Ferrites: A ‘Spintronics’ perspective

‘Spin’ based electronics (spintronics) also known as magneto electronics has emerged as new field of interest in solid state device technology where the intrinsic spin of an electron, with which the magnetic moment is associated, instead of or in addition to the fundamental electronic charge of the same is exploited. Conventional electronic devices including today’s integrated circuits rely on the transport of electrical charge carriers -electrons - in a semiconductor such as silicon where in the information processing is performed using transistors that work by transfer of electrons, but the storage is done by magnetic recording using spin of electrons in a separate ferromagnetic metal. By the accomplishment of spintronics, tomorrow's technology can be seen magnetism (spin) and semi conductivity (charge) combined in one device that exploits both charge and spin to process and store the information. We may then be able to use the capability of mass storage and processing of information in the same device. Such a device will be called as “Spintronic device”. The potential advantages of spintronic devices will be higher speed, greater efficiency, and better stability, in addition to the low energy required to flip a spin. Some of the spintronic devices are spin value transistors, spin light emitting diodes, non volatile memory, logic gates, optical isolators, ultra flat optical switches etc.

Figure : The charge and spin associated with an electron

In general, such devices explore mostly the spin of electrons to encode and process data rather than the charge. The advantage of spin over charge is that spin can be easily manipulated by externally applied magnetic fields, a property already in use in magnetic storage technology. Another significant property of spin is its long coherence, or relaxation, time (nanoseconds, compared to tens of femto seconds during which electron momentum decays) once created it tends to stay that way for a long time, unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or recombination. These characteristics open the possibility of developing devices that could be much smaller, consume less power and will be more powerful for certain types of computations which is not possible with electron-charge-based systems. We know that electrons are spin ½ fermions and therefore constitute a two- state system with spin up and spin down. To make a spintronic device, the primary requirements are, first a system that can generate a current of spin – polarized electrons comprising more of one spin spices-up or down than the other (called a spin injector), and secondly, a separate system sensitive to the spin polarization of the electrons (spin detector). There are metal based spintronic devices in which spin–polarized current is generated by passing current through magnetic material; the most common application based on this effect is a giant magnetoresistance (GMR) device. Another one is semiconductor based spintronic devices, which is of the greatest relevance.

 Figure: The two interstitial sites in ferrites

The current interest in spintronics is focused on (a) the search for new materials in which spin polarization of injected currents could be increased and (b) the identification of highly polarized materials aiming for increasing tunnel magnetoresistance. Based on their potential applications in spintronics, ZnO-based materials have received renewed interest. On the other hand, ferrites with spinel structure are known for their applications in high frequency devices, and are also promising candidate for spintronics because their magnetic properties could be engineered as a function of size. Among ferrites, magnetite (Fe₃O₄) has high Curie temperature, weak crystalline anisotropy, and high degree of spin polarization, which makes it a potential candidate for spin electronics devices. The spinel structure consists of two cation sites for metal cation occupancy, i.e. tetrahedral (A) and octahedral (B) sites, where metal ions are coordinated by oxygen. If ‘A’ sites are occupied by divalent metal cation and ‘B’ sites are occupied by trivalent Fe, the structure of ferrites is said to be normal spinel. The structure is known as inverse spinel when ‘A’ sites are completely occupied by Fe³⁺ cation whereas ‘B’ sites are randomly occupied by divalent cation and Fe³⁺. Based on the first principle calculations it has been suggested that ZnFe₂O₄ is small band gap insulator and MnFe₂O₄ is a low carrier density half-metal in fully ordered state could be a candidate for spintronics. Recent studies on Zn_1-xCo_xFe₂O₄ show its potential application for magnetoelectric devices in multilayer structure. Ferrite-based structures could be useful for spintronics applications, if they exhibit half-metallicity with small carrier concentration. In this perspective, our interest in studies related to the synthesis and characterization of such mixed spinel structures and their polymer coated nanocomposites is having obvious importance towards achieving an optimal device composition for spintronics.