  We systematically investigate the influence of annealing effect on terahertz
(THz) generation from CoFeB based magnetic nanofilms driven by femtosecond
laser pulses. Three times enhancement of THz yields are achieved in W/CoFeB
through annealing effect, and double boosting is obtained in Pt/CoFeB. The
mechanism of annealing effect originates from the increase of hot electron mean
free path induced by crystallization, which is experimentally corroborated by
THz transmission measurement on time-domain spectroscopy. Comparison studies of
the thickness dependent THz efficiency after annealing are also implemented,
and we eventually conclude that annealing and thickness optimization are of
importance for scaling up THz intensity. Our observations not only deepen
understanding of the spintronic THz radiation mechanism but also provide normal
platform for high speed spintronic opto-electronic devices.
