Spin-lattice relaxation phenomena in manganite La _0.7 Sr _0.3 MnO ₃ thin films

Abstract

2Time-resolved magneto-optics was used to study spin-lattice relaxation dynamics in thin epitaxial La 0.7 Sr 0.3 MnO 3 films. Two distinct recovery regimes of the ferromagnetic order can be resolved upon photoexcitation, which manifest themselves by two different relaxation times. A pump pulse energy independent spin-lattice relaxation time can be deduced. Due to a weak spin-orbit coupling in manganites this spin-lattice relaxation time is much longer than in ferromagnetic metals. Heat flow into the substrate sets the ultimate recovery speed of the ferromagnetic order and allows for a determination of heat diffusion properties of manganite films. PACS number(s): 78.47.+p , 75.40.Gb 3 Significant research effort has been focused on ultrafast magnetization and demagnetization dynamics in the recent years. An all-optical technique employing short laser pulses may be of interest, as field pulse assisted magnetization switching cannot be faster than a magnetization precession cycle 1 . However, a special demagnetization mechanism is required -an optical pulse can barely change the magnetization directly.Ultrafast magnetization dynamics has been studied in ferromagnetic 2-4 and various other material systems 5,6 . On the other hand, colossal magnetoresistive manganites are promising candidates for ultrafast spin control due to coupled spin, charge, orbital and lattice degrees of freedom 7 .Perovskite manganites exhibit an insulator to metal transition usually related to a paraferromagnetic phase transition at the Curie temperature. The origin of ferromagnetism is double-exchange interaction 8 yielding a strong correlation between magnetization and charge transport properties. Photoinduced effects on the picosecond timescale for the ferromagnetic phase of manganites have been previously studied by conventional pumpprobe spectroscopy [9][10][11] . The first report on laser-induced demagnetization was presented by Matsuda et al. 9 . A rapid change of the photoinduced absorption within 1-2 ps and a following gradual change up to about 200 ps was found. The ultrafast component is attributed to electron-phonon thermalization, whereas the subsequent slower component with a temperature dependent time constant to spin and lattice thermalization. However, the evaluated time scales for the demagnetization dynamics were based on changes in optical absorption. Thus, spin channels, such as spin-lattice relaxation, have not been discerned in the absorption changes. 4In the present work, magnetization dynamics in epitaxial La 0.7 Sr 0.3 MnO 3 (LSMO) films was investigated by time-resolved magneto-optics where the Kerr rotation was employed as a reliable probe of magnetization 12 . Special emphasis was put on the determination of the spin-lattice relaxation time, a key quantity to answer the question about the fundamental speed limit of spin manipulation in this material. Upon arrival of a short laser pulse a collapse of the ferromagnetic order takes place accompanied by spin-lattice thermalization. The deduced spin-lattice tim...

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