マンガン酸化物の光誘起磁気相転移とその超高速ダイナミックス

利用La_<0.7>Ca_<0.3>MnO_3薄膜（T_c= 262K）研究了光无序自旋体系的临界行为。与电荷转移激发T<T_c对应的光子能量为3.05eV的激光脉冲激发导致j隙跃迁的谱权增加，表明铁磁相的自旋相关受到抑制。我们已经研究了弛豫时间的温度变化。当温度为200K时，从低温侧接近T_c时，弛豫时间急剧增加为|T/T_c-1|^<-1.39>。在强激发条件下，相同的标度关系在~ 10K的宽温度范围内保持不变。用二阶相变的动力学标度理论解释了这些结果，并且观测到的指数与三维Hiesenberg模型很好地吻合。为了研究依赖于光激发强度的临界慢化的来源，我们用La_<0.7>Ca_<0.3>Mn_<1-x>Cr_x O_3薄膜进行了类似的实验。已知b位取代会破坏电荷的相干性和/或轨道的交替。我们观察到cr掺杂抑制了弛豫时间的临界行为，这表明由于b位杂质引起的光诱导电荷有序波动的相关长度减少。我们还研究了电子带宽度对临界慢化的影响。La_<0.7>Sr_<0.3>MnO_3的带宽比La_<0.7>Ca_<0.3>MnO_3的带宽大。在La_<0.7>Sr_<0.3>MnO_3中也观察到类似的临界弛豫现象。因此，我们发现光诱导的自旋无序和由此产生的跳跃幅度的随机性即使在宽带系统中也会引起电荷有序的波动。

Critical behaviors of a photo-disordered spin system have been investigated using a La_<0.7>Ca_<0.3>MnO_3 thin film (T_c= 262K). Laser pulse excitation with the photon energy of 3.05eV corresponding to the charge transfer excitation T<T_c induces increase in spectral weight of the J-gap transition, which indicates suppression of spin correlation in the ferromagnetic phase. We have investigated temperature variation of relaxation times. The relaxation time increases critically on approaching to T_c from the low temperature side as |T/T_c-1|^<-1.39> for T>200K. In a strong excitation condition, the same scaling relation holds in the wide temperature range down to 〜10K. These results are explained in terms of the dynamical scaling theory for the second-order phase transition, and the observed exponent is in good agreement with the three-dimensional Hiesenberg model.In order to investigate origin of the critical slowing down which is dependent on the photoexcitation intensity we have carried out similar experiments using La_<0.7>Ca_<0.3>Mn_<1-x>Cr_x O_3 films. B-site substitution is known to destroy the coherence of the charge and/or orbital alternation. We have observed that Cr-doping suppresses the critical behavior of the relaxation time, which indicates the reduction of correlation length of photoinduced charge-ordering fluctuations due to the B-site impurities.We have also investigated an effect of the electron band-width on the critical slowing down. The band-width is larger in La_<0.7>Sr_<0.3>MnO_3 than in La_<0.7>Ca_<0.3>MnO_3. The quite similar behavior of critical relaxation phenomena has been observed in La_<0.7>Sr_<0.3>MnO_3. Therefore, we have found that the photoinduced spin-disorder and resultant randomness of the hopping-amplitude can cause charge-ordering fluctuations even in the wide band-width system.