Spin-polarized current can excite the magnetization of a ferromagnet through the transfer of spin angular momentum to the local spin system. This pure spin-related transport phenomenon leads to alluring possibilities for the achievement of a nanometer scale, complementary metal oxide semiconductor-compatible, tunable microwave generator that operates at low bias for future wireless communication applications. Microwave emission generated by the persistent motion of magnetic vortices induced by a spin-transfer effect seems to be a unique manner to reach appropriate spectral linewidth. However, in metallic systems, in which such vortex oscillations have been observed, the resulting microwave power is much too small. In this study, we present experimental evidence of spin-transfer-induced vortex precession in MgObased magnetic tunnel junctions, with an emitted power that is at least one order of magnitude stronger and with similar spectral quality. More importantly and in contrast to other spintransfer excitations, the thorough comparison between experimental results and analytical predictions provides a clear textbook illustration of the mechanism of spin-transfer-induced vortex precession.

We fabricated a spin-torque oscillator (STO) having a nanopillar-shaped magnetic tunnel junction with perpendicularly magnetized FeB free and in-plane magnetized CoFeB reference layers. The perpendicular magnetization of the FeB was stabilized by strong perpendicular magnetic anisotropy induced at both the MgO tunnel barrier/FeB and FeB/MgO cap interfaces. Under a perpendicular field (3 kOe), the STO exhibited a large emission power (0.55 W), a high frequency (6.3 GHz) and a high Q factor (135) simultaneously, all of which are the largest to date among nanopillar-shaped STOs. The bias voltage dependence of the oscillation property was well explained by the macrospin model.

We investigated the effect of voltage on the perpendicular magnetic anisotropy in an ultrathin Fe80B20 layer sandwiched between two MgO barrier layers with different thicknesses, in which the bias voltage is predominantly applied to one of the MgO layers. The application of both positive and negative bias voltages enhanced the perpendicular anisotropy, in contrast with the odd function dependence previously observed in a MgO/ferromagnetic metal/non-magnetic metal structure. Moreover, for positive bias voltages, the large anisotropy change slope of 108 fJ/Vm was demonstrated. These results indicate that the MgO-sandwich structure has high potential to extend the availability of the voltage effect.

Phase locking experiments on vortex based spin transfer oscillators with an external microwave current are performed. We present clear evidence of phase locking, frequency pulling, as well as fractional synchronization in this system, with a minimum peak linewidth of only 3 kHz in the locked state. We find that locking ranges of the order of 1/3 of the oscillator frequency are easily achievable because of the large tunability ∂f /∂I dc observed in our vortex based systems. Such large locking ranges allow us to demonstrate the simultaneous phase locking of two independent oscillators connected in series with the external source.Injection of a direct electrical current through spinvalve structures or magnetic tunnel junctions offers the possibility to induce microwave steady-state magnetization precession by the action of the spin transfer torque. Due to the magneto-resistive effects, these oscillations are converted to an a.c. electrical signal at microwave frequencies [1,2]. Such spin transfer nano-oscillators are advantageous for applications in wireless telecommunications, but despite significant progress in increasing the power of these oscillators and reducing the linewidth [3], these parameters however do not yet match requirements for practical applications. Synchronization of many oscillators is a solution to overcome these issues [4,5]. One of the mechanisms of synchronization of many oscillators discussed in Ref. [6,7] is based on their ability to adapt their frequency to the frequency of an injected a.c. current. Synchronization of a single oscillator to a microwave source has been demonstrated experimentally for systems in which the motion of a quasi-uniform magnetization has been excited [8,9]. In our work, we study synchronization to a microwave current of oscillators having a vortex in the free layer of a magnetic tunnel junction (MTJ). We have demonstrated that these spin transfer vortex oscillators (STVOs) result in large emitted power with low linewidth compared to oscillators based on a quasi-uniform magnetization precession [3]. We find that such STVOs can be locked to the fractional frequencies of the microwave current source as well as to its main frequency. The ratio of the locking range to the emission frequency can be very large, and it allows us to observe experimentally the synchronization of two separate STVOs to a common external microwave current.In this letter, we perform phase-locking experiments using STVOs made of circular shape nanopillars (diameter D = 170 nm) from the same MTJ wafer as in Ref.[3]. The magnetic stacks, grown by sputtering, are composed of PtMn 15 / CoFe 2.5 / Ru 0.85 / CoFeB 3 / MgO 1.075 / NiFe 15 / Ru 10 (nm). The ratio thickness over diameter of the free NiFe layer is chosen in such a way that a magnetic vortex is stabilized as a remanent magnetic state. The top layer of the synthetic antiferromagnet (SAF) PtMn/CoFeB/Ru/CoFeB serves as a po-FIG. 1: (a) Frequency, (b) integrated power, (c) linewidth evolution of the emitted signal as a function of the applied ...