The chapter discusses the structural and magnetic properties of equiatomic ternary RTX compounds, where R represents rare earth, T is transition metal, and X belongs to the p block elements. RTX compounds exhibit a variety of crystal structures, which leads to a range of magnetic phenomenon ranging from long range antiferromagnetic-ferromagnetic ordering, unconventional superconductivity, magnetic frustration to spin ices. Coexistence of various magnetic phenomenon result in many exotic properties, which make these materials promising for next generation technological applications. In RTX family of compounds, most transition metals, except manganese, Mn do not contribute significantly to the magnetic moment and behave as non-magnetic. The lack of magnetic behaviour in the transition metal sublattices of these ternary compounds may be attributed to hybridization between p electron states of X atom and d electron states of the transition metal, resulting in the filling of the d band. The dominant interaction in these compounds is of Ruderman Kittel Kasuya Yosida type due to the localized nature of the 4f electrons in the rare earths. The magnetic transition temperature in these materials varies from ultra-low to high temperatures, making them suitable for integration into devices operating at room temperature. The RTX series discussed here encompasses all rare earth elements, a range of transition metals, and various p-block elements such as Al, Ga, In, Si, Ge, Sn, Sb, and Bi. Most of the materials studied are arc melted polycrystalline materials with some in the form of single crystal or thin films.
Spin-orbit torques (SOT) provide a versatile tool to manipulate the magnetization of diverse classes of materials and devices using electric currents, leading to novel spintronic memory and computing approaches. In parallel to spin transfer torques (STT), which have emerged as a leading non-volatile memory technologie, SOT broaden the scope of current-induced magnetic switching to applications that run close to the clock speed of the central processing unit and unconventional computing architectures. In this paper, we review the fundamental characteristics of SOT and their use to switch magnetic tunnel junction (MTJ) devices, the elementary unit of the magnetoresistive random access memory (MRAM). In the first part, we illustrate the physical mechanisms that drive the SOT and magnetization reversal in nanoscale structures. In the second part, we focus on the SOT-MTJ cell. We discuss the anatomy of the MTJ in terms of materials and stack development, summarize the figures of merit for SOT switching, review the field-free operation of perpendicularly magnetized MTJs, and present options to combine SOT, STT and voltage-gate assisted switching. In the third part, we consider SOT-MRAMs in the perspective of circuit integration processes, introducing considerations on scaling and performance, as well as macro-design architectures. We thus bridge the fundamental description of SOT-driven magnetization dynamics with an application-oriented perspective, including device and system-level considerations, goals, and challenges.
The interest in the magnetic cooling devices has led to an intensive search for suitable well-performing magnetocaloric materials. High-throughput studies based on density functional theory (DFT) calculations can significantly simplify and increase the range of this search. In this chapter, an effective approach to the screening of magnetocaloric materials based on the information obtained from crystallographic databases is demonstrated. To identify systems of interest, several screening parameters were developed using properties of various well-known materials with magnetocaloric effect (MCE) as a reference. Along with magnetic properties, other factors important for practical applications are taken into consideration including price, availability, and toxicity of candidate materials. Combining these criteria, an algorithm for the screening process is suggested. It utilizes both information readily available in the database and additional ab-initio calculations. A step-by-step application of initial screening parameters to sort out unsuitable materials before performing more computationally heavy assessments allows fast processing of a large number of candidates. This results in a shortlist of promising compounds ranked by their potential which can serve as a guide for experimental research.
Magnetic materials for magnetoelectric coupling are reported. After an introduction of magnetoelectric effect and materials, an historical on the main developments in this field are presented. Then, the main concepts related to multiferroic and magnetoelectric materials are introduced, together with the description of the main types of magnetoelectric materials and structures. Finally, the magnetic materials used the development of magnetoelectric composites are presented and discussed, highlighting their main physico-chemical characteristics and processing methods. In this way, a complete account on concepts, materials and methods is presented in this strongly evolving research field, with strong application potential in the areas of sensors and actuators, among others.
An emerging direction in spintronics aims at discovering novel phenomena and functionalities originating from spin–orbit coupling. The new field of spintronics based on the spin–orbit coupling is called spin-orbitronics. The spin-orbitronics technology promises smaller, faster, and far more energy efficient spin-based devices. Of particular importance in this field is the manipulation of magnetization. The spin–orbit coupling in structures with broken inversion symmetry triggers the transfer of orbital angular momentum from the lattice to the spin system, providing a route to generate a new family of spin torques: spin–orbit torques. The current-induced spin–orbit torques provide efficient and versatile ways to control the magnetic state and dynamics. This chapter reviews the spin–orbit torques, focusing on basic concepts and recent progress in this field.
This chapter discusses the magnetic properties of ternary rare-earth transition-metal compounds. The magnetism of pure elements concerns the properties of about 20 metals, mostly from the 3d or 4f series. Binary intermetallic compounds are much more numerous. Magnetic binaries may involve one or both elements with magnetic moments. Composition adds a further dimension, with many binary diagrams exhibiting ranges of solid solubility and a number of intermetallic phases, each with its particular structure. Sometimes, the distinction is a matter of site preference, such as ordered substitution of one quarter of the sites of the fcc structure leads to a Cu3Au-type structure compound, whereas complete disorder produces an A75B25 fcc solid solution. The magnetic properties of binary intermetallic compounds, usually involving a 3d or 4f element, and sometimes both, are reviewed in the chapter. The order of decreasing transition-metal content the magnetic properties of ternaries with structures related to a binary structure type is discussed. The true ternary compounds are also discussed in the chapter.
Basic information about magnetocaloric effect (MCE) in ferromagnetic solids is expounded in brief. An overview of experimental data on MCE in shape memory alloys is presented with the emphasis on the giant MCE observed in the materials exhibiting magnetostructural phase transformations. The Landau-type theory of such phase transformations is used for the quantitative description of giant MCE and heat capacity of shape memory alloys. The difference between this approach to a theoretical description of giant MCE and the orthodox thermodynamic theory of MCE is discussed.
In this chapter, we review recent advances in Nd-Fe-B based permanent magnets that have been made in the last 10 years with a historical review of the early developments of Nd-Fe-B magnets in the 1980s. The basic technology in the commercial production of Nd-Fe-B permanent magnets was established within a decade after its invention in 1983. The overwhelmingly high energy density of Nd-Fe-B magnets at that time led to major applications in small motors and actuators in many electric devices such as hard disk drives and cell phones, where the miniaturization of motors and actuators is of priority over their relatively high cost. However, the current major applications have shifted to the traction mortars of hybrid and pure electric vehicles, for which higher coercivity is required against thermal demagnetization. The merit of permanent magnet generators for large scale wind turbines has also been established, and these new applications will boost the usage of Nd-Fe-B PM in the near future. Recent government policies in Europe, India and China have forced major automotive manufactures to accelerate the development of electric vehicles and the emerging area such as robotic, drones and future electric planes will further boost the demand for high performance permanent magnets. To achieve the required coercivity value for these applications, the partial substitution of Dy for Nd has been employed for the past three decades. However, recently emerging issue on the limited natural resources of heavy rare earth elements created a new technological challenge to develop high coercivity Nd-Fe-B magnets without using heavy rare earth elements for future mass supply of Nd-Fe-B permanent magnets. With this background, the researches on the structure-coercivity relationships of Nd-Fe-B permanent magnet have been revived recently mainly in Japan, Europe and China. With new analytical tools employed in recent studies, many new findings on the microstructural features of Nd-Fe-B permanent magnets have been made in the last five years and the ways to increase the coercivity substantially higher than the conventional values in commercial magnets have been proposed. This chapter will update these recent progresses made in Nd-Fe-B based permanent magnets.
Off-axis electron holography is a powerful technique that can be used to measure the phase shift of a high-energy electron wave in a transmission electron microscope. The phase shift can, in turn, be used to provide quantitative high-resolution information about the magnetic field within and around a specimen. This chapter introduces the theoretical and experimental basis of the technique. Representative results obtained from the measurement of magnetic fields of nanoparticles, nanowires, skyrmions, and domain walls are then presented.
We propose a review of the current knowledge about the synthesis, magnetic properties and applications of magnetic cylindrical nanowires and nanotubes. By "nano" we consider diameters reasonably smaller than a micrometer. At this scale, comparable to micromagnetic and transport length scales, novel properties appear. At the same time, this makes the underlying physics easier to understand due to the limiter number of degrees of freedom involved. The three-dimensional nature and the curvature of these objects contribute also to their specific properties, compared to patterns flat elements. While the topic of nanowires and later nanotubes started now decades ago, it is nevertheless flourishing, thanks to the progress of synthesis, theory and characterization tools. These give access to ever more complex and thus functional structures, and also shifting the focus from material-type measurements of large assemblies, to single-object investigations. We first provide an overview of common fabrication methods yielding nanowires, nanotubes and structures engineered in geometry~(change in diameter, shape) or material (segments, core-shell structures), shape or core-shell. We then review their magnetic properties: global measurements, magnetization states and switching, single domain wall statics and dynamics, and spin waves. For each aspect, both theory and experiments are surveyed. We also mention standard characterization techniques useful for these. We finally mention emerging applications of magnetic nanowires and nanotubes, along with the foreseen perspectives in the topic.
Abstract Optical manipulation of magnetization with femtosecond laser pulses has opened a new paradigm shift in magnetization and spin dynamics. Since the very first demonstration of femtosecond laser-induced demagnetization in 1996 by Beaurepaire et al., optical manipulation of magnetization has developed into a highly active area of research that has been growing at a breathtaking pace over the last decades. Scientific highlights include the discovery of femtosecond timescale laser-induced demagnetization, all-optical generation of coherent spin waves, photo-induced generation of magnetization, and many others. Undoubtedly, the most appealing is the recent demonstration of all-optical magnetization reversal and opto-magnetic recording with femtosecond laser pulses. All-optical magnetization switching (AOS) is emerging as a novel magnetic recording technology, its potential being fully recognized by the magnetic recording industry, which enlisted the AOS process on its roadmap toward ultrafast and ultrahigh densities magnetic recording beyond 1 Tb/in 2 . Here we review experimental work, models developed to explain these experiments, and introduce relevant theoretical concepts needed to construct these models. Validity of different involved methods and approximations is critically examined, employing ab initio calculation results where possible. Proposed explanations of ultrafast magnetization dynamics involve different underlying physical mechanisms, the last section of the review provides a discussion of the possible contributions of these mechanisms. We note that the ultrafast magnetism field has become very vast and the present review cannot exhaustively describe all the works related to laser-induced magnetization dynamics.
Abstract Molecular magnets containing lanthanide ions have been receiving an increasing level of attention during the last years. The applicability of molecular magnets on foreseeable devices and new technologies, such as spintronic components, spin valves, or quantum bits, relies on the achievement of magnetic bistability with long relaxation times, such that the molecule allows the designed operation. The use of lanthanides as ingredients in molecular magnetism facilitates the achievement of high-energy barrier for magnetization reversal, a parameter related to bistability, and long relaxation times, thanks to their unquenched orbital momentum which enhances the magnetic anisotropy of the molecule. The understanding and tailoring of the diverse magnetic relaxation mechanisms through adequate chemical design are at the core of this multidisciplinary research field. Nowadays, it is mandatory to use up-to-date ab initio computational tools, providing a new input for the next round of chemical design, which is nowadays involving concepts such as new topologies and interaction dimensionality. In this chapter, we describe the theoretical basis as well as the experimental and computational tools used in the determination of the magnetic relaxation times and the identification of the relevant relaxation processes in lanthanide-based magnetic molecules. We review the recent progress on this class of compounds, from single ion molecules, to homo- and heteronuclear dimers and clusters (both containing only lanthanides and combined with 3d magnetic ions). Recent developments on systems with higher magnetic dimension in the interactions, such as chains, planes, and 3D compounds, are also reviewed. Finally, we focus on the recent progress on lanthanide-based molecules grafted to different substrates and how the relaxation processes are affected by the molecule–surface interaction.
Rare-earth Mössbauer spectroscopy gives clear information concerning the crystal field effects and magnetic interaction. In this chapter a review will be given about the scientific results over the last 50 years. A reanalysis of Tm 2 O 3 Mössbauer results on old measurements gives new information about the crystal field. Further, the oxides Dy 2 O 3 and on PrO 2 will be discussed. For RMO 3 , its magnetic behavior will be explained based on the crystal field results. In the case of RMO 4 the crystal field determinations on TmMO 4 and YbMO 4 were shown in relation to the existence of a Jahn–Teller effect. Furthermore, the magnetic interplay between rare earth and chromium in RCrO 4 compounds will lead to some interesting features on magnetic and crystallographic behaviors. For the superconducting RBa 2 Cu 3 O 7 compounds, studies on the magnetic and crystal field behavior by a diversity of authors lead to quite interesting results on magnetic and crystal field behaviors. The determination of the intermediary valency of 3.4 with help of the isomer shift interpolation from the 141 Pr Mössbauer measurements on the Pr x O y compounds is an exceptional result. Crystal-field effect determination has been performed on the R 2 BaCuO 5 compounds to explain its magnetic behavior. At last the Gd and Yb Mössbauer results on magnetic dynamic behavior in pyrochlore and garnet compounds will be discussed in combination with μ SR results to explain the dynamic behavior of its magnetic spin fluctuations.
