High Magneto Research Articles

Exploring the effect of exchange coupling in (SrFe9.8Al2La0.2O19)1-x/(Co0.7Zn0.3Fe2O4)x nanocomposites synthesised by sol–gel auto combustion method

Extensive research has been conducted over the last decade to develop permanent magnets utilizing less rare earth materials. Exchange-coupled nanocomposite magnets are a fascinating development in the field of permanent magnets. They involve the combination of hard and soft ferrites, resulting in a unique and evolving magnet. Here we present an in-depth structural, morphological and magnetic characterization of ferrite-based nanostructures obtained through a bottom-up sol−gel approach. The combination of high coercivity of a hard phase SrFe9.8Al2La0.2O19 (SFALO) and high saturation magnetization of a soft phase, Co0.7Zn0.3Fe2O4 (CZFO), allowed us to develop exchange-coupled bi-magnetic nanocomposites by varying the mass percentage (x = 0, 0.1, 0.2, 0.3, 0.4, 1). Thermogravimetric analysis (TGA) up to 1200 °C was used to investigate the material’s thermal properties with the phase formation temperature. Detailed atomic/nano-scale structural characterizations of these nanocomposites were performed by X-ray diffraction and Transmission Electron Microscopy. The average particle size was calculated from the SEM analysis and it was observed that most of the composites with ratio x = 0.1, 0.3 & 0.4 was 145 nm and x = 0.2 was 165 nm. The Switching Field Distribution (SFD) curve revealed the exchange coupling between soft and hard magnetic particles. The findings indicate that the inclusion of a soft magnetic phase has a considerable impact on the exchange coupling between the hard and soft ferrite phases. As the spinel concentration increased, in the composite the saturation magnetization increased from 18 emu/g to 48 emu/g. From the stroner-wholfrathmodel, the decrease in the remanence ratio below 0.5 concludes the crystallites are randomly oriented. The nanocomposite with the ratio x = 0.1 was observed with the high magneto crystalline anisotropy of 2183.23 × 102 (J/m3) and a high energy product (BH)max of 5.5 kJ/m3. The aforementioned features of nanocomposites demonstrate their potential for use in permanent magnet applications.

Emergence of net magnetization by magnetic field biased diffusion in antiferromagnetic L10 NiMn

NiMn is a collinear antiferromagnet with high magneto crystalline anisotropy ($K_2=-9.7\times10^5\;\text{J m}^{-3}$). Through magnetic annealing of NiMn with excess Ni, strongly pinned magnetic moments emerge due to an imbalance in the distribution of Ni in the antiferromagnetic Mn-sublattices. The results are explained with a model of magnetic-field-biased diffusion, supported by ab initio calculations. Another observation is the oxidation of Mn at the surface, causing an enrichment of Ni in the sub-surface region. This leads to an additional ferromagnetic response appearing in the magnetization measurements, which can be removed by surface polishing.

Enhancement of magnetic properties of Sm-Co nano composite ribbons by heavy lanthanide doping

Sm-Co magnets becoming more and more interesting among research community since 1950′s due to its essential role in space and transport industry to be used in some precious instruments such as, sensors, actuators, motors, and gyroscope. The combination of Rare-earth (RE) and Transition metal (TM) is an ideal combination to make a high-performance magnet, as the RE provides the high magneto crystalline anisotropy (HA) and the TM provides the high magnetization (Ms) and high Curie temperature (Tc). SmCo5 compound possesses Tc as high as ∼ 700 °C. Previous studies have reported that HRE (Gd, Dy, Er and Ho) could be used as a temperature compensation material as unpaired 4f electrons in shell gives rise to maximum magnetic moment. In this study, effect of lanthanides (HREs - Gd and/ Dy) on microstructure and magnetic properties of Sm-Co-based rapidly solidified alloy ribbons have been reported. Phase analyses confirmed the presence of 1:5H phase which was prevalent along with a fraction of 2:17R phase in all the alloy ribbons. Hc value was increased from 10.9 to 13.3 kOe (1.09 to 13.3 T) and Br value was increased from 12.5 to 16.2 kG (1.25 to 1.62 T) with Dysprosium addition.

Feasibility Study of High-Efficiency Cooling of High-Temperature Superconducting Coils by Magnetic Refrigeration

As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration.

Effects of functionalization of carbon nanotubes on its spin transport properties

Spin-transport properties of in-plane fluorinated zigzag (8, 0) single-walled carbon nanotubes contacted by two CrO2 Half-Metallic-Ferromagnetic (HMF) electrodes are investigated using density-functional theory. Comparison of the results with pristine carbon nanotube show that fluorinated carbon nanotube (CNT) offers high TMR ∼100% and the transport phenomenon is tunneling, since there are no transmission states near Fermi level. However, for pristine CNT structure the transport phenomenon cannot be tunneling since there are significant number of transmission states near Fermi level, although high Magneto Resistance (MR) ∼97% is observed in it. Both TMR and Spin Injection Efficiency η (Spin-Filtration) are higher in fluorinated (8,0) carbon nanotube (CNT) structure, which is due to fluorinated carbon nanotube (CNT) acting as a perfect barrier in vertical (out-of-plane) arrangement resulting in negligible spin-down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

Spin transport in carbon nanotubes bundles: An ab-initio study

First principles investigations are performed on understanding the spin-polarized transport in carbon nanotubes and carbon nanotube bundles consisting of (8,0) and (17,0) SWCNTs kept in vertical (out-of-plane) arrangement and contacted by two CrO2 Half-Metallic-Ferromagnetic (HMF) electrodes. On comparison of the results for all the structures, it is observed that carbon nanotube bundle consisting of (17,0) CNT offers high TMR ∼100% and the transport phenomenon is tunneling, since there are no transmission states near Fermi level. However, in individual (8,0) and (17,0) CNT the transport is not because of tunneling, since there are significant number of transmission states near Fermi level. High Magneto Resistance (MR) 96% and 99% is observed in individual (8,0) and (17,0) CNTs respectively. Both TMR and Spin Injection Efficiency η (Spin-Filtration) are higher in (17,0) carbon nanotube bundle structure, which is due to carbon nanotube bundle acting as a perfect barrier in vertical (out-of-plane) arrangement resulting in negligible spin-down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

Magnetocrystalline properties of Iron-Platinum (L10-FePt) nanoparticles through phase transition

High magneto- crystalline anisotropy (ku=7×106j/m3) of L10-FePt nanoparticles are an excellent candidate for ultra high-density magnetic recording. The 4 nm FePt nanocrystals were prepared by superhydride reduction of FeCl2·4H2O and Pt(acac)2 precursors in the phenyl ether by reduction of the 1, 2-hexadecanediol and LiBEt3H superhydride. The crystal and magnetic structures were studied by XRD and VSM analysis. By TEM and EDS analyses the size distribution and molar concentration of Fe/Pt of the nanoparticles were determined. The results showed that the particles are first superparamagnetic before heat treatment and then a phase transition accrue from disorder fcc to order fct structure after annealing. Finally, the phase transition leads the magnetic anisotropy of hard FePt nanoparticles to increase to 7 kOe.

Martensite Transformation and Magnetic Properties of Ni-Fe-Ga Heusler Alloys

Compositional instability and phase formation in Ni-Fe-Ga Heusler alloys are investigated. The alloys are synthesized into two-phase microstructure. Their structures are identified as fcc and L21, respectively. The γ-phase formation could be suppressed with higher Ga-content in the alloy as Ga stabilizes austenite phase, but Ga lowers the martensite transformation temperature. The increase of Fe content improves the magnetization value and the increase of Ni from 52 to 55 at. pct raises the martensite transformation temperature from 216 K to 357 K (−57 °C to 84 °C). Magnetic properties and martensitic transformation behavior in Ni-Fe-Ga Heusler alloys follow opposite trends, while Ni replaces either Fe or Ga, whereas they follow similar trends, while Fe replaces Ga. Modulated martensite structure has low twinning stress and high magneto crystalline anisotropic properties. Thus, the observation of 10- and 14 M-modulated martensite structures in the studied Ni-Fe-Ga Heusler alloys is beneficial for shape memory applications. The interdependency of alloy composition, phase formation, magnetic properties, and martensite transformation are discussed.

Spallation reactions in shock waves at supernova explosions and related problems

The isotopic anomalies of some extinct radionuclides testify to the outburst of a nearby supernova just before the collapse of the protosolar nebula, and to the fact that the supernova was Sn Ia, i.e. the carbon-detonation supernova. A key role of spallation reactions in the formation of isotopic anomalies in the primordial matter of the Solar System is revealed. It is conditioned by the diffusive acceleration of particles in the explosive shock waves, which leads to the amplification of rigidity of the energy spectrum of particles and its enrichment with heavier ions. The quantitative calculations of such isotopic anomalies of many elements are presented. It is well-grounded that the anomalous Xe-HL in meteoritic nanodiamonds was formed simultaneously with nanodiamonds themselves during the shock wave propagation at the Sn Ia explosion. The possible effects of shock wave fractionation of noble gases in the atmosphere of planets are considered. The origin of light elements Li, Be and B in spallation reactions, predicted by Fowler in the middle of the last century, is argued. All the investigated isotopic anomalies give the evidence for the extremely high magneto- hydrodynamics (MHD) conditions at the initial stage of free expansion of the explosive shock wave from Sn Ia, which can be essential in solution of the problem of origin of cosmic rays. The specific iron-enriched matter of Sn Ia and its MHD-separation in turbulent processes must be taking into account in the models of origin of the Solar System.

Studies on Structural and Magnetic Properties in Co/Sm Multilayers

Co/Sm multilayer films with structure of 20 [Co (x nm)/Sm (1.2 nm)] where x = 1.1, 2.2 and 4.2 nm and 20[Co (4.2nm)/Sm (x nm)] where x=1.2 nm to 7.5 nm were fabricated using dc magnetron sputtering. Each multilayer filmconsisted of 20 bilayers of Co layers with various thicknesses sandwiched with Sm layers. The application of lowangle X-ray diffraction measurements to the characterization of these multilayers is described. The periodiclayered structure with sharp interfaces was observed for all multilayer films. The measured magnetization valuesare lower than the values calculated in terms of the nominal concentration of cobalt in the multilayers. This impliessignificant “mixing” at small film thickness. The formation of a high magneto crystalline anisotropy of CoSm alloyat the interfaces, as a result of interdiffusion between Co and Sm layers was considered to be responsible for theincrease of the coercivity for Co/Sm multilayer.

Enhanced magnetoresistance ratio and improved thermal stability of specular spin valve multilayer with capping layer

Si/Ta/Ni–Fe/Mn–Ir/Co–Fe/NOL/Co–Fe/Cu/Co–Fe/Cu/capping bottom specular spin valves were prepared by using the DC magnetron sputtering method. We studied the effect of capping layers on the magnetic and thermal properties of specular spin valves. We can obtain a high magneto resistance ratio, high exchange bias field, and low interlayer coupling field by using Ta and Al capping layers. The thermal aging experiment showed that the Ta capping layer was more stable than the Al capping layer. We can obtain low interlayer coupling filed about 1 kA/m by using Al 2O 3 oxide capping layer.

高密度磁気記録媒体用ＮｄＦｅＢ／Ｗ薄膜のＢ組成依存性

NdFeB thin films with perpendicular c-axis orientation texture have been studied as promising candidates for ultrahigh-density recording media because of their high magneto crystalline anisotropy and large saturation magnetization. Films with composition of Nd18Fe76B6 and a thickness of 15 nm exhibited perpendicular coercivity as high as 6.0 kOe. The grain size for this film is about 70 nm, as evaluated by SEM surface images. It was found that an increase in B percentage can decrease the grain size to 20 nm with B content up to 15 at.%. The perpendicular coercivity, nucleation field, and grain size for NdFeB films with 12.3 at.% of B and a thickness of 15 nm are 5 kOe, -2.5 kOe, and about 40 nm, respectively.

ＡｒとＫｒ混合ガススパッタによるＳｒフェライト薄膜の作製

Magnetoplumbite type Sr-ferrite (SrM) thin films with high magneto crystalline anisotropy were deposited on Pt underlayers. It was found that Pt underlayer is effective to enhance crystallization and the c-axis orientation perpendicular to the film plane. In order to get high crystallization and high coercivity and large squareness ratio, high substrate temperature (> 500 °C) and large thickness of magnetic layer (> 50 nm) were required. In this study, SrM/Pt bilayers were prepared using mixture gasses of Ar and Kr as sputtering gas. Ar and Kr mixture gasses was effective to deposit SrM films with high c-axis orientation and better crystallization even though their thickness below 20 nm. SrM layer sputter-deposited using Ar and Kr mixture gas revealed high perpendicular coercivity Hc⊥ around 3 kOe. SrM layer were able to be dethickness below 20 nm, and crystallized at substrate temperature 450 °C.

Nonlinear magneto-optical imaging of interface magnetic structures

We report the development of a new magnetic imaging technique: nonlinear magneto-optical microscopy. Magnetization-induced optical second harmonic generation (MSHG) is used to image the domain structure in ultrathin Pt/CoNi/Pt and Au/Co/Au/Fe/GaAs films. Due to both very high magneto optical contrast and enhanced interface sensitivity, fine details of magnetization several on film interfaces become visible with the MSHG imaging that are not detectable by usual magneto-optics.

Ferrimagnetic Cobalt Ferrite Films Prepared by the Sol-Gel Process

ABSTRACTFerrimagnetic oxide films have been shown to have potential for use as Magneto-optical information storage Materials. Cobalt ferrite films are particularly interesting for magneto optical information storage due to their high magneto optical rotation [1]. In this work, synthesized soluble cobalt (II) and iron (III) Methoxyethoxides were mixed in stoichiometric ratios for use as Co and Fe precursors in the preparation of CoFe2O4ferrimagnetic films. The decomposition of the precursors was characterized by thermogravimetric analysis.CoFe2O4 films were prepared by the dip coating technique using fused silica substrates. These films were then heat treated at temperatures ranging from 200°C to 600°C to study the transformation from an amorphous film to a crystalline film as determined by x-ray diffraction. The Magnetic hysteresis behavior of the films as a function of heat treating temperature and hence crystallinity was also studied. As a general trend, films having a greater degree of crystallinity exhibited larger values of saturation magnetization and remanent Magnetization. The amorphous film was found to exhibit the highest coercive field, but low values of saturation and remanent Magnetization. The effect of heat treating under the influence of a magnetic field of 1.88 kÖe was found to enhance crystallization only slightly and had very little effect on the magnetic properties of the film.

Hydrogen diffusion in metallic glasses studied by a magnetic method

A new method to measure H diffusion in high magneto strictive materials is reported. The behaviour of a characteristic magnetic parameter (i.e. the initial magnetic susceptibility) is continously monitored during the electrolytic H charge process of the sample. The H diffusion produces internal local stresses in the sample, which changes the magneto-elastic energy, altering the magnetic properties of the sample. The method is applied to investigate H diffusion in the metallic glass Fe 78B 13Si 9.

Recent developments in half-metallic magnetism

The compound PtMnSb is a half-metallic ferromagnet and its unusual band structure is intimately connected with the high Magneto Optical Kerr Effect (MOKE). In this paper it will be discussed how various types of substitutions can modify this band structure and lead to possible enhancement of the MOKE transitions. Novel half-metallic systems will be presented comprising half-metallic ferrimagnets and half-metallic antiferromagnets.