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In recent years, spin–orbit effects have been widely used to produce and detect spin currents in spintronic devices. The peculiar symmetry of the spin Hall effect allows creation of a spin accumulation at the interface between a metal with strong spin–orbit interaction and a magnetic insulator, which can lead to a net pure spin current flowing from the metal into the insulator. This spin current applies a torque on the magnetization, which can eventually be driven into steady motion. Tailoring this experiment on extended films has proven to be elusive, probably due to mode competition. This requires the reduction of both the thickness and lateral size to reach full damping compensation. Here we show clear evidence of coherent spin–orbit torque-induced auto-oscillation in micron-sized yttrium iron garnet discs of thickness 20 nm. Our results emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current.

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It is demonstrated that the threshold current for damping compensation can be reached in a 5  μm diameter YIG(20  nm)|Pt(7  nm) disk. The demonstration rests upon the measurement of the ferromagnetic resonance linewidth as a function of I(dc) using a magnetic resonance force microscope (MRFM). It is shown that the magnetic losses of spin-wave modes existing in the magnetic insulator can be reduced or enhanced by at least a factor of 5 depending on the polarity and intensity of an in-plane dc current I(dc) flowing through the adjacent normal metal with strong spin-orbit interaction. Complete compensation of the damping of the fundamental mode by spin-orbit torque is reached for a current density of ∼3×10(11)  A·m(-2), in agreement with theoretical predictions. At this critical threshold the MRFM detects a small change of static magnetization, a behavior consistent with the onset of an auto-oscillation regime.

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 spinwaves and exchange coupling in a common magnetic layer has been demonstrated. Here we show that the dipolar interaction is also an efficient mechanism to synchronize neighbouring oscillators. We experimentally study a pair of vortex-based spin transfer nano-oscillators, in which mutual synchronization can be achieved despite a significant frequency mismatch between oscillators. Importantly, the coupling efficiency is controlled by the magnetic configuration of the vortices, as confirmed by an analytical model and micromagnetic simulations highlighting the physics at play in the synchronization process.

The exfoliation of two naturally occurring van der Waals minerals, graphite and molybdenite, arouse an unprecedented level of interest by the scientific community and shaped a whole new field of research: 2D materials research. Several years later, the family of van der Waals materials that can be exfoliated to isolate 2D materials keeps growing, but most of them are synthetic. Interestingly, in nature, plenty of naturally occurring van der Waals minerals can be found with a wide range of chemical compositions and crystal structures whose properties are mostly unexplored so far. This Perspective aims to provide an overview of different families of van der Waals minerals to stimulate their exploration in the 2D limit.

International audienceWe study experimentally with submicrometer spatial resolution the propagation of spin waves in microscopic waveguides based on the nanometer-thick yttrium iron garnet and Pt layers. We demonstrate that by using the spin-orbit torque, the propagation length of the spin waves in such systems can be increased by nearly a factor of 10, which corresponds to the increase in the spin-wave intensity at the output of a 10 μm long transmission line by three orders of magnitude. We also show that, in the regime, where the magnetic damping is completely compensated by the spin-orbit torque, the spin-wave amplification is suppressed by the nonlinear scattering of the coherent spin waves from current-induced excitations

We present a study of the magnetoresistance of highly oriented pyrolytic graphite (HOPG) as a function of the sample size. Our results show unequivocally that the magnetoresistance reduces with the sample size even for samples of hundreds of micrometers size. This sample size effect is due the large mean free path and Fermi wavelength of carriers in graphite and may explain the observed practically absence of magnetoresistance in micrometer confined small graphene samples where quantum effects should be at hand. These were not taken into account in the literature yet and ask for a revision of experimental and theoretical work on graphite.PACS numbers: 81.05. Uw,72.80.Cw Graphitic systems are nowadays a field of intensive activity [1]. There have been observations of quantum Hall effect in HOPG [2,3] as well as very large anisotropy in the electrical conductance (ratio between current parallel to perpendicular to the graphene planes) larger than 10 4 at room temperature [1]. Graphite looks as a good conductor in plane and an insulator between planes leading to a weak screening to external electric fields [4]. This means that an external electric field penetrates by tens of nanometers in graphite, in contrast to a normal metal where the field is screened in the first atomic layers. In fact the dielectric constant of graphite at optical frequencies is positive, insulator like (ǫ plane = 5.6 + i7.0, ǫ c = 2.25) [5,6]. Electric field microscopy [7] detects that regions of graphite are more insulating than others upon the interconnections of graphite planes produced by defects and their overall density that influences the density of states and the Fermi level. The material exhibits a huge magnetic field driven metal-insulator transition [1,8]. Its band structure and interband transitions with electron and hole carriers manifest in a huge ordinary magnetoresistance (OMR) [2,9]. All these effects happen in macroscopic size samples of the order of millimeters.A large research activity has been recently started on a few graphene layers (FLG) [10] with typical size of a few microns. Strikingly, the OMR in FLG samples is practically suppressed even at T < 4 K, in contrast to bulk HOPG or Kish graphite where the OMR is 1000% at B 0.5 T at low temperatures [1]. Earlier work with graphite samples in the micrometer range showed similar behavior [11,12]. However, the question on what happens when the graphite sample size is reduced has not been correctly addressed and the experimental data may need a new interpretation. The size of the sample is very important for defining the properties of the system because the de-Broglie wavelength for massless Dirac6 m/s is the Fermi velocity and a typical Fermi energy k B E F 100 K) or for massive carriers with effective mass m ⋆ 0.01m, λ m = h/ √ 2m ⋆ E F , as well as the Fermi wavelength λ F ∼ 2πn −1/2 are of the order of microns or larger due to the low density of Dirac and massive fermions. Moreover, one can show that in such micron size systems new quantum mechanical oscillations ap...