Physics Of Manganites

This series of books, which is published at the rate of about one per year, addresses fundamental problems in materials science. The contents cover a broad range of topics from small clusters of atoms to engineering materials and involves chemistry, physics, materials science and engineering, with length scales ranging from Ångstroms up to millimeters. The emphasis is on basic science rather than on applications. Each book focuses on a single area of current interest and brings together leading experts to give an up to date discussion of their work and the work of others. Each article contains enough references that the interested reader can access the relevant literature. Thanks are given to the Center for Fundamental Materials Research at Michigan State University for supporting this series. M. F. Thorpe, Series Editor E mail: thorpe@pa. msu. edu V PREFACE This book records invited lectures given at the workshop on Physics of Manganites, held at Michigan State University, July 26 29, 1998. Doped manganites are an interesting class of compounds that show both metal insulator and ferromagnetic to paramagnetic transitions at the same temperature. This was discovered in the early 1950s by Jonker and van Santen and basic theoretical ideas were developed by Zener (1951), Anderson and Hasegawa (1955), and deGennes (1960) to explain these transitions and related interesting observations.

The physics of transition metal oxides has become a central topic of interest to condensed-matter scientists ever since high temperature superconductivity was discovered in hole-doped cuprates with perovskite-like structures. Although the renewed interest in hole-doped perovskite manganites following the discovery of their colossal magnetoresistance (CMR) properties, began in 1993 about a decade after the discovery of high temperature superconductivity, their first investigation started as early as 1950 and basic theoretical ideas were developed during 1951-1960. Experience in sample preparation and characterization, and in growth of single crystals and epitaxial thin films, gained during the research on high temperature superconductors, and the development of theoretical tools, were very efficiently used in research on CMR manganites. In early nineties it appeared to many condensed matter physicists that although the problem of high temperature superconductivity is a difficult one to solve, a quantitative understanding of CMR phenomena might be well within reach. This book is intended to be an account of the latest developments in the phys ics of CMR manganites. When I planned this book back in 2000, I thought that research on the physics of CMR manganites would be more or less consolidated by the time this would be published. I was obviously very optimistic indeed. We are now in 2003 and we still do not have a quantitative understanding of the central CMR effect. Meanwhile the field has expanded. It is still a very active field of research on both the experimental and theoretical fronts.

Mixed-valence manganites have attracted considerable attention since the late 19th century. The intriguing interplay of charge, spin and orbital ordering in the manganites systems superimposed by interface/surface effects provides a scientific platform beneficial for both fundamental and application-oriented research. The fundamental basis for understanding physical properties and designing new functionalities of manganites is the concept of the symmetry of order parameters which can be manipulated artificially by controlling the interplay of electronic degrees of freedom. This book explores the possibility of externally modifying the properties of manganite based thin films and heterojunctions by epitaxial strain or artificial boundaries, which could give us new insights to generating properties at the interface between film and heterointerface. The book describes some of the sophisticated concepts concerning manganite-based films and heterojunctions in clear detail, bringing us closer than ever to understanding the research progress and main trends. What we can learn from the book is highly thought-provoking: the explanation of the strain stabilisation and strain induced effects in manganite based films and heterointerfaces; the comparison between electrical transport in manganite based heterojunctions and semiconductor junctions; and, in particular, the interpretation of the photo-induced and photo-voltaic effect in manganite based films and heterojunctions. Furthermore, the magneto-tunability at the manganite based heterointerfaces leading to the design of new functionalities will be highlighted at last.

The fact that magnetite (Fe304) was already known in the Greek era as a peculiar mineral is indicative of the long history of transition metal oxides as useful materials. The discovery of high-temperature superconductivity in 1986 has renewed interest in transition metal oxides. High-temperature su perconductors are all cuprates. Why is it? To answer to this question, we must understand the electronic states in the cuprates. Transition metal oxides are also familiar as magnets. They might be found stuck on the door of your kitchen refrigerator. Magnetic materials are valuable not only as magnets but as electronics materials. Manganites have received special attention recently because of their extremely large magnetoresistance, an effect so large that it is called colossal magnetoresistance (CMR). What is the difference between high-temperature superconducting cuprates and CMR manganites? Elements with incomplete d shells in the periodic table are called tran sition elements. Among them, the following eight elements with the atomic numbers from 22 to 29, i. e. , Ti, V, Cr, Mn, Fe, Co, Ni and Cu are the most im portant. These elements make compounds with oxygen and present a variety of properties. High-temperature superconductivity and CMR are examples. Most of the textbooks on magnetism discuss the magnetic properties of transition metal oxides. However, when one studies magnetism using tradi tional textbooks, one finds that the transport properties are not introduced in the initial stages.

The study of the spontaneous formation of nanostructures in single crystals of several compounds is now a major area of research in strongly correlated electrons. These structures appear to originate in the competition of phases. The book addresses nanoscale phase separation, focusing on the manganese oxides known as manganites that have the colossal magnetoresistance (CMR) effect of potential relevance for device applications. It is argued that the nanostructures are at the heart of the CMR phenomenon. The book contains updated information on manganite research directed to experts, both theorists and experimentalists. However, graduate students or postdocs will find considerable introductory material, including elements of computational physics.

This book presents insights into manganese oxides, which are materials with important technological applications such as magnetic refrigeration and magnetic sensors. It provides many mathematical proofs carried out in a didactic and elegant way; some of them use, for example, the method of mathematical induction. The book will be of interest to both researchers of physical sciences and the general reader interested in the subject.

For environmental concerns, it is highly desirable to replace gas-based refrigeration by magnetic refrigeration. Magnetic refrigeration has significant advantages such as small volume, chemical stability, low cost, non-toxicity and not causing sound pollution. Among the pertinent magnetocaloric materials, perovskite manganites are of special interest because they exhibit extremely large magnetic entropy and adiabatic temperature variations, a small thermal or magnetic hysteresis, high chemical stability. Further, the Curie temperature and saturation magnetization can be tailored by changing doping element and doping concentrations. The book references 289 original resources and includes their direct web link for in-depth reading. Keywords: Magnetic Refrigeration, Magnetocaloric Effect, Perovskite Manganites, Perovskite Structure, Magnetic Entropy, Magnetic Hysteresis, Thermal Hysteresis, Chemical Stability, Curie Temperature, Saturation Magnetization, Lanthanides.