Spin Electronics (Spintronics)

8.2.2.2. Spin Electronics (Spintronics)

This subsection is a continuation of the previous subsection on magnetic semiconductors. Dur- ing the last decade or so, one of the new and highly potential inter-disciplinary fields that has emerged in the horizon of nano science is the ‘Spin Electronics’, which is popularly known as “Spintronics”. Apart from various electronic effects in materials in the quantum or nano level, the spin dynamics assumes a great importance, as written above. The main purpose of ‘spintronics’ is to understand this spin depend- ent property and its implications in the domain of newer potential applications, e.g. in ‘high density information storage media’ and the non-volatile ‘memory devices’, which is now known as “Magnetic Random Access Memories” or in short as MRAM, which would combine the advantages of magnetic memories and those of DRAM. It should be pointed out here that in the field of ‘ferroelectricity’, there is also a new class of materials like lithium niobate and lithium tantalate, which show high non-volatile memory called FRAM, and there is a possibility of 'infinite memory' in such materials [29].

This exciting new field of ‘Electronics and Magnetics’, namely ‘Spintronics’, has attracted great attention recently. This is based on the very basic fact that electrons have spin as well as charge. Within

293 the context of spin-electronics, the electron spin, as well as charge, is manipulated for the operation of

OTHER METHODS AND OTHER NANO MATERIALS

information processing circuits. The advantages of ‘spintronics devices’ over traditional ‘semiconduc- tor devices’ include the following :

(a) Much faster switching times, (b) Reduced power consumption, (c) Non-volatile memory, and (d) Increasing levels of miniaturisation. The research into ‘spintronics devices’ has mainly focused on methods of switching the mag-

netic configuration and there is a growing interest in the use of spin-polarised current to switch magnetic devices. There are several advantages associated with the use of spin-polarised current rather than exter- nal magnetic fields, i.e. the most important being no more (or much reduced) cross-talk and low power consumption. The mechanism involved in the current-induced (i.e. spin-polarised current) switching of magnetisation is not clear yet, and some new insights of the switching process need to be given in this area.

It is important to make devices, but the characterization of some properties need to be also done, particularly by novel techniques. With magnetic recording data rates approaching GHz range and the data density exceeding 150 Mbit/mm 2 (100Gbit/in 2 ), the ‘dynamic magnetization mapping’ at ‘nanometer- length’ scale and ‘sub-nanosecond’ temporal resolutions is demanded for material analysis and device characterization. This is beyond the limits of traditional imaging techniques. The 'Time-Resolved Mag- neto-Optical Scanning Kerr Effect Microscopy’, i.e. in short called TR-SKEM, has recently emerged and demonstrated its power in imaging fast dynamics of the sample magnetization directly, but it does not image the strain field around it. This is done at femtosecond and picosecond temporal resolution.

With the advent of TR-SKEM, the study of ‘spin dynamics’ in nano-scale contact ‘spintronic devices’, by polarisation current-induced switching at picosecond temporal resolution, the activity in this field is now taking shape. In TR-SKEM, the transient spin-polarized current pulses are generated by

a ‘photo-conductive switch’, which is triggered by ‘picosecond laser pulses’. The magnetization of the devices can be manipulated directly by feeding the ‘spin-polarized current’ into the devices themselves. The device materials for ‘spintronics’ are prepared by different methods of nano-fabrication, involving : (a) Optical Lithography, (b) Electron-Beam Lithography, and finally by (c) Focussed Ion-Beam (FIB) Lithography.

In the domain of ‘spintronics’, in the ‘advanced magnetic recording materials’, the study of ‘spin dynamics’ at ‘submicron spatial resolution’ and ‘picosecond temporal resolution’ is also done by using a high-field TR-SKEM. The magnetization is pumped by sub-nanosecond magnetic field pulses with high amplitude, which are generated by feeding high voltage pulses with picosecond rise time to a microstrip line structure. The ‘magnetic recording materials’ that are normally studied include the fol- lowing :

(a) Thin film with perpendicular anisotropy, (b) Patterned thin films with in-plane or perpendicular anisotropy, (c) Nano-scaled single crystals of Fe, Co and Ni elements. The ‘spintronics’ is quite a new area. With the emergence of newer materials and devices in the

horizon of ‘spintronics’ along with some powerful techniques of chatracterization of the devices through the study of ‘spin dynamics’, there is definitely a great future for ‘nano-magnetics’.

NANO MATERIALS