Spin-based electronics has evolved into a major field of research that broadly encompasses different classes of materials, magnetic systems, and devices. This review describes recent advances in spintronics that have the potential to impact key areas of information technology and microelectronics. We identify four main axes of research: nonvolatile memories, magnetic sensors, microwave devices, and beyond-CMOS logic. We discuss state-of-the-art developments in these areas as well as opportunities and challenges that will have to be met, both at the device and system level, in order to integrate novel spintronic functionalities and materials in mainstream microelectronic platforms.Conventional information processing and communication devices work by controlling the flow of electric charges in integrated circuits. Such circuits are based on nonmagnetic semiconductors, in Technologies based on GMR and MTJ devices are now firmly established and compatible with CMOS fab processes. Yet, in order to meet the increasing demand for high-speed, high-density, and low power electronic components, the design of materials, processes, and spintronic circuits needs to be continuously innovated. Further, recent breakthroughs in basic research brought forward novel phenomena that allow for the generation and interconversion of charge, spin, heat, and optical signals.Many of these phenomena are based on non-equilibrium spin-orbit interaction effects, such as the spin Hall and Rashba-Edelstein effects 6,8,23 or their thermal 24 and optical 25,26 analogues. Spin-orbit torques (SOT), for example, can excite any type of magnetic materials, ranging from metals to semiconductors and insulators, in both ferromagnetic and antiferromagnetic configurations 6 . This versatility allows for the switching of single layer ferromagnets, ferrimagnets, and antiferromagnets, as well as for the excitation of spin waves and auto-oscillations in both planar and vertical device geometries 10,11 . Charge-spin conversion effects open novel pathways for information processing using Boolean logic, as well as promising avenues for implementing unconventional neuromorphic 27,28,29 and probabilistic 30 computing schemes. Finally, spintronic devices cover a broad bandwidth ranging from DC to THz 31,32 , leading to exciting opportunities for the on-chip generation and detection of high frequency signals.

Current-driven magnetization dynamics in spin torque nano-oscillators ͑STNOs͒ is intensely investigated because of its high potential for high-frequency ͑HF͒ applications. We experimentally study current-driven HF excitations of STNOs for two fundamental magnetization states of the free layer, namely, vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STNO enables a direct comparison of the critical currents, agility, power, and linewidth of the HF output signals. We find that the vortex state has some superior properties, in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field. DOI: 10.1103/PhysRevB.80.054412 PACS number͑s͒: 72.25.Ba, 75.60.Jk, 75.70.Kw, 75.75.ϩa The proposal of spin-polarized current-induced magnetization dynamics of Slonczewski 1 and Berger 2 initiated a lot of theoretical and experimental studies in the last years. Promising applications have been found in spin-torque magnetic random access memory and spin torque nanooscillators ͑STNOs͒. The latter shows a steady precession of the magnetization of the free layer under the action of a spin-polarized dc current. Via the giant magnetoresistance or tunnel magnetoresistance ͑GMR or TMR͒ effect this precession generates a high-frequency ͑HF͒ voltage oscillation with frequencies in the GHz range and a rather wide tuning range by dc current and external magnetic field. Still, one drawback of STNOs is their low output power. To achieve useful power levels several groups work on the synchronization of arrays of STNOs.3-5 While this is a very promising approach, maximizing the output power of every single STNO is undeniably the first step to do.There are several possible arrangements for STNOs. Inplane magnetized free and fixed layers with in-plane 6 or outof-plane external fields, 7 in-plane magnetized free and perpendicularly magnetized fixed layers, 8 and free layer magnetized in a vortex state with in-plane magnetized fixed layer 9,10 have been studied experimentally. Comparing the characteristics-especially output power-of HF excitations of these arrangements from different experiments is not conclusive, because impedance and absolute resistance change, ⌬R = R AP − R P , of the samples have a very strong influence on the detected power. Here, we study HF excitations in two of the arrangements mentioned above that we are able to realize in the same sample. While the fixed layer is uniformly inplane magnetized, the free layer is either uniformly in-plane magnetized or in a vortex state. The direct comparison shows some advantages of the vortex state for the application in STNOs.The samples are fabricated by depositing 150 nm Ag/2 nm Fe/6 nm Ag/20 nm Fe/50 nm Au by molecular beam epitaxy on a cleaned and annealed GaAs͑100͒ substrate. All layers grow epitaxially as is confirmed by in situ low-energy electron diffraction measurements. The Fe layers adopt a bcc structure, which yields a cubic magnetocrystalline anisotropy. Bottom electro...

The authors report on current-induced magnetization switching ͑CIMS͒ in single-crystalline nanopillars. Fe͑14 nm͒ /Cr͑0.9 nm͒ /Fe͑10 nm͒ /Ag͑6 nm͒ /Fe͑2 nm͒ multilayers are deposited by molecular-beam epitaxy. The central Fe layer is coupled to the thick one by interlayer exchange coupling over Cr, while the topmost Fe layer is decoupled. Nanopillars with 150 nm diameter are prepared by optical and e-beam lithographies. The opposite spin scattering asymmetries of the Fe/ Cr and Fe/ Ag interfaces enabled the authors to observe normal and inverse CIMS for the two subsystems, which are combined in a single device. At high magnetic fields, steplike resistance changes are measured at positive currents and are attributed to current-driven magnetic excitations.

We investigate current-perpendicular-plane giant magnetoresistance ͑CPP-GMR͒ and current-induced magnetization switching in single-crystalline Fe/ Ag/ Fe nanopillars of 70 nm diameter. The interplay between the in-plane, fourfold magnetocrystalline anisotropy of the Fe͑001͒ layers and the spin-transfer torque ͑STT͒ gives rise to a two-step switching behavior, which allows an investigation of the angular dependences of CPP-GMR and spin-transfer torque. Both behave asymmetrically with respect to the perpendicular alignment of the two Fe layer magnetizations as theoretically predicted due to strong spin accumulation at the Fe/ Ag͑001͒ interfaces ͓M. D. Stiles and D. R. Penn, Phys. Rev. B 61, 3200 ͑2000͔͒. The asymmetry parameter determined from the STT data quantitatively agrees with calculated spin-dependent interface resistances, whereas CPP-GMR yields a smaller degree of asymmetry.

Spin-torque oscillators (STOs) are a promising application for the spin-transfer torque effect. The major challenge lies in pushing the STO’s microwave output power to useful levels, e.g., by operating an array of STOs in a synchronized, phase-locked mode. Our experiment on metallic, giant magnetoresistance-type nanopillars focuses on the influence of external high-frequency signals on the current-driven vortex dynamics and demonstrates the injection locking of the gyrotropic mode. We find a gap of about three orders of magnitude between the high-frequency power emitted by one oscillator and the power needed for phase-locking.