Hard magnetic materials based on iron nanoparticles

The processing of ferric oxide Fe₂O₃ in a high-energy planetary-type ball mill and its subsequent postannealing create properties characteristic of hard magnetic materials. The additional introduction of iron powder into the mixture increases magnetization, resulting in great coercive force. Computer simulations show that the highly coercive state of the material is due to its nanocrystalline iron particles. Press-compacting and thermomagnetic treatment enhance the magnetic texture in samples.     

Mössbauer and magnetic study of mechanical alloying of Fe3Si

Using Mössbauer spectroscopy and measurement of hysteresis loops and thermomagnetic curves, phase composition and magnetic parameters of Fe₃Si mechanically alloyed powders were studied in dependence on milling time and subsequent heat treatment at a thermomagnetic experiment. Samples of as-prepared powders show high value of coercivity, the saturation magnetization and the content of amorphous Fe₃Si phase raise with increasing time of milling, the content of α-Fe diminishes. Heat treatment of samples with long enough milling time can produce almost perfect Fe₃Si alloy.     

Structure and magnetic properties of iron-and cobalt-based amorphous alloys versus nanocrystallization conditions

The effect of the structural state of Fe₅Co₇₀Si₁₅B₁₀, Fe₆₀Co₂₀Si₅B₁₅, and Co_81.5Mo_9.5Zr₉ amorphous alloys on their magnetic properties is studied under different nanocrystallization conditions. A permanent magnetic field applied during thermomagnetic treatment is found to affect structuring in the amorphous alloys at the initial stage of devitrification. The fine structure of the devitrified amorphous alloys is shown to correlate with the field shifting the hysteresis loop. A mechanism accounting for a hysteresis loop shift in amorphous alloys is discussed.     

Structure and phases of low-neodymium NdFeB permanent magnets

Structures and phase compositions of two low-neodymium magnetically hard materials, differing by the way of preparation — centrifugal atomization and melt spinning — were compared using Mössbauer spectroscopy, X-ray diffraction and measurements of thermomagnetic curves. Better hard magnetic characteristics of the melt-spun material are explained on the basis of the differences in the content of surface and/or interface Fe(Nd,B) phases. Their remarkable presence in the centrifugally atomized material lowers the content of Fe₃B, Fe₂B, α-Fe, and Nd₂Fe₁₄B phases that are responsible for the magnetic quality of the material. There are only subtle differences in the phase compositions of both materials after thermomagnetic measurement, where the α-Fe phase prevails as a product of the thermal decomposition.     

Effect of thermal annealing on the magnetic properties of thin films of Co-Pd alloys

The results of studies of the effect of thermal annealing on the magnetic properties of thin films of Co-Pd alloys are described, along with a method for obtaining easy-axis perpendicular anisotropy in these materials. The method consists of depositing layers of the alloy on a substrate whose thermal expansion coefficient is considerably smaller than that of the film, and then annealing it in a vacuum. This method is used to prepare samples with rectangular hysteresis loops for magnetization perpendicular to the plane of the film, coercive forces of 750 Oe, and Kerr rotation angles of 0.21°. Also presented are the results of experiments on thermomagnetic recording on the samples. Zh. Tekh. Fiz. 68, 66–70 (April 1998)     

Synthesis and properties of Fe-W powder

This paper deals with the study of Fe-W powders, which were prepared by electrolyzing, and their stabilized properties. The synthesized Fe-W powders were studied using X-ray diffraction, thermal analysis, Mössbauer spectroscopy and thermomagnetic measurement. A poor hydrogen absorption occurred. After the heat treatment at 800°C, the amorphous and α-Fe(W) phases were decomposed into prevailing α-Fe with approx. 3 at.% W accompanied by the W(Fe), λ-Fe₂W, and Fe²⁺ in oxide phases.     

Formation of the magnetic domain structure and properties of magnetic cores under conditions of ordered laser deformations

The methods of laser processing and mechanical scribing are used for improving the functional characteristics of transformer magnetic cores from anisotropic Fe–3%Si alloys. The effect is achieved due to controlling the micromagnetic state of elements of the magnetic core by optimizing the type, the dimensional characteristics of their domains, and the magnetic-anisotropy type. These results lead to a substantial decrease in magnetic losses by 9–12% and, with subsequent thermal or thermomagnetic processing, by 14–16% due to decreasing the vortex-current component of magnetic losses.     

Mössbauer spectroscopy of creep-annealed soft magnetic amorphous alloys

To produce magnetic anisotropy in amorphous alloys, stress-annealing above the Curie temperature was substituted for the common thermomagnetic treatment. A tilting three-angle Mössbauer method was applied to see the changes of the⁵⁷Fe hyperfine field directions and intensities due to this procedure. The resulting pictures of the domain orientations distribution in the Fe₈₀Cr₂B₁₄Si₄ and Fe₄₀Ni₄₀B₂₀ amorphous alloys are compared with the measured magnetic anisotropies and domain structures.     

Thermomagnetic and transport properties of metglas 2605 SC and 2605

Thermomagnetic and transport properties of Metglas 2605 (Fe₈₁B_13.5Si_3.5C₂) and 2605 (Fe₈₀B₂₀) have been studied in order to find out the results of partial replacement of one metalloid by others. It is concluded that 2605 SC is more stable than 2605 as an amorphous ferromagnet. The thermomagnetic properties are seriously affected by the replacement of B by Si and C while the transport properties have changed only quantitatively.     

Investigation of nanostructural,thermal and magnetic properties of yttrium iron garnet synthesized by mechanochemical method

This paper focuses on the magnetic, structural and thermal properties of mechanically alloyed Y₂O₃/α-Fe₂O₃ mixed powders and investigates the effects of the mechanical milling and heat treatment on the synthesis of yttrium iron garnet from the primary materials. The morphological and structural studies were carried out by scanning electron microscope and X-ray diffraction, respectively. The thermal activities were measured by differential thermal analysis. The magnetic properties were studied by vibrating sample magnetometer. The results showed that high-energy milling does not lead to the garnet formation and even does not decrease the temperature of the garnet formation. Furthermore, the orthoferrite phase can be achieved slightly during the milling process (up to 96 h) and completely by the heat treatment at lower temperatures (850 °C).     

Structural,magnetic and electrical study of nano-structured α-Fe1.4Ti0.6O3

Nano-structured α-Fe_1.4Ti_0.6O₃ has been synthesized using a simple technique of mechanical alloying. Doping of Ti atoms into α-Fe₂O₃ structure has been characterized by XRD, SEM-EDX, FTIR, VSM and impedance spectroscopy. Magnetic and electrical studies have revealed important changes at the interfaces of grains and grain boundaries, and at rhombohedral planes during formation of α-Fe_1.4Ti_0.6O₃ alloy. The alloy has shown the properties of ferromagnetic semiconductor with substantial increase of magnetic moment and electrical conductivity with the decrease of grain size. The 100 h milled sample has been air annealed to study the effects of thermal activated grain size, lattice structure, magnetism and electrical properties. This work has demonstrated some recent issues of the synthesis of ferromagnetic semiconductor, e.g., structural phase stability, grain size effects, magnetic ordering and electrical conductivity at different stages of the mechanical alloying and subsequent annealing at 700 °C.     

Unusual flow behavior of Fe-based soft magnetic amorphous ribbons under high temperature tensile loading

The viscous thermal flow behavior and mechanical property of [Fe_0.6Co_0.15B_0.2Si_0.05] _(100−x)Ta_x (x = 0, 1, 2, 3, 4, and 5) soft magnetic amorphous ribbons were studied. The characteristics of melt-spun amorphous ribbons were measured by using vibration sample magnetometer (VSM), nanoindentation, differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA) to study the effects of Ta content variation on the thermal stability, mechanical, and soft magnetic properties. We observed that the nanoindentation hardness, Young's modulus, and glass transition and crystallization temperatures were improved by the addition of Ta. Using dilatometry measurement, TMA, by heating at a constant rate under tension mode, we examined not only the glass transition and crystallization behaviors but also the possibility of coexistence of multiple amorphous phases.     

Effect of thermomagnetic and thermomechanical treatments on the magnetic properties and structure of the nanocrystalline soft magnetic alloy Fe81Si6Nb3B9Cu1

The structural and magnetic states of ribbon samples of the soft magnetic alloy Fe-Si-Nb-B-Cu (6 at % Si) have been investigated after the nanocrystallization at a temperature of 550°C in a constant magnetic field (thermomagnetic treatment), in a field of mechanical tensile stresses (thermomechanical treatment), and without external effects. It has been shown that exposure to a constant magnetic field or a field of mechanical tensile stresses gives rise to a longitudinal anisotropy of magnetic properties. The mag- netic hysteresis loop transforms and becomes close to rectangular. This is accompanied by a significant increase in the residual magnetic induction, which approaches the saturation magnetic induction. While the time required to complete the processes of nanocrystallization is as short as 20 min and, under thermome- chanical treatment, the magnetic anisotropy is induced for 20 min, the time it takes to decrease significantly the coercive force of the alloys under thermomagnetic treatment is substantially longer (up to 60 min). After the thermomagnetic treatment, no lattice strains of α-FeSi nanocrystals have been found. Either they do not exist at all, or their values are within the error of the X-ray diffraction experiment. In the samples subjected to annealing under tensile loading, anisotropic lattice strains of nanocrystals with the values increasing pro- portionally to the applied stress have been revealed. The highest strains reaching 1% have been observed after the annealing under a stress of 860 MPa.     

Synthesis of copper-nickel nanoparticles prepared by mechanical milling for use in magnetic hyperthermia

Mechanical alloying of a mixture of copper and nickel powders has been applied for the preparation of copper-nickel alloy particles in the nanometer range. The particles were designed to be used for controlled magnetic hyperthermia applications. The milling conditions were optimized using the desired alloy composition. Utilizing a ball-to-powder mass ratio of 20, we could obtain a nanocrystalline Cu_27.5Ni_72.5 (at%) alloy with a crystallite size of around 10 nm and a Curie temperature of 45 °C.Thermal demagnetization in the vicinity of the Curie temperature of the nanoparticles was determined by thermomagnetic measurements using an adapted TGA-SDTA apparatus. The size and morphology of the particles were determined by XRD measurements and TEM analyses. The magnetic properties were also examined with a VSM. The magnetic heating effects were measured for the powdered material.     

Mechanical properties,lattice thermal conductivity,infrared and Raman spectrum of the fullerite C24

Lightweight carbon materials with excellent thermal and mechanical properties have important applications in aerospace industry. In this study, the stability, mechanical properties, lattice thermal conductivity, electronic structure, infrared and Raman spectrum of sp³ hybridized low-density fullerite C₂₄ were investigated according to density functional theory (DFT) calculations. It was found that the fullerite C₂₄ was both thermodynamic and dynamic stable. Quasi-harmonic approximation and Grüneisen parameter calculations clarified why the fullerite C₂₄ had a positive thermal expansion coefficient at low temperature. The fullerite C₂₄ also exhibited excellent mechanical properties. Interestingly, the Vickers hardness of carbon allotropes was found to almost be linear proportional to the density of a carbon material. HSE06 electronic structure calculations showed that it was a semiconductor with direct bandgap of 2.56 eV. Anharmonic lattice dynamic calculations showed that its thermal conductivity was higher than semiconductor silicon. Besides, Raman and infrared active modes as well as the corresponding spectra were presented.     

一级相变材料Mn1.2Fe0.8P0.4Si0.6的热磁发电性能

本文报道把热能直接转换电能的热磁发电技术所用一级相变新材料Mn_1.2Fe_0.8P_0.4Si_0.6的磁性和热磁发电性能.用高能球磨机械合金化技术和固相烧结合成方法制备了Mn_1.2Fe_0.8P_0.4Si_0.6化合物.磁性测量结果表明,该化合物呈现从铁磁状态变为顺磁状态的一级相变,居里温度为337K,并伴随巨大的磁化强度的变化.根据该材料的这一特性,设计制作了热磁发电演示装置,测定了热流引起材料的相变而产生的电流,并研究了固定磁场中热致磁转变产生的电流随热流温度和样品质量的变化.研究结果表明Mn_1.2Fe_0.8P_0.4Si_0.6化合物具有很好的热磁发电性能,可作为热磁发电材料.     

Preparation for exchange-coupled permanent magnetic composite between α-Fe (soft) and Nd2Fe14B (hard)

Soft magnetic α-Fe nanoparticles were prepared by a coprecipitation route and hard magnetic Nd₁₅Fe₇₇B₈ nanoparticles were prepared by ball milling for 20 h by using a shaker mill. A mechanical ball-mill technique was applied to build up exchange-coupled nanoparticles. A mixture of Nd₂Fe₁₄B and α-Fe nanoparticles in a stainless steel boat was milled for 2 h and annealed in a vacuum furnace under vacuum (∼10⁻⁵ Torr) at 650 °C for 30 min. The crystal structure of the nanoparticles was confirmed by using X-ray powder diffraction (XRD). The surface morphology was identified by FE-SEM. The magnetization curve was measured with a vibrating-sample magnetometer (VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis (TMA) curve showing the downward magnetic force versus temperature.     

Preparation and characterization of ternary magnetic semiconductors

Summary Crystal growth processes, the characterization of properties and the application prospects for ternary magnetic semiconductors (TMS) are reviewed. The regulation of the TMS properties can be achieved either by composition change while growing crystals or by further thermal treatment. The compounds CdCr₂Se₄, CdCr₂S₄, Cu _y Cr₂Se_4−z Br _x have been selected for discussion. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Thermal magnetic hysteresis of nickel-doped pyrrhotites

The magnetic properties of polycrystalline natural pyrrhotite samples with low-nickel contents and synthesized Fe_0.675Ni_0.2S and Fe_0.475Ni_0.4S pyrrhotites are investigated. All of the investigated pyrrhotites are shown to have a thermal magnetic hysteresis in the region of 298–623 K. As a result, there is not only an increase in the difference between the magnetic moment values of the materials after a heating-cooling regime, but also a change in their magnetic state.