Narrow Results

We report a theoretical study on the bias dependence of the current-induced torque in the noncollinear ferromagnet/barrier/ferromagnet (FM) tunnel junction. By using the Keldysh–Green function technique, we derived an analytic expression of the spin transfer torque (STT) based on a continuum model. It was found that for finite bias, the torque induced by current increases monotonically with bias and is nearly antisymmetric with regard to the current direction, which agrees with the experimental observation on the STT in the spin valve system.

In this paper, a power-efficient self write-terminated hybrid full-adder (SWTHFA) has been developed using the self write-terminated write driver and an improved version of the sense amplifier already reported in the literature. The SWTHFA is designed using hybrid spin transfer torque-magnetic tunnel junction (STT-MTJ)/CMOS circuit based on logic-in-memory architecture. The use of a modified sense amplifier improves the power dissipation and output response on one hand, whereas, on the other hand, self-write-terminated write driver cuts off the unnecessary flow of write current in the driver circuit, thereby eliminates power wastage in SWTHFA. Proposed SWTHFA shows improvement in power saving, output response, read and write power delay product by 38.87%, 26.45%, 40.86% and 36.53%, respectively, compared to conventional write hybrid full-adder (CWHFA). Further, we performed Monte-Carlo simulations by incorporating process and mismatch variations for CMOS and extracted parameters of MTJ to demonstrate the feasibility of SWTHFA in low-power VLSI circuits.

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.

The use of polarized currents as tools for influencing the magnetic switching of magnetic materials was an important step in the evolution of modern recording technologies. A new paradigm was opened and new technologies based on this new idea have been developed. However, the modeling effort did not followed at the same pace the technological developments. In this paper, we are analyzing the effect on switching critical curves of the polarized current, which seems to be the most sensitive tool for the moment in evidencing these effects. The most important point of this paper is that we took into consideration the effect of higher order terms in the series expansion of the uniaxial anisotropy free energy and we analyze how these terms are influencing the critical curve shape. The results are discussed in terms of the efficiency of the polarized currents on the switching behavior of macrospins.

Instant-on nonvolatile electronics, which can be powered on/off instantaneously without the loss of information, represents a new and emerging paradigm in electronics. Nonvolatile circuits consisting of volatile CMOS, combined with nonvolatile nanoscale magnetic memory, can make electronics nonvolatile at the gate, circuit and system levels. When high speed magnetic memory is embedded in CMOS logic circuits, it may help resolve the two major challenges faced in continuing CMOS scaling: Power dissipation and variability of devices. We will give a brief overview of the current challenges of CMOS in terms of energy dissipation and variability. Then, we describe emerging nonvolatile memory (NVM) options, particularly those spintronic solutions such as magnetoresistive random access memory (MRAM) based on spin transfer torque (STT) and voltage-controlled magnetoelectric (ME) write mechanisms. We will then discuss the use of STT memory for embedded application, e.g., replacing volatile CMOS Static RAM (SRAM), followed by discussion of integration of CMOS reconfigurable circuits with STT-RAM. We will then present the scaling limits of the STT memory and discuss its critical performance parameters, particularly related to switching energy. To further reduce the switching energy, we present the concept of electric field control of magnetism, and discuss approaches to realize this new mechanism in realizing low switching energy, allowing for implementation of nonvolatility at the logic gate level, and eventually at the transistor level with a magnetoelectric gate (MeGate). For nonvolatile logic (NVL), we present and discuss as an example an approach using interference of spin waves, which will have NVL operations remembering the state of computation. Finally, we will discuss the potential impact and implications of this new paradigm on low energy dissipation instant-on nonvolatile systems.

We report that the current induced translational motion of transverse domain wall and domain wall switching in ferromagnetic nanowire is primarily governed by the configuration and polarization of the domain wall. The out-of-plane torque due to nonadiabacity and spin relaxation is found to be the dominant mechanism in the configuration and polarization dependent translational motion of the domain wall. The transverse domain wall undergoes a damped periodic back and forth translational motion along with a periodic switching from +ve polarization to -ve polarization and vice versa under the application of a pulsed current. The domain wall switching occurs at the start of each period of translation.

Some of Co-based full Heusler alloys have remarkable properties in spintronics, that is, high spin polarization of conduction electrons and low magnetic damping. Owing to these properties, magnetic tunnel junctions (MTJs) using Co-based full Heusler alloys are potentially of particular importance for spintronic application such as magnetoresistive random access memories (MRAMs). Recently, we have first demonstrated spin transfer torque (STT) switching and perpendicular magnetic anisotropy (PMA), which are required for developing high-density MRAMs, in full-Heusler Co₂FeAl alloy-based MTJs. In this review, the main results of the experimental demonstrations are shown with referring to related issues, and the prospect of MTJs using Heusler alloys is also discussed.

With the growing concerns of standby power in sub-100-nm CMOS technologies, alternative computing techniques and memory technologies are explored. Spin transfer torque magnetoresistive RAM (STT-MRAM) is one such nonvolatile memory relying on magnetic tunnel junctions (MTJs) to store information. It uses spin transfer torque to write information and magnetoresistance to read information. In 2012, Everspin Technologies, Inc. commercialized the first 64Mbit Spin Torque MRAM. On the computing end, nanomagnetic logic (NML) is a promising technique with zero leakage and high data retention. In 2000, Cowburn and Welland first demonstrated its potential in logic and information propagation through magnetostatic interaction in a chain of single domain circular nanomagnetic dots of Supermalloy (Ni₈₀Fe₁₄Mo₅X₁, X is other metals). In 2006, Imre et al. demonstrated wires and majority gates followed by coplanar cross wire systems demonstration in 2010 by Pulecio et al. Since 2004 researchers have also investigated the potential of MTJs in logic. More recently with dipolar coupling between MTJs demonstrated in 2012, logic-in-memory architecture with STT-MRAM have been investigated. The architecture borrows the computing concept from NML and read and write style from MRAM. The architecture can switch its operation between logic and memory modes with clock as classifier. Further through logic partitioning between MTJ and CMOS plane, a significant performance boost has been observed in basic computing blocks within the architecture. In this work, we have explored the developments in NML, in MTJs and more recent developments in hybrid MTJ/CMOS logic-in-memory architecture and its unique logic partitioning capability.

From historical obscurity, antiferromagnets are recently enjoying revived interest, as antiferromagnetic (AFM) materials may allow the continued reduction in size of spintronic devices. They have the benefit of being insensitive to parasitic external magnetic fields, while displaying high read/write speeds, and thus poised to become an integral part of the next generation of logical devices and memory. They are currently employed to preserve the magnetoresistive qualities of some ferromagnetic based giant or tunnel magnetoresistance systems. However, the question remains how the magnetic states of an antiferromagnet can be efficiently manipulated and detected. Here, we reflect on AFM materials for their use in spintronics, in particular, newly recognized antiferromagnet Mn₂Au with its in-plane anisotropy and tetragonal structure and high Néel temperature. These attributes make it one of the most promising candidates for AFM spintronics thus far with the possibility of architectures freed from the need for ferromagnetic (FM) elements. Here, we discuss its potential for use in ferromagnet-free spintronic devices.

The continuous downscaling in CMOS devices has increased leakage power and limited the performance to a few GHz. The research goal has diverted from operating at high frequencies to deliver higher performance in essence with lower power. CMOS based on-chip memories consumes significant fraction of power in modern processors. This paper aims to explore the suitability of beyond CMOS, emerging magnetic memories for the use in memory hierarchy, attributing to their remarkable features like nonvolatility, high-density, ultra-low leakage and scalability. NVSim, a circuit-level tool, is used to explore different design layouts and memory organizations and then estimate the energy, area and latency performance numbers. A detailed system-level performance analysis of STT-MRAM and SOT-MRAM technologies and comparison with 22nm SRAM technology are presented. Analysis infers that in comparison to the existing 22nm SRAM technology, SOT-MRAM is more efficient in area for memory size $\ge 128$KB, speed and energy consumption for cache size $\ge 32$KB. A typical 256KB SOT-MRAM cache design is 27.74% area efficient, 2.97 times faster and consumes 76.05% lesser leakage than SRAM counterpart and these numbers improve for larger cache sizes. The article deduces that SOT-MRAM technology has a promising potential to replace SRAM in lower levels of computer memory hierarchy.

Magnetic tunnel junction with perpendicular magnetic anisotropy (p-MTJ) is the core component in spintronics-based memory, logic and sense devices, the synthetic anti-ferromagnetic (SAF) has been used to fix the reference layer and cancel the stray field in free layer and few works report the SAF-based reliability analysis of p-MTJ nanopillars. Here, we developed a complete set of method for single p-MTJ devices using CMOS compatible process, and manufactured 80-nm diameter p-MTJs with lower performance degrade. The results of our p-MTJs also revealed that anomalous transitions in major resistance versus external magnetic field ($R-H$) curves can be assisted with the uncompensated SAF flipping after nanofabrication, even though well magnetostatic matched during film stack design. We have also given a reliability analysis of magnetization states in anomalous major $R-H$ curves and implied that SAF parameters must be designed for etched nanopillars.

We investigate the role of magnetic impurities in the barrier region in enhancing the torque transferred to the free ferromagnet layer in a non-collinear model tunnel junction. The tunnel junction is modeled using the non-equilibrium Green’s function formalism within the tight-binding approximation. The position of the impurity which is described by the single impurity Anderson model is found to have a profound impact on the trends observed in the spin transfer torque. The parallel ($T_{\parallel }$) torque is observed to have a switch on-off step-like bias behavior while the perpendicular ($T_{\perp }$) torque exhibits a change in sign between the switch-on bias voltages. The spin transfer torque is found to be dramatically enhanced when the impurity is positioned at the interface of a five-layer barrier which is three orders of magnitude larger even at temperatures as high as 200K. With this formulation the effect of Coulomb repulsion at the impurity site on the torque could be deduced and the obtained results are explained in terms of the region-wise electronic density of states in the junction.

Based on the spin-polarized free-electron model, we derive the spin transfer torque in the magnetic tunnel junctions with the synthetic ferrimagnetic layers by analyzing the spin and charge transports in the ballistic regime. In the realistic junctions, the spin torque exerted on the magnetizations of two ferromagnetic layers in the synthetic ferrimagnetic layer shows the unidirectional rotation. It is suggested that, through the antiferromagnetic interlayer coupling in the synthetic ferrimagnetic layer, this unidirectional rotation induces the cooperative reversal of magnetizations in two ferromagnetic layers, and expected that this cooperative rotation reduces the critical current for the magnetization reversal in the synthetic ferrimagnetic layer.