SPIN

This paper reviews the recent progress made to realize reliable write operations in spin transfer torque magnetic random access memory. Theoretical description of write error rate (WER) based on macro-spin models are discussed with comparison to experimental data. Recent studies on the phenomena that can lead to abnormal WER behaviors which include back-hopping and low probability bifurcated switching are reviewed with emphasis on underlying mechanism. The studies on the WER in perpendicular magnetic tunnel junction (MTJ) are also reviewed. It is demonstrated that, for both in-plane and perpendicular MTJ, reliable and error-free write operations can be achieved with thorough understanding of the underlying physics and innovative design/process solutions.

Electric-field-control of magnetism can dramatically improve the energy efficiency of spintronic devices and enhance the performance of magnetic memories. More generally, it expands the range of applications of nonvolatile spintronic devices, by making them energetically competitive compared to conventional semiconductor solutions for logic and computation, thereby potentially enabling a new generation of ultralow-power nonvolatile spintronic systems. This paper reviews recent experiments on the voltage-controlled magnetic anisotropy (VCMA) effect in thin magnetic films, and their device implications. The interfacial perpendicular anisotropy in layered magnetic material stacks, as well as its modulation by voltage, are discussed. Ferromagnetic resonance experiments and VCMA-induced high-frequency magnetization dynamics are reviewed. Finally, we discuss recent progress on voltage-induced switching of magnetic tunnel junction devices and its potential applications to magnetic random access memory (MRAM).

We review recent developments in magnetic tunnel junctions with perpendicular easy axis (p-MTJs) for nonvolatile very large scale integrated circuits (VLSIs). So far, a number of material systems such as rare-earth/transition metal alloys, L1₀-ordered (Co, Fe)–Pt alloys, Co/(Pd, Pt) multilayers, and ferromagnetic-alloy/oxide stacks have been proposed as electrodes in p-MTJs. Among them, p-MTJs with single or double ferromagnetic-alloy/oxide stacks, particularly CoFeB–MgO, were shown to have high potential to satisfy major requirements for integration.

Multiferroic materials and devices have attracted intensified recent interests due to the demonstrated strong magnetoelectric (ME) coupling in new multiferroic materials and devices with unique functionalities and superior performance characteristics. Strong ME coupling has been demonstrated in a variety of multiferroic heterostructures, including bulk magnetic on ferro/piezoelectric multiferroic heterostructures, magnetic film on ferro/piezoelectric slab multiferroic heterostructures, thin film multiferroic heterostructures, etc. Different multiferroic devices have been demonstrated, which include magnetic sensors, energy harvesters, and voltage tunable multiferroic RF/microwave devices which are compact, lightweight, and power efficient. In this progress report, we cover the most recent progress on multiferroic heterostructures and devices with a focus on voltage tunable multiferroic heterostructures and devices with strong converse ME coupling. Recent progress on magnetic-field tunable RF/microwave devices are also covered, including novel non-reciprocal tunable bandpass filters with ultra wideband isolation, compact, low loss and high power handling phase shifters, etc. These novel tunable multiferroic heterostructures and devices and tunable magnetic devices provide great opportunities for next generation reconfigurable RF/microwave communication systems and radars, Spintronics, magnetic field sensing, etc.

This paper reviews the current status of maximizing output power in spin transfer nano-oscillators (STNOs). The key factors affecting output power and the methods to maximize output power in STNOs are briefly introduced. The recent development trends for STNOs are also reviewed in this paper. This article is one of a series devoted to the subject of Latest Progress on Spintronics Devices.

Over the past few years, several novel nanoscale computing concepts have been proposed as potential post-complementary metal oxide semiconductor (CMOS) computing fabrics. In these, key focus is on inventing a faster and lower power alternative to conventional metal oxide semiconductor field effect transators. Instead, we propose a fundamental shift in mindset towards more functional building blocks, replacing simple switches with more sophisticated information encoding and computing based on alternate state variables to achieve a significantly more efficient and compact logic. Specifically, we propose wave computation enabled by magnetic spin wave interactions called as spin wave functions (SPWFs). In SPWFs, computation is based on wave interference and information can be encoded in a wave's phase, amplitude and frequency. In this paper, we provide an update on key fabric concepts and design aspects. Our analysis shows that circuit design choices can have a significant impact on overall fabric/device capabilities required and vice versa. Thereby, we adapt an integrated fabric-circuit exploration methodology. Control schemes for wave streaming and synchronization are also discussed with several SPWF circuit topologies. Our estimations show that significant area and power benefits can be expected for SPWF-based designs versus CMOS. In particular, for a 1-bit adder up to 40X area benefit and up to 304X power consumption reduction may be possible with SPWF-based implementation versus 45 nm CMOS.

As one promising candidate for next-generation nonvolatile memory technologies, spin-transfer torque random access memory (STT-RAM) has demonstrated many attractive features, such as nanosecond access time, high integration density, nonvolatility, and good CMOS process compatibility. In this paper, we address the asymmetry in the write operations of STT-RAM cells: the mean and the deviation of the write latency for the switching from low- to high-resistance state is much longer or larger than that of the opposite direction. Some special design concerns, e.g., the data-dependent write reliability, are raised by this observation. We systematically analyze the root reasons for the asymmetric switching of MTJs, including the thermal-induced statistical MTJ magnetization process, the asymmetric biasing condition of NMOS transistors, and the device variations of both NMOS and MTJ. Their impacts on STT-RAM write operations were also investigated. At last, we explore the design spaces of different STT-RAM cell structures by considering the asymmetry of write operations.

Spin-transfer torque random access memory (STT-RAM) has demonstrated great potentials as a universal memory for its fast access speed, zero standby power, excellent scalability and simplicity of cell structure. However, large process variations of both magnetic tunneling junction and CMOS process severely limit the yield of STT-RAM chips. In this paper, we propose a novel voltage-driven non-destructive self-reference sensing scheme (NDRS) to enhance the STT-RAM chip yield by significantly improving sense margin. Monte-Carlo simulations of a 16 Kb STT-RAM array shows that our proposed scheme can achieve the same yield as the previous NDRS scheme while improving the sense margin by 5 × with the similar access performance and power.

Long time magnetization thermal switching under small amplitude high frequency excitation is analyzed. Approaches based upon conventional time-dependent energy barrier are not sufficient to describe magnetization nonvolatility under GHz excitations. Methods based upon large angle nonlinear magnetization dynamics are developed for both coherent and noncoherent magnetization switching. This dynamic approach is not only important for fundamental understanding of magnetization dynamics under combined radio frequency excitations and thermal fluctuations, but also critical for practical design of emerging spintronic devices. When applied to spin torque random access memory read operations, as sensing current duration reaches nanosecond, dynamic approach gives a switching probability estimation orders of magnitude different from that obtained from conventional time-dependent energy barrier approach.