Dynamical torque in Co_xFe_3–xO₄ nanocube thin films characterized by femtosecond magneto-optics: a π-shift control of the magnetization precession

For spintronic devices excited by a sudden magnetic or optical perturbation, the
torque acting on the magnetization plays a key role in its precession and damping. However the torque itself can be a dynamical quantity via the time dependent anisotropies of the system. A challenging problem for applications is then to disentangle the relative importance of various sources of anisotropies in the dynamical torque, such as the dipolar field, the crystal structure or the shape of the particular interacting magnetic nanostructures. Here, we take advantage of a range of colloidal cobalt ferrite nano-cubes assembled in 2D thin films under controlled magnetic fields to demonstrate that the phase φprec of the precession carries a strong signature of the dynamical anisotropies. Performing femtosecond magneto-optics, we show that φprec displays a π-shift for a particular angle θH of an external static magnetic field H. θH is controlled with the cobalt concentration, the laser intensity as well as the inter-particles interactions. Importantly it is shown that the shape anisotropy, which strongly departs from the one of equivalent bulk thin films or individual non-interacting nanoparticles, reveals the essential role played by the interparticles collective effects. This work shows the reliability of a non-invasive optical approach to characterize the dynamical torque in high density magnetic recording media made of organized and interacting nanoparticles.