Hexaferrite Permanent Magnetic Materials

Transcription

1 Hexaferrite Permanent Magnetic Materials by Sami H. Mahmood Ibrahim Abu-Aljarayesh The relatively high metallicity of magnetic materials for practical applications imposes limitations for their efficient use due to their unfavorable characteristics. Accordingly, magnetic oxides with ferromagnetic properties emerged as the most widely used magnetic materials for practical applications, owing to their characteristic high resistivity and low eddy current losses, chemical stability, simplicity of production in mass quantities, and other favorable characteristics. An important class of these oxides is the class of hexagonal ferrites developed in the early 1950 s, which dominated the world market of permanent magnet applications since the end of the 1980 s. Among these ferrites, the magnetoplumbite (M-type) hexaferrite, is produced nowadays in large quantities at very competitive low prices, thus providing the permanent magnet market with probably the most cost-effective magnetic material. This concise book is intended to provide an overview of the basic concepts of magnetism and magnetic properties pertinent to permanent magnetic materials, importance of these materials in terms of their market share and versatility of practical use, synthesis techniques, and routes adopted for the modification and tuning of their magnetic properties. Emphasis is placed on hexaferrite materials for permanent magnet applications, with M-type ferrites as the focal point. The discussion is kept brief, in an attempt to provide a wide spectrum of knowledge for quick reference to specialized scientists and engineers in this ever increasing industry.

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3 Hexaferrite Permanent Magnetic Materials By Sami H. Mahmood 1 and Ibrahim Abu-Aljarayesh 2 1 The University of Jordan, Amman, Jordan 2 Yarmouk University, Irbid, Jordan

4 Copyright 2016 by Sami H. Mahmood and Ibrahim Abu-Aljarayesh Published by Materials Research Forum LLC Millersville, PA 17551, USA All rights reserved. No part of the contents of this book may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Published as part of the book series Materials Research Foundations Volume 4 (2016) ISSN (Print) ISSN (Online) Print ISBN ??? epdf ISBN ??? This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Distributed worldwide by Materials Research Forum LLC 105 Springdale Lane Millersville, PA USA Manufactured in the United State of America

5 Table of Contents Foreword Chapter 1 Basics of Magnetism... 1 Chapter 2 High Performance Permanent Magnets Chapter 3 Properties and Synthesis of Hexaferrites Chapter 4 Ferrites with High Magnetic Parameters Chapter 5 Permanent Magnet Applications Chapter 6 Magnetic Recording Keywords About the author

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7 Foreword The utilization of magnetic materials, especially those with ferromagnetic or ferrimagnetic properties, in a wide range of industrial and technological applications, have contributed largely to convenience and prosperity in our modern life. The range of applications of these materials have extended from their limited use in compasses in the ancient era, and in lifting iron scrap pieces in the eighteenth century, to almost uncountable number of devices and machines used in the automated industries, power conversion, traction, electronics and mechatronics, home and office appliances, telecommunications, space technology, and medical applications, to mention a few. Such applications demanded great efforts dedicated to the development of new magnetic materials with tunable properties to fit the wide variety of applications. The inherent relation between developing new magnetic materials with high performance, and the developments in the technological and industrial sectors, can be clearly demonstrated by the ever increasing demands of these sectors for higher and higher efficiencies on the one hand, and the role of newly developed materials in promoting new emerging technologies, on the other hand. Relatively high metallicity of magnetic materials for practical applications impose limitations on their efficient use due to their unfavorable characteristics, such as high dielectric and magnetic losses, corrosion, and possibly poor mechanical properties. Accordingly, magnetic oxides with ferromagnetic properties emerged as the most widely used magnetic materials for practical applications, owing to their characteristic high resistivity and low eddy current losses, chemical stability, simplicity of production in mass quantities, and other favorable characteristics. An important class of these oxides is the class of hexagonal ferrites developed in the early 1950 s, which dominated the world market of permanent magnet applications by the end of the 1980 s. Among these ferrites, the magnetoplumbite (M-type) hexaferrite, is produced nowadays in large quantities at very competitive low prices, thus providing the permanent magnet market with probably the most cost-effective magnetic material. In addition, the possibility of tuning the magnetic properties of these materials over a wide range by suitable metallic substitutions for iron have contributed significantly to the numerouse recent advances in technologies such as magnetic recording and microwave applications. This concise book is intended to provide an overview of the basic concepts of magnetism and magnetic properties pertinent to permanent magnetic materials, importance of these materials in terms of their market share and versatility of practical use, synthesis techniques, and routes adopted for the modification and tuning of their magnetic properties. In the course of presentation, emphasis is made on hexaferrite materials for

8 permanent magnet applications, with M-type ferrites as the focal point. The discussion, however, is brief, in an attempt to provide a wide spectrum of knowledge in a book with limited volume for quick reference to specialized scientists and engineers. While the scope and depth of the discussion is not comprehensive in terms of subject matter or rigor of the theoretical background, the authors-selected material for this book in an attend to give a broad prospective of the subject matters addressed, referring to the most relevant scientific findings, according to authors point of view, in a small portion of a voluminous literature in the respective fields of study. Chapter 1 is dedicated to a brief discussion of the basic principles of magnetism pertaining to the subject matter of the book, including the origin of magnetism and magnetic interactions, magnetic anisotropies, and the performance of a permanent magnet in a magnetic circuit. At the end of the chapter, magnetic units are discussed in detail, relying on derivations of the various quantities from the basic electromagnetic equations. Chapter 2 is concerned with the historical development of permanent magnets, with emphasis on the main magnets in practical use nowadays, namely, alnico, ferrite, and rare-earth permanent magnets. Statistics pertaining to the market share of these materials, and level of interest of both scientists and technologists in the development and improvement of their properties, occupied a good portion of this chapter. Also, some of the basic concepts related to magnet performance are addressed. Chapter 3 is devoted to the structural and magnetic properties of some hexaferrites, with emphasis placed on M-type hexaferrites. The major routes of synthesis adopted for the preparation of the magnetic ferrite powders are described in this chapter. Chapter 4 is mainly concerned with the methods of modifying and tuning the magnetic properties of M-type hexaferrites. Here, effects of the synthesis routes and experimental conditions, as well as the role of the various strategies of cationic substitutions are addressed. Emphasis in this chapter was directed toward improving the magnetic properties for hard permanent magnet and microwave applications on the one hand, and for magnetic recording applications on other hand. Chapter 5 and chapter 6 are devoted to the discussion of some most important applications of hard ferrite magnets, where chapter 6 was devoted to magnetic recording. The separation of the applications into two different chapters is due to the different requirements of magnetic recording, and other types of applications requiring hard permanent magnet properties. In chapter 5, applications in the field of power conversion actuators and transducers, with emphasis on motors and speakers which have the largest market share of permanent magnet applications, were briefly discussed. Also, some of

9 the important passive microwave devices where hexaferrites play an important role in improving device operation were discussed. The discussion of all devices, however, was neither exhaustive in terms of addressing all aspects of a given application, nor complete in terms of addressing all types of devices. Thus these chapters are intended to give the reader a general idea about the importance of ferrite magnets for practical applications. Sami Mahmood The University of Jordan, Amman July 2016

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