O. Gutfleisch

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.

Giant magnetocaloric effect driven by structural transitions

Magnetic cooling could be a radically different energy solution substituting conventional vapour compression refrigeration in the future. For the largest cooling effects of most potential refrigerants we need to fully exploit the different degrees of freedom such as magnetism and crystal structure. We report now for Heusler-type Ni–Mn–In–(Co) magnetic shape-memory alloys, the adiabatic temperature change ΔT(ad) = −3.6 to −6.2 K under a moderate field of 2 T. Here it is the structural transition that plays the dominant role towards the net cooling effect. A phenomenological model is established that reveals the parameters essential for such a large ΔT(ad). We also demonstrate that obstacles to the application of Heusler alloys, namely the usually large hysteresis and limited operating temperature window, can be overcome by using the multi-response to different external stimuli and/or fine-tuning the lattice parameters, and by stacking a series of alloys with tailored magnetostructural transitions.

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.

Novel Design of La(Fe,Si)13 Alloys Towards High Magnetic Refrigeration Performance

www.MaterialsViews.com C O M M Novel Design of La(Fe,Si) 13 Alloys Towards High Magnetic Refrigeration Performance U N IC By Julia Lyubina , * Rudolf Schäfer , Norbert Martin , Ludwig Schultz , and Oliver Gutfl eisch A IO N Magnetic refrigeration near room temperature has great potential to become an energy-effi cient alternative to the conventional vapor-compression technology. [ 1–3 ] Attractive candidates for use as magnetic refrigerants are materials that undergo a fi rstorder magnetic phase transition, [ 4–7 ] which is accompanied by a change of crystal symmetry and/or volume. The latter leads inevitably to fracture during thermal and magnetic fi eld cycling and is a signifi cant challenge for the integration of these materials in the emerging magnetic cooling technology. [ 8 ] Here, we present a novel approach to controlling the magnetocaloric effect (MCE) in these materials by introducing porosity. In particular, we show that LaFe 13 − x Si x magnetic refrigerants with porous architecture do not degrade mechanically during cycling, and thus maintain their excellent cooling performance. This very signifi cant improvement is attributed to partial removal of grain boundaries that restrain volume expansion in the bulk material. We demonstrate that the fi rst-order magnetic phase transition is retained in porous LaFe 13– x Si x and the volume effects lead to particle size–dependent transition parameters. These results provide insight into how size-tuning can enhance the MCE and have exciting technological implications for the development of high-performance magnetic refrigerants. Magnetic cooling relies on the reversible magnetization and demagnetization of a magnetic material (magnetic refrigerant) and the resultant change in temperature in the refrigerant material under adiabatic conditions, Δ T ad . [ 1 , 9 ] The latter phenomenon is known as the magnetocaloric effect (MCE) and is maximized at temperatures corresponding to a magnetic phase transition, for example, the Curie point in ferromagnets. Usually the Curie transition is a phase transition of second order, but there exist magnetic materials in which the transition from one magnetic (disordered or ordered) state to another is a fi rst-order phase transition. [ 10–13 ] The fi rst-order magnetic phase transition gives rise to an abrupt change of the magnetization near the transition point and this, in turn, results in a large (giant) MCE. [ 2–7 ]

Hydrogen storage in different carbon nanostructures

Carbon nanostructures of different kinds have been synthesized by chemical vapor deposition. By modifying the deposition temperature, the catalyst material, and the hydrocarbon, nanofibers with herringbone structure, multi-walled nanotubes with tubular structure, and single-walled nanotubes were deposited. The nanostructures were purified with different treatment methods. The carbon nanostructures were characterized using scanning and transmission electron microscopy. The hydrogen storage capability was investigated for all obtained nanostructures. The measurements show that the storage capacity of hydrogen is very limited in all the carbon nanostructures.

Large magnetocaloric effect in melt-spun LaFe13−xSix

A very large value of magnetic entropy change ∣ΔS∣=31J∕kgK was obtained at 201K under 5T in LaFe11.8Si1.2 melt-spun ribbons subjected to a very short-time annealing (2h∕1050°C). This value is much higher than that of a bulk LaFe11.44Si1.56 in this temperature range. The large ∣ΔS∣ is attributed to the first-order thermally induced transition at the Curie temperature TC, and is enhanced even further due to a more homogenous element distribution. With increasing Si concentration, TC is increased and ∣ΔS∣ is decreased due to a weakening or an even disappearance of the first-order magnetic phase transition.

Reversibility of magnetostructural transition and associated magnetocaloric effect in Ni–Mn–In–Co

By analyzing isothermal magnetization curves under magnetic field cycling, the reversibility of the magnetostructural transition was investigated in Ni–Mn–In–Co in form of bulk sample and melt-spun ribbons. The hysteresis of thermally/magnetically induced martensitic transformation plays an important role in the reversibility of the magnetostructural transition. In ribbons with a large hysteresis of 18K, residual field-induced austenite is present after removing the magnetic field, while, in the bulk sample, the magnetostructural transition is reversible at moderate temperatures due to a relatively smaller hysteresis of 8K. Additionally, the magnetocaloric effect strongly depends on the sample history due to the occurrence of the irreversible magnetostructural transition, especially for the ribbons.

Effects of hydrostatic pressure on the magnetism and martensitic transition of Ni-Mn-In magnetic superelastic alloys

We report magnetization and differential thermal analysis measurements as a function of pressure accross the martensitic transition in magnetically superelastic Ni-Mn-In alloys. It is found that the properties of the martensitic transformation are significantly affected by the application of pressure. All transition temperatures shift to higher values with increasing pressure. The largest rate of temperature shift with pressure has been found for Ni

Large reversible magnetocaloric effect in Ni-Mn-In-Co

_{50}

The role of local anisotropy profiles at grain boundaries on the coercivity of Nd2Fe14B magnets

Mn