A comparative study for two different series,La 0.67 Ca 0.33 Mn 1-x Cr x O3 and La 0.67+x Ca 0.33-x Mn 1- x Cr x O 3 ,is performed with x changing from 0.04 to 0.08 through measurements of zero-field resistivity(ρ), magnetoresistance(MR)and ac susceptibility(χ)as functions of temperature(T).For the second group,a single insulator-metal transition was characterized by the resistivity maximum appears inρvs.T curve.For the first group,besides the resistivity peak appearing at higher temperatures,the sample shows evidence for the existence of another peak appearing at lower temperatures.Compared to the first group,the second group shows a significant enhancement in MR effect,while the measurement ofχ-T dependence does not display obvious differences for the two series.Different behaviors observed in the two series are discussed by considering possible exchange interaction between Mn3+ion and Cr3+ion.
The effects of Pr on the structure and magnetic properties of PrxFe60.5-xPt39.5 alloys (x = 0, 0.5, 1.0, and 1.5) were investigated. X-ray diffraction data indicated that the phase transition temperature of FePt based alloys from disordered face-centered-cubic to ordered face-centered-tetragonal cubic decreases with the increase in Pr concentration. Pr plays the role of a grain refiner and it can enhance the exchange coupling between soft magnetic phase and hard magnetic phase. The results indicate that the replacement of Fe by Pr can significantly improve the remanence and coercivity of the Fe60.5Pt39.5 alloy. These results can be explained on the basis of phase transformation and microstructure. Both the remanence ratio and coercivity of the FePt based alloy as a function of the Pr content are increased by the optimum addition of 0.5 at.% Pr.
The effect of Nd addition on the structure, phase transformation and magnetic properties of FePt based alloys was investigated. The results indicated that the transition temperature from ordered FCT to disordered FCC phase decreased with increasing Nd concentration, but for alloys quenched rapidly from the γ phase region into ice-water, it increased with increasing Nd. The Nd element not only effectively reduced the grain size of the ordered phase but also decreased the degree of the ordered phase and refined the grains of the FCC matrix phase. The remanence ratio and coercivity of the FePt based alloy as a function of the Nd content had maximum values, respectively.
Bulk samples with nominal composition Zn0.95Co0.05O and Zn0.92Co0.05Mn0.03O were fabricated by a solid-state reaction method at 600℃.X-ray diffraction experiment showed that the peaks of secondary phase Co3O4 with a cubic structure were visible in both samples,besides the main peaks of wurtzite structure as ZnO.Magnetization measurement indicated that doping Co alone can induce ferromag- netism in ZnO itself,while the introduction of Mn significantly enhances ferromagnetism.However, both samples showed different magnetic behavior at temperatures below 50 K.It was also noted that ferromagnetic coupling interaction was weakened due to the presence of antiferromagnetic Co3O4.
WANG YongQiang YUAN SongLiu SONG YunXing LIU Li TIAN ZhaoMing LI Pai ZHOU YuanMing LI YunLong YIN ShiYan
The electrical and magnetic properties of bilayer manganites La1.4Sr1.6Mn1.96TE0.04O7 (TE=Mn, Fe, Ti, Nb) were investigated. Doping caused obvious changes in electrical and magnetic behaviors such as decrease of insulator-metal transition and magnetic transition temperatures, increase of peak resistivity, and different magnetoresistance effect. These changes had a significant degree of correlation with the valence of doped ions. From Fe, Ti to Nb doping, the effect was doubly stronger. The results could be well understood by considering the different destructions on double-exchange interaction and different influences on lattice distortion caused by Fe, Ti and Nb doping. The temperature dependence of magnetization measured at high field showed that the influence of doping was greatest near three-dimensional magnetic transition temperature of parent phase.
