Effect On Magnetic And Electrical Transport Properties In Manganite Granular System Due To The Introduction Of Different Magnetic Phases

In the past two decades, substantial attention has been focused on colossalmagnetoresistance(CMR) and extrinsic magnetoresistance due to their potential practicalapplication. As a matter of fact, extrinsic magnetoresistance gained much more attentionrecently as its driving magnetic field is lower compared with the high magnetic fielddriving CMR. However, the effect of grain boundary plays an important role in theextrinsic mangnetoresistance and electrical and magnetic properties. Therefore, we tend tomodify the gain boundaries of manganites by introduction of the second phase, in order toinvestigate the effect of the modified gain boundaries on the electrical and magnetictransport properties. The main investigations are shown as follows:1) The composites La_2/3Ca_1/3MnO₃(LCMO)/CuMn₂O₄ were fabricated. We applied theexperimental formulaρ(T)=ρ₀+ρ₁Tⁿ to fit the low temperature region electricaltransport: for x=0 and 0.04, n=2; For x=0.1 and 0.2, n=3. The extrinsicmagnetoresistance is enhanced substantially. We consider with the increasing amountof the ferrimagnetic insulator CuMn₂O₄, the magnetic disorder of the grain boundariesincreases and have a great effect on the electric and magnetic properties.2) The ferromagnetic insulator CuFe₂O₄ was introduced into the LCMO matrix tofabricate the LCMO/CuFe₂O₄ composites. For x=0.1, a substantial thermal hysteresisbehavior is observed at the vicinity of the metal-insulator transition temperature T_IM.This thermal hysteresis is able to be suppressed by applied magnetic field. Applyingcooling field 0.1 T on the x=0.1 composite from room temperature to 110K, we foundthe exchange bias behavior: the coactivity H_C=100 Oe, and the bias field H_ex=10 Oe.This exchange bias behavior supplies the proof to the antiferromagnetic couplingbetween LCMO and CuFe₂O₄. At H=3 T, there are two platform magnetoresistancefor all the composites; we also consider this behavior origins from theantiferromagnetic coupling between LCMO and CuFe₂O₄. 3) We fabricate the LCMO/LaMnO₃ composites where LaMnO₃ is antiferromaganeticinsulator. We applied the experimental formulaρ(T)=ρ₀+ρ₁Tⁿ to fit the lowtemperature region electrical transport: for x=0, n=3; for x=0.05, n=4; for x=0.15,n=6; for x=0.25, n=8; the n=6 and n=8 are contradictions with the max valueof n is 4.5(the electron-magnon) as far as we know. At 0.3 T and 3 T, the lowtemperature region magnetoresistance of all the composites enhance substantiallycompared with the pure LCMO.4) Through sol-gel and different sintering temperature, we gained LCMO particles ofdifferent size. The unusual low temperature minimum is observed in the electricaltransport of LCMO particles of small size. As grain size is closely related to Coulombblockade(CB), corresponding changes are made to a phenomenological model whichfailed to fit the low-temperature behavior before, and this modified model is inexcellent accord with the experiment date upon a field or zero field. This result givesa strong support to consider CB as one of the crucial origins of this unusual minimumbehavior.5) Samples of La_1-xBa_xMnO₃(0.33≤x≤0.95) have been fabricated by standard solid statereaction. Microstructural studies show that manganites La_1-xBa_xMnO₃(0.33≤x≤0.95)begin structural phase separation into La_0.67Ba_0.33MnO₃ and BaMnO₃ for x＞0.33.These composites form a cellular-like structure when the volume faction ofLa_0.67Ba_0.33MnO₃(f_LBMO) is near the percolation threshold(f_C). The percolationthreshold(f_C) for our composites is 0.18. This result is not Consistent with theprevious results which prefer smaller percolation threshold value. This could beattributed to the contribution of grain boundaries. This gain-boundary contributionalso induces the large low-temperature bump in electrical transport. The criticalexponent t gained from the good fitting for the experimental data is 1.6 at 150 K and1.7 at 300 K, which is in good agreement with the previous universal result: t=1.6-2.0 for the three dimensional space.