Abstract
The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for example as a cell for a magnetic random access memory and spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarised electrons efficiently into a non-magnet. We have accordingly identified 2 key properties to be achieved by developing new magnetic materials for future spintronic devices: (1) Half-metallicity at room temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For the first property, 5 major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: Heusler alloys, ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level EF only for its minority spins, achieving 100% spin polarisation at EF. We have also evaluated L10-alloys and D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarisation. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations and interdisciplinary applications within 35 years time scale.
Highlights
H EUSLER alloys are ternary alloys originally discovered by Heusler [1]
A great deal of effort has been devoted to achieve the half-metallicity at room temperature (RT) using a Heusler alloy
Using spinel ferrite-based magnetic tunnel junction (MTJ) consisting of ferrite/NM barrier/ferrite structure, >100% RT tunneling magnetoresistance (TMR) is expected within 10 years through the development of high-quality spinel ferrite thin films and the selection of a proper NM barrier
Summary
H EUSLER alloys are ternary alloys originally discovered by Heusler [1]. He demonstrated the ferromagnetic behavior in an alloy consisting of nonmagnetic (NM) atoms, Cu2MnSn. Block et al [3] measured a large tunneling magnetoresistance (TMR) in bulk full-Heusler Co2(Cr, Fe)Si alloy, followed by a similar measurement in a thin-film form [4] Among these Heusler alloys, Co-based full-Heusler alloys are the most promising candidates to achieve the RT half-metallicity due to their high Curie temperature. By reflecting on the development over the last five years, one can expect that the Heusler-alloy GMR junctions can achieve 100% GMR ratios within three years This will satisfy (m1.1) and will lead to device applications as HDD read heads. The other device application expected is to fabricate all Heusler junctions consisting of antiferromagnetic/ ferromagnetic/NM/ferromagnetic Heusler-alloy layers Such junctions can offer a template to avoid any crystalline disorder at the interfaces as the lattice matching and symmetry can precisely be controlled by atom substitution in these alloy layers. Milestones and their associated roadmaps for three half-metallic oxide ferromagnets, (A) spinel ferrites, (B) rutiles, and (C) perovskites are discussed
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