A study of magnetic domain formation and motion in perpendicularly magnetized ultrathin film using the magnetic ac-susceptibility

The Complex, low-frequency ac magnetic susceptibility $\chi $ has been used to study magnetic domain formation and motion in perpendicularly magnetized ultrathin Fe films on a 2ML Ni/W(110) substrate. Analysis of the real and imaginary part of $\chi $ reveals that below a characteristic temperature T$_{sat}$, the stripe-domain density cannot change quickly enough to maintain equilibrium. This is due to the fact that domain wall creation and/or annihilation is itself an activated process, with nucleation energy E$_{n}$, distinct from the pinning of existing domain walls by defects in film structure, with activation energy E$_{a}$. T$_{sat}$, is set by the time scale of the measurements, which is determined by the rate of change of temperature (R). The Magnetic susceptibility was recorded as a function of temperature at different heating rates between 0.03 (K/s) to 1 (K/s). Our results show that below heating rate R$_{c }$(= 0.2 K/s for 1.5ML Fe film) the susceptibility peak temperature (T$_{peak})$ decreases as dT/dR = -200.0 (s). Above R$_{c}$, the susceptibility peak temperature increases as dT/dR = 16.6 (s). Preliminary model calculation show the movement of T$_{peak}$ is due to the change in T$_{sat}$ as the heating rate changes. R$_{c }$is set by the relative values of E$_{n }$and E$_{a}$.