Jian-Gao Zhao

Colossal magnetoresistance of spin-glass perovskite La0.67Ca0.33Mn0.9Fe0.1O3

The magnetic and magnetotransport properties of the perovskite La0.67Ca0.33Mn0.9Fe0.1O3 have been investigated, and the spin-glass behavior with a spin freezing temperature of 42 K has been well confirmed for this compound. A metal-to-insulator transition and colossal magnetoresistance have been observed near its spin freezing temperature; besides, the insulator behavior has been found to reappear at lower temperature. The formation of ferromagnetic and antiferromagnetic clusters and the competition between them with the introduction of Fe3+ ions, which do not participate in the double-exchange process, have been suggested to explain the experimental results.

Pressure effects on mechanical properties of bulk metallic glass

The effects of high pressure (up to 5GPa) on the mechanical properties of a typical Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG) have been investigated. It is found that the high-pressure pretreatment at room temperature can significantly improve the mechanical performance of the BMG. Particularly, the compressive plasticity of the BMG can be increased as large to as 12% by 4.5GPa pressure pretreatment. The origin of the pressure effect on mechanical properties is studied.

Colossal magnetoresistance in cluster glass-like insulator La0.67Sr0.33(Mn0.8Ni0.2)O3

Substitution of Ni for Mn in La0.67Sr0.33MnO3 (LSMO) lowers the Curie temperature TC from 365 K for LSMO to 194 K for La0.67Sr0.33(Mn0.8Ni0.2)O3, which exhibits a cluster glass-like state. The oxide is insulating under both zero field and 60 kOe, but application of the magnetic field induces colossal magnetoresistance (CMR) especially at low temperature. Far below TC, the compound’s field dependence of resistivity has a very similar shape with that of metallic perovskite manganite at relatively high temperature when the spin fluctuation grows stronger. The results indicate that the ferromagnetic superexchange between Ni and Mn, is helpful for the overall ferromagnetic exchange components just overcoming the generic antiferromagnetic exchange components, which leads to the presence of the cluster glass-like state. The reduction of the magnetic disorder and the suppression of the spin fluctuation in Mn–O layer by the external magnetic field are suggested to explain the CMR effect in this insulating compoundSubstitution of Ni for Mn in La0.67Sr0.33MnO3 (LSMO) lowers the Curie temperature TC from 365 K for LSMO to 194 K for La0.67Sr0.33(Mn0.8Ni0.2)O3, which exhibits a cluster glass-like state. The oxide is insulating under both zero field and 60 kOe, but application of the magnetic field induces colossal magnetoresistance (CMR) especially at low temperature. Far below TC, the compound’s field dependence of resistivity has a very similar shape with that of metallic perovskite manganite at relatively high temperature when the spin fluctuation grows stronger. The results indicate that the ferromagnetic superexchange between Ni and Mn, is helpful for the overall ferromagnetic exchange components just overcoming the generic antiferromagnetic exchange components, which leads to the presence of the cluster glass-like state. The reduction of the magnetic disorder and the suppression of the spin fluctuation in Mn–O layer by the external magnetic field are suggested to explain the CMR effect in this insulating compound

The formation and magnetic properties of Sm2Fe15Al2Cx (x=0-2.0) compounds prepared by arc melting

The structural and magnetic properties of compounds in the series Sm2Fe15Al2Cx have been investigated by means of x-ray diffraction (XRD) and magnetization measurements. Samples with x=0, 0.5, 1.0, 1.5, and 2.0 were prepared by the arc-melting method. XRD patterns indicate that these carbides are single phase with a rhombohedral Th2Zn17-type structure, except for Sm2Fe15Al2, which contains a few per cent of BCC alpha -Fe. The saturation magnetization for all samples is about 110 emu g-1 and has a small dependence on C concentration. The Curie temperature Tc enhancement is about 190 K for x=1.5 compared with that of Sm2Fe17. All compounds exhibit an easy c-axis anisotropy at room temperature and the anisotropy field increases to about 110 kOe when x<or=1.5. The substitution of Al, like Ga and Si, is found to not only help the formation of high-C-concentration rare-earth-Fe compounds with 2:17-type structure, but also increase the anisotropy field.

GA-CONCENTRATION DEPENDENCE OF MAGNETOCRYSTALLINE ANISOTROPY IN GD2FE17-XGAX COMPOUNDS

The structure and magnetic properties of arc‐melted Gd2Fe17−xGax compounds were studied by means of x‐ray diffraction and magnetization measurements. X‐ray diffraction patterns demonstrate that all samples have a rhombohedral Th2Zn17‐type structure and the substitution of Ga leads to an approximately linear increase in the unit cell volumes. The Curie temperature is found to first increase and then decrease with increasing Ga concentration. It is noteworthy that the substitution of Ga has a significant effect on the magnetic anisotropy of iron sublattice. The easy magnetization direction of Gd2Fe17−xGax compounds changes from basal plane to c axis with increasing Ga concentration. The sample with x=7 exhibits a uniaxial anisotropy.

Structure and magnetic properties of Dy2Co17−xGax (x=0–6) compounds

The structure and magnetic properties of arc-melted Dy2Co17-xGax compounds were studied. These compounds have the hexagonal Th2Ni17-type structure for 0 less than or equal to x less than or equal to 3 and the rhombohedral Th2Zn17-type structure for 3 less than or equal to x less than or equal to 6. The unit cell volume shows an approximately linear increase at a rate of about 7.7 Angstrom(3) per Ga atom. The Curie temperature was found to decrease with increasing Ga concentration. The saturation magnetization decreases linearly with increasing Ga content from 7.0 mu(B), f.u.(-1) for x=0 to -7.9 mu(B), f.u.(-1) for x=6. The exchange coupling constants J(DyCo) and J(CoCo) have been derived by analyzing both the field dependence and the temperature dependence of the magnetization. An approximately constant value of J(DyCo) indicates that the substitution of Ga for Co has only a small influence on the exchange coupling between the Dy and Co atoms. The effect of Ga substitution on J(CoCo) is strong, leading to a linear decrease of T-C in the Dy2Co17-xGax compounds

Asymmetry of coercivity dependence on temperature in exchange-biased FeMn/Co bilayers

The temperature dependence of the coercivity in biased FeMn/Co bilayers is investigated. An asymmetric behavior of the left and right coercivity with varying temperature is observed. The asymmetry can be understood by taking into account the variation of the spin arrangements in the antiferromagnetic layer. The calculated results are in agreement with the experimental ones qualitatively. All these results suggest that the magnetization reversal mechanisms are different for increasing and decreasing fields.

STRUCTURE AND MAGNETIC-ANISOTROPY OF SM2FE17-XALXC (X=2-8) COMPOUNDS PREPARED BY ARC MELTING

In previous work it was discovered that the 2:17‐type rare‐earth–iron compounds with high carbon concentration could be formed by the substitution of Ga, Si, or Al, etc., for Fe in R2Fe17Cx. The effect of Al substitution for Fe on the structure and magnetic anisotropy of Sm2Fe17C has been investigated. Alloys with the composition of Sm2Fe17−xAlxC (x=2, 3, 4, 5, 6, 7, and 8) were prepared by arc melting. The carbides are single phase with rhombohedral Th2Zn17‐type structure except for Sm2Fe17C which contains a small amount of α‐Fe. The addition of Al results in an approximately linear increase in the lattice constants and the unit‐cell volumes. The Curie temperature Tc is found to increase slightly when x≤3, then decrease rapidly with increasing Al concentration, while the room‐temperature saturation magnetization decreases monotonically with the addition of aluminum. X‐ray‐diffraction and magnetization measurement studies of magnetic‐field‐oriented powders demonstrate that the samples with x≤6 exhibit an ...

Effects of Ga substitution on the hard magnetic properties of the Sm2Fe17C1.5 compounds

Abstract A systematic investigation of the structure and magnetic properties of arc-melted and as-quenched Sm 2 Fe 17− x Ga x C 1.5 samples with 0 ≤ × ≤ 6 has been made by X-ray diffraction and magnetic measurements, X-ray diffraction studies have shown that these samples crystallize in the rhombohedral Th 2 Zn 17 -type structure. The unit-cell volumes υ of Sm 2 Fe 17− x Ga x C 1.5 compounds increase monotonically with increasing Ga concentration. The Ga-concentration dependence of the Curie temperature T C exhibits a maximum value of 633 K at about x = 2. The room-temperature saturation magnetization M s decreases monotonically with increasing Ga concentration. The Sm 2 Fe 17− x Ga x C 1.5 compounds exhibit an easy c -axis anisotropy at room temperature. The anisotropy field is found first to increase and then to decrease with increasing x , having a maximum value of 130 kOe at about x = 2. The coercivities of 13.0–16.0 kOe are obtained in as-quenched Sm 2 Fe 17- x Ga x C 1.5 ( x = 2 and 3) ribbons prepared at the speeds of 15–30 m/s at room temperature. The substitution of Ga in Sm 2 Fe 17 C 1.5 not only stabilizes the hard magnetic phase but also leads to the increase in coercivity.

Magnetic properties of Fe/Pd multilayers grown by electron‐beam evaporation

The magnetic properties of [Fe(20 A)/Pd(x A)]25 multilayers prepared by ultrahigh vacuum electron‐beam evaporation are presented. It is found that the crystal structure of Fe layers change from bcc to fcc when the thickness of Pd layers dPd≥36 A. The saturation magnetization per unit Fe volume at 5 K is enhanced and oscillates with the increment of the thickness of Pd layers, due to the polarization of Pd atoms. The magnetic hysteresis loops of samples indicate low coercive forces, and ferromagnetic coupling between the Fe layers for all Pd thicknesses (6–60 A). The conversion electron Mossbauer spectra measurements proved that the magnetic moment of fcc Fe is the same as that of the bcc Fe. It is also found that the magnetic anisotropy dependence on dPd is similar to that of the saturation magnetization, and relates to the Fe layer structure transition. The Curie temperature of Fe/Pd multilayers decreases monotonously with the increasing of dPd. The low temperature magnetization measurement of Fe/Pd mult...