respect to crystal orientation in the nanoscale regime. Importantly, this can be achieved on virtually any type of substrate.

Bi2O3 liquid phase assisted and Mn substituted permeability and magnetic properties of Ni-Cu-Zn ferrite for multilayer chip inductor application

In the present work, low-temperature fired MnxNi0.5−xCu0.25Zn0.25Fe2O4 + Bi2O3 (x = 0.0 to 0.45) was synthesized by the sol-gel auto combustion technique at a temperature of 1073 K. Bi2O3 (with 2 wt. %) was used as a sintering aid for improving the density, microstructure and lowering the sintering temperature of Mn-Ni-Cu-Zn ferrites. Synthesized samples were investigated for their structural, magnetic, and electrical properties such as density, porosity, saturation magnetization, and permeability as a function of Mn2+ substitution. Permeability and saturation magnetization were found to increase with the replacement of Ni2+ with Mn2+. Bulk density of 4.5 g/cm3 was observed for Mn-Ni-Cu-Zn ferrite with Bi2O3 additives which is greater than that of pure Mn-Ni-Cu-Zn ferrite (4.1 g/cm3). Saturation magnetization of NiCuZn ferrite increased from 37 to 63 emu/g with increase in Mn2+ substitution. Mn substituted NiCuZn ferrite shows higher complex permeability (μ′ = 672) than that of the pure NiCuZn ferrite (μ′...

Variations in the saturation magnetization of nanosized NiFe2O4 particles on adsorption of carboxylic acids

Abstract This work investigated magnetization changes in NiFe2O4 nanoparticles induced by the adsorption of a series of carboxylic acids. The application of formic acid resulted in a significant 8.6% decrease in the magnetization of NiFe2O4 nanoparticles at 18,000 Oe. With increasing carbon bond number in the saturated carboxylic acids, reductions in the magnetization of NiFe2O4 nanoparticles became around 4%. All unsaturated carboxylic acids produced approximately equivalent reductions in the magnetization, regardless of their double bond content. Based on these results, the observed NiFe2O4 magnetization changes appear to depend on either the polarity or the molecular size of the carboxylic acids and are believed to be caused by canting or pinning of spins in the vicinity of particle surfaces following adsorption of the acids.

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The role of Bi₂O₃ addition on the structural and magnetic properties of NiCuZn-Tb ferrites with composition Ni_0.5Cu_0.25Zn_0.25Tb_0.1Fe_1.9O₄+xBi₂O₃ has been investigated, where values of x (in wt %) are 0, 1, 2 and 3. Samples were prepared by the sol-gel auto combustion synthesis. The crystallographic, structural and magnetic properties of the samples were investigated by X-ray diffraction (XRD), scanning electron micrography, vibrating sample magnetometer and permeability measurements. Cubic spinel structure was observed from the XRD pattern for all the samples, implying that the Bi₂O₃ additives did not form a visible second phase in ≤ 2 wt% of Bi₂O₃. Microstructural studies showed that Bi₂O₃ forms a liquid-like phase and promotes the grain growth. The average grain size increased for ≤ 2 and then decreased for > 2 wt% of Bi₂O₃ content. The maximal grain size appears with 2 wt% Bi₂O₃. These characteristics evidently affect the magnetic properties, such as saturation magnetization (Ms) and permeability of the NiCuZn-Tb ferrites. Bi₂O₃ increase the Ms, bulk density (d_B) and permeability effectively, while excessive Bi₂O₃ concentration results in the abnormal grain growth and decrease of Ms and d_B.

{\hbox{Bi}}_{2}{\hbox{O}}_{3}

A series of Li0.5MnxCr0.5Fe2-xO4 (x = 0.0-1 in the steps of 0.25) were synthesized by sol-gel auto combustion method, aiming to study the structural, magnetic and dielectric properties of ordered and disordered Li-ferrite related to the concentration of the substituted Mn3+ ions. X-ray diffraction measurements were used for analyzing the structural parameters of the prepared samples. Superstructure lines are observed for Li0.5Cr0.5Fe2O4 and disappear at higher levels of Mn3+ substitution. TEM images demonstrate that the particle size is uniform and are in good crystalline form. Saturation magnetization decreased from 51 to 24 emu/g, whereas coercivity varies from 90-170 Oe with Mn3+ substitution. Permeability measurements were carried out as a function of temperature. Initial permeability (μi)decreased with the increasing Mn3+ substitution. The dc resistivity increased with the Mn3+ substitution. Dielectric constant decreased with the increasing frequency and it also decreased with the increasing Mn3+ substitution.

Additives on the Microstructure Development and Magnetic Properties of NiCuZn-Tb Ferrites

The magneto-optical (MO) properties of perpendicular magnetic nanostructures consisting of CoPt layers and isolated fine grains of Ag or Au were investigated. The nanostructures exhibited a polarization reversal on Kerr rotation, and the Kerr loop depended on the external environment. These MO behaviors are attributed to localized plasmon resonance.

Structural and Magnetic Properties of Mn

Ordered, two-dimensional, self-organized Au nanoparticles were fabricated using radiofrequency (RF) magnetron sputtering. The particles were uniformly spherical in shape and ultrafine in size (3-7 nm) and showed an ultrahigh density in the order of ∼10(12) inch(-2). A custom-developed sputtering apparatus that employs low sputtering power density and a minimized sputtering time (1 min) was used to markedly simplify the preparation conditions for Au nanoparticle fabrication. The spatial distribution of Au nanoparticles was rigorously controlled by placing a Ta interfacial layer between the Au nanoparticles and substrate as well as by post-annealing samples in an Ar atmosphere after the formation of Au nanoparticles. The interfacial layer and the post-annealing step caused approximately 40% of the Au nanoparticles on the substrate surface to orient in the (111) direction. This method was shown to produce ultrafine Au nanoparticles showing an ultrahigh surface density. The crystal orientation of the nanoparticles can be precisely controlled with respect to the substrate surface. Therefore, this technique promises to deliver tunable nanostructures for applications in the field of high-performance electronic devices.