Magnetic properties of epitaxial CoCr films with depth-dependent exchange-coupling profiles

Abstract

We present a study of the compositional and temperature-dependent magnetic properties of epitaxial CoCr thin films whose composition has a bathtublike depth profile $\mathrm{Co}/\mathrm{C}{\mathrm{o}}_{1\ensuremath{\rightarrow}1\text{\ensuremath{-}}{x}_{\mathrm{c}}}\mathrm{C}{\mathrm{r}}_{0\ensuremath{\rightarrow}{x}_{\mathrm{c}}}/\mathrm{C}{\mathrm{o}}_{1\text{\ensuremath{-}}{x}_{\mathrm{c}}}\mathrm{C}{\mathrm{r}}_{{x}_{\mathrm{c}}}/\mathrm{C}{\mathrm{o}}_{1\text{\ensuremath{-}}{x}_{\mathrm{c}}\ensuremath{\rightarrow}1}\mathrm{C}{\mathrm{r}}_{{x}_{\mathrm{c}}\ensuremath{\rightarrow}0}/\mathrm{Co}$ with the highest Cr concentration $({x}_{\mathrm{c}})$ at the center of the sample. Polarized neutron reflectometry (PNR) shows that the effective Curie temperature varies as a function of depth and exhibits a minimum in the center of the structure. Correspondingly, we observe that the effective coupling between the two outer Co layers is strongly dependent on the magnetization of the graded CoCr spacer and can be continuously tuned via ${x}_{\mathrm{c}}$ and temperature $T$. In particular, for ${x}_{\mathrm{c}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}0.28$, magnetometry reveals a transition from one-step to two-step reversal behavior for temperatures $T$ > 260 K, indicating a transition from a fully correlated magnetic film structure to an uncoupled system containing effectively two independent magnetic sublayers. Corroborating evidence of the temperature-dependent coupling of the top and bottom regions for ${x}_{\mathrm{c}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}0.28$ was revealed by PNR, which demonstrated the field-dependent occurrence of antiparallel magnetization alignment on opposite interfaces at sufficiently high temperatures only.