Preface
This book seeks to provide an up-to-date version, in a reasonable number of chapters, of a summary of the current state on the magnetism of wires from the nano- to the micrometric scale. In fact, this is the second edition of the book on Magnetic Nano- and Microwires after the first one, which was published in 2015 and was successfully accepted by magneticians working on that topic. In this edition, special effort has been made to collect information from different points of view and to discuss each topic as developed in the very recent years.
As shown in the Table of Contents, there is a broad scope of topics where magnetic wires are relevant. Magnetic nanowires with a rectangular cross section, sometimes called nanostrips, have been considered in detail in many books and series of review articles. A number of magnetic phenomena have been discovered and investigated in these nanostrips and in ultrathin multilayer films, including the dynamics of domain walls with relevance to spintronics. In this edition, we have kept several representative chapters devoted to nanostrips, but it is not our objective to consider them in full depth.
On the other hand, increasing interest is emerging on the effects of curvature on the magnetism at the nano- and micrometer scales. Curvature, a defining feature of magnetic wires, induces novel fundamental aspects including magnetochiral behavior or topological spin textures, while it also launches novel functionalities. This second edition pays particular attention to magnetic wires having cylindrical symmetry whose intrinsic curved nature facilitates the appearance of those and related effects. Cylindrical nanowires with diameter in the range of tens to a few hundred nanometers are specially considered. While these are electrochemically synthesized in most cases, alternative routes to synthesize nanotubes or heterostructured wires are also considered.
The magnetic domain structure and the magnetization reversal mechanism of these materials are dominated by their cylindrical shape; current important challenges include determination of the effects of the magnetic singularity existing at the nanowire axis, imaging the internal magnetization distribution, and understanding singular effects at the microwave frequency range. In addition, magnetic nanowires offer a wide spectrum of quite interesting properties to enable many technological applications ranging from arrangements for 3-D architectures to sensors and actuators, thermopower, or robotics.
Finally a number of chapters are devoted to magnetic microwires fabricated by the quenching and drawing technique, possessing a ferromagnetic core with a diameter typically in the range from less than one to tens of micrometers. Microwires with large magnetostriction reverse their magnetization in a unique way by the motion of a single domain wall. In this area, new alloy compositions have been recently achieved that offer opportunities for advanced sensing applications or for microwave-sensitive functionalities.
This book contains 31 chapters and is organized into three main parts devoted to the following: (I) Design, Synthesis, and Properties; (II) Magnetization Mechanisms, Domains, and Domain Walls; and (III) Technological Applications. In Part I, chapters are mainly devoted to various technologies to fabricate families of nanowires and nanotubes. Additionally, Part I also describes very important properties concerning interconnected nanowires, modulations in nanowire diameter, optoelectronics and spintronics, permanent magnets, and Heusler alloys.
In Part II the domain wall dynamics and interface engineering in nanostrips are first considered. Following this, investigations on emergent curvature effects and micromagnetic modeling are introduced. Chapters on relevant experimental aspects are later considered, including imaging of the magnetization configurations in cylindrical nanowires and nanotubes, ferromagnetic resonance, and spin waves. Toward the end of Part II, quite recent characteristics derived from magneto-optical properties and from the dynamics of the single domain wall propagation in magnetic microwires are discussed.
Finally, Part III presents a number of technologies where the used of magnetic nanowires is proposed and, in many cases, is already incorporated. Phenomena include magnetostrictive effects for sensing devices; thermopower and thermoelectric effects for spin caloritronics; magnetic nanoswimmers for robotics; and transducers for biomedical applications. Further, magnetic microwires are being considered as sensing elements in advanced orthogonal fluxgates and in a broad spectrum of sensor applications, as well as in microwave absorption and tunable microwave functionalities.
To properly cover the very wide spectrumof recent developments concerning magnetic nano- and microwires, the most recognized experts in their fields have been invited to write the chapters of this second edition. I very sincerely appreciate their outstanding contributions that have enabled this compilation.
The book is dedicated to all of you, physicists, chemists, engineers, and biomedical professionals, with an interest in the exciting field of magnetism of nano- and microwires. I hope you will enjoy reading it at least as much as I have.
3D porous alumina: A controllable platform to tailor magnetic nanowires
Olga Caballero-Calero, Marisol Martı´n-Gonza´ lez
Micro and Nanotechnology Institute, INM-CNM, CSIC (CEI-UAM + CSIC), Tres Cantos, Spain
1.1 Introduction
Understanding the magnetic properties of nanostructures is a field under continuous evolution. Not only the low-dimensional characteristics of the materials confer unique properties to these structures, but also the high degree of freedom when fabricating them increases the possibilities of tailoring their magnetic properties. The recent development of novel technologies that provide ways of both fabrication and characterization at the nanoscale have opened the door to the obtainment of nanostructures with specifically designed magnetic properties. In this chapter, we reviewed some of these tailored nanostructures that are sometimes referred as three-dimensional (3D) nanostructures, such as diameter-modulated nanowires and composition-modulated nanowires. Then, we compared these structures to actual 3D interconnected nanostructures, those that take advantage of their design in three spatial dimensions.
Finally, we have discussed in depth a fascinating 3D self-sustained magnetic nanostructure which has been developed within our research group, based on template growth inside 3D tailored alumina, which is obtained using only industrially scalable and highly tunable fabrication methods.
The importance of fabricating nano-sized structured magnetic materials, discussed in different sections of this book, has attracted wide attention because they not only present different properties than bulk materials due to their low dimensions, but also have a great potential for miniaturizing technological applications. Moreover, the fabrication of low-dimensional structures presents extra degrees of freedom, which affect their characteristics, such as the nanometer diameter in the case of nanowires, the aspect ratio of the structures, the different behavior of an isolated nano-sized entity compared to their collective behavior (a single nanowire and an array of nanowires, for instance), multilayer arrangements combining different materials, etc. The control over nano-magnetism has a wide range of applications [1] and therefore has received a great deal of attention in recent years. Small-sized magnetoresistive devices or permanent magnets, miniaturized sensors, advanced information storage means, such as perpendicular recording media, are quite relevant nowadays. Controlling their magnetic properties such as anisotropy, coercive field, remanent, and saturation magnetization is fundamental to understanding the basis of magnetism and also to proceed with their applications. For instance, the control of the movements of the magnetic domain walls has been proposed as a way to store information. Moreover, magnetic nanowires fabricated via template-assisted techniques can be directly implemented into conventional devices, while embedded in the matrices, making their usage much more straightforward than using isolated nanowires. Nevertheless, the ultimate aim of using arrays of magnetic nanowires in most cases is to achieve ultrahigh density magnetic recording media by using each of the nanowires as a storage entity or by controlling individually the wall motion of the magnetic domains. Therefore, state-of-the-art nano-fabrication tools and nano-electronics are needed in order to implement these next-generation storage devices and much research is being devoted to the development of such technology. It is important to say that these cutting-edge technological developments will not be the subject of this chapter. Here we will focus on different advances made in the fabrication of 3D magnetic nanowires themselves. Most of these developments have been envisaged as a way of controlling and understanding the magnetic domain wall dynamics, which are affected by the shape anisotropy, dipolar interaction, wire diameter, inhomogeneities, etc. To date, there are still many open questions and these implementations are in most cases looking for the perfect candidates to gain control over the creation and movement of the walls.