Synthesis and Orbital structure of structure- and valence-controlled layered manganites

我们系统地研究了结构和价控锰氧化物的电学、磁学和光学性质，这些材料表现出光致磁跃迁。以下是主要成果1.通过控制单电子带宽，测量了典型的半金属铁磁体R_&lt；0.6&gt；Sr_&lt；0.4&gt；MnO_3的电阻率和比热的温度变化。我们观察到了电阻率对T^3的依赖关系，这可以用半金属系统中的反常单磁振子散射过程来解释。本文研究了La_&lt；0.7&gt；Ca_&lt；0.3&gt；MnO_3的光致自旋无序及其动力学行为。我们发现，当温度接近相变的临界温度时，S自旋有序态的恢复时间从5K时的30 ns延长到30μ。这一结果清楚地证明了相变过程中弛豫时间的临界减慢。研究了La&lt；0.6&gt；Ca&lt；0.4&gt；MnO_3薄膜的光致电阻率随激发光子能量和温度的变化。在315K的转变温度以上，随着温度的升高，薄膜的光致电阻率变化幅度急剧减小。由于向下自旋载流子的注入增强了光致信号，因此将这种变化归因于铁磁金属态光致自旋无序。这一结果表明，人们可以利用电阻率信号来检测光致自旋无序。

We have systematically investigated electric, magnetic and optical properties of structure- and valencecontrolled manganites which show photoinduced magnetic transitions. The following is a major result.1. The temperature variations of the resistivity and specific heat have been measured for prototypical half-metallic ferromagnets R_<0.6>Sr_<0.4>MnO_3 by controlling the one-electron band width. We have observed the T^3 dependence of the resistivity which is interpreted in terms of the anomalous single-magnon scattering process in the half-metallic system.2. Photoinduced spin disorder and its dynamical behavior have been investigated for La_<0.7>Ca_<0.3>MnO_3. By optical injection of down-spin electrons into the Mn e_g orbital the spin disorder in the ferromagnetic phase is induced. We have found that the recovery time to the spin ordered state is lengthened from 30 ns at 5 K to 30 μs when the temperature approaches to the critical temperature of the phase transition. This result clearly demonstrates the critical slowing down of relaxation time in the phase transition.3. Photoinduced resistivity change in a La_<0.6>Ca_<0.4>MnO_3 film has been investigated with variation of the excitation photon energy and temperature. The magnitude of the photoinduced resistivity change steeply decreases with increasing temperature above the transition temperature of 315 K.Since the photoinduced signal is enhanced by injection of the down spin carriers, the resistivity change is ascribed to the photoinduced spin disorder in the ferromagnetic metallic state. This result indicates that one can detect photoinduced spin disorder with the resistivity signal.