双层钙钛矿(La_(1-x)Gd_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.05)的相分离

Phase separation of bilayered perovskite manganite(La_(1-x)Gd_x)_(4/3)Sr_(5/3)Mn_2O_7(x = 0, 0.05)

采用传统固相反应法制备(La1-xGdx)4/3Sr5/3Mn2O7(x=0,0.05)多晶样品,并通过测量样品的磁化强度与温度的变化曲线(M-T曲线)、电子自旋共振谱(ESR谱)和电阻率与温度的变化曲线(ρ-T曲线),研究了x=0和x=0.05样品的相分离现象.研究结果表明,两样品在低温部分出现了反铁磁与铁磁相互竞争的现象,体现出团簇自旋玻璃态的特征.x=0和x=0.05样品分别在125—375 K和100—375 K范围内观察到类Griffiths相,同时发现掺杂使得三维铁磁有序温度(T3Dc0≈125 K和T3Dc1≈100 K)降低,而对类Griffiths温度(TG≈375 K)没有明显影响.在TG温度以上两样品均表现出纯顺磁特性.其电特性表明,x=0样品在整个测量范围内出现两次绝缘-金属转变,这是由钙钛矿锰氧化物共生现象所致.而x=0.05样品只出现一次绝缘-金属转变,表明掺杂能抑制共生现象的产生.通过对ρ-T曲线的拟合发现两样品在高温部分的导电方式基本都遵循三维变程跳跃的导电方式.

La1-xGdx）4/3Sr5/3Mn2O7（x = 0, 0.05） polycrystalline samples have been prepared by solid state reaction method, and the phase separation phenomena in this samples are investigated by measuring the magnetization-temperature （M- T） curve, electron spin resonance （ESR） curve and resistivity-temperature （ρ-T） curve. For both samples, experimental results suggest there exists competition between ferromagnetic and antiferromagnetic interactions in low temperature range, which reflects a characteristic of cluster spin glass. A Griffiths-like phase is observed in temperature ranges 125-375 K and 100-375 K for x = 0 sample and x = 0.05 sample, respectively. It is found that doping contributes to the decrease of three-dimensional long-range ferromagnetic ordering temperature （from T3D c0≈125 K for x = 0 to T3D c1≈ 100 K for x = 0.05）, but has no obvious effect on the Griffiths-like temperature （TG ≈ 375 K）. Above TG ≈ 375 K, a pure paramagnetic phase appears in both samples. The ρ-T curves reveal two insulator-metal transitions in the entire temperature range for x =0 sample, which is caused by coexistence of the two phases in perovskite manganese oxides. For x = 0.05 sample, however, there exhibits a single insulator-metal transition, indicating that doping can hinder the coexistence phenomenon. It can be seen from the fitted ρ-T curves that the electron conduction mechanism in high temperature range is in accordance with the three-dimensional variable range of hopping conduction.