J. Ping Liu

Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient

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 into 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, conditioning, conversion, transportation, and other energy-use sectors of the economy. This review 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, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure-property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy efficiency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

Nanoscale magnetic materials and applications

Spin Dynamics: Fast Switching of Macro-spins.- Core-Shell Magnetic Nanoclusters.- Designed Magnetic Nanostructures.- Superconductivity and Magnetism in Silicon and Germanium Clathrates.- Neutron Scattering of Magnetic Materials.- Tunable Exchange Bias Effects.- Dynamics of Domain Wall Motion in Wires with Perpendicular Anisotropy.- Magnetic Nanowires for Domain Wall Logic and Ultrahigh Density Data Storage.- Bit-Patterned Magnetic Recording: Nanoscale Magnetic Islands for Data Storage.- The Magnetic Microstructure of Nanostructured Materials.- Exchange-Coupled Nanocomposite Permanent Magnets.- High-Temperature Samarium Cobalt Permanent Magnets.- Nanostructured Soft Magnetic Materials.- Magnetic Shape Memory Phenomena.- Magnetocaloric Effect and Materials.- Spintronics and Novel Magnetic Materials for Advanced Spintronics.- Growth and Properties of Epitaxial Chromium Dioxide (CrO2) Thin Films and Heterostructures.- FePt and Related Nanoparticles.- Magnetic Manipulation of Colloidal Particles.- Applications of Magnetic Nanoparticles in Biomedicine.- Nano-Magnetophotonics.- Hard Magnetic Materials for MEMS Applications.- Solid-State Magnetic Sensors for Bioapplications.

Advances in nanostructured permanent magnets research

This paper reviews recent developments in research in nanostructured permanent magnets (hard magnetic materials) with emphasis on bottom-up approaches to fabrication of hard/soft nanocomposite bulk magnets. Theoretical and experimental findings on the effects of soft phase and interface conditions on interphase exchange interactions are given. Synthesis techniques for hard magnetic nanoparticles, including chemical solution methods, surfactant-assisted ball milling and other physical deposition methods are reviewed. Processing and magnetic properties of warm compacted and plastically deformed bulk magnets with nanocrystalline morphology are discussed. Prospects of producing bulk anisotropic hard/soft nanocomposite magnets are presented.

Monodisperse face-centred tetragonal FePt nanoparticles with giant coercivity

Monodisperse face-centred tetragonal (fct) FePt nanoparticles with high magnetic anisotropy and, therefore, high coercivity have been prepared via a new heat treatment route. The as-synthesized face-centred cubic FePt nanoparticles were mixed with salt powders and annealed at 700uC. The salts were then removed from the particles by washing the samples in water. Monodisperse fct FePt particles were recovered with the particle size and shape being retained. Coercivity of the isolated particles up to 30 kOe at room temperature has been obtained. (Some figures in this article are in colour only in the electronic version)

Zinc ferrite nanoparticles as MRI contrast agents

Mixed spinel hydrophobic ZnxFe1-xO x Fe2O3 (up to x = 0.34) nanoparticles encapsulated in polymeric micelles exhibited increased T2 relaxivity and sensitivity of detection over clinically used Feridex.

Fabrication of bulk nanocomposite magnets via severe plastic deformation and warm compaction

We demonstrate that a SmCo/FeCo based hard/soft nanocomposite material can be fabricated by distributing the soft magnetic α-Fe phase particles homogeneously in a hard magnetic SmCo phase through severe plastic deformation. The soft-phase particle size can be reduced from micrometers to smaller than 15 nm upon deformation. Up to 30% of the soft phase can be incorporated into the composites without coarsening. A warm compaction process of the plastically deformed powder particles then produces bulk nanocomposite magnets of fully dense nanocomposites with energy product up to 19.2 MGOe owing to effective interphase exchange coupling, which makes this type of nanocomposite magnets suitable for high energy-density applications at high temperatures.

High Energy Product Developed from Cobalt Nanowires

Cobalt nanowires with high aspect ratio have been synthesized via a solvothermal chemical process. Based on the shape anisotropy and orientation of the nanowire assemblies, a record high room-temperature coercivity of 10.6 kOe has been measured in Co nanowires with a diameter of about 15 nm and a mean length of 200 nm. As a result, energy product of the wires reaches 44 MGOe. It is discovered that the morphology uniformity of the nanowires is the key to achieving the high coercivity and high energy density. Nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.

SmCo5∕Fe nanocomposites synthesized from reductive annealing of oxide nanoparticles

Hard magnetic nanocomposites SmCo5∕Fex (x=0–2.9) are synthesized by a simultaneous calcium reduction of Sm–Co–O and Fe3O4 nanoparticles. The composites consist of nanostructured SmCo5 and Fe with their average grain sizes at 29 and 8nm, respectively. The magnetic properties of the composites can be tuned by controlling Fe composition. SmCo5∕Fe1.5 shows an enhanced remanent magnetization at 56emu∕g (45emu∕g for SmCo5). The largest coercivity value of 11.6kOe is achieved with SmCo5∕Fe0.23. The synthesis represents a general process toward SmCo-based exchange-spring nanocomposites for high performance permanent magnet applications.

Hard magnetic FePt nanoparticles by salt-matrix annealing

Proceedings of the 50th Annual Conference on Magentism and Magnetic Materials. L10 and Other Hard Magnetic Materials L10 and Other Hard Magnetic Materials L10 and Other Hard Magnetic Materials. L10 and Other Hard Magnetic Materials.

Bulk FePt-based nanocomposite magnets with enhanced exchange coupling

Department of Physics, University of Texas at Arlington. NanoTech Institute, University of Texas at Dallas. School of Materials Science and Engineering, Georgia Institute of Technology.