News / Highlights

N°23 - 23 September 2016 - SPIN TORQUE DRIVEN DYNAMICS OF A COUPLED TWO LAYER STRUCTURE: INTERPLAY BETWEEN CONSERVATIVE AND DISSIPATIVE COUPLING

The general concepts of spin wave theory are adapted to the spin torque driven dynamics of a self-polarized system based on two layers coupled via interlayer exchange (conservative coupling) and mutual spin torque (dissipative coupling). An analytical description of the non-linear dynamics is proposed and validated through numerical simulations. In contrast to the single layer model, the phase equation of the coupled system has a contribution coming from the dissipative part of the LLGS equation.

N°22 - 23 September 2016 - CURRENT DRIVEN MAGNETIZATION DYNAMICS OF A SELF-POLARISED SYNTHETIC FERRIMAGNET

Spin torque driven excitations in spin valves and tunnel junctions are often investigated for a two magnetic layer system for which a polarizer (fixed magnetization) and a free layer can be distinguished. In the search for improved microwave properties and to understand the role of different coupling mechanisms between the magnetic layers, here the excitation spectrum of an exchange coupled two layer synthetic ferrimagnet (SyF) system is investigated numerically with spin momentum transfer acting on both layers simultaneously.

N°21 - 23 September 2016 - MUTUAL SYNCHRONIZATION OF SPIN TORQUE NANO-OSCILLATORS THROUGH A NON-LOCAL AND TUNABLE ELECTRICAL COUPLING

The concept of spin torque driven high frequency magnetization dynamics has opened up the field of spintronics to non-linear physics, potentially in complex networks of dynamical systems. In the scarce demonstrations of synchronized spin-torque oscillators, the local nature of the magnetic coupling that is used have largely hampered a good understanding and thus the control of the synchronization process. Here we take advantage of the non-local nature of an electrical coupling to mutually synchronize spin-torque oscillators through their self-emitted microwave currents.

N°20 - 23 September 2016 - SELF-INJECTION LOCKING OF A VORTEX SPIN TORQUE OSCILLATOR BY DELAYED FEEDBACK

The self-synchronization of spin torque oscillators is investigated experimentally by re-injecting its radiofrequency (rf) current after a certain delay time. We demonstrate that the integrated power and spectral linewidth are improved for optimal delays. Moreover by varying the phase difference between the emitted power and the re-injected one, we find a clear oscillatory dependence on the phase difference with a 2π periodicity of the frequency of the oscillator as well as its power and linewidth.

N°19 - 23 September 2016 - SPIN TORQUE RESONANT EXPULSION OF THE VORTEX CORE FOR AN EFFICIENT RADIO-FREQUENCY DETECTION SCHEME

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state.

N°18 - 23 September 2016 - CONTROLLING THE CHIRALITY AND POLARITY OF VORTICES IN MAGNETIC TUNNEL JUNCTIONS

Static and dynamic control of the chirality and polarity of a magnetic vortex confined in a magnetic tunnel junction is demonstrated. The modes associated with the four chirality/polarity vortex configurations are first explored by resonant excitation with a low power rf current. When the rf power is increased, both the chirality and polarity of the vortex can be resonantly switched, which—as shown by micromagnetic simulations—involves vortex expulsion and renucleation.

N°17 - 23 September 2016 - SPIN-WAVE-BEAM DRIVEN SYNCHRONIZATION OF NANOCONTACT SPIN-TORQUE OSCILLATORS

The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field.

N°16 - 23 September 2016 - HOMODYNE-DETECTED FERROMAGNETIC RESONANCE OF IN-PLANE MAGNETIZED NANOCONTACTS: COMPOSITE SPIN-WAVE RESONANCES AND THEIR EXCITATION MECHANISM

This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo-spin-valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current, and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes.

N°15 - 23 September 2016 - Enhanced modulation rates via field modulation in spin torque nano-oscillators

Spin Transfer Nano-Oscillators (STNOs) are promising candidates for telecommunications applications due to their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning is of interest for Frequency Shift Keying (FSK) concepts to be used in wireless communication schemes or in read head applications. For these technological applications, one important parameter is the characterization of the maximum achievable rate at which an STNO can respond to a modulating signal, such as current or field.

N°14 - 23 September 2016 - EFFICIENT SYNCHRONIZATION OF DIPOLARLY COUPLED VORTEX-BASED SPIN TRANSFER NANO-OSCILLATORS

Due to their nonlinear properties, spin transfer nano-oscillators can easily adapt their frequency to external stimuli. This makes them interesting model systems to study the effects of synchronization and brings some opportunities to improve their microwave characteristics in view of their applications in information and communication technologies and/or to design innovative computing architectures. So far, mutual synchronization of spin transfer nano-oscillators through propagating spin-waves and exchange coupling in a common magnetic layer has been demonstrated.

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