Dream for a computer storage device which performs as fast as static RAM (SRAM), delivers the cost effectiveness of dynamic RAM (DRAM), promises the non-volatility of flash memory, combined with added benefits of longevity, denser storage, durability and stability. Let’s discuss whether spin transfer torque based Random access Memory (STT-RAM) devices can help to realize this dream.
What is spin transfer torque?
Charge carriers like electrons carry an intrinsic form of angular momentum called spin. As we know, electric current is a continuous flow of charge carriers such as electrons or ions. The electric current in general consists of 50 % spin up and 50 % spin down electrons give rise to an unpolarized stream of charge carriers. But we can make this unpolarized current to spin polarized (more electrons possess spin in either one of the spin up or spin down direction as demonstrated in figure) by passing the unpolarized current through a magnetic layer whose spin state is made fixed with the application of an external magnetic field called fixed layer.
If we allow the resultant spin polarized light to pass through another magnetic layer of comparatively smaller thickness with respect to fixed layer, the angular momentum can be transferred to this layer with a flip to direction of orientation. As this layer change its spin states in response to magnetic state of fixed layer, it is often called as a free layer. This phenomenon is achieved by passing electric current from the fixed layer to free layer through a thinner insulating layer separating it. Such structures comprised of two ferromagnetic layers (fixed layer and free layer discussed) separated by a thinner insulating layer to allow polarized electrons to tunnel through it, called magnetic tunnel junctions (MTJ). This effect which can be realized in MTJs in which magnetic orientation of a particular magnetic layer (free layer) is modified using a spin polarized current transferred via an insulating tunnel junction from a fixed magnetic layer is called spin transfer torque. When spin polarized current reaches the free layer, majority of the spin tries to relax into lower energy state of opposite spin resulting to an effective torque in the free layer. However, the key concept behind this effect is, if a magnetic layer can act as a spin filter, then it must cause to produce a torque to hold good spin angular momentum conservation. When unpolarized electric current pass through the fixed layer, the electrons will align in the same direction of magnetization of fixed layer. Suppose, if the fixed layer magnetic spin state and free layer magnetic spin states are aligned antiparallel with respect to each other initially and we want to change both layers spin parallel to each other. In such a switching requirement from antiparallel to parallel state, we send current from fixed layer to free layer. The polarized current from fixed layer reach the free layer and provide spin transfer torque to free layer to let the magnetization direction of free layer to change to parallel state with respect to magnetization direction of fixed layer. In case, if the magnetization of fixed layer and free layer are parallel and we want to change it to antiparallel alignment with respect to each other, we pass electrons from the free layer to fixed layer and so ,the electrons get polarized by the fixed layer. The polarized current will reach the free layer with majority of electrons with spin aligned in parallel direction and a fewer number of electrons with spin oriented in anti-parallel direction. The electrons with antiparallel spin get reflect at the interface between fixed layer and barrier before it reach to fixed layer and get polarised. These reflected electrons with anti-parallel spin reach the free layer and provide spin transfer torque to let the magnetization change to anti-parallel state.
Role of spin transfer torque in memory devices
The memory devices based on charge of electrons such as SRAM and DRAM has a major disadvantage of charge leakage and high-power consumption. On the other hand, STT-RAM are based on spintronics with a promise of near zero charge leakage. In addition, STT-RAM has better scalability compared to other memory devices which use magnetic fields such as magnetoresistive RAM (MRAM). In STT-RAM spin polarized torque is used to achieve change in direction of orientation of spin in active elements whereas MRAM use magnetic field to obtain flip in active elements. The spin transfer torque (STT) technology can offer low current requirements in MRAM. Significant progress in research front has been witnessed recently in view of reduction in current density in devices based of STT technology.
Qualitative model of STT-RAM
The fundamental building block of a conventional memory device based on electron charge is a tiny circuit with one or more transistor and capacitor. These building blocks are called cells. Here, in the case of STT-RAM, the capacitor connected to the word line is replaced by an MTJ structure. The cells are organized into rows and demonstrate a bit line structure and the path which connect to a memory address is called word line. The word line provides an electrical path for the memory cells to be activated either for write or read operations, often called as storage and retrieval respectively.
The cost effectiveness of memory devices directly proportional to the density and chip size. Also, current available for data storage scales with transistor size. In view of this, device modeling of STT-RAM has to focus on to overcome these challenges. In addition, different applications may require different retention time, which is a measure of duration till data has to be retained in the absence of power. Hence application specific modeling will be an add on advantage to STT-RAM.