Experimental Evidence for a Dynamic Phase Transition in Ultra-thin Magnetic Films*

We present the first convincing evidence for an experimental observation of a Dynamic Phase Transition (DPT) in a magnetic system: an ultra-thin [Co(0.4nm)/Pt(0.7nm)]$_3$ multilayer, which is well modeled by a two-dimensional kinetic Ising system. This DPT, as a function of the period $P$ of an applied alternating magnetic field, has been observed previously in simulations of magnetic systems [1]. For several values of $P$ and bias field $H_b$, the magnetization was measured for 50 cycles of the field [2]. The order parameter, which was identified in simulations as the magnetization averaged over the {\it i}th cycle, $Q_i$, was obtained from the experimental data as a time series. Kinetic Monte Carlo simulations produced close agreement with the experimental data for the order parameter averaged over the final 30 cycles, $$, as a function of $P$ and $H_b$. The experimental fluctuations in the order parameter are also consistent with a DPT. [1] S.W. Sides, P.A. Rikvold, and M.A. Novotny, Phys. Rev. Lett. {\bf 59}, 2710 (1999). [2] D.T. Robb, Y.H. Xu, O. Hellwig, A. Berger, M.A. Novotny, and P.A. Rikvold, submitted to PRL.