Abstract
From time immemorial, magnetism and magnetic materials (the materials which can respond to an applied magnetic field) have been a fascination for mankind. Later, scientists and engineers successfully employed this ever attracting property into numerous machines and devices, while most of those devices were based on electromagnetic induction. On the other hand, a classical analogue to the magnetic properties, ferroelectricity [1-3], the generation of spontaneous polarisation in the presence of an applied electric field, began to be a hot topic of research on at a later stage. Other closely related properties like piezoelectricity [4-6] (conversion of mechanical stress in to electric signals), pyroelectricity [7] (conversion of temperature difference into voltage) etc. were also investigated widely. As magnetism and electricity have always been treated on similar lines, scientists started dreaming about a material possessing these two properties together in the same phase bearing in mind the potential of such materials in microelectronics and micromechanics. However, these two properties are hard to be observed in the same phase at normal temperatures as they are governed by mutually exclusive physical laws [8, 9]. The most exciting and prevalent property of multiferroics and magnetoelectrics is controlling and manipulating the ferromagnetism with electric fields which paved way to the areas of information storage, actuation and sensing [10-19]. Rapid advancement of the property of electric field control of the spin of carriers has broadened the applications of multiferroics and magnetoelectrics to the field of spintronics [20]. Spin filter junction is one of the special manifestations of the device architecture in the above aspect using multiferroic tunnel junction [21-27]. As a part of this discovery, multiferroic thin films deserve an important role because critical thickness is a strongly dependent parameter for determining the properties of an electrode [28-30]. A strong focus on spin wave devices leads to the invention of spin wave amplifiers [31] and magnetoelectric generators, where the ferroelectric oscillations are controlled by magnetic field and vice versa. Recent discoveries reveal that the potential applications of multiferroics are beyond the electrical control of magnetic ordering which includes the invention of electrically controllable logic elements or nanoscale storage devices. Multiferroic Magnetoelectric composite nanostructures have been developed as a prerequisite for micro fabrication techniques due to their interesting interface design and atomic arrangements [32, 33]. It could be used as magnetoelectric read heads where the output waveforms followed the wave function of AC magnetic excitation signals [34]. Artificially engineered multiferroics can be made possible by properly designing magnetoelectric heterostructures which are the composites of piezo/ferro electric structures and a career mediated magnet. A diluted magnetic semiconductor or a double exchange ferromagnet can serve the purpose. The magneto electric coupling in this case is effected through the interface electric field [28, 35, 36]. 1-D Heterostructures or multiwalled structures are advantageous because of their large interfacial surface area compared to the conventional heterostructures [28, 37] and hence allow strong magneto electric coupling. Additionally, this geometry can reduce lattice deformation induced suppression of the magnetic as well as piezoelectric response. The concept of multiferroic behavior can be extended to other physical phenomena, such as electron transport, which can expand the application potential [38]. This edition of Recent Patents on Materials Science is dedicated to magnetoelectrics and multiferroics. Recent patents and highly relevant publications are analysed in depth so as to give the readers better insight into the later developments in the device level applications of these materials. The issue covers wide range of applications of single phase as well as composite type multiferroics. Chapter 1 is a lead article prepared by reviewing the recent patents and most relevant publications in the field of multiferroics and magneto electrics. Chapter 2 gives a detailed investigation on spintronic applications of multiferroics and magnetoelectrics. Different synthesis mechanisms for thin film multiferroics is dealt in the following two chapters. In one of these chapters, thrust is given on the synthesis of thick and thin film multiferroics by electrophoretic deposition, while the other chapter describes the synthesis of multiferroic and magnetoelectric composite thin films. There exists enough scope for further improving the properties of these materials and unravelling novel applications, fabrication of novel devices and explaining the deep underlying physical laws governing the properties. Hence I wish this special issue can guide researchers working in this area, especially those focusing on device level research towards unseen applications.
Keywords: Ferroelectricity, magnetism, magnetoelectrics, multiferroics, piezoelectricity, sensors, spintronics.
21