International audienceTime-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. Spin-polarized current-induced magnetization switching (CIMS) has now been reported in many experimental works involving a wide variety of geometries including point contacts, nanopillars (spin valves or tunnel junctions), and nanowires with or without notches. 1 These systems are extensively studied, in part, because they hold the potential for applications in spin-transfer magnetic random access memory. 2 Interest in materials with perpendicular magnetic anisotropy (PMA) has grown considerably as a pathway for lowering the critical current required to switch the magnetization while maintaining thermal stability as compared with in-plane systems. 3,4 This interest has, to a large degree, stemmed from calculations of the switching behavior using a macrospin approximation.

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The spin-torque switching of metallic nanopillar spin-valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the DC current applied to the device. A state diagram presents a comprehensive graph the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagram are studied for perpendicular devices and the influence of lateral size, temperature and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results whereas if the symmetry is broken a qualitative agreement is obtained. Finally the possible origins of the symmetry break are discussed in the light of analytical model and numerical simulations.2