Hui-qun Guo

Positive colossal magnetoresistance in a multilayer p–n heterostructure of Sr-doped LaMnO3 and Nb-doped SrTiO3

A positive colossal magnetoresistance (CMR) has been discovered in an epitaxial multilayer p–n heterostructure fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by laser molecular-beam epitaxy. In contrast to the negative CMR of the LaMnO3 compound family, positive CMR is observed in the temperature range from 100 to 300 K. The largest value of the magnetoresistance (MR) ratio (ΔR/R0,ΔR=RH−R0), 517%, is one order of magnitude larger than that of simple p–n junctions of the same materials previously reported. A very large MR ratio, 297%, remains in a low field of 0.01 T. Even at a temperature as high as 300 K, a MR ratio as large as 17.3% is still observed.

Structure and magnetocrystalline anisotropy of R2Fe17−xGax compounds with higher Ga concentration

The effect of Ga substitution for Fe in R2Fe17 (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Tm) compounds on the structure and magnetocrystalline anisotropy has been studied by means of x‐ray diffraction and magnetization measurements. Both iron sublattice anisotropy and rare earth sublattice anisotropy are found to be modified by the introduction of the gallium atoms. A uniaxial anisotropy is shown in R2Fe17−xGax (for R=Y, Gd, Tb, Dy, Ho, Er, and Tm) compounds with high Ga concentration, whereas a reversal change in the easy magnetization direction is observed in the samples for R=Sm. The contributions to the uniaxial orientation of the magnetization in these compounds result from not only the rare earth sublattice, but also the iron sublattice.

Influence of Al substitution on the structure and Co-sublattice magnetocrystalline anisotropy of Gd2Co17 compounds

The structure and magnetic properties of Gd2Co17−xAlx compounds were investigated. All samples have a rhombohedral Th2Zn17-type structure and the replacement of Co by Al results in an approximately linear increase in the unit cell volumes at a rate of 8.2 A3/Al. The Curie temperature decreases monotically with the increase of Al concentration. On the basis of magnetization curves at the compensation temperature, the intersublattice-molecular-field coefficient, nRT, and the RT exchange-coupling constant JRT have been determined. It is noteworthy that the substitution of Al has a significant effect on the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd2Co17−xAlx compounds from the basal plane to the c axis.

HIGH-COERCIVITY SM-FE-GA-C COMPOUNDS WITH TH2ZN17 STRUCTURE BY MELT-SPINNING

The magnetic hardening of the Sm2Fe14Ga3Cx was investigated by melt spinning. It was found that high coercivities can be achieved by direct quenching at the optimum substrate velocity. The coercivities of 12.6–15.0 kOe were obtained in Sm2Fe14Ga3Cx ribbons with carbon contents x from 1.0 to 2.5. The as‐quenched Sm2Fe14Ga3C1.5 ribbons had a coercivity exceeding 13 kOe within a large range of quench rates between 18 and 30 m/s. X‐ray diffraction experiments indicate that the ribbons are almost entirely comprised of the Th2Zn17 phase. It is concluded that Ga not only stabilizes the hard magnetic phase but also is very effective in raising the coercivity in the Sm2Fe14Ga3Cx melt‐spun ribbons.

Effects of Co addition on magnetic properties and nanocrystallization in amorphous Fe84Zr3.5Nb3.5B8Cu1 alloy

The structure, Curie temperature, and magnetization of (Fe1−xCox)84Zr3.5Nb3.5B8Cu1 (x=0.0–0.8) alloys were investigated by x-ray diffractometer, differential scanning calorimeter, magnetic balance, and extracting sample magnetometer, respectively. The bcc grains are embedded in the amorphous matrix after controlled annealing. The crystallization temperature Tx of the amorphous alloy decreases with the increasing Co content, whereas the Curie temperature Tc of the amorphous alloy is enhanced by an increase in Co content. The magnetization measured under the field of 60 kOe at a temperature of 1.5 K is enhanced first by the increase of Co addition, and exhibits a maximum at the composition where x=0.2, then decreases while Co content increases.

The more shearing, the thicker shear band and heat-affected zone in bulk metallic glass

In a compression test for a Zr-based bulk metallic glass, a dominant shear band was preserved before fracture by a cylindrical stopper. A heat-affected zone (HAZ) ∼10 m thick together with shear band was discovered in the center of the deformed sample by preferential ion milling. By using a low aspect ratio sample for compression, diverse micron-scaled HAZs among multiple shear bands were also revealed. Based on above experimental results and the isothermal source model, it was found that the thickness of shear band and its HAZ increased continuously with the progression of shear deformation.

Effects of Ga substitution in Y2Fe17 compounds on the magnetocrystalline anisotropy

The heating effects on the structure and fine structure of Cu-11.9Al-5Ni-1.6Mn-1Ti (wt%) shape memory alloy have been studied by means of transmission electron microscopy and microscopic compositional analysis. With the increase of the heating temperature and time, the stabilized M18R(1) martensite, transformed from the parent phase of L2(1) structure type, underwent the following structural variations: M18R(1) --> N18R(1) --> disorder N9R. Disorder N9R bainite precipitates and retained parent phases were observed in specimens heated at 573 and 623 K for 7.2 ks or longer. When the alloy was heated at 673 K for 133.2 ks, equilibrium alpha, gamma(2) and Ni,Al-rich phases were formed without any retained beta(1) parent phase. While the size of the X(s) particles decreased in the specimen heated at 623 K for 7.2 ks and they disappeared completely in the specimen heated at 673 K, the X(L) particles were coarsened after heating. With the increase of the heating temperature and time the growth of the antiphase domains was observed.

Structural, magnetic and magnetostrictive studies of Tb0.27Dy0.73(Fe1 − xAlx)2

Abstract Measurements of magnetic properties, X-ray diffraction and magnetostriction were made on Tb 0.27 Dy 0.73 (Fe 1 − x Al x ) 2 ( x = 0.1, 0.2, …, 0.7) compounds. It was found that the system has the cubic MgCu 2 structure over almost the whole (Fe,Al) concentration range investigated, except for a narrow intermediate range ( x = 0.4–0.6) where the hexagonal MgZn 2 structure appears. With increasing Al content x , the lattice constant a increases linearly with x . The first replacement of Fe results in a marked decrease in the Curie temperature, which is followed by a slight decrease in T C with x . A linear decrease in magnetostriction of | λ | − λ ⊥ | at room temperature with x was also observed from 1530 × 10 −6 for x =0 to 36×10 −6 for x =0.3. The saturation magnetization σ s exhibits a complex concentration dependence in the Tb 0.27 Dy 0.73 (Fe) 1 − x Al x ) 2 system: in the range x σ s increases linearly with x and, for x = 0.5–0.6, σ s decreases and then increases again. An enhancement of the magnetic ‘hardness’ in this system was also observed at low temperature.

Fluctuation electron microscopy of Al-based metallic glasses: effects of minor alloying addition and structural relaxation on medium-range structural homogeneity

Al-based metallic glasses with relatively high glass-forming ability have been studied using fluctuation electron microscopy (FEM). These glasses exhibited well-defined glass transition and remained fully amorphous throughout the FEM experiments. Peaks were observed in the normalized intensity variance curves, evolving with sample composition and thermal history, while diffraction patterns remained identical. Specifically, minor alloying of Co into the Al85Ni5Y10 glass improved its medium-range homogeneity. This is in accord with, and partly explains, the increase in the glass-forming ability. After annealing of Al85Ni5Y8Co2 below its glass transition temperature, FEM suggests structural homogenization or intensified medium-range fluctuations, depending on the annealing temperature. Such dissolution versus growth of quenched-in heterogeneity accompanying structural relaxation was not detectable using other techniques.

MAGNETIC AND MAGNETORESTRICTIVE PROPERTIES OF SM1-XDYXFE2 COMPOUNDS AND ITS MOSSBAUER-EFFECT STUDIES

Measurements of x‐ray diffraction, magnetization, magnetostriction, and Mossbauer effect were performed on a series of Sm1−xDyxFe2 samples (x=0, 0.1, 0.15, 0.25, 0.45, and 0.65). It is shown that the system retains the cubic MgCu2 structure over the whole range and the lattice constant decreases linearly with increasing x. Results of Mossbauer effect study at room temperature show that for a small amount of Dy substitution for Sm, x<0.15, easy axes of the magnetization keep in the [111] direction. For x≳0.15, the direction of easy axes rotates gradually from [111] for x=0, and 0.1 to [100] for x=0.45 and 0.65. The concentration dependence of the saturation magnetization exhibits a minimum at x=0.30 for 1.5 K and at x=0.45 for room temperature, reflecting the occurrence of the compensation of magnetic moments at various Sm/Dy ratios. For x=0.45 and 0.65, the pinning phenomenon of narrow domain walls was observed at low temperature. The magnetostriction was found to decrease with the increase in the Dy cont...