Studies of hysteresis loop criticality in ultrathin Gd(0001)/W(110)-films

We report an experimental study on the disorder dependence of hysteresis loop behavior for ultrathin epitaxial Gd-films. The use of ultrathin films with in-plane orientation of the magnetization allows the suppression of dipolar effects and enables us to mimic theoretical models of collective magnetization reversal. The sample disorder is tuned by varying the annealing temperature in the range of T-anneal = 470 K-870 K after room temperature deposition of Gd(0001)/W(110)-films. In accordance with theoretical predictions, we find two regimes for the magnetization reversal: (a) for large disorder, the magnetization changes gradually, i.e., the reversal processes of individual sample parts are uncorrelated and (b) for small disorder, the magnetization reversal is jump-like, i.e., major parts of the sample switch collectively. The transition between these two regimes is abrupt, which confirms the theoretically predicted existence of a critical point in the non-equilibrium phase diagram.