Relationship of Polaron Exchange with Ferromagnetic and Insulator--Metal Transitions in Doped Manganites

We report the experiment results and data analyses based on a polaron exchange model for La0.7Ca0.3MnO3 and Pr0.7（Sr1-xCax）0.3MnO3 epitaxial thin films. In the polaron exchange model with an energy balance condition, critical temperature of Tc for stable ferromagnetic （FM） ordering should depend on △E as kBTc = E0 exp（-△E/kBTc）, where A E denotes the potential barrier for the exchange polarons to overcome. Using the small polaron hopping model, the resistivity peak temperature Tp is a function of the hopping energy Ehop. The dependence of Tp on Ehop is similar to the dependence of Tc on AE, which reveals that the polaron exchange relates to FM and insulator-metal transitions. The result indicates that the polaron exchange model is a simple way for describing the FM ordering, and is very helpful for understanding of complex doped manganites.     

热处理中Pr0.5Sr0.5MnO3薄膜的电学性能变化和应力弛豫

我们用脉冲激光法在(001)LaAlO3衬底上制备了Pr0.5Sr0.5MnO3薄膜,在900℃的空气中进行后退火处理,研究了退火前后薄膜的电磁性能和晶格结构变化.随着温度降低原位生长薄膜在低温段电阻率迅速上升,表现为绝缘体性质.磁化强度-温度曲线表明薄膜具有顺磁-铁磁-反铁磁的相变,并且存在相分离现象.相比之下,退火后薄膜随着温度降低只出现了顺磁绝缘体-铁磁金属相变,在低温区域一直表现为铁磁金属性.x光衍射实验发现退火使得薄膜在垂直于衬底表面方向的晶格伸长量明显减小,表明退火过程中发生了应力弛豫.为了搞清楚后退火中氧含量和应力弛豫的不同作用,我们进行了(110)SrTiO3衬底上Pr0.5Sr0.5MnO3薄膜的后退火对比实验,结果表明退火中的应力弛豫是导致退火前后薄膜电磁性质差异的主要原因.     

Deposition of MgB2 Superconducting Films on Different Metal Substrates

By a method of hybrid physical-chemical vapour deposition （HPCVD） on three metal substrates of stainless steel, copper and niobium, we deposit MgB2 superconducting films over 1 μm thickness. All of them have zero resistance temperatures To（O） 〉 36 K and critical current densities Jc （IOK, OT） 〉106 A/cm^2. Meanwhile, in the bending test, all the MgB2 superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB2 superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential applications in future.     

不锈钢衬底MgB2厚膜韧性的研究

我们用混和物理化学气相沉积法(HPCVD)在不锈钢衬底上原位制备了以MgB2厚膜(约10微米厚)为过渡层的MgB2超导厚膜(约20微米厚)样品,两层膜总厚约30微米.X光衍射实验表明过渡层为(101)取向的织构膜,表层MgB2超导厚膜接近多晶膜,但有较强(101)取向,两者均含有少量的Mg和MgO杂相.此超导厚膜样品的Tc(onset)=37.8K,Tc(zero)=36.6K,ΔT=1.2K.对此样品的弯曲实验表明,以曲率半径500微米弯曲到180°后,样品仍具有Tc(onset)=37.8K,Tc(zero)=36.4K的超导电性.样品剖面SEM观测表明该膜结构致密,表面厚膜层和过渡层之间连接紧密.样品表面的SEM观测表明虽然样品弯曲导致表面MgB2超导厚膜面出现了裂缝,甚至有小部分膜面的脱落,但过渡层始终紧紧附着在不锈钢衬底上.这表明过渡层MgB2厚膜的存在大大地提高样品整体的柔韧性,展现了不锈钢衬底上的MgB2厚膜超导带(线)巨大的开发潜力和诱人的广阔应用前景.