Magnetic materials in sustainable energy

Oliver Gutfleisch
Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden)

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 (e.g. hydrogen storage, batteries, thermoelectrics, etc.).

The lecture 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 analysed.

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Magnetic Soft X-Ray Microscopy: A path towards imaging magnetism down to fundamental length and time scales

Peter Fischer
Lawrence Berkeley National Laboratory

One of the scientific and technological challenges in nanomagnetism research is to image magnetism down to fundamental magnetic length and time scales with elemental sensitivity in advanced multicomponent materials. Magnetic soft X-ray microscopy is a unique analytical technique combining X-ray magnetic circular dichroism (X-MCD) as element specific magnetic contrast mechanism with high spatial and temporal resolution. Fresnel zone plates used as X-ray optical elements provide a spatial resolution down to currently 10nm thus approaching fundamental magnetic length scales such as magnetic exchange lengths. Images can be recorded in external magnetic fields giving access to study magnetization reversal phenomena on the nanoscale and its stochastic character with elemental sensitivity. Utilizing the inherent time structure of current synchrotron sources fast magnetization dynamics such as current induced wall and vortex dynamics in ferromagnetic elements can be performed with a stroboscopic pump-probe scheme with 70ps time resolution, limited by the lengths of the electron bunches.

With a spatial resolution approaching the <10nm regime, soft X-ray microscopy at next generation high brilliant fsec X-ray sources will make snapshot images of nanosclae ultrafast spin dynamics become feasible.

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Spin caloritronics

Axel Hoffmann
Materials Science Division, Argonne National Laboratory, Argonne, IL, USA.

As semiconducting electronic devices are miniaturized to ever-smaller dimensions, power dissipation becomes an ever-increasing problem due to leakage charge currents.  Spintronics may help addressing some of these issues by utilizing besides the charge degree of freedom also the electron spin.  Conventional spintronics approaches are used for non-volatile devices, such as magnetic random access memory, where spin currents are mainly considered as spin-polarized charge currents and as a result the spin and charge currents are in parallel and directly coupled.  Looking further into the future, the question arises, whether eliminating charge currents altogether could provide additional benefits for applications.  Towards addressing this question, non-local device geometries allow for separating spin and charge currents, which in turn enables the investigation and use of pure spin currents [1].  This approach opens up new opportunities to study spin-dependent physics and gives rise to novel approaches for generating and controlling angular momentum flow.

This lecture will discuss different approaches for generating pure spin currents, such as non-local electrical injection from a ferromagnet, charge-to-spin current conversion via spin Hall effects, and spin pumping from ferromagnetic resonance.  Furthermore, examples will be shown for how spin currents can be used for gaining new insights into spin dependent phenomena.  In particular, the temperature dependence of spin and charge relaxation times allows to identify different spin relaxation mechanisms [2].  In addition, spin pumping facilitates the generation of macroscopically large pure spin currents.  This permits to quantify spin Hall effects with great precision, even in materials where these effects are relatively weak [3,4].  Finally, the lecture will conclude with a brief outlook on the current scientific and future technological opportunities for pure spin currents.

[1] A. Hoffmann, Phys. Stat. Sol. (c) 4, 4236 (2007).
[2] G. Mihajlović, J. E. Pearson, S. D. Bader, and A. Hoffmann, Phys. Rev. Lett. 104, 237202 (2010).
[3] O. Mosendz, J. E. Pearson, F. Y. Fradin, G. E. W. Bauer, S. D. Bader, and A. Hoffmann, Phys. Rev. Lett. 104, 046601 (2010).
[4] O. Mosendz, V. Vlaminck, J. E. Pearson, F. Y. Fradin, G. E. W. Bauer, S. D. Bader, and A.Hoffmann, Phys. Rev. B 82, 214403 (2010).

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Growth-control and microstructure characterization of magnetic thin films, application to high density perpendicular magnetic recording media

Masaaki Futamoto
Chuo University

Various magnetic thin films are used for recording media and heads of hard disk drives. The magnetic properties have been greatly improved to cope with a continuous areal density increase of more than 10⁴ times over the past quarter century. The improvement has been realized by tailoring the composition and the microstructure of magnetic thin films.

This lecture covers the technology and the physics for controlling the microstructure of magnetic thin films, focusing mainly on perpendicular recording media and related magnetic materials. Initially, technological developments will be briefly reviewed and then the following topics will be discussed: (1) nucleation and growth of magnetic thin films through heteroepitaxy on nonmagnetic underlayers, (2) nanostructure and nano-composition characterization, (3) application to perpendicular magnetic recording media, (4) magnetization structure analysis, (5) epitaxial growth of single-crystal magnetic thin films with metastable and ordered crystal structures, and (6) patterned-type perpendicular recording media for higher densities. The relationships between film microstructure and magnetic properties will also be discussed.

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