  The transition frequency between nonthermal coherent magnetic precessions and
ultrafast demagnetization is arguably the most sought after answer in magnetism
science and technology nowadays. So far, it is believed to be in the terahertz
(THz) range. Here, using an ultra-intense low frequency THz bullet, and thin
magnetic layers, we report on experimental evidences that fully coherent
nonthermal THz magnetic switching may never be reachable in conventional
ferromagnetic thin films. At high excitation intensities, while the spins still
coherently precess with the THz magnetic field, the deposited THz energy
initiates ultrafast demagnetization and ultimately material damage. These
series of phenomena are found to take place simultaneously. The reported
experiments set fundamental limits and raise questions on the coupling between
electronic and magnetic systems and the associated structural dynamics on the
ultrafast time scale.
