Zhi-Qiang Jin

Preparation and magnetic property of Fe nanowire array

Fe was electrodeposited into the holes of porous anodic aluminium oxide (AAO) which was prepared electrochemically. X-ray diffraction (XRD) spectra illustrated that the deposited material was α-Fe. TEM observation showed that the aspect ratio of the nanowires was larger than 1000. The diameter of the wire was about 35 nm corresponding to that of the holes in the AAO. Magnetic measurements of the Fe nanowire array showed that its easy magnetization direction is perpendicular to the sample plane. This kind of nanowire array has potential applications in perpendicular magnetic recording.

Magnetic properties of isotropic SrFe12O19 fine particles prepared by mechanical alloying

Abstract M-type Sr-hexaferrites fine particles have been prepared by mechanical alloying. The influences of milling conditions on magnetic properties of SrFe12O19 compounds have been systematically investigated. It can be found that annealing treatment at low temperature can play similar role in structural transformation as compared with long time milling process. For those samples annealed at about 750–800°C, it is clear that saturation magnetization σs values are more sensitive to milling conditions than coercivity values Hc. The maximum σs of 74.8 emu/g and Hc of 5.6 kOe have been obtained in the isotropic powders annealed at 1000°C after milling for 30 h by using high energy mill. With prolonged annealing time, little changes of magnetic saturation and coercivity in powders can be observed and short annealing time results in low properties. The coercive field Hc, as determined from demagnetization curve, corresponds to the pinning or nucleation field.

Synthesis and magnetostriction of melt-spun Pr1−xTbx(Fe0.6Co0.4)2 alloys

Pseudobinary Pr1−xTbx(Fe0.6Co0.4)2 (0⩽x⩽0.4) cubic Laves single phases have been synthesized by melt spinning and subsequent annealing. Their structure, magnetic properties and stability have been investigated. The composition, at which the anisotropy of Pr1−xTbx(Fe0.6Co0.4)2 is compensated, is close to x=0.1. The spontaneous magnetostrictions λ111 of Pr0.9Tb0.1(Fe0.6Co0.4)2 and Pr0.8Tb0.2(Fe0.6Co0.4)2 are larger than 1500×10−6 and 1900×10−6, respectively. Pr1−xTbx(Fe0.6Co0.4)2 (0.1⩽x⩽0.4) ribbon-based materials with 3% epoxy resin combine high magnetostriction with significant magnetic coercivity. Pr0.9Tb0.1(Fe0.6Co0.4)2 is a promising magnetostrictive material.

New modified Sr-ferrite particles for high density magnetic recording

Platelike particles of SrFe12O19 with epitaxially grown Fe3O4 overlayer were prepared by a wet method. The experimental results have shown that modified sample has an exchange coupling between epitaxial layer and Sr-ferrite core. As Fe2+ quantity added, coercive force decreased and saturation magnetization increased, and these values can be controlled by changing ferrous ions ratio with respect to core material, simultaneously, reflection intensity of magnetoplumbite phase has been weaker and that of spinel phase stronger. Moreover, the variations of particle morphology changing from the hexagonal platelet of Sr-ferrite to the square platelet of Fe3O4 were observed by transmission electron microscope. The origin of coercive force decrease quickly may be understood by the nucleation and growth of reverse magnetic domains when Fe2+ content x<0.4 wt %.

Crystallization dynamics and magnetoresistance of perovskite-like manganate synthesized by mechanical alloying

Nanocrystalline ferromagnetic La0.7 Ca0.3 MnO3 perovskites with magnetoresistance effect have been successfully prepared by mechanical alloying. Thermal transformation of the amorphous phase, resulting from ball milling, to perovskite structure was studied by differential scanning calorimetry. Following the law of mass action, we discuss the crystallization dynamics of the amorphous phase. The activation energy for crystallization transformation is calculated to be about + 1.25 kJ g-1 . The characterization of resistivity (T ) for La0.7 Ca0.3 MnO3 perovskites has also been investigated. At low temperature T , (T ) has a direct-proportion dependence on T 2 . With increasing annealing temperature, the slope of the -T 2 curve decreases. The temperature dependent magnetoresistance effect at temperature far below the Curie temperature can be well expressed as the equation / 0 = p 1 - p 2 T 3/2 - p 3 T 5/2 .

Structure and magnetic properties of melt-spinning Pr(Fe0.6Co0.4)2 alloys

Pr(Fe0.6Co0.4)2 ribbons were prepared by melt spinning with different wheel speeds from 35 to 45 m/s. Their structure, magnetic properties, and thermal stability are investigated. At a wheel speed of 35 m/s, the ribbon consists of a mixture of Pr(Fe,Co)2 cubic Laves phase and some noncubic phases. An almost Pr(Fe,Co)2 nanocrystalline single phase with a Curie temperature of 305 °C is obtained at a wheel speed of 40 m/s. Except for Pr(FeCo)2 phase a small amount of amorphous phase is observed with increasing wheel speeds to 45 m/s. Pr(Fe,Co)2 phase becomes unstable and decomposes above 770 °C. The resin-bonded Pr(Fe,Co)2 nanocrystalline phase which is obtained at a wheel speed of 40 m/s combines high magnetostriction (λ∥−λ⊥=140 ppm), with significant coercivity, iHc=5 kOe.

Structural and magnetic evolution of rapidly quenched Sm–Fe–Si–C alloys

Sm2Fe15.5Si1.5C1.5 samples with a rhombohedral Th2Zn17-type structure have been prepared by high frequency induction melting. The crystallization, crystal structure, and magnetic properties of melt-spun and annealed ribbons have been studied by means of x-ray diffraction, scanning electron microscopy, differential thermal analysis, and magnetic measurements. The results show that the as-spun ribbons consist mainly of an amorphous phase, and that they crystallize in two steps. Upon annealing at 660 °C the ribbons form a metastable phase of the TbCu7-type and an α-Fe phase. Upon further annealing above 750 °C the metastable phase transforms to a 2:17-type phase coexisting with α-Fe. A saturation magnetization of up to 108 emu/g with a coercivity of up to 3.5 kOe is obtained in the ribbon annealed at 660 °C for 30 min. However, the reduced remanence of the ribbon σr/σs is only 0.6 and its hysteresis loop exhibits two independent magnetic components. Additionally, the grain size of the ribbons obtained by ann...

Evaluation of exchange interactions in (GdxY1-x)3Co11B4 compounds

Abstract Two-sublattice molecular field theory (MFT) is employed to describe the temperature dependence of magnetization for (Gd x Y 1- x ) 3 Co 11 B 4 compounds. Three molecular field coefficients, n RCo , n CoCo , n RR , have been calculated by a numerical fitting process. MFT with a single coefficient provides quite a reasonable description for Y 3 Co 11 B 4 . For x > 0, we find that n CoCo is the largest coefficient, implying that the magnetic interactions are dominated by exchange between cobalt 3d electrons.

Phase transformation and magnetic properties of mechanically alloyed NdFe12-xMoxNy

The structure and permanent magnetic properties of isotropic powders of NdFe12-xMox (1 less than or equal to x less than or equal to 2) and their nitrides were investigated by using mechanical alloying. It was found that heat treatment of as-milled powders at 850 similar to 900 degrees C resulted in the formation of the Nd(Fe,Mo)(12) phase. The metastable Nd(Fe,Mo)(7) phase and a large amount of alpha-Fe were present at 800 degrees C. After nitrogenation, the volume expansion of NdFe12-xMox phases was about 3.5% and the Curie temperature of the samples was enhanced by 157 similar to 207 degrees C. In NdFe12-xMoxNy compounds the values of T-c fall in the range of 272 similar to 75 degrees C. It has been found that the intrinsic coercivities strongly increased with increasing Mo content. The maximum coercivity, H-i(c)=7.8 kOe, was achieved at 850 degrees C for the sample with x=2. The difference between the coercivities of the mechanically alloyed Nd(Fe,Mo)(12)N-y powders with different Mo content was attributed to their different crystallization behavior

Nonequlibrium phase transition of Nd3(Fe,Ti)29 compound during mechanical milling

A nonequilibrium phase transition has been found during mechanical milling of monoclinic Nd3(Fe,Ti)29 compound. The transition process and the mechanically milled products have been investigated by X-ray diffraction and AC initial susceptibility. The results show that mechanical milling results in the transformation of Nd3(Fe,Ti)29 compound into a nanostructure during the early stage of milling, and then generates it to decompose to the hexagonal TbCu7-type Nd(Fe,Ti)7 disorder phase and α-Fe(Ti), and finally transforms the material to a mixture of the amorphous phase and α-Fe(Ti). The structure of mechanically milled Nd(Fe,Ti) 7 phase is similar to but not exactly identical with that of annealed Nd(Fe, Ti)7 phase. The magnetic-ordering transition of the former is broader than that of the later. The broadening of the magnetic-ordering transition can be attributed to the effect of the nanostructure, and the inhomogeneity of the concentration of Nd(Fe,Ti)7 phase.