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Microwave Magnetic Materials: from Ferrites to Metamaterials

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Microwave Magnetic Materials: from Ferrites to Metamaterials Microwave magnetic materials: from ferrites to metamaterials Soft magnetic materials and ferromagnetic metals are widely used in microwave applications. The newly-developed metamaterials, instead of replacing them, can combine with them to extend their potential. Two other systems – thin layers and magnetic microwires – offer equally remarkable microwave properties Pyramidal radar-absorbent materials in the Aquitaine science and engineering research centre (CEA/Cesta) anechoic chamber at Le Barp, near Bordeaux. Ferrites, which absorb electromagnetic radiation in certain frequency ranges, can be fitted under the pyramid-shaped radiation-absorbent foams lining the walls of the anechoic chambers to further increase the absorbent capacities. Philippe Labèguerie/CEA agnetic materials have long been used for micro- antenna substrates, radar absorbents, tunable fil- Mwave applications. Inductors, antenna cores ters, etc. and ferrite filters are also widely employed. Finally, when these magnetic materials are magne- Microwave permeability µ(f) is a fundamental phy- tized, they become non-reciprocal, which means their sical unit when working with these inductive appli- characteristics depend on the direction of motion of cations, as it can be used to gauge the performance the wave crossing through them. This non-recipro- of the material. Permeability describes the response city is put to use to build circulators and isolators of induction b to a magnetic field h oscillating at a employed in radar systems, mobile telephone relay frequency f, as: b = µ(f).µ0.h, where µ0 is the vacuum stations, etc. Ferrites and garnets are still the first- permeability. Therefore, it is the materials with high choice materials for these applications. Ferrites have permeability µ(f) that are used, as they can generate a long history as microwave materials. Louis Néel, strong induction from the field created by a current. working through the CEA, played a major role in Among the various classes of magnetic materials, it developing our understanding of these materials. is the soft magnetic materials that offer the highest Later on, there was a strong drive in the development permeabilities. Their magnetisation, in contrast with of ferromagnetic metals for these applications. Over permanent magnets, is highly responsive to small- the last few years, the focus has turned to metama- CEA scale outside magnetic fields. terials as a totally novel approach for the synthesis Louis Néel, who was awarded These magnetic materials are also used for elec- of materials presenting novel microwave magnetic the 1970 Nobel Prize in tromagnetic applications. While in most materials, responses. Physics, played a major role propagation, reflection and transmission to an inter- in developing our understanding of magnetic face depend on a sole parameter, the behaviour of High-permeability ferromagnetic materials. Instrumental in magnetic materials depends on two independent materials the development of scientific parameters – permittivity and permeability. This research in the Grenoble extra degree of freedom makes it possible to obtain As early as the late 1940s, it was discovered that there area in the latter half of the twentieth century, Louis Néel properties beyond the reach of a dielectric mate- was a certain compromise between the achievable pioneered the creation of rial, which is why magnetic materials are used as microwave permeability and the maximum frequency CEA Grenoble. CLEFS CEA - No.56 - WINTER 2007-2008 19 Magnets and magnetic materials The versatility of metamaterials This kind of material, an example of which illustra- ted in Figure 2 is etched onto a printed circuit board substrate, offers a permeability peak at around 1.5 gigahertz (GHz) despite having no magnetic component! Since metamaterials first arrived on the scene less than a decade ago, they have sparked an enormous amount of interest within the electroma- gnetism community. They offer exceptional versati- CEA lity in the design and fabrication of materials pre- A handful of ferrite components for senting two independent electromagnetic parameters. high-frequency at which this level can be achieved. According to This added flexibility in design has been exploited applications widely used Snoek's law, the product of these two units is pro- to produce different types of lenses that are not limi- in radio and electronic portional to the saturation magnetisation. This rela- ted by diffraction aberration. More recently, scien- controllers: inductor cores (at left), ferrite filter tionship clearly establishes the advantage of working tists have demonstrated 'cloaks of invisibility'. (at right) and antenna core with materials whose saturation magnetisation is Another line of development offering huge potential (at bottom). higher than that of ferrites, i.e. ferromagnetic metals is to integrate these copper patterns into electronics and alloys. However, ferromagnetic alloys are highly to produce 'controllable' materials. 10 nm conductive, and microwaves can only penetrate At the CEA's Le Ripault centre (in conductors at an extremely low thickness, cal- the Indre- et-Loire), the Materials led skin depth. This means these high-frequency Science Department was first to materials can only be used if they are in the demonstrate this principle at form of thin layers, wires, or composites inte- work, by producing a mate - grating ferromagnetic materials in the form of rial with voltage-tunable powders or flakes. Research conducted at the microwave permeability. CEA in tandem with Paris VII University has Metamaterials make it pos- led to the synthesis of submicron-scale pow- sible to synthesize proper- ders with remarkable properties (Figure 1). ties beyond the capacities These powders, which have a grain size less than of conventional magnetic the skin depth, can interact fully with the microwave materials. Man-made electromagnetic field. Their low granulometric magnetic materials have dispersion made it possible to see how these pow- been produced that ope- ders show remarkable permeability behaviour, with CEA rate in the visible frequency quantified electromagnetic excitation states in each spectrum. However, CEA sphere. Thin layers and magnetic microwires are two Figure 2. research teams have also Metamaterial built other examples of systems presenting remarkable from periodical copper shown that metamaterials microwave properties (see High-permeability magne- patterns etched into are unable to reproduce tic thin layers on p. 21, and Ferromagnetic microwi- a printed circuit wideband performances substrate and adapted res on p. 24). to coaxial line function. on a par with ferromagne- While the conductivity of the materials heavily tic materials once the influences their microwave response, and negatively frequencies drop below so in the case of ferromagnetic metals, it is still pos- around the 10 gigahertz mark. By combining cop- sible to create a high-frequency magnetic response per patterns and conventional magnetic materials, without a magnetic component, if conductive pat- it becomes possible to combine the advantages affor- terns can be crafted. What is being created is a meta- ded by the two approaches: high levels of permea- material. bility with relatively straightforward engineering. Extraordinary perspectives The advantages presented by ferromagnetic metals mean that they are able to edge out ferrites in a cer- tain number of applications. The more recently deve- loped metamaterials represent a novel approach for obtaining microwave magnetic properties, and they are opening up extraordinary perspectives. Although they cannot fully replace conventional magnetic mate- rials, they can combine with them to extend their potential. > Olivier Acher Figure 1. Scanning electron Military Applications Division microscope image CEA Le Ripault Centre of a submicron-scale Fe0.13[Co80Ni20]0.87 powder synthesized by 200 nm the polyol process. CEA/DAM 20 CLEFS CEA - No.56 - WINTER 2007-2008 High-permeability magnetic thin layers Magnetic thin layers are a perfect illustration of convergence between materials, components and systems-specific research disciplines by considerably shortening the distance between physicists and applications engineers. agnetic thin layers are integrated into an extre- Mmely diverse range of microwave applications including the read-write heads in magnetic disk drives, spin electronics systems (see Data storage: achievement and promises of nanomagnetism and spintronics, p. 62), band-pass filters(1), planar induc- tors for mobile phones, or anti-theft marking sys- tems. Thin layer engineering has employed newly- developed homogenous or composite materials to tailor the properties of the layers to individual appli- cations. The main challenges involved are to build systems that are 'frequency-agile' and/or that work at high operating frequencies. This is a field in which the CEA provides worldwide state-of-the-art exper- tise, with strong multidisciplinary inputs from pure Two-wire etched antenna (2 GHz) geared to use physics to technological applications engineering with radiofrequency and back to microwave instrumentation. There are ferromagnetic thin two overriding objectives: overcoming the cons- C. Delaveaud/CEA-LETI/DCIS layers. traints involved in working with extremely small devices, and meeting new applications requirements as sample size decreases together with the appea- in terms of frequencies to be employed. rance of additional resonance peaks at frequen- cies below the frequencies
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