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

用La_<0.7>Ca_<0.3>MnO_3薄膜（T_c= 262 K）研究了光无序自旋系统的临界行为。在对应于电荷转移激发T&lt;T_c的3.05eV光子能量的激光脉冲激发下，J-能隙跃迁的光谱权重增加，表明铁磁相的自旋关联被抑制。我们研究了弛豫时间的温度变化。弛豫时间从低温侧接近T_c时临界增加，|T/T_c-1| ^<-1.39>对于T&gt; 200 K。在强激发条件下，相同的标度关系在下至10 K的宽温度范围内成立。用二级相变的动力学标度理论解释了这些结果，所观察到的指数与三维Hiesenberg模型符合得很好.为了研究依赖于光激发强度的临界慢化的起源，我们用La_<0.7>Ca_<0.3>Mn_<1-x>Cr_xO_3薄膜进行了类似的实验.已知B位点取代会破坏电荷和/或轨道交替的相干性。我们观察到Cr掺杂抑制了弛豫时间的临界行为，这表明B位杂质导致光生电荷有序涨落的相关长度减小，我们还研究了电子带宽对临界慢化的影响。La_ Sr_ MnO_3的禁带宽度<0.7><0.3>大于La_<0.7>Ca_<0.3>MnO_3。在La_Sr_MnO_3中也观察到类似的临界弛豫现象<0.7><0.3>。因此，我们发现，即使在宽带宽的系统中，光致自旋无序和由此产生的跳跃振幅的随机性也会引起电荷有序涨落。

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.