Magnetic Entropy Dynamics in Ultrafast Demagnetization

Development of femtosecond laser sources and magneto-optical pump-probe techniques enabled measurements of ultrafast demagnetization in ferromagnets. It is necessary to measure two quantities to thermodynamically describe a magnetic state. In particular, measuring the magnetic entropy dynamics of a material in addition to its magnetization dynamics is crucial for fully understanding the transient magnetic state.

Time-resolved magneto-optical Kerr effect measurements of magnetization and magnetic entropy dynamics were made for ferromagnetic Co/Au/glass thin films. An intense 1030 nm pump pulse excited the film and a weak 515 nm time-delayed probe pulse was detected after reflecting off the sample. An external magnetic field was applied along the film surface. Different powers of the pump were used, increasing the film temperature to Curie temperature. Measurements revealed unexplored properties of the magnetic state at ultrafast timescales. Results will be compared to predictions of the Landau model and computer simulations.