What is Spintronics?
(Schematic illustration of spin transport through a nano-scale system Figure taken from https://physics.aps.org/articles/v4/28)
“Spintronics” (named by S. Wolf in 2001), a sub-discipline of Condensed Matter Physics, has emerged out as a promising field in the last two decades that aims to manipulate the spin degree of freedom of an electron to explore a lot more rich and intriguing physics. Until few years back, when we talked about the mainstream ‘Electronics’, it was ‘charge’ based, whereas in Spintronics, it is not the charge, rather the ‘spin’ of the electron, that carries the information. The possibility of manipulating electron ‘spin’ was initiated by the discovery of Giant Magnetoresistance (GMR) effect independently by Peter Grünberg and Albert Fert in 1988, which awarded them the Noble prize in 2007.
Giant Magnetoresistance (GMR) Effect:
GMR is the drastic change in electrical resistance of a multilayer formed by alternating magnetic and non-magnetic metallic ones, when an external magnetic field is applied. In the absence of any external magnetic field, the exchange coupling between the magnetic layers through the non-magnetic one aligns the magnetization vector anti-parallel to each other. Then when a magnetic field, strong enough to overcome the anti-ferromagnetic coupling is applied, all the magnetization vectors align themselves along the field direction. This new parallel configuration exhibits an electrical resistance much smaller than the anti-parallel one. This dramatic change in electrical resistance is commonly known as the GMR effect. The discovery of GMR has far reaching consequences as not only the interplay between transport and magnetism was demonstrated here, but it also established the fact that the longly ‘neglected’ spin degree of freedom can play a crucial role in transport phenomena and hence in electronics.
(Schematic illustration of GMR Effect. Figure taken from https://simple.wikipedia.org/wiki/Giant_magnetoresistance)
Application and Possibilities:
Today GMR based magnetic data storage devices are in every computer which has already created a multi-billion industry. GMR effect provides a key idea to device a magnetic field sensor. As the magnetic field can change the orientation of the magnetic moments in one layer of the structure, it disrupts the relative orientation between the layers and hence changes the electrical resistance.
Another significant application is Spin Dependent Tunneling (STD) Device which is almost similar like GMR setup, the only change being here the non-magnetic metallic layer is substituted by an insulator. In this case, the resistance difference of the two configurations (parallel and anti-parallel) is quite large compared to the GMR setup. The key concept is to encode the ‘low resistance’ state as ‘1’ and the ‘high resistance state’ as ‘0’, and it is used in Magnetoresistive Random Access Memory (MRAM).
One of the most notable advances in the area of spin based device making is the experimental discovery of Spin Momentum Transfer (SMT) effect in 2002, in which the spin polarized current can exert a torque on the magnetization of a magnetic film. SMT has the potential to offer orders of magnitude smaller switching current and consequently much small energy is needed to write per bit. In contrast to the ordinary MRAM, SMT-MRAM provides advantages in speed, energy and endurance. There are various other significant discoveries in spin based device making (like Spin FET, Spin RTD, etc.) which makes this field a promising as well as a challenging one.
Successful spin based device making involves three pertinent requirements
(i) Spin injection
(ii) Spin transport with long spin coherence length
(iii) Spin detection.
Recent efforts in designing and manufacturing spintronic devices involve two different approaches.
- The first one is improving the existing GMR-based technology by either developing new materials with larger spin polarization of electrons or making perfections in the existing devices that behave as an efficient spin filter.
- The second approach focuses on finding novel ways of generation and utilization of spin-polarized currents. These include investigation of spin transport in semiconductors and looking for ways in which semiconductors can function as spin polarizer and spin valves.
Spintronics at Nano-scale:
Spintronic devices offer several advantages over the conventional bulk semiconductors such as
Non-volatility (i.e., the information is retained even after being switched off)
- Increased Data Processing
- Decreased Electrical Power Consumption
- Increased Integration Density
After the discovery of GMR effect remarkable advancement has taken place in case of data processing, device making technologies and quantum computation. The ultimate target is to reach beyond ordinary binary logic using ‘qubits’ and ‘spin entanglement’ for new quantum computing strategies, which in turn requires measured control of spin dynamics even on a single spin scale. It demanded merging of two rapidly evolving fields, ‘Spintronics’ and ‘Molecular Electronics’. So, in short a deeper understanding is needed in spin dynamics and spin transport at atomic and molecular length scales.
Several path breaking research findings are there like discovery of Spin-FET, Spin Hall Effect (SHE), Spin polarization in Double stranded DNA, Non-volatile Spin Logic gates, Increasing spin coherence length considerably by applying sound waves etc., to name a few. For example, in Datta-Das Spin-FET, Spin-Orbit coupling (Rashba type) was used to modulate the spin orientation of the electrons in the conduction band of a 2DEG. Rashba coupling strength can be tuned externally by applying gate voltage and it provides an advantage over using ferromagnetic leads as the later case generates high mismatch in resistivity. Or for that matter, in SHE, longitudinal flow of unpolarized charge current through a sample with SO coupling induced non-equilibrium spin accumulation at the lateral edges of the sample and thus a pure Spin Current got established. Here, just like Classical Hall effect, due to Spin-Orbit interaction, a spin dependent transverse force acts on up and down spin electrons and deflects them in opposite directions.
Conclusion:
In short, the subject ‘Spintronics’ undoubtedly brings new challenges to the researchers across the globe. We all are hopeful that the newly developing spin based electronic devices will replace soon the traditional ones. Researchers anticipate that establishing the principles of inter-relation for different disciplines of Physics like Electromagnetism, Heat, and even Mechanics may provide an opportunity to extend Spintronics remarkably, which may direct us to a new paradigm of Physics and its applications.
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