Yucheng Sui

Pore structure, barrier layer topography and matrix alumina structure of porous anodic alumina film

Different anodic voltages and methods were adopted to produce porous anodic alumina films (PAAF) in an aqueous solution of oxalic acid. Carbon tube growth by chemical vapor deposition (CVD) in the films was used to copy the internal pore structure and was recorded by transmission electron microscopy (TEM) photos. Atomic force microscope (AFM) was employed to obtain the topography of the barrier layer of the corresponding films. When the anodic voltage was 40 V and the two-step method adopted, the barrier layer of the film had domains with highly ordered hexagonal cell distribution, and the corresponding pores were straight. When the anodic voltage increased to 60 V, the barrier layer showed random cell distribution with an obvious difference in cell size and form, and the corresponding pores exhibited multi-branch features. When the anodic voltage increased further to 110 V, the barrier layer also showed a random cell distribution. Additionally, smaller protrusions connected to bigger cells were found, which can be correlated to the formation of branches with smaller diameters. Most of the branches of carbon tubes grown in the film anodized at 110 V have a saw-tooth like feature. X-Ray diffraction analysis shows that all the anodic films are amorphous, regardless of the anodic voltage. However, unoxidized aluminum particles in the film anodized at 110 V was observed by TEM.

FePt nanodot arrays with perpendicular easy axis, large coercivity, and extremely high density

Ordered FePt nanodot arrays with extremely high density have been developed by physical vapor deposition using porous alumina templates as evaporation masks. Nanodot diameter of 18nm and periodicity of 25nm have been achieved, resulting in an areal density exceeding 1×1012dots∕in.2. Rapid thermal annealing converts the disordered fcc to L10 phase, resulting in (001)-oriented FePt nanodot arrays with perpendicular anisotropy and large coercivity, without the need of epitaxy. High anisotropy and coercivity, perpendicular easy axis orientation and extremely high density are desirable features for future magnetic data storage media applications.

Magnetic nanotubes produced by hydrogen reduction

FePt and Fe3O4 nanotubes are produced by hydrogen reduction in nanochannels of porous alumina templates and investigated by electron microscopy, x-ray diffraction, and superconducting quantum interference device magnetometry. Loading the templates with an Fe chloride and Pt chloride mixture, followed by hydrogen reduction at 560 °C, leads to the formation of ferromagnetic FePt nanotubes in the alumina pores. An Fe nitrate solution, thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h at the same temperature, yields Fe3O4 tubes. The versatility of the method indicates that materials with a wide range of parameters can be produced.

CoPt hard magnetic nanoparticle films synthesized by high temperature chemical reduction

The synthesis of hard-magnetic CoPt nanoparticle films by hydrogen reduction of Co nitride and Pt chloride mixture is reported. Thin porous alumina films are adopted as the carrier of the initial aqueous solution and of the final reducing products of CoPt nanoparticles. It is found that chemical ordering of L10–CoPt occurs above 400 °C. Partial phase transformation occurs in the alumina substrate when the treatment temperature is higher than 600 °C. The film coercivity increases with increasing annealing temperature and reaches a maximum value of 24.2 kOe when the reduction is carried out at 700 °C for 2 h.

Nd - Fe - (C, B) permanent magnets made by mechanical alloying and subsequent annealing

The structure, phase transformation and magnetic properties of mechanically alloyed (MA) alloys Nd16Fe76-mCm (7 less than or equal to m less than or equal to 11) and Nd16Fe84-mCm-yBy (0 less than or equal to y less than or equal to m, m = 7, 8, 9) have been studied systematically. The Nd2Fe14C compound can be formed within a wide window for the range of composition and annealing temperature. More carbon than the stoichiometric content for Nd2Fe14C is necessary to stabilize the tetragonal structure. Substitution of boron for carbon can accelerate the phase transformation from Nd2Fe17Cx to Nd2Fe14(C, B) and increase the magnetic properties drastically. There is an fee structure Nd-rich phase in the mechanically alloyed Nd-FeC alloy. The lattice constant of the Nd-rich phase decreases as the boron content increases. Nd2Fe14(C, B) compounds formed in the alloys with same boron content and different carbon contents have different Curie temperatures. The Curie temperatures also vary with the variation of annealing temperature. The best magnetic properties achieved in Nd16Fe76B5C3 alloy are H-i(c) = 1480 kA m(-1), M(r) = 0.71 T and(BH)(max) = 91.5 kJ m(-3).

CRYSTALLOGRAPHIC TRANSFORMATIONS OF RAPIDLY QUENCHED SM10FE90-XTIX AND MAGNETIC PROPERTIES OF THEIR NITRIDES

Sm10Fe90-xTix alloys (x=O, 1, 3, 4, 5, 6, 8, 10) were prepared by rapid quenching and sequent annealing. Structural transformations from amorphous, metastable-to-equilibrium phases, and magnetic properties of their nitrides were systematically investigated. A CaCu5-type metastable phase was formed for 0 less than or equal to x less than or equal to 10 as the crystallization of the amorphous alloys occurred below 700 degrees C. The CaCu5-type structure is transformed into a TbCu7-type one with increased annealing temperature (above 700 degrees C). These alloys have different structures including Th2Zn17 type for 0 less than or equal to x less than or equal to 3, TbCu7 type for 3<x<6, and ThMn12 type for 6 less than or equal to x less than or equal to 10, at high annealing temperatures (above 950 degrees C). Nitrogenation was carried out at 460 degrees C for 6 h. The best hard magnetic properties are obtained after nitriding for x=3. It gives H-i(c)=17.6 kOe, 4 pi M-r=7.7 kG, and (BH)(max)=6.8 MGOe for the TbCu7-type phase and H-i(c)=8.6 kOe, 4 pi M-r=8.4 kG, and (BH)(max)=10.2 MGOe for the CaCu5-type one. Although the coercivity of the CaCu5-type phase is lower than chat of the TbCu7-type one, enhanced remanence of the former has been observed

Structure, phase transformation and magnetic properties of Nd-Fe-C alloys made by mechanical alloying and subsequent annealing

Abstract Structure, phase transformation and magnetic properties of mechanically alloyed (MA) Nd y Fe 100−1.5 y C 0.5 y , Nd 16 Fe 84− m C m and Nd n Fe 92− n C 8 , alloys depend sensitively on the compositions and the annealing temperatures. Nd 2 Fe 14 C can be formed by a solid state reaction of α-Fe with Nd 2 C 3 as well as by a reaction of Nd 2 Fe 17 C x with Nd–C compounds. The thermal stability of Nd 2 Fe 14 C increases with the increase of carbon content, reaches a maximum and then decreases. Nd 2 Fe 14 C is stable in a triangular shaped area in a status diagram with axes of annealing temperature and composition. To make Nd 2 Fe 14 C the main phase, the Nd:C ratio must be kept within a certain range, because too little carbon leads to Nd 2 Fe 17 C x as the main phase, and too much carbon increases the α-Fe content, inhibiting the formation of Nd 2 Fe 14 C.

Growth and magnetism of FePt:C composites in nanoscale channels

Nanochannels of porous alumina films were used as nanoreactors for the reaction of hydrogen gas with a mixture of Fe nitrate and Pt chloride. This results in the formation of of FePt clusters within the nanochannels. Both the sizes of the clusters and the coercivity the cluster assembly increase with the increase of annealing temperature. In order to reduce excessive agglomeration at high temperature, carbon was introduced by chemical vapor deposition and FePt:C composites in nanoscale channels were created. When FePt clusters were synthesized with carbon and heat treated at high temperatures, cluster sizes were much smaller than those without carbon, suggesting that the introduced carbon serves effectively to block the agglomeration of clusters. The coercivity of the FePt:C composite containing the smaller clusters is as high as 29.0 kOe.

Template-mediated assembly of FePt L10 clusters under external magnetic field

FePt L10-structured clusters were synthesized by hydrogen reduction of Fe(NO3)3∙9H2O and H2PtCl6∙6H2O mixtures within the pores of alumina. They were released by dissolving the alumina matrix and capped with organic surfactants. After a series of chemical treatments, clusters with an average diameter of about 11nm were precipitated. The clusters were drop casted onto ordered nanoporous arrays. Clusters reaching the bottom of the pores formed an ordered magnetic array. The corresponding magnetic film exhibits distinct anisotropic behavior caused by the external magnetic field.

Structural and magnetic properties of Laves compounds Dy1-xPrx(Fe0.35Co0.55B0.1)(2) (0 <= x <= 1)

Dy1-xPrx(Fe0.35Co0.55B0.1)(2) (0 <= x <= 1) Laves compounds with a cubic MgCu2-type structure were synthesized by arc melting and subsequent annealing. The lattice parameter of the Laves compounds linearly increases, while the Curie temperature T-C decreases with increasing Pr content. The saturation magnetization M-s at 5 K or 295 K for the Dy1-xPrx(Fe0.35Co0.55B0.1)(2) alloys decreases to reach a minimum, then increases with increasing Pr content. The composition for magnetic moment compensation is about x=0.55 at 295 K and x=0.65 at 5 K, respectively. The magnetostriction lambda(parallel to) or lambda(perpendicular to) at room temperature was investigated either parallel or perpendicular to the applied field using a standard strain gauge technique