Experimental observation of disorder-driven hysteresis-loop criticality

We have studied the effect of magnetic disorder on the magnetization revers al process in thin Co/CoO films. The antiferromagnetic CoO layer allows a r eversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature: T-c, we observe a discontinu ous magnetization reversal, whereas smooth magnetization loops occur for T < T-c. Our measurements establish the existence of a disorder-driven critic al point in the nonequilibrium phase diagram. In addition, we observe scali ng behavior in the vicinity of the critical point and determine the critica l exponents <beta> = 0.022 +/- 0.006 and beta delta = 0.30 +/- 0.03 for thi s two-dimensional system.