A very short introduction to spintronics

The concept of spintronics:   The information revolution, which has surprised us over the last few decades was started with the discovery of transistor action in Ge (observed at Bell Labs in December 1947). In recent times the density of information that can be processed, stored, and transferred per unit area of the device has been increased exponentially, thus demanding for faster processing speed. The miniaturization of these devices has proven to be among the most important developments toward processing more information more quickly. However, experts believe that the silicon-based microchips will reach the physical limits of miniaturization within the next 10 years [1]. The traditional semiconductor based microelectronics industry is rapidly approaching a point where device fabrication can no longer be simply scaled to progressively smaller sizes. In order to continue at the current rate of miniaturization, and to continue to increase the computing capability of electronic computers, fundamentally new technologies must be introduced in the design and manufacturing of computing elements. This has triggered a substantial amount of research based on new ideas, such as the exploitation of quantum mechanical spin of the electron. The discovery of the giant magnetoresistance effect (GMR) [2,3] which is a quantum mechanical effect is just one of them. The storage capacity of magnetic materials has increased dramatically in recent years following this discovery. Another similar phenomenon called the tunnelling magnetoresistance effect [4] is already implemented in the latest magnetic random access memory devices. However, semiconductor manufacturers are still ignoring the electron spin, in spite of these advances in the magnetic recording industry. In semiconductors, the spin degree of freedom can be exploited to develop new logic devices with enhanced functionality, higher speeds and reduced power consumption. These devices could be fabricated with many of the tools already used in the electronics industry, and the idea holds the promise to speed-up the development in microelectronics. The concept of spintronics [5,6,7,8,9,10,11,12,13] is based on the exploitation of the quantum mechanical spin of the electron, which is used to differentiate electrical carriers into two different types according to whether their spin projection onto a given quantization axis is �[ 1/2]. Spintronics offer opportunities for a new generation of devices combining standard microelectronics with the spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material.

One of the first proposal of using spin injection in semiconductors which stimulated a worldwide research effort in semiconductor spintronics was proposed by S. Datta and B. Das [14], shortly after the discovery of GMR. They proposed a new type of electro-optic device, that is similar to a field effect transistor (FET). When a voltage is applied to the gate electrode of a FET, the resulting electric field creates a conducting channel between the source and the drain electrodes. Datta and Das suggested that the field could also be used to control the orientation of the spin so that it modulates the current. The beauty of their idea is that the "spin-FET" can be fabricated using the standard equipment in microelectronics. In fact the spin of the electron can be controlled by the electric field of the gate electrode of the FET using Rashba's idea that the ballistically travelling electron will feel the gate voltage as an effective magnetic field. Though the "spin-FET" conceived by Datta and Das continues to be developed by both theorists and experimentalists, the realization of a working prototype of the Datta-Das spin FET is rather difficult. The implementation of the Datta-Das spin FET faces several challenges which can be arranged into three distinct categories; (i) spin injection, (ii) spin transport/manipulation, and (iii) spin detection. There have been significant experimental and theoretical developments in each of these areas, yet there still exist a multitude of problems to overcome. In other words, before spin can become a big business, researchers need to fulfil some fundamental requirements in semiconductors; to create, transport, manipulate, store, and detect spin. The technological challenge for manufacturers is to combine the technology in the semiconductor industry with the completely different techniques used in the magnetic recording industry to produce devices on the nanometer scale.

Experimental approaches to spintronics:  

In order to utilize the spin degree of freedom in semiconductors we need to fabricate appropriate materials, understand the spin-dependent phenomena, and control the spins. This thesis is related to the development and fabrication of materials useful for spintronics. The development of semiconductor-based materials with magnetic or spin-related properties can be broadly divided into two categories namely, (i) magnetic semiconductors or diluted magnetic semiconductors (DMS) and their heterostructures, and (ii) ferromagnetic-metal/semiconductor heterostructures (FM/SC). A basic obstacle for the use of DMS in room-temperature (RT) spintronic devices, however, is their relatively low Curie temperature. This thesis is linked with the second approach, the FM/SCs, which include magnetic 3d-transition-metals or their alloys with SC. These systems offer Curie temperatures well above RT. Successful spin injection has been reported for both the DMS [15,16] and FM/SC [17,18,19,20] though the efficiency remained low in the latter (at low temperatures). However, researchers have the opinion that a significant increase in spin injection efficiency can be achieved by optimizing the interface structure, because the spin injection process is strongly influenced by the details of the FM/SC interface. For example, it has been shown that for the case of Fe/AlGaAs structures a decrease in interface roughness significantly increases the spin injection efficiency [21]. For this reason, FM/SC have experienced a tremendous boost of research activities. A detailed discussion of recent progresses in FM/SC, especially Fe/GaAs and related systems can be found in the recent review article by Wastlbauer, and Bland [22].

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