A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy effi ciency 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 effi ciency 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. Specifi cally, 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 effi ciency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

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Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.

Magnetic Properties of Nanostructured Materials

Exchange-spring magnets are nanocomposites that are composed of magnetically hard and soft phases that interact by magnetic exchange coupling. Such systems are promising for advanced permanent magnetic applications, as they have a large energy product--the combination of permanent magnet field and magnetization--compared to traditional, single-phase materials. Conventional techniques, including melt-spinning, mechanical milling and sputtering, have been explored to prepare exchange-spring magnets. However, the requirement that both the hard and soft phases are controlled at the nanometre scale, to ensure efficient exchange coupling, has posed significant preparation challenges. Here we report the fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly. In this approach, both FePt and Fe3O4 particles are incorporated as nanometre-scale building blocks into binary assemblies. Subsequent annealing converts the assembly into FePt-Fe3Pt nanocomposites, where FePt is a magnetically hard phase and Fe3Pt a soft phase. An optimum exchange coupling, and therefore an optimum energy product, can be obtained by independently tuning the size and composition of the individual building blocks. We have produced exchange-coupled isotropic FePt-Fe3Pt nanocomposites with an energy product of 20.1 MG Oe, which exceeds the theoretical limit of 13 MG Oe for non-exchange-coupled isotropic FePt by over 50 per cent.

Exchange-coupled nanocomposite magnets by nanoparticle self-assembly

https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

It is proposed that permanent magnets can be made of composite materials consisting of two suitably dispersed ferromagnetic and mutually exchange-coupled phases, one of which is hard magnetic in order to provide a high coercive field, while the other may be soft magnetic, just providing a high saturation J/sub s/, and should envelop the hard phase regions in order to prevent their corrosion. A general theoretical treatment of such systems shows that one may expect, besides a high energy product (BH)/sub max/, a reversible demagnetization curve (exchange-spring) and, in certain cases, an unusually high isotropic remanence ratio B/sub r//J/sub s/, while the required volume fraction of the hard phase may be very low, on the order of 10%. The technological realization of such materials is shown to be based on the principle that all phases involved must emerge from a common metastable matrix phase in order to be crystallographically coherent and consequently magnetically exchange coupled. >

The exchange-spring magnet: a new material principle for permanent magnets

The anhysteretic initial susceptibility \chi_{ar} of recording tape like materials is calculated from a dynamic mean field model and is given as an explicit function of the static magnetic tape parameters M s (spontaneous magnetization of the magnetic material), p (volume fraction of the magnetic material), m R (remanence to saturation ratio of the tape), and H R (remanence coercivity of the tape) for the case of particles with predominant shape anisotropy. The result, \chi_{ar} = \frac{p(1-p) M\min{s}\max{2}}{2H\min{R}\max{2}(1-m\min{R}\max{2})} is compared with experimental data on a large number and great variety of commercial and laboratory tape samples made of γ-Fe 2 O 3 , CrO 2 , or Fe acicular particles covering wide ranges of all of the above parameters, and is completely verified by these data. In particular, since H_{R}\proptoM_{s} for shape anisotropy, \chi_{ar} is independent of M s , and hence of temperature T;\chi_{ar} is also independent of H R as long as the orientation m R and the particle dimensions are kept constant. Further, the above theoretical relation is remarkably insensitive against variations of the underlying model, and thus reflects the known independence of chi ar upon many structural details of a material. Thus, the present theory seems to account completely for all known features of \chi_{ar} .

Relation between anhysteretic and static magnetic tape parameters

The anhysteretic remanence \bar{M}_{ar}(H_{o},T) of solidified suspensions of magnetic particles with predominant shape anisotropy is calculated from first principles for small dc fields H o and arbitrary temperature T (blocking temperature), describing the particle interactions by a mean field and assuming constant decrement of the ac field, 2H_{d} per cycle. For H_{d} , the anhysteretic distribution of particle magnetizations is found to be subject to the condition that the net internal dc field \bar{H}_{i} is a minimum, and, for small H o , to the condition, \bar{H}_{i} = 0 . The theory yields \bar{M}_{ar}(H_{o},T) as a unique function of independently measurable static magnetic material properties, i.e., it contains no adjustable parameters and is hence quantitatively related to experimental data. Further, according to theory, if \bar{M}_{ar}(H_{o},T,T_{m}) denotes \bar{M}_{ar} as acquired in H o at T and measured at T_{m}, \bar{M}_{ar}(H_{o},T,T_{m} = T) is independent of T for H_{d} \ll 2H_{o} , and \bar{M}_{ar}(H_{o},T,T_{m} \neq T) = [M_{s}(T_{m})/M_{s}(T)] \cdot \bar{M}_{ar}(H_{o},T,T_{m} = T) . The thermoremanent magnetization acquired in H o and measured at a temperature T_{m} \ll T_{B} , \bar{M}_{thr}(H_{o},T_{m}) , is related to \bar{M}_{ar}(H_{o},T = T_{m}, T_{m}) by \bar{M}_{thr}(H_{o},T_{m}) = [M_{s}(T_{m})/M_{s}(T_{B})]\bar{M}_{ar}(H_{o},T=T_{m},T_{m}) , where T B is the blocking temperature below which \bar{M}_{thr} becomes thermally stable. Up to a constant factor of about 2, the theoretical results agree quantitatively with the experimental data on all materials that correspond to the premises of the theory, i.e., solidified suspensions, tapes in particular, of particles having predominant shape anisotropy.

E. Kneller

Papers

The exchange-spring magnet: a new material principle for permanent magnets

Relation between anhysteretic and static magnetic tape parameters

Static and anhysteretic magnetic properties of tapes