The Role of Microstructure and Processing on Magnetic Properties of Materials

Abstract

Magnetic materials can be viewed in light of their utility for the betterment of humanity, for underlying physics as a basis for new discovery, or simply for the wonderment of the phenomena that is magnetism. The practical application of our knowledge of magnetism and magnetic materials relies heavily on materials scientists and engineers. Principally, this is due to our understanding of the processing– structure–property relationships that are essential to making progress in development of improved performance. Specifically, the role of processing and its effect on microstructure evolution have significant impact on extrinsic magnetic properties, including coercivity, remanent magnetization, and permeability. This optimization of (magnetic) properties by processing and control of the microstructure is deeply linked to the general interests of TMS, and there have been some specific activities addressing this topic; for instance, the symposium on Processing to Control Morphology and Texture in Magnetic Materials at the 2012 TMS annual meeting was one of the most recent. The following articles aim to provide a broad overview of some current trends in this field of research. The materials science behind advances in heatassisted magnetic recording media is highlighted in the contribution from Varaprasad et al. This article describes continuing design and development of FePt alloys with refined, textured, and atomically ordered grains for the next generation of magnetic storage media. The need of each of the foregoing microstructural features is described with discussion of the important alloy design and processing challenges that allow their preparation. The recent efforts to reduce reliance on critical materials have resulted in some interesting research in the field of magnetic materials. One of these topics is devoted to understanding and improving the rareearth free Alnico alloys. The contribution from Palasyuk et al. describes work on the microstructure and magnetic domain structure of Alnico grades 5–7 and 9. The interplay of microstructure and domain structure is described through the analysis of domain images, revealing pinning mechanisms related to microstructural features. Although soft magnetic nanocrystalline alloys are usually obtained by melt spinning and subsequent annealing, mechanical alloying has been shown as a very versatile ‘‘one-step’’ technique to produce nanocrystalline and amorphous metastable systems, including those compositions with interesting soft magnetic properties. The article by Blazquez et al. gives an overview of how studies of the dynamics of milling can be used to optimize the microstructure and magnetic properties of the obtained materials, stressing how the materials’ properties depend on multiscale parameters (grain size versus crystal size). Magnetic oxide materials are used in a wide range of high switching frequency (over 500 MHz) applications due in part to their large resistivities. Such applications include military and commercial radar, communication electronics, and passive electronics. The article by Harris et al. focuses on the role of magnetic, microstructure, and structural anisotropies in determining the radio-frequency (RF) properties of the magnetic oxides with spinel, magnetoplumbite, and garnet structures. The important role of crystallographic texture used to modify the magnetic anisotropy enabling a tuning of the operating frequency, bandwidth, and loss, among other properties, is discussed. Although these four previous articles rely on frequently used processing and microstructure evolution techniques, in this series of articles we also highlight some other possibilities of magnetic materials fabrication, which are not so familiar. The first of these articles, by Torrejon et al., is devoted to the fabrication of multilayer magnetic microwires, in which a core of magnetic material JOM, Vol. 65, No. 7, 2013