Current-induced magnetic superstructures in exchange-spring devices

We investigate the potential to use a magnetothermoelectric instability that may be induced in a mesoscopic magnetic multilayer (F/f/F) to create and control magnetic superstructures. In the studied multilayer two strongly ferromagnetic layers (F) are coupled through a weakly ferromagnetic spacer (f) by an “exchange spring” with a temperature-dependent “spring constant” that can be varied by Joule heating caused by an electrical dc current. We show that in the current-in-plane configuration a distribution of the magnetization, which is homogeneous in
the direction of the current flow, is unstable in the presence of an external magnetic field if the length L of the sample in this direction exceeds some critical value Lc ~ 10 µm. This spatial instability results in the spontaneous formation of a moving domain of magnetization directions, the length of which can be controlled by the bias voltage in the limit L Lc. Furthermore, we show that in such a situation the current-voltage characteristic has a plateau with hysteresis loops at its ends and demonstrate that if biased in the plateau region the studied device functions as an exponentially precise current stabilizer.