Chiral Magnetization States In Continuously Exchange Modulated Ferromagnets

We have recently shown that compostional grading can be used to fabricate a ferromagnetic film with a continuous distribution of "local" Curie temperatures T_C, corresponding to variations in exchange strength [1]. This led to interesting functionality, a quasi phase-bondary between strongly and weakly magnetized regions that can be continuously and reversibly moved along the growth axis. Here we consider the response of such an exchange graded structure to magnetic field. Since coercivity generally scales with T relative to T_C, if the magnetic properties are sufficiently localized, the structure should reverse magnetization in a non-collinear fashion. We have fabricated Co_1-xRu_x films with x continuously modulated in a triangular waveform along the growth direction. The T_C of Co_1-xRu_x scales with x, and we have used polarized neutron reflectometry (PNR) to show that the local T_C is indeed modulated across distances as small as 4.9 nm. Field-dependent measurements at 300 K show that the sample magnetization of a 10 nm modulated sample does not reverse uniformly, instead forming a chiral state with the magnetizations of the highest local T_C regions lagging those of the lowest local T_C regions by approximately 20 degrees.
[1] B. J. Kirby et al., PRL, 116, 047203 (2016).