Ultrafast demagnetization of ferromagnetic iron following optical vs terahertz excitation

Ultrafast laser-induced demagnetization of ferromagnets is of fundamental relevance for ultrafast spintronics. An important open question is how angular-momentum transfer from the electron spins to the crystal lattice depends on the shape (nonthermal vs thermal) of the electron distribution directly after excitation [1]. To tackle this issue, we perform a direct comparison of magnetization dynamics of an iron thin film triggered by an ultrashort optical pump pulse (photon energy 3 eV, strongly nonthermal perturbation) and a terahertz pump pulse (4 meV, quasi-thermal perturbation) [2]. By varying the polarity of the magnetic field and the THz pump field, we are able to extract the signals related solely to the demagnetization process in the regime linear in the pump fluence. Our results show that on time scales of 100 fs and slower, the dynamics of ultrafast demagnetization doesn’t rely on the initial distribution of the excited electrons. This observation is consistent with an analytical model, in which the driving force of the magnetization change is proportional to the transient excess energy of the electronic system, independent of the precise shape of the electron distribution.
[1]B. Koopmans et al., Nat. Materials 9, 259-265 (2010)
[2]S. Bonetti et al., PRL 117, 087205 (2016)