Spin electronics (also called spintronics, magnetoelectronics or magnetronics) is “A branch of physics concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics.” Spin-based electronics focuses on devices whose functionality is based primarily on the spin degree of freedom of the carriers. This is in contrast to conventional electronics, which exploits only the charge of the carriers. Spin wave based devices, which utilise collective excitations of electronic spins in magnetic materials as a carrier of information, have huge potential as memory devices that are more energy efficient, faster, and higher in capacity.
Spin-based electronic (spintronic) devices offer a significant improvement to the limits of conventional charge-based memory and logic devices which suffer from high power usage, leakage current, performance saturation, and device complexity.
In particular, magnetic materials are highly promising for use in spintronic devices because their electron spin orientation can be readily manipulated through external magnetic fields. The difference in mobility and the population of spin up and spin down electrons in magnetic materials induce a net flow of spin-polarized current. Due to the intrinsic hysteresis properties of ferromagnetic materials, a spintronic device can ideally “remember” the set state for an indefinite time. Consequently, these non-volatile spin-based materials allow a significant increase in device density and energy savings while also offering new functionalities.
Current spintronic applications focus primarily on magnetic storage and sensing, using giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) effects. GMR is observed in thin films with alternate ferromagnetic and non-magnetic metallic spacer layers. The enhanced resistance observed when ferromagnetic layers are magnetically aligned anti-parallel to each other, rather than aligned parallel, forms the basis of GMR.
A TMR device, in its simplest form, consists of two ferromagnetic metallic electrodes on either side of a very thin non-magnetic insulator layer. Electrons are transmitted from one ferromagnetic electrode to the other through quantum mechanical tunneling across the insulating barrier. The insulator is only a few nanometers thick, which is thin enough to allow tunneling of electrons from one metallic electrode to the other. When the magnetizations of the ferromagnetic layers are aligned, the tunneling current is large and the device resistance is low. When the magnetizations of the ferromagnetic layers are anti-aligned, the tunneling current is small and the device resistance is large.
Similar to GMR, the resulting tunneling conductance depends on relative alignment of the ferromagnetic layers and is the underlying principle behind TMR. The GMR and TMR effects have been exploited and widely used in hard-drive read heads and, more recently, in magnetic random-access memory (MRAM) devices.
The control of magnetism and spin has far-reaching potential outside its conventional applications. This is confirmed by many rapidly advancing spintronic areas such as current-induced torque, spin Hall effect, spin caloritronics, silicon spintronics, and the spintronics of graphene and topological insulators
Despite so many advantages, there are lots of challenges of spintronics devices.
The major difficulty is the realization and production of spin-based magnetic devices. To overcome the difficulties or challenges scientists and researchers are trying to develop novel materials based on magnetism or spin of electrons and novel devices as well.
Spintronic Challenges and Material Requirements
A central goal of spintronics has been to discover new materials with prominent novel spin-based mechanisms. For large-scale practical realization of spintronic devices, they must outperform the switching time, energy per switching event, and output signal of current semiconductor devices. Enhanced efficiency for generation, transport, and detection of spin-polarized carriers is essential to meet this goal. Consequently, the development of novel materials with high spin-polarized carriers, efficient transport, and enhanced room temperature GMR and TMR effects is critical for the exploration of new directions in spintronics.
Ultrafast switching magnetism is required for spintronic memory and logic devices. However, there are several hurdles to incorporating ultrafast magnetic phenomena into spintronics devices. To obtain ultrafast magnetization, electrical current flow is required and the current comes from the semiconductor transistor, which has a minimum gate delay on the order of picoseconds.
If the magnetization of one electrode is fixed, for example by exchange coupling to a neighboring antiferromagnet, and the other layer can switch depending on an applied magnetic field, the MTJ exhibit magnetoresistance, in which the resistance state of the device depends on the sign of the applied field. MTJs are already used as sensors in the read heads of magnetic hard disk drives.
Reading of the magnetic state is done electrically, which necessitates a large magnetoresistance from an MTJ. The switching energy needs to be minimized, and it needs device dimensions in the nanoscale regime. Another challenge is getting a very highly precise uniform spread of the magnetic tunnel transistor’s junction because high-density MRAM will require at least 50-ohm resistance. The basic building block of MRAM is MTJ.
Spin-based devices, materials, and concepts are developed at RT conditions. But there is a growing demand for its application in the high-temperature regime also. Spintronics
devices are made of several magnetic materials, such as nickel (Ni), iron (Fe), cobalt (Co), etc., and their alloys but these are not generally used with conventional semiconductor devices. Hence, it becomes very troublesome in pattern and adding the magnetic materials into the silicon-based
processing and fabrication systems.
One of the major roadblocks in developing better spintronic devices is an effect called “damping,” in which the magnetic energy essentially leaks out of the materials, causing them to be less efficient. Traditionally, scientists have blamed this property on the interaction between the spin of the electron and its motion. However, the University of Minnesota-led team has proven that there is another factor—magnetoelastic coupling, which is the interaction between electron spin, or magnetism, and sound particles.
The study’s findings provide a more holistic picture of what causes damping. This will allow engineers to develop magnetic materials with “ultralow” damping that are more energy efficient, ultimately leading to higher quality computers of the future.
Emerging Materials for spintronic devices
A class of intermetallic solids, referred to as Heusler compounds, represents a remarkable class of materials with more than 1,500 members, with new compounds synthesized on a regular basis.
The two primary classes of intermetallics are the Full-Heusler with X2YZ composition (X, Y: transition metal atoms and Z: semiconductor or non-magnetic metal) having an L21 crystal structure, and the Half- Heusler with XYZ composition and C1b structure. Heusler alloys of both compositions are attractive candidates for spintronics applications and have been extensively studied over the past decade. In particular, room temperature (RT) materials with essentially 100% spin polarization (referred to as half-metallic) and high magnetocrystalline anisotropy are ideal for next-generation spintronic devices. Crystalline disorders like atomic displacement, misfit dislocation, and symmetry breaks near the surface of the film are primary obstacles to meeting this target in the Heusler compounds.
The second material type are Huesler alloys, which are intermetallic phases in the L21 structure. These materials are half metals: only one spin band crosses the Fermi level, so the current is in principle 100% spin polarized, which has the potential to enable devices with extremely high magnetoresistance ratios. The particular Huesler alloy we have studied is Co2MnSi, in MTJs with a MgO tunnel barrier. The device performance of a Co2MnSi depends on the interface termination with the MgO. Only the MnMn/O termination preserves 100% spin polarization in the tunneling current. A CoCo/O interface has 50% spin polarized tunneling, and the MnSi/O loses the spin polarization of the tunneling current completely. Based on Z-contrast STEM images, we have demonstrated devices with MnMn/O termination for the first time. Learning to control that interface through processing will be critical to future devices.
Two major criteria can be identified to assess the RT half-metallicity and, consequently, the potential of Heusler alloys as spintronic materials. First, the ability of the Heusler alloy to show over 100% GMR ratio at RT and second the demonstration of >1,000% TMR. In 2011, a 74.8% GMR ratio was reported with a Co2Fe0.4Mn0.6Si/Ag/Co2Fe0.4Mn0.6Si junction, which is a significant improvement over previously obtained values.3 With this GMR stack and other recent developments, the Heusler alloys are frontrunner contenders to satisfy the 2 Tbit/in2 areal density in hard disk drives. Based on recent technology roadmapping, these materials are expected to achieve the target 100% GMR ratio in the coming years. In terms of TMR ratio, magnetic tunnel junctions (MTJ) with MgO spacers exhibit the highest TMR effect, resulting from intrinsic spin-filtering behavior of MgO-junctions as opposed to simple tunneling. For example, a tri-layer stack of CoFeB/MgO/CoFeB showed a TMR of 604% at RT. While this is the largest TMR so far, this material’s low thermal stability and scalability may limit its potential.
Magnetic Oxide Materials
Transition metal oxides exhibit a wide range of tunable magnetic and electrical properties that are very attractive for spintronics. For example, the doped perovskite system La1-xSrxMnO3 (LSMO, x=0.3–0.4) is both metallic and ferromagnetic, with near-unity spin polarization. In combination with the perovskite material SrTiO3 as a tunnel barrier, LSMO has exhibited a TMR ratio as high as 1,850%.13 But such a high TMR value is observable only at low temperatures. In order to achieve high TMR at RT, magnetic double perovskites that have a higher Curie temperature have been explored.
Multiferroic Spintronic Materials
Multiferroics are materials in which at least two of the ferroelectric, ferro/antiferro magnetic, and ferroelastic phases co-exist. The technological appeal of multiferroic materials is huge, particularly for designing multifunctional hybrid spintronic devices, as these materials have the potential to control magnetism with electric fields.
Several promising multiferroic materials like Y-type hexaferrite [or (Ba, Sr)2Zn2Fe12O22, Ba2Mg2Fe12O22], Z-type hexaferrites (or Sr3Co2Fe24O41 and CuO), GdFeO3, CdCr2S4, and indium perovskites have been reported thus far. However, RT multiferroicity in single-phase materials has been challenging to achieve. The general focus in the field is shifting toward advanced characterization techniques, better thin-film fabrication tools, device architecture, and understanding of the domain-interface effects.
In parallel with the boom of spintronics, two-dimensional (2D) van der Waals (vdW) materials have been at the frontier of material research since the isolation of graphene. Distinct from their bulk materials, 2D vdW materials exhibit many novel physical phenomena. Some 2D materials have already shown great potential for the engineering of next-generation 2D spintronic devices.
For example, graphene exhibits high electron/hole mobility, long spin lifetimes, and long diffusion lengths, which make it a promising candidate for a spin channel. However, due to its characteristics of zero gap and weak spin–orbit coupling (SOC), graphene has limitations in building graphene-based current switches.
In contrast, 2D transition metal dichalcogenides (TMDCs) have varied band gaps, strong SOC effect, and, especially, unique spin-valley coupling, providing a platform to manipulate spin and valley degrees of freedom for nonvolatile information storage.
Topological insulators (TIs) with topologically protected surface states have strong spin–orbit interactions to achieve spin-momentum locking, which can suppress scattering and enhance spin and charge conversion efficiency.