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.
Spintronic Challenges and Material Requirements
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.