Novel Functional Magnetic Materials

Applications of the shape memory alloys (SMAs) are based mainly on their unusual deformational properties, caused by the structural phase transformation of martensitic type. That is why a special attention is paid in the literature to the achievement of a giant deformation of the ferromagnetic SMAs under a moderate magnetic field or under mechanical load in both the ferromagnetic and nonmagnetic SMAs. This task has both the technological and physical aspects. In the present chapter, the physical aspects of the giant deformation of SMAs are analyzed focusing on the following issues: (1) the reduction of a magnetic field needed for the creation of a giant magnetically induced deformation, (2) lowering of hysteresis of the deformation processes in the ferromagnetic and nonmagnetic SMAs, and (3) an improvement of the fatigue properties of SMAs subjected to a cyclic deformation. The relevant physical problems such as an influence of the crystal defects and volume magnetostriction on the martensitic phase transformation and elastic properties of the ferromagnetic SMA are considered.

In this review, we will survey recent experimental results on magnetic, magnetocaloric, magnetotransport, and magneto-optical properties of Ni–Mn–In-based Heusler alloys in bulk polycrystalline samples, melt-spun ribbons, and glass-coated microwires. These ternary Ni–Mn–In and doped, quaternary alloys comprise a novel class of multifunctional magnetic materials with exceptional properties related to the magnetostructural martensitic transformation. We will focus on recent developments that have led to a better understanding of properties that are promising for applications, possible routes for improvements, and the identification of unsolved problems.

We outline the microstructure, crystal structure, first-order martensitic transformation, and magnetic properties observed in selected Heusler Ni–Mn–Z (Z = In, Sn) alloys produced in ribbon shape by melt spinning. Along with a detailed description of Heusler alloy ribbon production and structural, calorimetric, and magnetic characterization, we highlight various characteristic features associated with the disorder influence on the magnetostructural martensitic transformation related to phase coexistence, metastability, supercooling, and superheating as a consequence of its first-order nature. Magnetic field and annealing effect on the martensitic phase transformation are also analyzed. The understanding of that transition process helps us to explain the exchange bias effect observed in the martensite phase of Ni–Mn–In and Ni–Mn–Sn systems.

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a solid state cooling technology that offers significant energy saving potential of 20 – 30% as compared to the conventional gas compression technology. In efforts to design high-performance magnetic refrigerators it has been recognised that such refrigerators will require magnetic materials that must fulfil a number of requirements. This chapter provides an overview of the technologically relevant parameters of magnetocaloric materials, methods used to assess their suitability for cooling purposes and gives an outline on how the understanding of the fundamental phenomena helps to design the materials and improve their performance.

In this chapter, we will survey early and recent experimental results on magnetic properties of dilute magnetic oxide semiconductors, focusing on TiO_2-δ:Co and TiO_2-δ:V. Room temperature ferromagnetism was observed in both types of thin film samples fabricated by RF sputtering, but their magnetic properties appeared to be quite different. Magnetic moments in case of TiO_2-δ:Co are mostly associated with local polarization of Co ions and induced defects. There is an evidence of intrinsic ferromagnetism in the case of low Co content (<1 at.%). Room temperature ferromagnetism was observed in TiO_2-δ:V at V content from 3 up to 18 at.% in the whole resistivity range from 10⁻³ up to 10⁶ Ω cm. Positron annihilation spectroscopy revealed a correlation between magnetization and concentration of the negatively charged defects in TiO_2-δ:V thin films. The origin of room temperature ferromagnetism in these systems is discussed. Besides, the recent research findings in ZnO-based magnetic semiconductors are briefly discussed with focus on defect-induced ferromagnetism.

First amorphous materials using rapid quenching from the liquid state were prepared nearly 50 years ago [1–4]. Development of the rapid-quenching technique allowed obtaining of new materials with metastable crystalline, amorphous, nanocrystalline, granular structures with a new combination of physical properties (mechanical, magnetic, electrochemical, etc.) and opening of new fields of research in material science, magnetism, and technology. During the next years, few rapid-quenching technologies allowing preparation of different types of rapidly quenched materials have been developed. At the beginning most attention has been paid to studies of planar rapidly quenched materials: rapidly quenched ribbons produced by quenching on the drum [4–6].

Bimagnetic microwires are cylindrically multilayered systems consisting of two magnetic metallic microlayers, a cylindrical nucleus, and an external shell, separated by an insulating layer. Such microwires are synthesized by combined quenching and drawing, sputtering, and electrodeposition, and the magnetic configuration of each phase can be suitably tailored to result in soft/soft, soft/hard, or hard/soft biphase microwires. Several families of alloy composition for each phase are considered in this overview: magnetostrictive Fe-based and non-magnetostrictive CoFe-based amorphous alloys for the nucleus and soft FeNi and harder CoNi alloys with polycrystalline character for the shell. The phenomenology of the magnetic behavior of the different microwires under low-frequency applied field is firstly described. Particularly the influence of the thickness of layers and that of thermal annealing are presented. A specific study is performed as a function of the measuring temperature in the range of below (15–300 K) and above (300–1000 K) room temperature. Magnetic and structural phase transitions are determined.

Special attention is paid to the ferromagnetic resonance and microwave absorption experimentally investigated with the help of network analyzer, NA-FMR, as a function of applied field in the frequency range up to 14 GHz and with perturbation cavity at X-band (9.5 GHz) and K-band (69 GHz) under different applied fields. The network analyzer-FMR allows us to conclude the presence of multipeak resonance spectra for soft/soft biphase systems while no absorption is detected for the hard phase. In addition, the observed non-Kittel absorption in bimetallic microwires allows us to confirm the equivalency to a capacitor.

The analysis in perturbation cavity is carried out as a function of the measuring dependence (X-band) where the role of the different contributing phases is determined. A correlation between FMR absorption data with that obtained through NA-FMR is performed. Further analysis at room temperature allows us to conclude the need of magnetic saturation of the hard phase to observe properly its FMR absorption. In addition, the screening effect induced by the external metallic shell on the nucleus is observed.

Metamaterials, traditionally in the form of artificial structures with surprising electromagnetic properties, have triggered unprecedented opportunities to achieve those fascinating applications that previously only exist in science-fiction works, for example, Harry Potter’s cloak. Nevertheless, their massive manufacturing costs incurred by their complicated structures restrict the scale-up and mass production. The ultimate properties are primarily (if not solely) determined by the intrinsic structures of metamaterials that make them merely ‘meta-structures’. In response to these issues, it is desirable to have a genuine engineering composite yet with metamaterial characteristics. Thus, ‘metacomposite’ has been proposed to account for a real piece of composite material. This has subsequently become a nascent area where metamaterial properties are attained under wider operating frequencies with certain tunability towards external magnetic fields or mechanical stresses. In this chapter, we start with an overview of metacomposites containing various dielectric and/or magnetic fillers following the fillers’ dimensions from 0D, 1D to 2D. We then critically discussed some progresses in metacomposites containing ferromagnetic microwires together with unparalleled advantages in microwave sensing and cloaking areas. Finally, the chapter is closed with an outlook of strategies for improving existing metacomposites and some future perspectives.

Recent developments in permanent magnetism are summarized, considering both intrinsic and extrinsic properties. After a general introduction to permanent magnetism, several classes of materials are discussed in the light of future improvements. Emphasis is on magnets rich in Fe, Co, and Mn. The search for new magnetic compounds with improved magnetization, Curie temperature, and anisotropy is accompanied by the need to realize a microstructure that ensures high coercivity. This need refers to both bulk magnets, where hcp Co and tetragonal FeNi are briefly discussed as negative and positive examples, respectively, and to aligned hard–soft nanocomposites. A very recent concept is imaginary magnetic hardness, which reflects easy-plane magnetism and may be exploited in some ferromagnetic compounds. In aligned two-phase nanostructures, soft-in-hard geometries are better than hard-in-soft geometries, and different shapes behave different in the first and second quadrants of the hysteresis loops. Both intrinsically and extrinsically, the most important task is to maximize the hard phase anisotropy while maintaining a high magnetization. Anisotropy field and magnetic hardness can be maximized by choosing a small magnetization, but this strategy is detrimental to the energy product. The last section deals with the behavior of permanent magnets above room temperature, with emphasis on nanoscale effects. Throughout the chapter, current research trends are critically evaluated, and several common misconceptions are dispelled.

Natural glasses are formed in various materials, for example, oxides, and polymers, while commercial metallic alloys have a crystalline structure either after slow or rapid cooling on casting. Metallic glassy alloys from the melt were first produced in Au–Si system [1] by using a rapid solidification technique at a very high cooling rate of 10⁶ K/s. Pd–Cu–Si and Pd–Ni–P system alloys were first macroscopic metallic glassy articles produced in the shape of 1–2 mm diameter rods [2]. Larger-size Pd–Ni–P samples were obtained later after flux treatment which helps to suppress heterogeneous nucleation of crystals [3].