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

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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.