The Dynamic Phase Transition in Ultrathin Co/Pt-Multilayer Films: Experimental Evidence and Comparison with Simulations

The dynamic phase transition (DPT), observed in numerical simulations of magnetic systems [1,2], manifests itself by the spontaneous occurrence of a non-vanishing period-averaged magnetization (the order parameter $Q$) when the frequency $f$ of an applied alternating magnetic field exceeds a critical value $f_c$. Near $f_c$, the DPT shows all common characteristics of a second-order phase transition. Our experimental studies of ultrathin Co/Pt-multilayers provide the first strong experimental evidence of a DPT. The multilayer structure results in perpendicular anisotropy and negligible demagnetizing effects [3]. We measure out-of-plane magnetization time series by the polar-Kerr effect as a function of $f$ and an applied bias field $H_b$, observing a sharp increase in $Q$ as $f$ is increased above $f_c$. In addition, we see sharp switching of $Q$ as $H_b$ is changed from positive to negative values. The data sets allow the assembly of an experimental phase diagram. Detailed comparison with simulations of a kinetic Ising model provides strong evidence that our data represent the first unequivocal experimental observation of the DPT. [1] S.W.\ Sides et al., Phys.\ Rev.\ Lett.\ {\bf 81}, 834 (1998) [2] B.\ Chakrabarti and M.\ Acharyya, Rev.\ Mod.\ Phys.\ {\bf 71}, 847 (1999) [3] Y.Yafet and E.M.\ Gyorgy, Phys.\ Rev.\ B {\bf 38}, 9145 (1988).