Session Y2: Invited Session: Magnetic Materials and Magnetism Research for Energy Applications

Magnetic Materials in sustainable energy

A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency in the total energy lifecycle, has accelerated research in energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conversion and transportation. Magnetic materials are essential components of energy applications (i.e. motors, generators, transformers, actuators, etc.) and improvements in magnetic materials will have significant impact in this area, on par with many ``hot'' energy materials efforts. The talk focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials with an emphasis on their optimization for energy applications. Specifically, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, will be discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure-property relationships, will be examined in the context of their respective markets as well as their potential impact on energy efficiency. Finally, considering future bottle-necks in raw materials and in the supply chain, options for recycling of rare-earth metals will be analyzed.\footnote{O. Gutfleisch, J.P. Liu, M. Willard, E. Bruck, C. Chen, S.G. Shankar, Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient (review), Adv. Mat. 23 (2011) 821-842.}   

Spintronics Device for Stand-by Power Free Nonvolatile CMOS VLSI

Recent progress in perpendicular magnetic-easy axis magnetic tunnel junctions (MTJs), a spintronics device, offers a high potential building block for constructing not only stand-alone fast and nonvolatile RAMs in the 30 nm feature size and beyond but also nonvolatile CMOS VLSI employing logic-in-memory architecture [1]. The shift from in-plane to perpendicular is prompted by the need for a high crystalline anisotropy that is available in perpendicular materials for reducing the device size. In addition, current-induced switching is inherently more efficient with perpendicular easy axis. However, satisfying both high tunnel magnetoresistance (TMR) ratio over 100{\%} and low switching current was a challenge, because of the mismatch between MgO (100) - CoFe(B) bcc (100) structure needed to obtain high TMR and the crystal structure of perpendicular materials. It was shown that a strong perpendicular interface anisotropy exists at the MgO-CoFeB interface [2, 3], strong enough ($K_{i}$ = 1.3 mJ/m$^{2})$ to overcome demagnetization energy and make the easy axis perpendicular when the ferromagnetic electrode thickness is thin enough. First principle calculation by Nakamura \textit{et al.} showed that the perpendicular anisotropy is due to the oxygen-iron bond that reduces contribution of in-plane crystalline anisotropy [4]. By the use of this perpendicular easy axis, a 40 nm$\phi $ MgO-CoFeB MTJ with high TMR ($>$100 {\%}) and low switching current of 49 $\mu $A was realized [2]. It was also pointed out that activation volume for reversal plays an important role in determining the thermal stability of the MTJs [5]. I will discuss how the MTJs are incorporated in CMOS VLSIs to make them nonvolatile and stand-by power free. \\[4pt] [1] S. Ikeda, \textit{et al.} IEEE Trans. Electron Devices, \textbf{54}, 991, 2007. \\[0pt] [2] S. Ikeda, \textit{et al.} Nature Mat., \textbf{9}, 721, 2010. \\[0pt] [3] M. Endo, \textit{et al.} Appl. Phys. Lett., \textbf{96}, 212503, 2010. \\[0pt] [4] K. Nakamura \textit{et al}., Phys. Rev. B, 81, 220409(R), 2010 \\[0pt] [5] H. Sato, \textit{et al. }Appl. Phys. Lett. \textbf{99}, 042501, 2011.   

Optimized Magnetocaloric Materials

The discovery of the giant magnetocaloric effect in Gd$_{5}$Si$_{2}$Ge$_{2}$ and other R$_{5}$T$_{4}$ compounds (R = rare earth metal and T is a Group 14 element) generated a broad interest in the magnetocaloric effect and magnetostructural transitions. Reports on the giant magnetocaloric effect in other systems soon followed. These include MnFeP$_{x}$As$_{1-x}$ and related compounds, La(Fe$_{1-x}$Si$_{x})_{13}$ and their hydrides, Mn(As$_{x}$Sb$_{1-x})$, CoMnSi$_{x}$Ge$_{1-x}$ and related compounds, Ni$_{2}$MnGa and some closely related Heusler phases, and a few other systems. A common feature is the enhancement of the magnetic entropy effect by the overlapping contribution from the lattice, regardless whether it is a massive structural change like in R$_{5}$T$_{4}$ compounds, or only a phase volume change as in La(Fe$_{1-x}$Si$_{x})_{13}$. Both the magnetic and lattice entropies are, therefore, important and each contribution must be maximized in order to have the optimum magnetocaloric effect. Both of these entropy terms and the potential pathways towards a further enhancement of the giant magnetocaloric effect will be discussed.   

High-Performance Permanent Magnets for Energy-Efficient Devices

Permanent magnets (PMs) are indispensable for many commercial applications including the electric, electronic and automobile industries, communications, information technologies and automatic control engineering. In most of these applications, an increase in the magnetic energy density of the PM, usually presented via the maximum energy product (\textit{BH})$_{max}$, immediately increases the efficiency of the whole device and makes it smaller and lighter. Worldwide demand for high performance permanent magnets has increased dramatically in the past few years driven by hybrid and electric cars, wind turbines and other power generation systems. New energy challenges in the world require devices with higher energy efficiency and minimum environmental impact. The potential of 3d-4f compounds which revolutionized the PM science and technology is almost fully utilized, and the supply of 4f rare earth elements does not seem to be much longer assured. This talk will address the major principles guiding the development of PMs and overview state-of-the-art theoretical and experimental research. Recent progress in the development of nanocomposite PMs, consisting of a fine (at the scale of the magnetic exchange length) mixture of phases with high magnetization and large magnetic hardness will be discussed. Fabrication of such PMs is currently the most promising way to boost the (\textit{BH})$_{max}$, while simultaneously decreasing, at least partially, the reliance on the rare earth elements. Special attention will be paid to the impact which the next-generation high-(\textit{BH})$_{max}$ magnets is expected to have on existing and proposed energy-saving technologies.   

Soft Magnetic Materials for Improved Energy Performance

A main focus of sustainable energy research has been development of renewable energy technologies (e.g. from wind, solar, hydro, geothermal, etc.) to decrease our dependence on non-renewable energy resources (e.g. fossil fuels). By focusing on renewable energy sources now, we hope to provide enough energy resources for future generations. In parallel with this focus, it is essential to develop technologies that improve the efficiency of energy production, distribution, and consumption, to get the most from these renewable resources. Soft magnetic materials play a central role in power generation, conditioning, and conversion technologies and therefore promoting improvements in the efficiency of these materials is essential for our future energy needs. The losses generated by the magnetic core materials by hysteretic, acoustic, and/or eddy currents have a great impact on efficiency. A survey of soft magnetic materials for energy applications will be discussed with a focus on improvement in performance using novel soft magnetic materials designed for these power applications. A group of premiere soft magnetic materials -- nanocrystalline soft magnetic alloys -- will be highlighted for their potential in addressing energy efficiency. These materials are made up of nanocrystalline magnetic transition metal-rich grains embedded within an intergranular amorphous matrix, obtained by partial devitrification of melt-spun amorphous ribbons. The nanoscale grain size results in a desirable combination of large saturation induction, low coercivity, and moderate resistivity unobtainable in conventional soft magnetic alloys. The random distribution of these fine grains causes a reduction in the net magnetocrystalline anisotropy, contributing to the excellent magnetic properties. Recently developed (Fe,Co,Ni)$_{88}$Zr$_{7}$B$_{4}$Cu$_{1}$ alloys will be discussed with a focus on the microstructure/magnetic property relationship and their effects on the energy efficiency of these materials for AC applications.