Aspects
of the optoelectronic performance of thin-film ferromagnetic
materials are evaluated for application in ultrafast devices. Dynamics
of photocarriers and their associated spin polarization are measured
using transient reflectivity (TR) measurements in cross linear and
circular polarization configurations for La0.7Sr0.3MnO3 films with a range of thicknesses. Three spin-related
recombination mechanisms have been observed for thicker films (thickness
of d ≥ 20 nm) at different time regimes (τ),
which are attributed to the electron–phonon recombination (τ
< 1 ps), phonon-assisted spin–lattice recombination (τ
∼ 100 ps), and thermal diffusion and radiative recombination
(τ > 1 ns). Density functional theory (DFT+U) based first-principles
calculations provide information about the nature of the optical transitions
and their probabilities for the majority and the minority spin channels.
These transitions are partly responsible for the aforementioned recombination
mechanisms, identified through the comparison of linear and circular
TR measurements. The same sets of measurements for thinner films (4.4
nm ≤ d < 20 nm) revealed an additional
relaxation dynamic (τ ∼ 10 ps), which is attributed to
the enhanced surface recombination of charge carriers. Our DFT+U calculations
further corroborate this observation, indicating an increase in the
surface density of states with decreasing film thickness which results
in higher amplitude and smaller time constant for surface recombination
as the film thickness decreases.
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