A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits tunnel magnetoresistance (TMR) due to ...spin-dependent electron tunnelling. Since the 1995 discovery of room-temperature TMR, MTJs with an amorphous aluminium oxide (Al-O) tunnel barrier have been studied extensively. Al-O-based MTJs exhibit magnetoresistance (MR) ratios up to about 70% at room temperature (RT) and are currently used in magnetoresistive random access memory (MRAM) and the read heads of hard disk drives. MTJs with MR ratios significantly higher than 70% at RT, however, are needed for next-generation spintronic devices. In 2001 first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(0 0 1) barrier would be over 1000% because of the coherent tunnelling of fully spin-polarized Delta1 electrons. In 2004 MR ratios of about 200% were obtained in MTJs with a single-crystal MgO(0 0 1) barrier or a textured MgO(0 0 1) barrier. CoFeB/MgO/CoFeB MTJs for practical applications were also developed and found to have MR ratios up to 500% at RT. MgO-based MTJs are of great importance not only for device applications but also for clarifying the physics of spin-dependent tunnelling. In this article we introduce recent studies on physics and applications of the giant TMR in MgO-based MTJs.
The concept of spin-torque-driven high-frequency magnetization dynamics, allows the potential construction of complex networks of non-linear dynamical nanoscale systems, combining the field of ...spintronics and the study of non-linear systems. In the few previous demonstrations of synchronization of several spin-torque oscillators, the short-range nature of the magnetic coupling that was used has largely hampered a complete control of the synchronization process. Here we demonstrate the successful mutual synchronization of two spin-torque oscillators with a large separation distance through their long range self-emitted microwave currents. This leads to a strong improvement of both the emitted power and the linewidth. The full control of the synchronized state is achieved at the nanoscale through two active spin transfer torques, but also externally through an electrical delay line. These additional levels of control of the synchronization capability provide a new approach to develop spin-torque oscillator-based nanoscale microwave-devices going from microwave-sources to bio-inspired networks.
It has been proposed that high-frequency detectors based on the so-called spin-torque diode effect in spin transfer oscillators could eventually replace conventional Schottky diodes due to their ...nanoscale size, frequency tunability and large output sensitivity. Although a promising candidate for information and communications technology applications, the output voltage generated from this effect has still to be improved and, more pertinently, reduces drastically with decreasing radiofrequency (RF) current. Here we present a scheme for a new type of spintronics-based high-frequency detector based on the expulsion of the vortex core in a magnetic tunnel junction (MTJ). The resonant expulsion of the core leads to a large and sharp change in resistance associated with the difference in magnetoresistance between the vortex ground state and the final C-state configuration. Interestingly, this reversible effect is independent of the incoming RF current amplitude, offering a fast real-time RF threshold detector.
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 MgO-based 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 spin-transfer excitations, the thorough comparison between experimental results and analytical predictions provides a clear textbook illustration of the mechanism of spin-transfer-induced vortex precession.
Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO. In this study, we shed light on the origin of large voltage-induced anisotropy change in ...Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For Cr/Fe (5.9 Å)/Cr (0.7 Å)/MgO with an effective PMA of 0.8 MJ/m
, a maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of 370 fJ/Vm was demonstrated due to Cr insertion.
Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high ...performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.
Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling ...via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation Delta mu super(ch) induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of Delta mu super(h) and the spin accumulation Delta mu super(ls) in the localized states. In the regime where Delta mu super(ls)/ Delta mu super(h) is largest, the detected spin signal is insensitive to Delta mu super(ls) and the ferromagnet probes the spin accumulation in the semiconductor channel.
A method for the reduction of the propagation loss of surface plasmons was proposed and experimentally demonstrated. A plasmonic structure, which contains a metal and two dielectric layers of ...different refractive indexes, is proposed in order to optimize the optical confinement and to reduce the propagation loss of the surface plasmons. Long-distance propagation of a surface plasmon on the surface of a ferromagnetic metal was demonstrated. A low propagation loss of 0.17 dB/μm for a surface plasmon in a Fe/MgO/AlGaAs plasmonic structure was achieved.
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