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Frequency modulation experiments were performed on a spin torque vortex oscillator for a wide range of modulation frequencies, up to 10% of the oscillator frequency. A thorough analysis of the intermodulation products shows that the key parameter that describes these experiments is the deviation sensitivity, which is the dynamical frequency-current dependence. It differs significantly from the oscillator tunability discussed so far in the context of spin-transfer oscillators. The essential difference between these two concepts is related to the response time of the vortex oscillator, driven either in quasisteady state or in a transient regime. [ABSTRACT FROM AUTHOR]

Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel junctions with very low-resistance area product were investigated. A high power of excitations has been obtained characterized by a power spectral density containing a very sharp peak at the fundamental frequency and a series of harmonics. The observed behavior is ascribed to the combined effect of spin-transfer torque and Oersted-Ampère field generated by the large applied dc current. We furthermore show that the synchronization of a vortex oscillation by applying an ac bias current is mostly efficient when the external frequency is twice the oscillator fundamental frequency. This result is interpreted in terms of a parametric oscillator. [ABSTRACT FROM AUTHOR]

Microwave-assisted switching of the magnetization is an efficient way to reduce the magnetic field required to reverse the magnetization of nanostructures. Here, the phase sensitivity of microwave-assisted switching of an individual cobalt nanoparticle is studied using a pump-probe technique. The pump microwave pulse prepares an initial state of the magnetization, and the probe pulse tests its stability against switching. Precession states are established, which are stable against switching. Their basin of attraction is measured and is in qualitative agreement with numerical macrospin calculations. The damping parameter is evaluated using the variable delay pump-probe technique. [ABSTRACT FROM AUTHOR]

While the usual approach to tailor the behavior of condensed matter and nanosized systems is the choice of material or finite-size or interfacial effects, topology alone may be the key. In the context of the motion of magnetic domain walls (DWs), known to suffer from dynamic instabilities with low mobilities, we report unprecedented velocities >600 m/s for DWs driven by spin-transfer torques in cylindrical nanowires made of a standard ferromagnetic material. The reason is the robust stabilization of a DW type with a specific topology by the Œrsted field associated with the current. This opens the route to the realization of predicted new physics, such as the strong coupling of DWs with spin waves above >600 m/s. [ABSTRACT FROM AUTHOR]

Voltage noise measurements were performed in the 3-7 GHz frequency range on magnetic tunnel junctions biased with a dc current. Magnetic noise associated with ferromagnetic resonance excitations is either amplified or reduced depending on the direction of the bias current. This effect is interpreted as the influence of spin transfer torque on the magnetization fluctuations and described using Gilbert dynamics equation including spin transfer torque and effective field terms.

Using a new micro-SQUID setup, we investigate magnetic anisotropy in a single 1000-atom cobalt cluster. This system opens new fields in the characterization and understanding of the origin of magnetic anisotropy in such nanoparticles. For this purpose, we report three-dimensional switching field measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix. We are able to separate the different magnetic anisotropy contributions and evidence the dominating role of the cluster surface.