  We investigate the impact of non-collinear dual optical excitation on the
magnetization precession in a permalloy thin film using two ultrashort laser
pulses. By analyzing the magnetization dynamics using time-resolved
magneto-optical methods, we find that the excitation with two ultrashort
optical pulses introduces a long-lasting modification of the electron system
seen as a sizable decrease of the precession frequency and significant increase
(about 25%) of the decay time. Our results reveal that the observed effect
strongly depends on the respective polarization of the two excitation pulses
and the time delay between two optical pulses. Our findings indicate the
occurrence of a new nonlinear opto-spin effect during photoexcitation with two
interfering optical pulses which can potentially be observed in various
materials and at different photon wavelengths.
