Structure and magnetic properties of nanoparticles encapsulated in carbon shells

Ni₂Y and Nd–Fe–Nb–B catalysts were used for the processing of nanoparticles by arc discharge between graphite electrodes. The products were collected from the cathode (deposit and collar) and reactor walls (soot). The ferromagnetic nanoparticles have size in the range of 10–50 nm and are encapsulated in carbon shells. The chemical composition, structure and magnetic properties of the nanoparticles have been studied. For the Ni₂Y catalyst we found that the arc discharge results in decomposition of the intermetallic Ni₂Y phase and formation of Ni nanoparticles encapsulated in carbon shells in the collar and soot, whereas yttrium oxide was found in the deposit. For the Nd–Fe–Nb–B catalysts the magnetic properties depend on the collection place and erosion rate. Fe and Fe–Nd–Nb nanoparticles were found in the soot and deposit, respectively.     

A structural,magnetic and microwave study on mechanically milled Fe-based alloy powders

Fe₉₀M₁₀ powders with M=Fe, Co, Ni, Si, Al, Gd, Dy and Nd were prepared by mechanical milling. Their structure and magnetic properties were investigated. Microwave measurements were performed on the mechanically milled Fe₉₀M₁₀ powders. The results were compared with those of carbonyl Fe powders and coarse Fe powder. It has been shown that fine nanocrystalline Fe-based alloy powders prepared by mechanical milling are promising for microwave applications.     

Nd2Fe14B的价电子结构分析和磁性计算

应用固体与分子经验电子理论计算了Nd₂Fe₁₄B的价电子结构、磁矩和居里温度,计算结果与实验值相符.计算表明:该合金的磁性与3d磁电子数成正比.从Fe(c)晶位到Fe(k₂)晶位磁矩增加,其机理源于价电子、哑对电子和3d磁电子之间的转化,有78%的哑对电子和18%的3d共价电子转化成了磁电子.居里温度和磁矩与Fe原子配位数成正比,与加权等同键数I^σ成反比,Nd原子 关键词： 2Fe₁₄B')" href="#">Nd₂Fe₁₄B 价电子结构 居里温度     

Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.     

Nd composition dependence of microstructure and magnetic properties for gradient sputtered NdFeB films

NdFeB films with Nd compositions varied from 13.34 to 24.30 at% were deposited by DC gradient sputtering using targets Nd_12.5Fe_71.5B₁₆ and Nd. The hard magnetic properties, grain growth direction and magnetic domain structures were dramatically influenced by Nd composition. The samples with intermediate Nd concentrations exhibited optimal magnetic properties and microstructures, such as large squareness ratio over 0.9, large energy product up to 174 kJ/m³, and vertical domain structure. However, the samples with higher and lower Nd compositions showed almost isotropic loops. (0 0 l) as main X-ray diffraction peaks in the optimal Nd composition region indicated most of Nd₂Fe₁₄B grains with c-axis perpendicular to the film plane, while NdFeB grains in other region are almost random growth. The good magnetic properties can be attributed to the vertical growth of Nd₂Fe₁₄B grains.     

Bioinspired fabrication of magneto-optic hierarchical architecture by hydrothermal process from butterfly wing

We developed a green solution to incorporate nano-Fe₃O₄ into the hierarchical architecture of a natural butterfly wing, thus obtaining unique magneto-optic nanocomposites with otherwise unavailable photonic features. Morphological characterization and Fourier Transform Infrared-Raman Spectroscope measurements indicate the assembly of Fe₃O₄ nanocrystallites. The magnetic and optical responses of Fe₃O₄/wing show a coupling effect between the biological structure and magnetic material. The saturation magnetization and coercivity values of the as-prepared magneto-optic architecture varied with change of subtle structure. Such a combination of nano-Fe₃O₄ and natural butterfly wing might create novel magneto-optic properties, and the relevant ideas could inspire the investigation of magneto-optical devices.     

Recent results in magnetic force microscopy

We present domain wall images obtained by using Magnetic Force Microscope (MFM) on magnetic samples like: double layer of permalloy alloy, magnetic hard disk, BaFe₁₂O₁₉ single crystal and YGdTmGa/YSmTmGa magnetic garnet. We have imaged topography and magnetic forces of the same area. The Fe double- and single-layer thin film tips have been prepared to achieve high sensitivity (10^–12N) and high resolution of MFM.     

Effects of applied magnetic field and pressures on the magnetic properties of nanocrystalline CoFe2O4 ferrite

Nanocrystalline CoFe₂O₄ ferrite with crystallite sizes of 30 nm have been successfully prepared by an emulsion method. X-ray diffractometer (XRD) shows that nanocrystalline CoFe₂O₄ ferrite possesses face center cubic structure. Crystal structure of the CoFe₂O₄ nanocrystals will not be changed by the applied magnetic field and pressures. The obtained CoFe₂O₄ nanocrystalline powders were pressed into thin columns with different pressures. Meanwhile, the dependences of the applied pressures and the direction of applied magnetic field on the magnetic properties of the CoFe₂O₄ nanocrystals were investigated in detail using vibrating sample magnetometer (VSM). The pressed CoFe₂O₄ nanocrystal gains the most excellent magnetisms in a parallel applied magnetic field.     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Magnetic properties of Nd–Fe–B system anisotropic HDDR powder made from segregated master ingots

In order to examine the possibility of applying the HDDR process to segregated master ingots, Nd–Fe–B system HDDR powders were made from ingots with different levels of homogeneity, and their structures and magnetic properties were evaluated in detail. HDDR powders made from segregated as-cast ingots displayed anisotropy and large coercivity. They had a nearly homogeneous Nd₂Fe₁₄B phase, although some large areas with α-Fe and Nd-rich regions of 30 μm in size were present after the HD process. With increasing in the homogeneity level of the master ingots, the anisotropy of HDDR-processed powders decreased and their coercivity increased. In addition, an intermediate Ar treatment was applied between the HD and DR processes to improve the magnetic properties. As a result, the effect of the IA treatment was clearly confirmed, and good magnetic properties of B_r=1.23 T, H_cJ=848 kA/m and (BH)_max=238 kJ/m³ were obtained.     

Studies on the magnetic viscosity and the magnetic anisotropy of γ-Fe2O3 powders

2 O₃ powders, acicular γ-Fe₂O₃, and CoFe–γ-Fe₂O₃ powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants K_E of granular γ-Fe₂O₃ powders at different temperatures are obtained by using the law of approach to saturation (LATS). K_E values of acicular γ-Fe₂O₃ and CoFe–γ-Fe₂O₃ powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of K_E with temperature for granular γ-Fe₂O₃ is about the same as that of shape magnetic anisotropy K_sh. Fluctuation field H_f and activation volume V_f of samples are measured. A theoretical expression of V_f is derived. For granular γ-Fe₂O₃ powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe₂O₃ powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe₂O₃ at different temperatures is close to homogeneous rotation. Received: 27 November 1996/Accepted: 18 April 1997     

Effects of precursor types of Fe and Ni components on the properties of NiFe2O4 powders prepared by spray pyrolysis

The effects of the precursor types of Ni and Fe components on the morphology, mean size, and magnetic property of NiFe₂O₄ powders prepared by spray pyrolysis from the spray solution, with citric acid were studied. The precursor powders with hollow and thin wall structure turned to the nano-sized NiFe₂O₄ powders after post-treatment at a temperature of 800 °C. The nickel ferrite powders obtained from the spray solution with ferric chloride had nanometer sizes and narrow size distributions irrespective of the types of nickel precursor. The nickel ferrite powders obtained from the spray solution with ferric nitrate and nickel chloride also had nanometer size and narrow size distribution. The saturation magnetizations of the NiFe₂O₄ powders changed from 37 to 42 emu/g according to the types of the Fe and Ni precursors. The saturation magnetizations of the NiFe₂O₄ powders increased with increasing the Brunauer-Emmett-Teller (BET) surface areas of the powders.     

Improvement of magnetic intergranular isolation and evaluation of read/write characteristics on SmCo5 perpendicular magnetic thin films

SmCo₅ alloy is a promising candidate for ultra-high-density perpendicular magnetic recording (PMR) media because of its high uniaxial magnetocrystalline anisotropy K_u of more than 1.1×10⁸ erg/cm³. Previously, we successfully achieved high K_u in a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, in order to apply the SmCo₅ films to practical PMR media, it is necessary to decrease medium noise. A granulated magnetic film comprising of small and magnetically decoupled grains is effective in reducing the medium noise. In this paper, we have proposed a new granular film that is fabricated by partial thermodiffusion of Cu between the Sm-Co continuous layer and the Cu underlayer, which is granulated using compositional segregation caused by the addition of Ta₂O₅. We have analyzed the magnetic properties, magnetic domain size, and magnetization reversal process of the proposed SmCo₅ film. The magnetic domain size decreased and the magnetization reversal process changed from the magnetic-wall-motion mode to a coherent rotation mode to some extent on isolation of magnetic grains. The read/write characteristics of granulated SmCo₅ double-layered media were also evaluated. The medium noise decreased and the signal-to-noise ratio increased for the granulated double-layered (PMR) medium.     

Effects of additives on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)12 alloys

Effects of such additives as Co, Zr, Nb or Ga on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)₁₂ alloys are investigated. It is found that Zr or Nb addition increases the disproportionation temperature of Nd(Fe,Mo)₁₂ alloys, and Co or Ga addition decreases the recombined temperature of its disproportionated products. This shows that Zr or Nb addition retards the disproportionation, while Co or Ga addition is effective for improving the recombination, which is similar to the effects of the additives on the hydrogen absorption and desorption characteristics of Nd₂Fe₁₄B alloys. However, according to X-ray diffraction (XRD) investigations for the magnetic-oriented samples, the final hydrogenation disproportiontation desorption recombination (HDDR) NdFe_10.5−XM_XMo_1.5 (M=Co, Zr, Nb or Ga) products are isotropic. The effects of additives on hydrogen absorption and desorption characteristics of Nd₂Fe₁₄B and Nd(Fe,Mo)₁₂ alloys are very similar, but the magnetic anisotropy of the final two HDDR products are different. In order to investigate this, similarities and differences of the two alloy systems and their corresponding HDDR phenomena are further studied. The results show that the formation of anisotropic powders may be related to the disproportionated products and crystal growth direction of the Nd₂Fe₁₄B and Nd(Fe,Mo)₁₂ system.     

Combustion synthesis of nickel ferrite powders: Effect of NaClO4 content on their characteristics and magnetic properties

Magnetic powders of nickel ferrite (NiFe₂O₄) were successfully synthesized by combustion synthesis in air using iron (Fe), iron oxide (Fe₂O₃), and nickel oxide (NiO) as reactants and sodium perchlorate (NaClO₄) as fuel (or oxidizing agent). The thermal behaviors were characterized using thermogravimetric analysis (TG) and differential thermal analysis (DSC). The as-combusted and final nickel ferrite powders were characterized in terms of chemical composition and morphology by X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) coupled with energy dispersive X-Ray spectroscopy (EDX). In addition, magnetic properties were examined by vibrating sample magnetometer (VSM). The results of TG/DSC analysis indicated that increasing the content of NaClO₄ increased the exothermicity of combustion reaction. XRD indicated that the final nickel ferrite powders formed a single spinel NiFe₂O₄ phase when the amount of NaClO₄ used was 0.08 or 0.10 mol. SEM revealed roughly octahedron particles with sizes in a sub-micrometer range (∼500 nm). All final products exhibited soft magnetism and, synthesis that included 0.1 mol of NaClO₄ produced pure NiFe₂O₄ powders that had a saturation magnetization (M_s) of 58.93 emu/g, which is higher than the reported value (55 emu/g) for the bulk product.     

Fabrication and laser performance of highly transparent Nd:YAG ceramics from well-dispersed Nd:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowders by freeze-drying

Well-dispersed Nd:Y₂O₃ powders with uniform particle size of about 60 nm were synthesized from freeze-dried precursors. Highly transparent 2 at.% Nd:YAG ceramics were fabricated from the as-synthesized Nd:Y₂O₃ powders and commercial Al₂O₃ powders by vacuum sintering at 1,750 °C for 5 h. Phase evolution, microstructures, and spectroscopic properties of the Nd:YAG transparent ceramics were investigated. Freeze-drying played an important role in the synthesis of high-quality Nd:Y₂O₃ nanosized powders, which were essential for the fabrication of highly transparent Nd:YAG ceramics. Optical transmittance of a 3-mm thick sample reached 82% in the wavelength range of 200–900 nm. 5.23 W output power was obtained with 14.3 W diode laser pumping, giving a slope efficiency of 36.5%.     

Study on ferroelectric polarization induced resistive switching characteristics of neodymium-doped bismuth ferrite thin films for random access memory applications

The present work reports on resistive switching (RS) characteristics of Neodymium (Nd)-doped bismuth ferrite (BFO) layers. The Nd (2–10 at%) doped BFO thin film layers were deposited using a spray pyrolysis method. The structural analysis reveals that a higher Nd doping concentration in BFO leads to significant distortion of the prepared Nd:BFO thin films from rhombohedral to tetragonal characteristics. The morphological analysis shows that all the deposited Nd:BFO thin films have regularly arranged grains. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the prepared Nd:BFO thin films have a higher Fe ³⁺/Fe ²⁺ratio and less oxygen vacancy (V_O) defects which enriches the ferroelectric characteristics in Nd:BFO layers. The polarization-electric field (P-E) and RS characteristics of the fabricated Nd:BFO-based RS device were examined. It was observed that the Nd (7 at%) doped BFO RS device shows large remnant polarization (P _r) of 0.21 μC/cm² and stable RS characteristics.     

Mössbauer study of the magnetic filler for suppositories

Mössbauer spectroscopy methods are discussed when applied to test the properties of magnetic suppositories used in medicine. The experiments were carried out on magnetic rectal suppositories containing parmadine and fine-dispersed ferrite powder (BaO·nFe₂O₃) as a magnetic filler. According to the data on the value of effective magnetic field on⁵⁷Fe nuclei in ferrite magnetic sublattices, the stoichiometricn-number equals approximately 5.5; this value corresponds to the composition range of optimal magnetic properties.     

Reaction mechanisms between Al and Fe3O4 powders in the formation of an Al-based metal matrix composite

The reaction mechanisms between Al and Fe₃O₄ powders were investigated. Differential thermal analysis revealed that a two-step displacement reaction between Al and Fe₃O₄ took place during sintering. Initially, the Fe₃O₄ was converted to amorphous FeO at ～720°C and some of the Al was oxidized to amorphous Al₂O₃. In the final stage, when the temperature reached ～840°C, crystalline Al₂O₃ particles were produced in the molten Al–Fe liquid. The effects of cooling rate on the microstructures were studied. When the Al–Fe liquid was furnace-cooled to room temperature, proeutectic Al₃Fe plates, plate-like divorced eutectic Al₃Fe and Al₂O₃ particles were in situ formed in the Al(Fe) matrix. While quenching from 700°C, nanometer-sized Al dendrites and Al–Al₆Fe eutectic lamellae were produced in the Al matrix. However, when it was rapidly quenched from 900°C, the size of the proeutectic Al₃Fe phases was further reduced and Al₆Fe nanorods were found in the Al–Al₆Fe eutectics. A model was proposed to describe the transformation of the Al–Fe intermetallics during solidification.