Magnetic localization limit in $T_{C}$ graded ferromagnetic thin films

We have recently demonstrated that the effective Curie temperature ($T_{C}$) of a ferromagnetic alloy thin film can be continuously varied as a function of depth via a corresponding compositional gradient.[1] This work showed that the effective $T_{C}$ can be made to vary continuously over tens of nm. However, over a short enough distance, the system must become localized, with exchange coupling dominating the effects of the compositional gradient. Understanding this localization limit is important for potential applications, as it dictates the length-scale below which this technique stops being a viable engineering tool (at least for itinerant ferromagnets and their thermodynamic properties). To determine the localization limit in this class of system, we have fabricated a series of Co[1-$x$]Cr[$x$] alloy alloy films, where $x$ varies sinusoidally between 0.28 (nominal $T_{C}$ $\approx$ 250 K) and 0.22 ($T_{C}$ $>$ 300 K), and have used polarized neutron reflectometry to study samples of differing oscillation wavelength. These measurements confirm the desired sinusoidal pattern was achieved, and reveal the temperature-dependence of the magnetic depth profile. Results will be presented in the context of mean-field simulations. [1] arXiv:1510.07535 [cond-mat.mtrl-sci].