What is Spintronics?
Spintronics, also known as spin electronics, is an emerging solid-state device technology that exploits the intrinsic spin properties of an electron and its associated magnetic moment, in addition to the electron charge. Conventional electronic and semiconductor devices rely on the transport of electron charge carriers. Whereas, spintronics deal with spin-charge coupling in metallic systems with implications in the efficiency of data storage and transfer. Spintronic systems are of particular interest in the field of quantum computing and neuromorphic computing.
Every electron can exist in one of the two spin states: spin-up and spin-down. In other words, electrons can rotate either clockwise or counterclockwise with constant frequency around its axis. They can represent 0 or 1 in logic operations. In ordinary materials, the spin-up magnetic moments cancel with spin-down magnetic moments and therefore are of no use for spintronics. Ferromagnetic materials are needed for spintronics which can provide a surplus accumulation of different spins in a tiny region called domains. These majority-up and majority-down domains are randomly scattered and with an externally applied magnetic field will line up the domains in the direction of the electric field.
Spintronics is the driving technology behind next-generation nano-electronic devices to increase their memory and processing capabilities while reducing power consumption. In these devices, the spin polarization is controlled either by magnetic layers or via spin-orbit coupling. Spin waves, also known as magnons, can be used to carry spin current without causing heat. Spintronics is also used in the semiconductor industry to manufacture different types of transistors, lasers and integrated magnetic sensors.
The miniaturization of microelectronic components is a basic necessity for semiconductor devices. However, over the years of miniaturization, the physical size of semiconductor electronics will soon approach a fundamental barrier. Therefore, device engineers and physicists feel that quantum mechanics can help in future miniaturization. After all, electronic spin is a quantum phenomenon. Spintronics combined with nanotechnology can be a perfect solution for the future miniaturized devices.
Types of Spintronics
- Metal-based spintronics: Giant-magneto resistance (GMR) in magnetic (metal) multilayers was discovered in 1988 and led to the birth of spintronics. GMR based metal spintronics became the standard technology for read-heads of hard disk drives. Later, a large tunnel-magnetoresistance (TMR) between two magnetic metals separated by a thin insulator was demonstrated at room temperature in 1994. Magnetic tunnel junction (MTJ) is currently the preferred choice for manufacturing of magnetic random-access memory (MRAM) devices.
- Semiconductor based spintronics: Despite rapid advancement in metal-based spintronics, a major focus was to find novel ways to generate and utilize spin-polarization currents in semiconductors. Doped semiconductor materials display dilute ferromagnetism, and strong ferromagnetism is essential for achieving spintronics. The selection of materials for semiconductor spintronics depends on the ability of the material to provide ferromagnetism at room temperature. Majority of the work is focussed on the use of GaAs (Gallium Arsenide) and InAs (Indium Arsenide) at semiconductor-semiconductor or semiconductor-metal interfaces. Spins in semiconductors can be easily manipulated and controlled. Spintronics based devices can easily integrate with existing semiconductor technology. Semiconductor spintronics combined with photonics and magnetics can provide multi-functional devices such as spin-transistors, spin-LEDs, memory devices, optical switches operating at terahertz frequencies and few other devices.
There are different ways to create spin polarisation and harness spin degree of freedom in metals and semiconductors. Few important ways are listed below.
- Spin-injection from a ferromagnetic material.
- A stray field (magnetic or electric) can induce population difference of spin polarised electrons in ferromagnetic materials.
- Electromagnetic waves such as circularly polarized light and microwave excite spin polarised electrons in semiconductors depending on optical selection rule. A spin-polarised electron current can be extended further to spin generation by electromagnetic waves, including spin pumping and high-frequency spin induction.
- A thermal gradient can also produce spin polarised carrier flow using the spin Seebeck and Nernst effect, and this can be useful in energy harvesting.
There are many spintronic based devices in the market ranging from transistors, oscillators, memory units to quantum computing. A few prominent devices are listed below.
- Spin transistor: The basic idea of a spin transistor is to control the spin orientation by applying a gate voltage. A spin field-effect transistor (FETs) consists of ferromagnetic source and drain electrodes, a semiconductor channel that contains a layer of electrons, and a gate electrode attached to the semiconductor. The spin-polarised electrons are injected from the source electrode. The spin precision angle controls the flow of current. The gate electrode controls the rotation of the electron spin after entering the semiconductor channel. The success of these transistors depends on efficient injection of spin currents from a ferromagnetic metal to a semiconductor.
- Quantum dots or Spin-based computers: In quantum dots, electron motion is quantized in all directions and conducting electrons are confined within the nano-meter distances. The charge and spin of electrons can be controlled in these dots. The spin of an electron confined to quantum dots can be used as quantum bits, and these arrays of quantum dots can serve to build quantum computers. Already, quantum dots are useful in electronic and optic devices such as quantum-dot lasers, memory chips, and also in quantum cryptography.
- Hard disk drive (HDD) read head: GMR based HDD was introduced by IBM in 1997. Later, TMR based HDD was introduced by Seagate in 2005. A new head assisted magnetic recording (HAMR) drive was demonstrated by Seagate in 2012.
- Magnetic Sensors: Magnetic sensors are used to detect position, angle, rotation and magnetic fields. These sensors are built mainly based on Hall, GMR and AMR effects. A highly sensitive magnetic sensor is used in magnetoencephalography to map the brain.
Spintronics is at the verge of becoming a major technology for microelectronics. Many devices have started entering the market with a recent launch of magnetic memory production at a large scale. However, there is a need for further improvements in spintronic device applications, and few are noted below.
- Development of low power devices
- Unconventional computing such as stochastic computing using spintronic devices
- Energy harvesting using spin-diodes or spin-caloritronics
- Need for the development of artificial neurons and synapses based on spintronic devices.