Material Systems for Spintronics

Very Recently, with the help of funding from Ministry of Communication and Information Technology (MCIT), Govt. of India, we initiated activities pertaining to development of Materials Systems for use in next generation Ultra High Magnetic Data Storage Density media (~1 Tera Bits/inch), Magnetic Random Access Memory (MRAMS), Multifunctional Spintronic Devices. It may be noted that hitherto in all the commercial electronic devices, the charge of the moving electron is manipulated, except in magnetic data storage where the spin of the electron is used to store bits in ‘1’s and ‘0’s. Thus, the spin degree of freedom of the electron is grossly under-utilized till so far.

It is envisaged that if we can simultaneously manipulate both the ‘charge’ and ‘spin’ of the moving electrons in a same semiconductor ship, this would open up path for newer (Spintronic) devices with improved device performance in many respects, e.g., lower operating powers, faster switching/processing speeds, compact chips, newer multifunctionalities leading to novel devices (e.g., SPIN FET, SPIN LEDs, SPIN SOLAR CELLS, POLARISED LIGHT EMITTERS, etc.). The most interesting among these would be computers that would get instantly start on touch of a button (requiring no booting delays). However, there are issues, which need to be resolved before such novel devices are yet to be realized. The spin of the carriers tend to go out of phase (both in time and space) in most of the materials thereby loosing the spin coherent current, which is required in order to manipulate and utilize the spin of the moving electrons. Thus, it is of great interest now to identify and engineer the materials so that they retain not only spin coherency, but also are capable of supporting maximum spin polarization. The potential materials systems which are believed to be possessing such characteristics are Half Metallic Ferromagnets and Room Temperature Ferromagnetic Semiconductors. While the former materials can inject and sustain electronic current composed of only one kind of spins (say, SPIN UP), the latter materials offer long spin coherence and current-amplification capabilities which are at the heart of any electronic application one would imagine.

It is important to understand/compare the difference between Half metallic Ferromagnets with conventional ferromagnets (e.g., Fe, Co and Ni, Gd, etc). While the spins are only partially polarized in later (~30-45%), half metallic ferromagnets are intrinsically 100% polarized. (As an analogy, the haphazard traffic at congested parts of city moving at slow pace on one hand and very fast moving vehicles along the dedicated road-lanes on High ways can be thought of the counterparts for usual ferromagnts and half metallic ferromagnets).

In our Thin Film Laboratory at IIT Delhi, we are using all variants of Sputtering for making synthetic Spintronic Systems - Pulsed DC, RF, and Ion Assist Ion Beam Sputtering. In particular, we are insitu fabricating the Magnetic tunnel junctions such as Fe/MgO/Fe using shadow masks. In the same Sputtering System, the Cu/Co Multilayers are also being synthesized for GMR applications. It has now been possible to control the thickness of thin films down to subatomic levels in pre-programmed fashion. Such a fine control is inevitable since the spin dependent tunneling is observed only in ultrathin barriers having thicknesses < 1-2 nm. Another important parameter of concern is the reduction of surface roughness and purity of the interface. Thus, not only good base vacuum, the lower operating pressures are key to achieve device quality multilayered thin films.

In addition, we are also actively engaged in exploring the possibilities of fabricating the Half Metallic Ferromagnets such as CrO₂, Fe₃O₄, etc., in different growth environments (e.g., nanowires) consistent with integrating with other materials. As an example, we are pursuing the growth of single phase and otherwise metastable CrO₂ by sputtering. We now plan to synthesize Fe₃O₄ by pulsed DC sputtering. It would be of immense importance to analyze the epitaxial growth of this Half metallic ferromagnet using RHEED.