Nitrided and carbided magnets

In this article, we review our recent studies on new rare-earth intermetallic compounds including the Ga, Si substituted 2:17-type compounds, their nitrides, carbides, and the Sm₃ (Fe,Ti)₂₉N₅ compounds. Much of our recent work is focused on the Sm₂(Fe,Ga)₁₇C_x alloys where we used melt spinning and subsequent annealing to obtain high coercivity. The highest coercivity obtained so far was in Sm₂Fe₁₄Ga₃C_2.5 with a value of 12.8 kOe at room temperature. The off-stoichiometric Sm₂Fe_14-xCo_xSi₂N_y nitrides maintain the Th₂Zn₁₇-type structure but with a unit-cell expansion ΔVV up to 5 % compared to the host materials. The Sm₂Fe_14-xCo_xSi₂C_z carbides maintain the Tr₂Zn₁₇-type structure when z = 1 and transform to the BaCd₁₁-type structure when z = 2. A very large anisotropy field with an applied magnetic field (H_a) value of 227 kOe for Sm₂Fe₁₄Si₂N_2.6 and 276 kOe for Sm₂Fe₁₀Co₄Si₂N_2.3 is observed at low temperature (1.5 K). The Sm₃(Fe,Ti)₂₉N₅ compound and its nitrides show very interesting magnetic properties. Both of these compounds exhibit ferromagnetic ordering with Curie temperature (T_c) of 486 and 750 K, respectively. The room temperature saturation magnetization is 119 emu/g for the parent compounds and 145 emu/g for the nitrides. The easy magnetization direction changes from planar to uniaxial upon nitrogenation. The anisotropy field for the nitrides is 12 T at room temperature and 25 T at 4.2 K.     

Magnetic properties of hard magnetic nanoparticles of Nd2Fe14B synthesized using self-assembled block copolymers

Nd₂Fe₁₄B nanoparticles are widely used in bonded magnets for actuators in many types of electric equipment, motors in mobile phones and hard disk drives. Magnetic nanoparticles are also used in magnetic storage devices, drug delivery systems, and ferrofluids. Improvement of the magnetic properties of Nd₂Fe₁₄B rare-earth magnets would allow for production of small, lightweight, high-torque motors for use in energy-saving household appliances. However, because of the chemical instability of Nd₂Fe₁₄B nanoparticles, it is challenging to synthesize them with high purity by chemical synthesis or by conventional metallurgy methods using Nd₂Fe₁₄B alloys. In addition, the high reduction potential of Nd³⁺ makes reduction difficult. In this study, block copolymers were used as templates for chemical synthesis of Nd₂Fe₁₄B nanoparticles. In the synthesis, tris(acetylacetonato) iron(III) as an iron precursor, neodymium tris(acetylacetonate) hydrate as a neodymium precursor, and 1,1-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ferrocene as a boron precursor were introduced into a polystyrene-b-poly(2-vinylpyridine) block copolymer template. Then, the block copolymer was removed by oxidation. After reduction with CaH₂ and washing with water to remove residual Ca species, highly pure Nd₂Fe₁₄B nanoparticles were obtained. Under the optimized experimental conditions, nanoparticles with a coercivity of 3.5 kOe and a saturation magnetization of 158 emu/g were obtained. The saturation magnetization was 93.5% of the theoretical value of Nd₂Fe₁₄B (169 emu/g).     

Synthesis and magnetic properties of nickel nanoparticles deposited on the silicon nanowires

Nickel (Ni) nanoparticles with sizes of ∼35 nm were deposited on the surface of silicon nanowires (SiNWs) by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking temperature (T_B) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC) plots. The M-H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value was ∼4.5 emu/g and the coercivity was ∼375.3 Oe for the loop at 5 K, respectively. While for the loop at 400 K, these values were of ∼2.6 emu/g and ∼33.3 Oe, respectively. The temperature dependence of coercivity followed by the relation H_C(T) = H_C0[1 − (T/T_B)^1/2], indicating a superparamagnetic behavior. The magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin function at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic defects on the surface of SiNWs.     

Facile synthesis and electromagnetic wave absorption properties of magnetic carbon fiber coated with Fe-Co alloy by electroplating

Magnetic carbon fiber coated with Fe-Co alloy was prepared by electroplating at 25 °C for 5 min. The obtained magnetic coatings show sheet-like morphology and the crystal structure of the uniform coating is Co₃Fe₇ with a thickness of about 0.5 μm. The saturation magnetization of the magnetic carbon fiber reaches 31.5 emu/g with a coercivity of 87.1 Oe. The complex permittivity and permeability of magnetic carbon fiber/paraffin (30 wt%) composite were measured in the 2-18 GHz frequency range. The reflection loss below −10 dB covers the whole frequency range while below −20 dB the absorption frequency bandwidth is 6.8 GHz, and the minimum value is −48.2 dB at a coating thickness of 1.7 mm. Magnetic carbon fiber exhibits excellent electromagnetic wave absorption properties.     

Fe3O4@SiO2磁性复合微球的制备与表征

采用化学共沉淀法制备Fe_3O_4颗粒,在其制备过程中控制Fe_3O_4核的长大时间,加入油酸钠作为表面修饰剂来控制Fe_3O_4核的尺寸,然后加入正硅酸乙酯(TEOS)生成纳米级Fe_3O_4@SiO_2复合纳米粒子和亚微米级Fe_3O_4@SiO_2复合微球。通过X射线衍射、透射电镜、能谱分析和红外光谱分析证实Fe_3O_4颗粒表面包覆有SiO_2,并研究了复合粒子的形貌和成分组成,然后进行了磁性能分析。结果表明,Fe_3O_4纳米颗粒、Fe_3O_4@SiO_2复合纳米粒子和亚微米级Fe_3O_4@SiO_2复合微球的饱和磁化强度分别为79.95、34.85和61.51 A·m2/kg,对应的剩磁分别为1.73、1.05和3.07 A·m2/kg,矫顽力分别为1083、755和2002 A/m,亚微米级复合微球的剩磁和矫顽力都显著增大。     

Effect of sintering temperature on the structure and magnetic properties of SmCo5/Fe nanocomposite magnets prepared by spark plasma sintering

Highly dense SmCo₅/Fe nanocomposite bulk magnets were prepared by spark plasma sintering of magnetic field-milled SmCo₅/Fe nanocrsytalline powders. The sintering experiments were conducted with varying temperatures of 973–1123 K. The resultant bulk materials had densities of 85–98% and mean grain sizes of 17–30 nm. The SEM analysis showed that the bulk samples prepared at higher sintering temperature exhibited dense and uniform microstructure. The XRD studies in complement with energy dispersive X-ray analysis revealed that the bulk magnets sintered at or above 1073 K exhibited Sm(Co,Fe)₅ as main phase, along with other secondary phases such as Sm₂(Co,Fe)₁₇ and α-Fe(Co). A single-phase behavior with high remanence ratios (0.67–0.77) for the nanocomposite magnets was demonstrated by the magnetic measurements. In the present study, the sintering temperature of 1073 K was found to be optimum in achieving relatively high coercivity (8.2 kOe), magnetization (97.5 emu/g) and energy product (278.7 kJ/m³) for the SmCo₅/Fe nanocomposite bulk magnets.     

Perpendicular magnetic anisotropy in 70 nm CoFe2O4 thin films fabricated on SiO2/Si(1 0 0) by the sol-gel method

Cobalt ferrite CoFe₂O₄ films were fabricated on SiO₂/Si(1 0 0) by the sol-gel method. Films crystallized at/above 600 °C are stoichiometric as expected. With increase of the annealing temperature from 600 °C to 750 °C, the columnar grain size of CoFe₂O₄ film increases from 13 nm to 50 nm, resulting in surface roughness increasing from 0.46 nm to 2.55 nm. Magnetic hysteresis loops in both in-plane and out-of-plane directions, at different annealing temperatures, indicate that the films annealed at 750 °C exhibit obvious perpendicular magnetic anisotropy. Simultaneously, with the annealing temperature increasing from 600 °C to 750 °C, the out of plane coercivity increases from 1 kOe to 2.4 kOe and the corresponding saturation magnetization increases from 200 emu/cm³ to 283 emu/cm³. In addition, all crystallized films exhibit cluster-like structured magnetic domains.     

One-step synthesis of MFe2O4 (M = Fe, Co) hollow spheres by template-free solvothermal method

Monodispersed magnetic MFe₂O₄ (M = Fe, Co) hollow spheres were synthesized by simple template free solvothermal method in ethylene glycol (EG) solution. The hollow spheres were in the same size with an average diameter of about 360 nm and the shells of these spheres were about 80 nm, consisted of closely packed nanocrystallines due to Ostwald ripening. EG plays the key role in the synthesis of hollow spheres in contrast with octahedral crystals synthesized in aqueous solution. The products synthesized in aqueous solution were calcined at 800 °C and 1000 °C. The amount of spinel ferrite products increased monotonically with the increase of temperature and appeared as a single phase at 1000 °C. The saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) for Fe₃O₄ hollow spheres was 74.47 emu/g, 2.59 emu/g and 32.503 Oe respectively whereas the reading of the same indicators for CoFe₂O₄ hollow spheres was 69.07 emu/g, 14.46 emu/g and 242.79 Oe, respectively. The magnetic variation between Fe₃O₄ and CoFe₂O₄ hollow spheres was caused by the radius difference of Fe²⁺ (3d⁶) and Co²⁺ (3d⁷) ions and it was also relevant with nanocrystal sizes of the spin disorder of crystal surface.     

Magnetic Properties and Nanocrystalline Phases in Sn Containing SmCo5 Alloys

SmCo₅ alloys with Sn additions (0.2-2.0 at.%) were prepared by mechanical milling of arc-melted samples. The nano-phase structures and magnetic properties of as-milled powders were investigated. The Sn additions resulted in development of nanocrystalline structures producing exchange-coupled magnets with better remanence magnetization to maximum magnetization ratios (M _r/M _max), typically 0.92 at 9.9 kOe coercivity. In addition, it was observed that the Sn concentrations lead to higher M _r/M _max ratios and maximum magnetization accompanying lower coercivity. X-ray diffraction revealed formation of 2:17 and 2:7 phases in 1:5 matrix, which were found to be dependent on Sn percentage. It appeared that higher Sn concentrations promoted 2:17 phase and helped in the formation of nano-sized phases.     

Structural and magnetic properties of the gas atomized Mn-Al alloy powders

Ferromagnetic Mn-Al powders were produced by a gas-atomization method followed by heating treatment. The gas-atomized powders were ?-phase, which is a high temperature phase in the Mn-Al system. The ?-τ phase transformation took place by subsequent heat treatment at temperatures from 500 to 700 °C. The gas-atomized powders with a smaller particle size formed the τ-phase faster and thus exhibited better magnetic properties. On the other hand, the annealing temperature and time similarly played important roles in determining the magnetic properties of the products. The Mn-Al powders of 25–38 μm annealed at 550 °C for 120 min exhibited a high coercivity of 3.2 kOe with a remanence of 38 emu/g.     

退火温度对钴铁氧体薄膜结构和性能的影响

采用溶胶-凝胶法结合匀胶旋涂工艺在复合基片(Pt/Ti/SiO2/Si)上制备了钴铁氧体(CoFe2O4)薄膜,利用XRD、SEM、VSM分析了薄膜的微结构以及磁性能,研究了不同退火温度对钴铁氧体薄膜的结构和磁性能的影响.结果表明,钴铁氧体在500℃时开始形成尖晶石相.随着退火温度的增高,钴铁氧体晶粒逐渐长大,饱和磁化...     

Magnetic properties of thermal plasma synthesized nanocrystalline nickel ferrite (NiFe2O4)

A rapid synthesis method is reported for magnetic nanoparticles of nickel ferrite involving thermal plasma assisted vapor phase condensation process. The as-synthesized samples were characterized by X-ray Diffraction, Transmission Electron Microscopy, Vibrating Sample Magnetometer and X-ray Photoelectron Spectroscopy techniques. The average particle size was determined from the TEM micrographs and found to be around 30 nm. The effects of reactor parameters on the magnetic and structural properties have been evaluated, to find the optimized parameters so as to achieve the highest values of saturation magnetization and coercivity. Reasonably high saturation magnetization (48 emu/g) has been assigned to the high degree of crystallinity, achieved on account of high temperature during the growth, and the cation redistribution. The high value of coercivity (115 Oe) is explained on the basis of possible lattice defects arising from the cation redistribution. Detailed analysis of cation distribution using the XRD line intensity data leads to the conclusion that these samples are iron deficit and nickel rich.     

Mn掺杂对快淬NdFeB永磁材料晶格与磁性能的影响

应用DTA、XRD、VSM，EXAFS对快淬Nd9Fe85-xMnxB6(x=0,0.5,1)纳米复合材料磁性能进行了研究。发现少量Mn的掺杂能够显著促进快淬样品的晶化并提高快淬样品的永磁性能，在合适的热处理条件下，得到的最佳矫顽力和剩磁比分别从339．6kA/m(4266.5Oe)和0．70提高到398．2kA/m(5002.5Oe)和0．72，最大磁能积（BH）max从84kJ/m^3(10.5MGOe)提高到88kJ/m^3(11MGOe)。认为永磁性能的提高是由于Mn的掺杂使快淬NdFeB具有更有序的晶体结构。     

Effect of annealing on the structural properties of AlN thin films containing Co particles

This paper describes a new approach for preparing AlN thin films containing various Co contents by using a two-facing targets type sputtering system. The as-deposited films exhibited a variable nature expected from the AlN-rich phase, as well as an amorphous-like phase, depending on the Co content in the films. The films were annealed isothermally at different temperatures and their microstructure, magnetic properties and resistivity were examined. The saturation magnetization of the as-deposited films was quite small and almost constant, irrespective of the Co content in the films, because Co was not in the crystalline state. At elevated annealing temperatures, the as-deposited AlN-Co amorphous films crystallized into two phases of AlN and Co. The saturation magnetization and resistivity of the films increased with increasing annealing time and temperature. The coercivity of the films was independent of the annealing time, but it increased with increasing annealing temperature due to the increase in grain size. A saturation magnetization, coercivity and resistivity of 360 emu/cm³, ～25 Oe and 2200 μΩ-cm, respectively, were obtained. Further improvement in the soft magnetic properties might lead to this material being applied as a high density magnetic recording head material.     

退火温度对非晶合金Fe69Al5Ga2P9.65B4.6Si3C6.75磁性能的影响

通过7种不同的温度对非晶合金Fe69 Al5 Ga2 P9.6s B4.6Si3 C6.75进行了退火处理,以研究退火温度对合金磁性能的影响.试验磁性能数据表明,合金的饱和磁化强度随退火温度的升高先增加后急剧减小,在450℃时的值最大,M.为628.76emu/g;而矫顽力曲线和剩磁比曲线的走势相近,且在490℃附近的值较小,其软磁性能较好.试验证明,合金的最大饱和磁化强度的退火温度与软磁性能最佳的退火温度不是同一温度值,其综合磁性能的退火温度应选择450～490℃之间的值.     

High permeability nano-crystalline FeSiBTaAg ribbons obtained by direct casting

Nano-crystalline soft magnetic ribbons, being extensively used as magnetic cores for switching power supplies, have been invariantly obtained by melt-spinning followed by post-annealing. Reported herewith are the attainment, by direct-casting without annealing, of nano-crystalline Fe_77.4−xSi_15.5B₇Ta_xAg_0.1 (x = 1, 2) ribbons with superior soft magnetic properties (named TAGMET after the addition of Ta and Ag). The grain size of nano-crystalline -FeSi, from 20 to 30 nm, varies with composition and quenching speeds. As-spun Fe_75.4Si_15.5B₇Ta₂Ag_0.1 ribbons consisting of 25 nm nano-crystals exhibit a saturation magnetization of 157 emu/g (1.45 T), an effective permeability of 56,000 at 1 kHz, and coercivity, 8 A/m. With the combination of easier manufacturing process and excellent soft magnetic properties, this alloy is competitive in industrial applications versus the well-known FINEMET.     

Structural and magnetic properties of nanostructured Fe50(Co50)-6.5 wt% Si powder prepared by high energy ball milling

This work investigates the effects of 6.5 wt% Si addition and milling times on the structural and magnetic properties of Fe₅₀Co₅₀ powders. For this purpose, at first the elemental Fe and Co powders were milled for 10 h to produce Fe₅₀Co₅₀ alloy and then Si was added and the new product was milled again for different times. The microstructural and magnetic properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The results show that the minimum crystallite size of the as-milled powders (∼12 nm) has been achieved after introducing Si and milled for 8 h (total milling time of 18 h). Also an amount of 188 emu/g has been achieved for M_s. This amount of M_s is higher than most of those which have been already reported for M_s of different Fe-Si systems.     

烧结(Nd,Tb)-Fe-B磁体的元素扩散、反磁化和矫顽力

采用双合金法将两种粉末混合制备烧结永磁体可提高磁体磁性能;但在烧结过程中两种粉末之间存在元素扩散,元素扩散对磁性能的影响程度需要进一步研究。本文将Nd13Fe81B6和TbHx粉末混合制备烧结磁体,Nd13Fe81B6磁体矫顽力为4.5 kOe。当TbHx混合量为3 wt.%,烧结磁体的矫顽力增加至20.0 kOe。通过热激活研究认为,磁畴壁的形核是反磁化需要经过的过程。由于热力学的原因Tb元素更容易扩散进入Nd2Fe14B主相而不是富集在晶间富稀土相。Tb元素进入主相替代Nd可形成具有更高各向异性场的(Nd,Tb)-Fe-B表层,在反磁化过程中晶粒表层磁畴壁的形核场会增加,因此矫顽力增加程度显著。但是,TbHx混合量超过5 wt.%,矫顽力增加幅度降低。对于TbHx混合量7 wt.%的磁体,元素分布显示在主相晶粒内部贫Tb区域明显增少,证实在烧结过程中更多Tb原子从晶粒表层扩散入晶粒内部,这样晶粒表层反磁化形核场的提高程度会减弱,因而磁体矫顽力增加幅度降低。本研究说明要提高双合金Nd-Fe-B磁体磁性能需进一步控制元素扩散并优化磁体的元素分布。     

The influence of residual stress and crystallite size on the magnetic properties of electrodeposited nanocrystalline Pd-Co alloys

Nanocrystalline Pd-Co alloys were obtained by electrodeposition from an ammoniacal chloride bath. The influence of the crystallite size and the residual stress on the magnetic properties of the alloys was investigated. The residual stress increased as the applied current density was increased. It was associated to the high nucleation rate during electrodeposition and correlated to the lattice strain, estimated from the XRD patterns. Also from the XRD patterns the average crystallite size and the lattice constant were determined by Scherrer's and Rietveld's methods, respectively. Both parameters were directly influenced by the applied current density. Magnetic properties such as coercivity, remanence, saturation magnetization and squareness showed strong dependence on the residual stress and crystallite size. Coercivity higher than 1 kOe was achieved when a high current density was applied. High coercivity was attributed to the presence of residual stress and to the small crystallite size of deposits.     

Sn、Si和C掺杂对MnAl合金相结构及磁性能的影响

研究了 Sn、Si和C元素掺杂对低Mn含量MnAl合金的相结构和磁性能的影响.结果表明:Sn元素的掺杂比Si元素的掺杂更容易稳定磁性τ相.C、Sn和C、Si双元素均比单元素掺杂有利于稳定磁性相.随着Mn含量的增加,1100℃热处理后,MnAl合金相结构中出现少量高温ε相,后续再进行500℃热处理,ε相转变成磁性τ相.在...