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Reversible and irreversible current induced domain wall motion in spin valve stripes

We present results on current-induced domain wall motion in Co/Cu/CoFeB and Co/Cu/NiFe trilayered stripes. We find that the threshold current densities in our CoFeB spin valve stripes are around 10^6 A/cm^2 at zero magnetic field i.e. about one order of magnitude smaller than in our NiFe spin valve stripes (that is about two orders of magnitude smaller than in single NiFe stripes). The motion of the domain wall can be reversible or irreversible depending on the experimental conditions. In the irreversible regime, we observe switching of the spin valve by irreversible domain wall displacement in the free layer. The domain wall displacement is field-assisted when the applied field acts in the same direction as the spin transfer torque. The switching current density decreases as the applied field increases. This feature can have interesting applications in spintronics devices such as magnetoresistive random access memories. When the applied field torque is opposed to the spin transfer one, and above a threshold field, we observe a reversible displacement of the domain wall (corresponding to a peak in the differential resistance measurements). This can be ascribed to the onset of domain wall fluctuations, which is confirmed by micromagnetic simulations, taking into account the non adiabaticity in the spin transfer model. To understand the origin of the reversible domain wall behavior, we have performed time dependent resistance measurements that show evidences of domain wall fluctuations dominated by telegraph noise. The domain wall fluctuations occur between two well-defined states, corresponding to domain wall pinning centers distant of 400 nm. The dwell times are measured in the two states as a function of both the magnetic field and the current. The main features of the fluctuations are probably linked to the energy form of the pinning potentials.

spintronics; domain wall; spin transfer torque

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