Zhongwu Liu

The effects of Co-Ti co-doping on the magnetic, electrical, and magnetodielectric behaviors of M-type barium hexaferrites

Magnetic, electrical and magnetodielectric properties have been studied in Co-Ti co-doped M-type hexaferrite BaCoxTixFe12-2xO19 (x = 0 ∼ 4). With the incorporation of Co-Ti, both their ferromagnetic magnetization and coercivity have been greatly changed. The temperature dependent magnetization curve shows a apparent hump at around 420 K, likely in association with more complicated cycloidal spin ordering, which is closely related to ferroelectric polarization. Interestingly, a significantly enhancement in resistivity (∼3 orders in magnitude) can be obtained in co-doped samples (x > 2), which is beneficial for magnetoelectric properties. The magnetoelectric effect were examined by dielectric tunibility under external magnetic field, which shows apparent tunability up to ∼−3% for sample with x = 2 at 1T magnetic field, further supporting it is a room temperature single phase mutliferroic material.

Enhanced adhesion and field emission of CuO nanowires synthesized by simply modified thermal oxidation technique.

Metal oxide nanowires (NWs) can be easily grown by the thermal oxidation method, but the low adhesion between the NWs and the substrate restricts their practical applications in functional devices. In this work, the conventional hotplate technique is simply modified by introducing one or two stainless steel plates to supply a more stable oxidation environment, which is found to be beneficial to the growth and adhesion of CuO NWs on the Cu substrate. In detail, the Cu foils were heated on the hotplate directly, on one plate over the hotplate, and between two plates over the hotplate at 400 °C in ambient condition. It is found that the NWs obtained between two plates exhibit large length and diameter with moderate density. The sufficient activated oxygen, stable temperature, and proper temperature gradient configuration caused by the two plates accelerate the formation of CuO NWs, and result in the longest NWs with enhanced adhesion. The grain-boundary diffusion and Kirkendall effect are proposed to explain the mechanism of NWs growth and the formation of cracks. The NWs obtained between two plates also showed the best field emission properties, with lowest turn-on field (5.31 V μm(-1)) and threshold field (9.8 V μm(-1)). Excellent field emission properties and enhanced NW-substrate adhesion indicate that these NW arrays could be potentially used as the cathode of field emission displays.

Synthesis, structure, morphology evolution and magnetic properties of single domain strontium hexaferrite particles

Single domain strontium ferrite particles (SrFe12O19) with hexagonal morphology were synthesized by conventional ceramic process. Effects of Fe/Sr mole ratio and milling time on structure, morphology and magnetic properties of the strontium ferrite particles have been systematically studied. Single phase SrFe12O19 was successfully synthesized in a large composition range of Fe/Sr ratio (Fe/Sr = 9–11). The particle size refinement effect and the morphology change were observed with the increase of Fe/Sr ratio. It was also found that the change of Fe/Sr ratio had little effect on the magnetization curve. However, the magnetization process was significantly influenced with different milling time. The optimal magnetic properties obtained at Fe/Sr = 11 with 6 h milling are 68.2 emu g−1 and 5540 Oe for saturation magnetization (M S) and intrinsic coercivity (H C), respectively. The high performance single domain strontium hexaferrite particles obtained in this paper would greatly facilitate the application in the permanent magnet industry.

Preparation of MnAlC flakes by surfactant-assisted ball-milling and the effects of annealing

Abstract An alternative approach for preparing anisotropic MnAlC powders is reported. [001] textured MnAlC flakes with various sizes and thicknesses were fabricated by surfactant-assisted ball-milling (SABM) of bulk ingots. After ball-milling for 8 h the flakes have thicknesses below 200 nm and aspect ratios as high as 102∼103. After being annealed at suitable temperatures, both bulk and SABMed powders can transform from ∊-phase to τ-phase completely. C-doping cannot prevent the decomposition of the τ-phase into the β+γ2 phases after annealing at above 500°C. The magnetic properties are strongly dependent on both the fraction of the τ-phase and the thickness. A high coercivity of ∼242.8 kA m−1 has been achieved in SABMed powders. The coercivity value is 128.8% higher than the experimental value for bulk materials.

High Coercivity FePtSiN Films With L1

FePtSiN films consisting of FePt nanoparticles embedded in Si-rich matrix were fabricated on silicon substrates by direct current (dc) reactive magnetron sputtering followed by vacuum annealing. The effects of Si-N additions and annealing temperature on the structure and magnetic properties were investigated. The as-deposited films had face-centered cubic (fcc) structure, which transforms into the face-centered tetragonal (fct) structure after thermal annealing at 600°C. The grain size of FePt increased with the annealing temperature but decreased with increasing Si-N content. Increasing Si content led to the formation of Si-N-rich amorphous phase distributed between the FePt nanograins, which reduced the lattice distortion and increased the coercivity. The fct-FePt films annealed at 700°C exhibited very high coercivity, up to 13.6 kOe at room temperature and about 17.5 kOe at 100 K. These FePtSiN films have shown promise for high-density magnetic recording medium.

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Abstract With the intention to improve its mechanical properties and corrosion resistance, the magnetocaloric metal Gd was alloyed with Zr by arc melting and heat treatment. All Gd 100– x Zr x ( x =0, 0.5, 1, 1.5, 2) alloys with various Zr dopings showed hexagonal crystal structure. The Curie temperatures ( T Cs ) of Zr doped alloys were lower than that of Gd by ∼2 K. The maximum magnetic entropy changes of Gd 99.5 Zr 0.5 alloy under magnetic field changes of 0–2 T and 0–5 T were 5.27 and 9.41 J/(kg·K), respectively, which were larger than that of pure Gd metal. The addition of a small amount of Zr increased the microhardness of Gd by at least 29.3%. The corrosion resistance of Gd was also improved by Zr doping, demonstrated by increased corrosion potential and reduced corrosion current density in electrochemical polarization curves measurements. These enhanced properties were beneficial to the potential applications of Gd 100– x Zr x alloys as magnetic refrigerants at room temperature.

–FePt Nanoparticles Embedded in a Si-Rich Matrix

The millimeter-sized Nd9.5Fe61.5Co10Ti2.5Nb0.5B16−x C x (x = 0–1.25) alloy rods with various compositions were fabricated by direct casting. Nano-sized hard phase Nd2(FeCo)14B, soft phase ɑ-FeCo, and amorphous phase were observed in all alloys. An optimized amount of carbon additions improved the magnetic properties by enhancing the glass forming ability and forming near single domain-sized Nd2(FeCo)14B grains around the rod surface. Various intergranular structures were observed in the alloys with x = 0.25–1. Micromagnetic simulation using the images obtained from the magnetic force microscope and transition electron microscope indicates that the distribution and magnetism of the intergranular phase have an important influence on the magnetic properties and demagnetization process of the alloys. A uniformly distributed nonmagnetic intergranular amorphous phase may enhance the magnetic properties, but the coercivity decreases when the amorphous phase is magnetic. It is important to modify the structure and distribution of the inter-grain amorphous phase in order to achieve high hard magnetic properties in these alloys.

Magnetocaloric properties, microhardness and corrosion resistance of Gd100-xZrx alloys

The Nd9.5Fe61.5Co10Nb20.5Ti0.5B15.5C0.5 nanocomposite magnets have been prepared by the copper-mold casting technique. The magnetic properties were calculated by the micromagnetic simulation using the magnetic force microscopy image. The influence of different saturation magnetization of amorphous grain boundary phase on the magnetization reversal process was investigated. The simulation results indicate that the coercivity increases with the reducing saturation magnetization of amorphous phase when the grain size varied from 300 to 1000 nm. The magnetization reversal process demonstrates that the magnetic moments of NdFeB phase show the prior inversion at the grain boundaries. Furthermore, the comparison between the anisotropic and isotropic model implies the importance of preparing anisotropic magnets.

Influences of intergranular structure on the magnetic properties of directly cast nanocrystalline NdFeCoTiNbBC alloys

Bulk amorphous or nanocrystalline Nd-Fe-based alloys show surprising hard magnetic properties. Its pronounced room temperature coercivity results from the high random anisotropy of ferromagnetic nanoclusters. In this paper, the magnetic characteristics of the mould-cast Nd70– x Fe30Al x (

Micromagnetic Simulation of Magnetization Reversal Process Using Magnetic Force Microscope Image

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