Stoner vs. Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization
Thomas J. Silva, Hans T. Nembach, Justin M. Shaw, Emrah Turgut, Dmitriy Zusin, Dominik Legut, Karel Carva, Ronny Knut, Cong Chen, Zhensheng Tao, Stefan Mathias, Martin Aeschlimann, Peter Oppeneer, Henry Kapteyn, Margaret Murnane, Patrik Grychtol
Understanding how the electronic band structure of a ferromagnetic material is modified during laser-induced demagnetization on femtosecond timescales has been a long-standing question in condensed matter physics. Here, we use ultrafast high harmonics to measure the time-, energy-, and angle-resolved M-edge magnetic asymmetry spectra for Co films after optical pumping to induce ultrafast demagnetization. This provides a complete data set that we can compare with advanced ab initio magneto-optical calculations. This allows us to explicitly identify and quantify the dominant mechanisms that contribute to ultrafast demagnetization on timescales up to several picoseconds, including a transient reduction in the exchange splitting and the excitation of ultrafast magnons. Surprisingly, we find that the magnon contribution to ultrafast demagnetization is already strong on sub-picosecond timescales, while the reduction in exchange splitting persists to several picosecond timescales.