Metamaterials, artificial electromagnetic media that are structured on the subwavelength scale, were initially suggested for the negative-index 'superlens'. Later metamaterials became a paradigm for engineering electromagnetic space and controlling propagation of waves: the field of transformation optics was born. The research agenda is now shifting towards achieving tunable, switchable, nonlinear and sensing functionalities. It is therefore timely to discuss the emerging field of metadevices where we define the devices as having unique and useful functionalities that are realized by structuring of functional matter on the subwavelength scale. In this Review we summarize research on photonic, terahertz and microwave electromagnetic metamaterials and metadevices with functionalities attained through the exploitation of phase-change media, semiconductors, graphene, carbon nanotubes and liquid crystals. The Review also encompasses microelectromechanical metadevices, metadevices engaging the nonlinear and quantum response of superconductors, electrostatic and optomechanical forces and nonlinear metadevices incorporating lumped nonlinear components.

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From metamaterials to metadevices.

Magnetic cooling could be a radically different energy solution that could replace conventional vapour compression refrigeration in the future. It is now shown that a Heusler-type magnetocaloric alloy exhibits a remarkable cooling capability due to the effect of a sharp structural transformation at a specific temperature. The finding may be of relevance beyond Heusler alloys and represents an important step towards the implementation of cooling systems based on magnetocaloric materials.

Giant magnetocaloric effect driven by structural transitions

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Transformation optics describes the capability to design the path of light waves almost at will through the use of metamaterials that control effective materials properties on a subwavelength scale. In this review, the physics and applications of transformation optics are discussed.

Transformation optics and metamaterials

Magnetocaloric effect: From materials research to refrigeration devices

Exploring La(Fe,Si)13-based magnetic refrigerants towards application

Magnetic cooling prototypes based on the magnetocaloric effect require the magnetic refrigerant to be shaped into geometries that enable quick and efficient heat-transfer to the transfer fluid. Simple solutions include a stack of parallel-plates or a bed of packed-spheres. Here, we demonstrate how more sophisticated geometries can be made by using selective laser melting, a rapid prototyping technique to form three-dimensional shapes. Using excellent magnetocaloric La(Fe,Co,Si)13 as starting-powder we made two geometries: a wavy-channel block with a high surface-to-volume ratio and an array of fin-shaped rods which eliminate unwanted heat conduction along the magnetic part. After annealing treatment, the geometries are intact and can survive more than 106 cycles of applying a magnetic field while still maintaining good magnetocaloric properties. This fabrication approach shows promise for making near-net shaped magnetic refrigerants with superior heat transfer properties and performance.

Selective laser melting of La(Fe,Co,Si) 13 geometries for magnetic refrigeration

Electromagnetic metamaterials are a class of materials which have been artificially structured on a subwavelength scale. They are currently the focus of a great deal of interest because they allow access to previously unrealisable properties like a negative refractive index. Most metamaterial designs have so far been based on resonant elements, like split rings, and research has concentrated on microwave frequencies and above. In this work, we present the first experimental realisation of a non-resonant metamaterial designed to operate at zero frequency. Our samples are based on a recently-proposed template for an anisotropic magnetic metamaterial consisting of an array of superconducting plates. Magnetometry experiments show a strong, adjustable diamagnetic response when a field is applied perpendicular to the plates. We have calculated the corresponding effective permeability, which agrees well with theoretical predictions. Applications for this metamaterial may include non-intrusive screening of weak DC magnetic fields.

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A DC magnetic metamaterial

Evidence of a magnetic glass state in the magnetocaloric material Gd 5 Ge 4

Simultaneous magnetization and sample temperature measurements were performed as a function of magnetic field and magnetic field sweep-rates to study the influence of these conditions on the hysteresis of the magnetocaloric transition in La(Fe1−x−yCoxSiy)13 samples. The large magnetocaloric effect in the compounds that show a first-order transition cause a significant departure from isothermal conditions leading to dynamic sweep-rate dependent magnetic hysteresis. Here we show how this deleterious effect can be greatly reduced by changing the sample geometry or by use of materials which show a second-order transition only. The key signatures of nonisothermal conditions in the magnetization data are highlighted.

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J. D. Moore

Papers

Exploring La(Fe,Si)13-based magnetic refrigerants towards application

Selective laser melting of La(Fe,Co,Si) 13 geometries for magnetic refrigeration

A DC magnetic metamaterial

Evidence of a magnetic glass state in the magnetocaloric material Gd 5 Ge 4

Reducing extrinsic hysteresis in first-order la (Fe,Co,Si)13 magnetocaloric systems