The ultimate aim of spintronics is to produce devices that manipulate the spin states of an electron in addition to the charge of an electron. There are two main spintronic devices in commercial use: hard drive read heads and magnetic random access memory (MRAM). Whilst hard drive read heads are ubiquitous, MRAM devices have only a narrow usuage due to problems in making them competitive with existing memory technology.
Hard drive read heads are the first major commercial application of a spin related technology. The GMR effect, discovered independently by Fert and Grünberg [1,2], is the phenomena whereby the resistance of a ferromagnetic-conductor-ferromagnetic junction decreases in the presence of an external magnetic field.
|Figure 1: Illustrative schematics of two simplistic designs for spinvalves. The design on the left makes use of effect of thickness on the coercivity of a magnetic layer, whilst the righthand design pins one ferromagnetic layer using the exchange bias from an adjacent antiferromagnetic layer.|
Figure 1 illustrates two simple designs principle graphically. Electrons of one spin are strongly scattered on leaving(entering) a ferromagnetic layer with opposite magnetisation. By controlling the polarisation in one of the ferromagnetic layers, the resistance of the configuration can be changed. For a constant voltage, this affects the current flowing through the device. This controllable resistance is an initial adjustable in the design of a spin transistor.
|Figure 2: Simplistic diagram showing the two configurations for spinvalve designs as used in modern spinvalve designs.|
Figure 2 shows the two types of spin valve used in read head applications. These are the current in plane (CIP) and current perpendicular to the plane (CPP) configurations. In the CIP valve the current flows along the plane, parallel to the layers and uses the GMR effect (where a change in relative polarisations causes a change in overall resistance). The CPP configuration makes use of room-temperature tunneling magnetoresistance (TMR), discovered by Parkin and Yuasa in 1994 \cite[3,4]. In TMR, a tunnel junction is introduced between the FM layers with the current flowing perpendicular to the layers. When the FM layers have different magnetisations, the probability of an electron tunneling across the barrier is suppressed and the resistance across the junction is increased. When the FM layers have the same magnetisation, then the tunneling current increases and the resistance decreases.
For hard drive read heads the larger the magnetoresistance change, the greater the sensitivity of the read head. An increased ability to detect smaller changes in applied external field, from smaller magnetic bits, results in greater bit densities in the hard drive. A disadvantage of smaller magnetic bits is a reduction in the detected current, this makes it more sensitive to noise in the system.
The transistor has had an understandably large impact on modern technology, spintronics aims to produce a spin analogue to this fundamental technology. One of the possible designs proposed is the Datta and Das spin FET , illustrated in Figure 3.
|Figure 3: Image of the Datta-Das SpinFET, reproduced from .|
In the spin FET, spin polarised electrons are injected through a ferromagnetic metal (FM), the source electrode and detected at the drain electrode. The device is constructed such that one FM layer has a fixed magnetisation (as with the exchange biased spinvalve), whilst the other is easy to manipulate through an applied field. Through the correct choice of narrow-gap semiconductor the electrons passing through this layer behave as a two-dimensional electron gas (2DEG). An appropriate choice of semiconducting material, such as InAs alloys, mean that the semiconductor layer can be made to behave as a quantum well. Application of a voltage at the gate electrode results in the electrons in the 2DEG rest in an effective electric field. In the reference frame of the electron this electric field is relativistically transformed into a magnetic field. This field causes the spin of the electron to precess and is known as the Rashba effect. Through an appropriate choice of drain magnetisation the resistance at this interface can be either high or low, corresponding to a high or low drain current. There are several difficulties in realising such a device. The two main issues are the requirement of having a highly spin-polarised current and the control of the spin precession in the 2DEG. These issues have so far prevented practical construction of spin FETs.
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