A system of fine particles of magnetic materials behaves differently than its bulk counterpart. This book provides a comprehensive background to understand this deviation of behaviour. After giving an introductory account of magnetism, the book presents a discussion on ferro, antiferro, ferri and weak ferromagnetism which belong to a class of cooperative phenomena. An elementary idea of domain structure which explains the behaviour of ferromagnets has been included. The fine particles of magnetic materials have been classified according to grain size into single and multidomain particles. In an assembly of fine single domain particles below a critical size, any cooperative alignment of dipole moments may not be possible because of thermal agitation. As a result, the assembly exhibits magnetic behaviour of a giant paramagnetic system above a critical temperature known as blocking temperature. This phenomenon known as superparamagnetism has been presented in detail. Magnetic materials comprising of fine grains can also carry a faithful record of the geomagnetic field at the time of cooling of an igneous rock or deposition of a sedimentary rock. This memory of the past geomagnetic field is maintained through their remanent magnetization, which is extremely stable. A detailed experimental study of a magnetic system exhibiting thermoremanent magnetization (TRM) due to its cooling below the critical temperature in the presence of the residual magnetic field of the experimental set up in the laboratory has been described. Different possible mechanisms on the origin of TRM in the samples including the formation of fine lamellae have been discussed. The last chapter presents an experimental study on magnetic systems comprising of very fine magnetic particles and exhibiting superparamagnetism.
Bibek Bandyopadhyay after earning his masters in Physics from Visva Bharati, Santiniketan obtained his doctorate from the University of Delhi in 1980. Since then he has worked in the Indian Institute of Technology, Delhi, Department of Science & Technology, Solar Energy Centre and Ministry of Non-Conventional Energy Sources, Government of India in various capacities. His research interest includes materials science and solar energy. He has authored more than 40 research articles and has lectured extensively as a guest faculty in the country and abroad. Currently, he is a director in the Ministry of Non-Conventional Energy Sources.
