Kewei Sun

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

One-Pot Synthesis of Urchin-like FePd–Fe3O4 and Their Conversion into Exchange-Coupled L10–FePd–Fe Nanocomposite Magnets

We report a one-pot synthesis of urchin-like FePd-Fe3O4 nanocomposites, spherical clusters of FePd nanoparticles (NPs) with spikes of Fe3O4 nanorods (NRs), via controlled thermal decomposition of Fe(CO)5 and reduction of Pd(acac)2. The FePd NPs with sizes between 6 and 9 nm self-aggregate into 60 nm superparticles (SPs), and Fe3O4 NRs grow on the surface of these SPs. Reductive annealing at 500 °C converts the FePd-Fe3O4 into exchange-coupled nanocomposites L1(0)-FePd-Fe with their Hc tunable from 0.8 to 2.6 kOe and Ms controlled from 90 to 190 emu/g. The work provides a general approach to L1(0)-FePd-Fe nanocomposite magnets for understanding exchange coupling at the nanoscale. The concept may be extended to other magnetic nanocomposite systems and may help to build superstrong magnets for magnetic applications.

Magnetic hardening of Ce1+xFe11−yCoyTi with ThMn12 structure by melt spinning

A recent study on the intrinsic magnetic properties of CeFe11−yCoyTi has revealed that substituting one Co for Fe retains the favorable magnetocrystalline anisotropy Ha found in the ternary Fe end member, while enhancing the Curie temperature Tc and saturation magnetization 4πMs. These findings warrant further optimization around Co substitution y = 1 to try to exploit the hard magnetic properties of these Ce-based magnets. Both Ce and Co concentrations in Ce1+xFe11−yCoyTi have been optimized in the range of x = 0 − 0.2 and y = 0 − 1.5. It was found that Co substitution effectively enhances all hard magnetic properties, although the values are still lower than those predicted from the intrinsic magnetic properties. Specifically, Tc increases from 210 °C to 285–350 °C; 4πM19 (magnetization at 19 kOe) from 8.9 kG to 10.5–11.5 kG, remanence Br from 3.1 kG to 4.1–4.5 kG, and most importantly, Hci from 1.1 kOe to 1.5 kOe. As a result, the room temperature energy product (BH)max has been increased by over 100% ...

Heat capacity (150–300 K) and anisotropic magnetic susceptibility (5–300 K) of single-crystal La2CuO4+x

Abstract Heat capacity measurements from 150 to 300 K were carried out on a single crystal of La 2 CuO 4+ x synthesized by subjecting an La 2 CuO 4 crystal to 3 kbar oxygen pressure at 575 °C. The data reveal three small (about 1%) anomalies at temperatures ( T ) of 206, 222 and 259 K. The first two are tentatively attributed to CuO inclusions in the crystal. The third is observed on warming, but not on cooling, and is attributed to the previously documented first-order transition from the orthorhombically distorted K 2 NiF 4 structure to a low T mixture of nearly stoichiometric La 2 CuO 4 and oxygen-rich superconducting La 2 CuO 4+ y ( y > x ). The size of the anomaly at 259 K is about one-seventh of that observed previously for a single crystal of La 2 CuO 4 at the second-order tetragonal-to-orthorhombic phase transition temperature of about 530 K. Magnetization measurements from 5 to 300 K and from 50 G to 50 kG are also reported for the La 2 CuO 4+ x crystal. The normal state magnetic susceptibility χ ( T ) is quite anisotropic, with χ ( T ) for H perpendicular to the CuO 2 layers ( χ c ) in good agreement with previous data on a different, but similarly prepared, crystal. The anisotropy in χ is nearly independent of T from 40 to 300 K and the magnitude of χ c - χ ab per CuO 2 layer is very similar to that at high T in YBa 2 Cu 3 O 6.1 , La 2 CuO 4 , Sr 2 CuO 2 Cl 2 and La 2− x M x CuO 4 (MSr, Ba).

Magnetic Hardening of CeFe11Ti and the Effect of TiC Addition

We report the magnetic hardening of CeFe11Ti by melt spinning and compare ribbons prepared with and without TiC additions for grain refinement. X-ray diffraction indicates that samples melt-spun at surface wheel speeds between vs = 10 and 35 m/s are multiphased. However, CeFe11Ti with a major ThMn12-type phase has been successfully obtained either by directly melt spinning at the optimum wheel speed vs = 10 m/s or by annealing the overquenched ribbons melt spun at vs = 35 m/s. To restrain the grain growth during annealing, 3 and 6 at% TiC have been added to the starting ingots, which were subsequently melt spun in the same range of wheel speed. For as-spun samples, adding TiC leads to much finer grains as well as much greater phase separation compared with samples without TiC. However, upon annealing, multiphased TiC added samples can be fully converted to the desired CeFe11Ti phase with ThMn12type crystal structure together with TiC precipitates. Because of the grain refining effect played by TiC, samples with TiC are subject to less grain growth during the heat treatment, and hence feature an enhanced Hci =1.3

High-Performance MnBi Alloy Prepared Using Profiled Heat Treatment

kOe and energy product (BH)max = 0.87

Highly responsive ground state of PbTaSe2 : Structural phase transition and evolution of superconductivity under pressure

MGOe that are 18% and 22% higher, respectively, compared with the best annealed samples without TiC.

Anisotropic hot deformed magnets prepared from Zn-coated MRE-Fe-B ribbon powder (MRE = Nd + Y + Dy)

The profiled heat treatment (PHT) method has been used to synthesize MnBi alloys with high-purity low-temperature phase (LTP). In the PHT method, the arc-melted MnBi alloy was remelted then slowly cooled by a pseudo-equilibrium solidification process to promote the formation of LTP phase. The PHT-treated MnBi alloys had an LTP phase up to 94 wt.% and a magnetization of 73 emu/g under a field of 9 T. Scanning electron microscopy showed that there exist some micrometer-sized Mn-rich inclusions in the LTP matrix of the PHT MnBi alloy. The PHT MnBi alloys were crushed into powders with an average size of ~3 μm by low-energy ball milling. These MnBi powders were aligned in an 18 kOe field and warm compacted into a bulk magnet at 300 °C for 30 min. The magnet had a density of 8.2 g/cm3 and magnetic properties of Ms = 6.7 kG, Mr = 5.3 kGs, i Hc = 5 kOe, and (BH)max = 6.1 MGOe.

Structural and ferromagnetic properties of an orthorhombic phase of MnBi stabilized with Rh additions

Transport and magnetic studies of PbTaSe

Heat capacity of high-purity polycrystalline YBa2Cu3O7 from 0.4 to 400 K in applied magnetic fields of 0 and 70 kG.

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