Nanoscale Magnetic Materials and Applications

Recent progress on the theoretical studies of fast magnetization reversal of Stoner particles is reviewed. The following results are discussed: (1) The Stoner–Wohlfarth (SW) limit becomes exact when the damping constant is infinitely large. Under the limit, magnetization moves along the steepest energy descent path. (2) For a given magnetic anisotropy, there is a critical damping constant, above which the minimal switching field is the same as that of the SW-limit. (3) The field of a ballistic magnetization reversal should be along a certain direction window in the presence of energy dissipation. (4) Since a time-dependent magnetic field can be an energy source, two new reversal strategies are possible. One is to use a field following magnetization motion, and the other is to use a circularly polarized microwave near the ferromagnetic resonance frequency. The critical switching fields of both strategies are substantially lower than that of precessional reversal for realistic materials. (5) The theoretical limits for both field-induced and current-induced magnetization reversal are presented for uniaxial Stoner particles.

Nanoclusters, aggregates of a few tens to millions of atoms or molecules, have been extensively studied over the past decades. Core–shell nanoclusters have received increasing attention because of their tunable physical and chemical properties through controlling chemical composition and relative sizes of core and shell. The magnetic core–shell nanoclusters are of particular interests because these heterogeneous nanostructures offer opportunities for developing devices and cluster-assembled materials with new functions for magnetic recording, bio, and medical applications.

The purpose of this review is to report latest progress in the experimental and theoretical studies of bimetallic magnetic core–shell nanoclusters (e.g., at least one component of the constitution is magnetic). Due to page limit, a concise survey of synthetic techniques and main experimental characterizations for magnetic properties is presented. A more detailed overview is given to previous theoretical work.

The fabrication, structure, and magnetism of a variety of designed nanostructures are reviewed, from self-assembled thin-film structures and magnetic surface alloys to core–shell nanoparticles and clusters embedded in bulk matrices. The integration of clusters and other nanoscale building blocks in complex two- and three-dimensional nanostructures leads to new physics and new applications. Some explicitly discussed examples are interactions of surface-supported or embedded impurities and clusters, the behavior of quantum states in free and embedded clusters, the preasymptotic coupling of transition-metal dots through substrates, inverted hysteresis loops (proteresis) in core–shell nanoparticles, and nanoscale entanglement of anisotropic magnetic nanodots for future quantum information processing.

Clathrates are materials containing closed polyhedral cages stacked to form crystalline frameworks. With Si, Ge, and Sn atoms populating these frameworks, a wide variety of electronic and vibrational properties can be produced in these materials, by substitution upon framework sites or through incorporation of ions in cage-center positions. Commonly formed structures include the type I, type II, and chiral clathrate types, whose properties will be described here. Ba₈Si₄₆ with the type-I structure has been found to exhibit superconductivity with T _c as high as 9 K. The enhanced T _c in this compound has been shown to arise predominantly from very sharp features in the electronic densities of states associated with the extended sp ³-bonded framework. Atomic substitution can tailor these electronic properties; however, the associated disorder has been found to inevitably lower the T _c due to the disrupted continuity of the framework. Efforts to produce analogous Ge-based superconductors have not been successful, due to the appearance of spontaneous vacancies, which also serve to disrupt the frameworks. The formation of these vacancies is driven by the Zintl mechanism, which plays a much more significant role for the structural stability of the Ge clathrates. The sharp density of states features in these extended framework materials may also lead to enhanced magnetic features, due to conduction electron-mediated coupling of substituted magnetic ions. This has led to magnetic ordering in Fe- and Mn-substituted clathrates. The largest number of clathrates exhibiting magnetic behavior has been produced by substitution of Eu on cage-center sites, with a ferromagnetic T _c as high as 38 K observed in such materials.

Neutron scattering is a comprehensive tool for condensed matter research. After a brief description of the interaction of neutrons with matter, the usefulness of neutrons to probe the physical properties of magnetic materials is illustrated using examples taken from different research areas. Then a description of the crystal structure investigation, including in situ and time-resolved studies is given. The use of polarized or unpolarized neutrons to study magnetic structures or magnetic phase transition is also illustrated. The potential of techniques such as small-angle neutron scattering or neutron scattering on magnetic surfaces is presented showing that neutron scattering now offers a wide range of useful techniques to probe the structural and magnetic properties of magnetic materials whatever their state: polycrystalline, single crystal, amorphous, bulk, or thin films. Examples are taken from a wide range of research fields: hard magnetic materials, nanocomposite soft magnets, multilayers, superlattices, geometrically frustrated magnetic materials, etc. The experimental aspects are not covered in detail but relevant references are given throughout the chapter.

Extrinsic control mechanisms of the interface magnetization in exchange bias heterostructures are reviewed. Experimental progress in the realization of adjustable exchange bias is discussed with special emphasis on electrically tunable exchange bias fields in magnetic thin film heterostructures. Current experimental attempts and concepts of electrically controlled exchange bias exploit magnetic bilayer structures where a ferromagnetic top electrode is in close proximity of magnetoelectric antiferromagnets, multiferroic pinning layers, or piezoelectric thin films. Various experimental approaches are introduced and the potential use of electrically controlled exchange bias in spintronic applications is briefly outlined. In addition, isothermal magnetic field tuning of exchange bias fields and extrinsically tailored exchange bias training effects are reported. The latter have been studied in a variety of systems ranging from conventional antiferromagnetic/ferromagnetic bilayers and core–shell nanoparticles to all ferromagnetic heterostructures where soft and hard ferromagnetic thin films are exchange coupled across a non-magnetic spacer. Such ferromagnetic bilayers show remarkable analogies to conventional exchange bias systems. At the same time they have the experimental advantage to provide direct access to the magnetic state of the pinning layer by simple magnetometry. A large number of exchange-coupled magnetic systems with qualitative differences in materials composition and coupling share a common physical principle that gives rise to training or aging phenomena in a unifying framework. Deviations from the equilibrium spin configuration of the pinning layer generate a force that drives the system back toward equilibrium. The initial nonequilibrium states can be tuned by temperature and applied set fields providing control over various characteristics of the training effect ranging from enhancement to complete quenching.

Recently, much attention has been focused on new concepts of highly integrated spintronics devices based on magnetic domain walls driven by a spin-polarized current. However, several fundamental questions must be answered before the technology can be considered as feasible. This review covers the current understanding of DW propagation along sub-micronic size wires with ultra-thin films having perpendicular magnetic anisotropy. These films exhibit very narrow domain walls that interact strongly with pinning defects, making them model systems to study the dynamics of a 1D interface in a 2D weakly disorder medium. Three important issues are addressed: the peculiarities of domain wall motion driven by magnetic fields in nanoscale devices, the manipulation of the pinning potential for the control of efficient field induced domain wall motion, and the physics of current-driven domain wall motion.

Spintronics describes the concept of attempting to use both the charge and the spin on the electron in microelectronic devices [1, 2]. One of the most highly sought after functionalities in microelectronics is non-volatility, i.e. the ability to retain memory even when power is removed. This is particularly true as the popularity of mobile electronic communication and computing devices grows. In principle, ferromagnetic materials could provide this functionality, due to the hysteresis, and hence memory, that accompanies most ferromagnets. Unfortunately, no suitable room temperature ferromagnetic semiconductor material has yet been identified [3]; the most common ferromagnetic materials are metals. The aim of this research has been to see how far we can push the properties of basic ferromagnetic metallic alloys, which are usually considered to have relatively simple magnetic and electrical properties, towards highly functional devices which mimic and complement the digital logic functions and non-volatile data storage functions of semiconductor microelectronics. Using the concept of the domain wall conduit, we show how information can be represented, moved, processed and stored in networks of ferromagnetic nanowires.

Bit-patterned recording shows potential as a route to thermally stable data recording at densities greater than 1 Tbit/in², provided that a number of challenging requirements can be met. Micromagnetic modeling of the write process shows that high write-field gradient (>350 Oe/nm) and tight tolerances on island fabrication and write synchronization (both in the range of ∼1 nm sigma) are required for addressability (the ability to write a given island without detrimentally affecting neighboring islands). Magnetically uniform islands are also required, with tight island switching-field distribution (5−10% of H _k ). We show that magnetic multilayer films with perpendicular anisotropy (e.g., Co/Pd multilayers and laminated films of Co/Pd with other materials) are promising candidates for magnetic layer deposition onto pre-patterned substrates. A suitable strategy for patterned media fabrication begins with master pattern generation using electron beam lithography to create chemical contrast guiding patterns for self-assembly; this approach produces higher quality and higher density patterns than e-beam alone. Patterns are replicated over large volumes of disks by UV-cure nanoimprint lithography, followed by etching of the substrate or magnetic layer. Integration of bit-patterned media into a functional recording system requires write synchronization, in which the timing of current switching in the write head is synchronized with the passage of individual islands under the write head. Write synchronization may be implemented using a sector synchronization system, in which the write clock is frequency- and phase-locked to timing bursts read from the disk during periodic interruptions in the writing process.

The magnetic microstructure and magnetization processes of nanostructured materials are reviewed in a phenomenological way, mainly based on domain observation by magneto-optical Kerr microscopy. This covers nanocrystalline soft magnetic ribbons and films as well as nanostructured hard magnetic materials. For comparison also the domain structure in coarse-grained material and amorphous ribbons are briefly touched to provide the frame for the nanostructured materials.

In nanocrystalline ribbons or films, the random magnetocrystalline anisotropy of the ultrafine grain structure is largely averaged out by exchange coupling. The soft magnetic properties are rather controlled by uniaxial, induced anisotropies that are uniform on a scale much larger than the exchange length. The interplay between these uniform and the random anisotropy results in a different degree of microscopic magnetization disorder which is reflected in the magnetization processes.

In high-anisotropy materials with exchange-coupled grains in the 10 nm regime (exchange-enhanced nanocrystalline permanent magnets), a highly irregular domain structure is found, consisting of immobile and high-coercive patch domains. If exchange coupling between the grains is interrupted, the so-called interaction domains are observed due to the predominance of magnetostatic interactions between the (single domain) grains.

Exchange-coupled nanocomposite magnets are a new type of permanent magnetic materials. Large amounts of theoretical and experimental research have been carried out in the past two decades in understanding the inter-phase exchange interactions and in processing bulk nanocomposite magnets with enhanced energy products. This chapter reviews recent advancements in both the fundamental research and the materials processing technologies. Details in the new findings about the effects of soft phase properties and interface conditions on the hard/soft phase exchange interactions are presented. Various methods for characterizing the inter-phase exchange coupling are reviewed. In materials processing aspects, the development of the bottom-up approaches is discussed. Novel methodology for nanoparticle synthesis including the salt-matrix annealing and surfactant-assisted ball milling is described. Unconventional compaction techniques including warm compaction and dynamic compaction are recommended because they can be used to retain the desired nanoscale morphology for effective exchange coupling in the nanocomposites. At the end of this chapter, perspectives on fabrication of anisotropic nanocomposite magnets are given.

This chapter reviews the development of SmCo-type magnets over the last 40 years. First, the physical metallurgy and crystal structures are considered; then the focus is on the recent developments in high-temperature Sm(Co_balFe _w Cu _x Zr _y ) _z magnets suitable for operation temperatures up to 500°C. It is elucidated that the evolution of coercivity and microchemistry in the respective phases of the heterogeneous nanostructure as well as magnetic domain structure is very sensitive to details of the processing procedure, especially to the slow cooling ramp as the last step where the hard magnetic properties evolve. These changes give rise to rather complex pinning mechanisms in a three-phase precipitation structure, which again depend in a subtle manner on the microchemistry of the 1:5-type cell boundary phase in the 2:17-type magnets. It is the amount and distribution of Cu in and at the cell boundary phase which is the prevalent factor determining the pinning strength and which can yield a non-monotonic temperature dependence of coercivity. The chapter concludes with an overview of novel non-equilibrium processing routes used to obtain SmCo-type nanocomposites.

Reduction of the grain size to less than 20 nm has provided major advances in soft magnetic materials performance, including reduced core losses and coercivities. These promising results have stimulated research efforts, worldwide, in the areas of nanocrystalline alloy design, alloy processing, materials performance evaluation, and transition to various applications. This chapter presents recent advances in nanocrystalline soft magnetic alloy processing methods, phase transformations, microstructure evaluation, magnetic property measurement and analysis, and applications.

Giant magnetically induced strain up to 50 times larger compared to the strain of giant magnetostriction was observed in some Heusler alloys, particularly in Ni–Mn–Ga. In analogy with the shape memory phenomenon this effect was called magnetic shape memory effect. The effect includes two different phenomena: a magnetically induced structural phase transformation (usually a martensitic transformation) and a magnetically induced structural reorientation occurring in the martensitic phase. Transformation behavior, structure of the martensite, and phenomenology of the magnetically induced reorientation are described. The description is based mainly on the well-studied compound Ni–Mn–Ga.

A brief review for magnetocaloric effect (MCE), including its potential application to magnetic refrigeration and the corresponding magnetic materials, has been given. Focuses are recent progresses in the exploration of magnetocaloric materials which exhibit a first-order phase transition, thus a giant MCE. Special issues such as proper approaches to determine the MCE associated with the first-order transition and the effects of lattice and electronic entropies are discussed. The applicability of the giant MCE materials to the magnetic refrigeration near ambient temperature is evaluated.

This chapter contains both the description of advanced spintronic devices for logic and memory applications and the synthesis and characterization of some new magnetic materials that would lead to new paradigms in spintronics. The first part gives a brief introduction to spintronics and its history. First-generation spintronics has entered the mainstream of information technology through its utilization of the magnetic tunnel junction in applicable devices such as read head sensors for hard disk drives and magnetic random access memory. We also discuss the conceptual spintronic devices, including spin torque transfer random access memory, spin-polarized field-effect transistor, and spin-based qubit quantum processor, and their potential impacts on information technology. The future of spintronic devices requires next-generation spintronic materials. The second part of the chapter is dedicated to the synthesis and characterization of some novel magnetic materials, including ferromagnetic oxides and diluted magnetic Group IV semiconductors.

\({\rm CrO}_2\) is a remarkable ferromagnetic material that is simultaneously an excellent metal for majority spin electrons and an insulator for minority spin electrons [1–3]. For this reason, \({\rm CrO}_2\) is called a half-metal, and in fact, it is the only one experimentally demonstrated [4–6]. Because of this, \({\rm CrO}_2\) has received considerable interest for spintronic applications in recent years. Band structure calculations have shown that the conduction bands in the spin minority channel of this system are completely shifted away from the Fermi level, resulting in 100% spin polarization. This makes it an attractive choice as a ferromagnetic material for spin-dependent devices such as spin injectors and spin detectors. In this chapter, we briefly describe the bonding characteristics in \({\rm CrO}_2\), based on first principles band structure calculations, as well as discuss some of its intrinsic structural, electrical, and magnetic properties. The strain-induced magnetic anisotropy resulting from lattice mismatch with the substrates is also discussed. Finally, we provide some details regarding the fabrication of epitaxial rutile-based heterostructures and their transport properties in micron-sized tunnel junction and GMR devices.

This chapter reviews recent studies of chemically synthesized FePt and related nanoparticles. Various methods for synthesizing the nanoparticles and controlling their shape are described. Thermal effects in nanoparticles near the superparamagnetic limit are discussed. Some of the methods for reducing sintered grain growth during annealing to obtain the L1₀ phase are described, including the use of a hard shell, annealing in a salt matrix, and flash annealing. The effect of metal additives on the ordering temperature and on sintered grain growth is discussed. Additive Ag and Au significantly not only reduce the ordering temperature but also the grain growth temperature in close-packed 3-D arrays. Preliminary experiments that show additive Ag also reduces the ordering temperature when sintering is prevented. Easy-axis alignment of L1₀ FePt nanoparticles can be achieved by drying a nanoparticle dispersion in a magnetic field, and the effect of thermal fluctuations on orientation is discussed. Large particle-to-particle compositional distributions in chemically synthesized FePt nanoparticles have been measured. A method of determining the anisotropy distribution is described. Theoretical and experimental works showing the size effect on chemical ordering of FePt nanoparticles are discussed.

We review some recent advances in the field of magnetic manipulation techniques, with particular emphasis on the manipulation of mixed suspensions of magnetic and nonmagnetic colloidal particles. We will first discuss the theoretical framework for describing magnetic forces exerted on particles within fluid suspensions. We will then make a distinction between particle systems that are highly dependent upon Brownian influence and those that are deterministic. In both cases, we will discuss the type of structures which are observed in colloidal suspensions as a function of the size and type of particles in the fluid. We will discuss the theoretical issues that apply to modeling the behavior of these systems, and we will show that the recently developed theoretical models correlate strongly with the presented experimental work. This chapter will conclude with an overview of the potential applications of these magnetic manipulation techniques.

In recent years, magnetic nanoparticles have played an increasing role in biomedical applications and have been the subject of extensive research investigations. Physical properties, including nanoparticle size, composition, and surface chemistry, vary widely and influence their biological and pharmacological properties and, ultimately, their clinical applications. Among different magnetic nanoparticles, superparamagnetic iron oxide nanoparticles (SPIOs) were found nontoxic and used as magnetic resonance imaging (MRI) contrast agents, in molecular and cellular imaging applications. SPIOs are used in detection of liver metastases, metastatic lymph nodes, and inflammatory and/or neural degenerative diseases. In addition, drug delivery via magnetic targeting, hyperthermia, and labeling/ tracking of stem cells have also been explored as potential therapeutic options.

Data-processing and optical communication systems that allow controlling intensity and polarization state of light via external magnetic fields require compact, efficient, and low-cost magnetic materials. It is these properties of magnetic materials that are the subject of the present chapter. Owing to their strong linear and nonlinear magneto-optical responses along with unique optical characteristics, magnetophotonic crystals have already found applications in electronics. As a particular example, film-type optical isolator/circulator devices have been proposed. Recent renewed interest in magneto-optical spatial light modulators has resulted from the development of optical volumetric recording using holography, particularly, collinear holography. Here we focus on reviewing experimental and theoretical studies of light coupling to various artificial magnetic nanostructured media and nanocomposites providing strong magneto-optical responses and having miniature dimensions. We first examine properties of different types of MPCs. Then, the magnetorefractive effect of various materials is considered, an enhancement of the magnetorefractive response is demonstrated for structures fabricated in the use of the magnetophotonic crystals’ concept. Finally, the influence of localized surface plasmon resonances on optical and magneto-optical properties of bismuth-substituted yttrium iron garnet films impregnated with nanoparticles of noble metals is discussed. Another promising way to enhance Faraday rotation is to exploit the regime of extraordinary transmission for systems comprising a perforated noble-metal film supporting transmission resonances and a magnetic material.

Micro-magnets of thickness in the range 1−500 μm have many potential applications in micro-electro-mechanical-systems (MEMS) because of favorable downscaling laws and their unique ability to produce long range bi-directional forces. The advantages and disadvantages of a number of “top-down” routes, which use bulk processed precursors (magnets or magnetic powders), to produce μ-magnets of thickness in the range 10−500 μm will be discussed. Progress in the fabrication and patterning of thick film magnets (1−100 μm) using “bottom-up” deposition techniques will be reviewed. In particular, recent results concerning high-rate triode sputtering and micro-patterning of high-performance NdFeB and SmCo films will be presented.

During the last decade, intensive research efforts have been expended to develop solid-state magnetic sensors for applications such as biomolecular sensing and single molecule detection. This chapter reviews sensors proposed thus far, including (a) GMR and spin valve sensors based on the giant magnetoresistance (GMR) effect; (b) magnetic tunnel junction (MTJ) sensors based on tunneling magnetoresistance (TMR); (c) anisotropic magnetoresistance (AMR) ring sensors and planar Hall effect sensors based on the AMR effect; (d) Hall sensors based on the classical Hall effect; and (e) giant magnetoimpedance (GMI) sensors based on the frequency-dependent variation of the skin depth in magnetic wires with field. Two different types of sensors are highlighted: the ones with large sensing areas (hundreds of μm²) intended to provide statistical counting of a large number of magnetic micro- or nanoparticles and the others with micro- or submicrometer-sized sensing areas that focus on single particle detection.