It is important to note that everybody has already a spintronic device in their desktop/laptop, since the read heads of the hard disc drives use the giant magnetoresistance (GMR) phenomenon to read the magnetic information written on the disc. Magnetoresistance (MR) is a term widely used to mean a change in the electrical resistivity due to the externally applied magnetic field. GMR [1,2] exploits the influence of the spin of the electrons on the electrical resistivity in a magnetic multilayer films composed of alternate ferromagnetic (FM) and non-magnetic (NM) layers (For example, Fe and Cr). Before the discovery of GMR, the investigations on the charge and spin of the electrons were usually considered to be independent of each other and hence little attention was paid to probe a correlation between charge and spin. However, the influence of the spin on the mobility of the electrons in FM metals, first suggested by Mott [3], had been experimentally demonstrated and theoretically described in early works [4] more than ten years before the discovery of GMR.

Nevertheless, how to enhance the MR effect is an attractive challenge and further scope of progress for scientists in fundamental physics and also for researchers in industries. There are several strategies to obtain larger GMR effects: (1) taking the current flowing perpendicular to plane GMR (CPP – GMR), (2) using the tunneling current (Tunneling MR (TMR)), (3) using a new class of materials called half-metal as the magnetic constituent and (4) using the ballistic current (Ballistic MR (BMR)). Usually resistance measurements in thin metallic specimens are carried out in a conventional geometry to use an electric current flowing in the film plane. Such configuration is called as Current in Plane (CIP) geometry. In contrast, resistance measurements in the other geometry (i.e., CPP) are very inconvenient for thin metallic films. However, an enhancement of MR ratio is expected in the CPP geometry compared to the CIP geometry because the GMR effect is associated with electrons passing through interfaces.

Apparently, realization of GMR was the first step on the road of utilization of the spin degree of freedom, which triggered the development of the active field of research of spintronics. Today, this field is extending largely with very promising new axes like the phenomena of spin transfer, spintronics with semiconductors, molecular spintronics and single-electron spintronics. In addition, the recent advent of quantum computing has added a new dimension to spintronics. The spin polarization of a single electron can exist in a coherent superposition of two orthogonal spin polarizations (i.e., mutually anti-parallel spin orientations) for a relatively long time without losing the phase coherence, compared to the charge degree of freedom. Therefore, spin has become the preferred vehicle to host a quantum bit. The potential application of spin to scalable quantum logic processors has provided a tremendous boost to the field of spintronics.

(b). Classes of Magnetic Materials:

As the origin of magnetism lies in the orbital and spin motions of electrons and how the electrons interact with one another, the right way of introducing different types of magnetism is to demonstrate how the materials respond to an external applied magnetic fields.