microwave magnetic materials: from ferrites to metamaterials · two other systems – thin layers...

CLEFS CEA - No. 56 - WINTER 2007-2008 19

Soft magnetic materials and ferromagnetic metals are widely used in microwaveapplications. The newly-developed metamaterials, instead of replacing them, can combinewith them to extend their potential. Two other systems – thin layers and magnetic microwires– offer equally remarkable microwave properties

Microwave magnetic materials: from ferritesto metamaterials

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Magnetic materials have long been used for micro-wave applications. Inductors, antenna cores

and ferrite filters are also widely employed.Microwave permeability µ(f) is a fundamental phy-sical unit when working with these inductive appli-cations, as it can be used to gauge the performanceof the material. Permeability describes the responseof induction b to a magnetic field h oscillating at afrequency f, as: b = µ(f).µ0.h, where µ0 is the vacuumpermeability. Therefore, it is the materials with highpermeability µ(f) that are used, as they can generatestrong induction from the field created by a current.Among the various classes of magnetic materials, itis the soft magnetic materials that offer the highestpermeabilities. Their magnetisation, in contrast withpermanent magnets, is highly responsive to small-scale outside magnetic fields.These magnetic materials are also used for elec-tromagnetic applications. While in most materials,propagation, reflection and transmission to an inter-face depend on a sole parameter, the behaviour ofmagnetic materials depends on two independentparameters – permittivity and permeability. Thisextra degree of freedom makes it possible to obtainproperties beyond the reach of a dielectric mate-rial, which is why magnetic materials are used as

Pyramidal radar-absorbentmaterials in the Aquitainescience and engineeringresearch centre (CEA/Cesta)anechoic chamber at Le Barp, near Bordeaux.Ferrites, which absorbelectromagnetic radiation in certain frequency ranges,can be fitted under the pyramid-shapedradiation-absorbent foamslining the walls of the anechoic chambers to further increase theabsorbent capacities.

Louis Néel, who was awardedthe 1970 Nobel Prize inPhysics, played a major rolein developing ourunderstanding of magneticmaterials. Instrumental inthe development of scientificresearch in the Grenoblearea in the latter half of thetwentieth century, Louis Néelpioneered the creation ofCEA Grenoble.

antenna substrates, radar absorbents, tunable fil-ters, etc.Finally, when these magnetic materials are magne-tized, they become non-reciprocal, which means theircharacteristics depend on the direction of motion ofthe wave crossing through them. This non-recipro-city is put to use to build circulators and isolatorsemployed in radar systems, mobile telephone relaystations, etc. Ferrites and garnets are still the first-choice materials for these applications. Ferrites havea long history as microwave materials. Louis Néel,working through the CEA, played a major role indeveloping our understanding of these materials.Later on, there was a strong drive in the developmentof ferromagnetic metals for these applications. Overthe last few years, the focus has turned to metama-terials as a totally novel approach for the synthesisof materials presenting novel microwave magneticresponses.

High-permeability ferromagneticmaterials

As early as the late 1940s, it was discovered that therewas a certain compromise between the achievablemicrowave permeability and the maximum frequency

CLEFS CEA - No. 56 - WINTER 2007-200820

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 boardsubstrate, offers a permeability peak at around1.5 gigahertz (GHz) despite having no magneticcomponent! Since metamaterials first arrived on thescene less than a decade ago, they have sparked anenormous amount of interest within the electroma-gnetism community. They offer exceptional versati-lity in the design and fabrication of materials pre-senting two independent electromagnetic parameters.This added flexibility in design has been exploitedto produce different types of lenses that are not limi-ted by diffraction aberration. More recently, scien-tists have demonstrated 'cloaks of invisibility'. Another line of development offering huge potentialis to integrate these copper patterns into electronics

to produce 'controllable' materials.At the CEA's Le Ripault centre (inthe Indre- et-Loire), the Materials

Science Department was first todemonstrate this principle at

work, by producing a mate -rial with voltage-tunablemicrowave permeability. Metamaterials make it pos-sible to synthesize proper-ties beyond the capacitiesof conventional magneticmaterials. Man-mademagnetic materials havebeen produced that ope-rate in the visible frequencyspectrum. However, CEAresearch teams have alsoshown that metamaterialsare unable to reproducewideband performanceson a par with ferromagne-tic materials once the frequencies drop below

around the 10 gigahertz mark. By combining cop-per patterns and conventional magnetic materials,it becomes possible to combine the advantages affor-ded by the two approaches: high levels of permea-bility with relatively straightforward engineering.

Extraordinary perspectives

The advantages presented by ferromagnetic metalsmean 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 forobtaining microwave magnetic properties, and theyare opening up extraordinary perspectives. Althoughthey cannot fully replace conventional magnetic mate-rials, they can combine with them to extend theirpotential.

> Olivier AcherMilitary Applications Division

CEA Le Ripault Centre

at which this level can be achieved. According toSnoek's law, the product of these two units is pro-portional to the saturation magnetisation. This rela-tionship clearly establishes the advantage of workingwith materials whose saturation magnetisation ishigher than that of ferrites, i.e. ferromagnetic metalsand alloys. However, ferromagnetic alloys are highlyconductive, and microwaves can only penetrateconductors at an extremely low thickness, cal-led skin depth. This means these high-frequencymaterials can only be used if they are in theform of thin layers, wires, or composites inte-grating ferromagnetic materials in the form ofpowders or flakes. Research conducted at theCEA in tandem with Paris VII University hasled to the synthesis of submicron-scale pow-ders with remarkable properties (Figure 1).These powders, which have a grain size less thanthe skin depth, can interact fully with the microwaveelectromagnetic field. Their low granulometricdispersion made it possible to see how these pow-ders show remarkable permeability behaviour, withquantified electromagnetic excitation states in eachsphere. Thin layers and magnetic microwires are twoother examples of systems presenting remarkablemicrowave properties (see High-permeability magne-tic thin layers on p. 21, and Ferromagnetic microwi-res on p. 24).While the conductivity of the materials heavilyinfluences their microwave response, and negativelyso in the case of ferromagnetic metals, it is still pos-sible to create a high-frequency magnetic responsewithout a magnetic component, if conductive pat-terns can be crafted. What is being created is a meta-material.

Figure 1. Scanning electronmicroscope image

of a submicron-scaleFe0.13[Co80Ni20]0.87

powder synthesized by the polyol process.

A handful of ferritecomponents for high-frequency

applications widely usedin radio and electronic

controllers: inductor cores(at left), ferrite filter

(at right) and antenna core(at bottom).

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Figure 2. Metamaterial built from periodical copperpatterns etched into a printed circuitsubstrate and adapted to coaxial line function.

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Magnetic thin layers are integrated into an extre-mely diverse range of microwave applications

including the read-write heads in magnetic diskdrives, spin electronics systems (see Data storage:achievement and promises of nanomagnetism andspintronics, 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 totailor the properties of the layers to individual appli-cations. The main challenges involved are to buildsystems that are 'frequency-agile' and/or that workat high operating frequencies. This is a field in whichthe CEA provides worldwide state-of-the-art exper-tise, with strong multidisciplinary inputs from purephysics to technological applications engineeringand back to microwave instrumentation. There aretwo overriding objectives: overcoming the cons-traints involved in working with extremely smalldevices, and meeting new applications requirementsin terms of frequencies to be employed.

Overcoming dimensional limitationsThe ferromagnetic layers being built are designedto be incorporated into very small devices wherethe material close to the film edges no longer hasa negligible effect in relation to the material at thecore of the system. This makes it important to cha-racterise the response of the film in these poten-tially disturbed zones whose magnetic propertiesdeviate from the gyromagnetic behaviour of a thinlayer of supposedly infinite dimensions. Thus, inorder to keep its self-energy to a minimum, micro-metric layers adopt magnetic domain-based struc-tures that, depending on the conditions of excita-tion, will induce an overall drop in energy levels

as sample size decreases together with the appea-rance of additional resonance peaks at frequen-cies below the frequencies of the main peak. Thishas been observed in polycrystalline alloy micro-structures like NiFe alloys for the polar parts ofread heads or micro-inductor cores where the wallsthat form side-closing domains (at 90° to the axisof magnetisation) enter into resonance well beforethe main gyromagnetic phenomenon occurs.Amorphous alloys like CoZr or CoFeSiB also adopta strongly non-homogenous "needle-shaped" struc-ture formed of interlaced alternating triangular-shaped domains with antiparallel magnetisation(Figure 1). In both cases, as CEA research has shown,the number and position of these 'secondary peaks'is a function of the thickness deposited and themain magnetic characteristics of the layer (magne-tisation, anisotropy and exchange constant).A further dimensional limitation is the thicknessdeposited. At below the skin depth, modifications

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Two-wire etched antenna(2 GHz) geared to usewith radiofrequencyferromagnetic thinlayers.

Figure 1. Kerr microscope imagesof needle-shapeddomains and associatedpermeability spectra forCoZr layers of increasingthickness, running from0.2 μm to 1.9 μm.

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.

High-permeability magnetic thin layers

(1) Band-pass filter: a device that only allows electromagneticwaves within a certain range bracket to pass through, based ontheir wavelength.

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in the layer's magnetic structure can completelylose the uniform gyromagnetic state being targe-ted. In fact, during the deposition process, porous-ness gradient or constraint effects can drive thedevelopment of anisotropy perpendicular to thelayer and lead the layer to take on a magnetic struc-ture called 'in-band domains' where a layer magne-tisation component is out of plane. The microwaveresponse of this structure is highly chaotic, com-prising a series of extremely narrow peaks. This isa phenomenon studied under a project led in tan-dem with Dassault Aviation. A digital approachwas employed to account for these spectra, withexcellent agreement between theoretical and expe-rimental results (Figure 2).

Application to planar inductorsOne of the heaviest constraints to miniaturisingRF circuits (CMOS or BiCMOS) is the integrationof the inductors which are one of the least space-efficient of the passive components. A highly pro-mising way forward is to integrate high-permea-bility magnetic films on silicon. The CEA-Leti hasintegrated amorphous CoZr and FeCoSiB filmsdeposited at the Le Ripault centre into a high-qua-lity-factor RF inductor system. This research madethe breakthrough discovery of a potential effectivereduction of around 10% to 15% in the footprintof the induction coils without losing electrical pro-perties. This pioneering research has paved the wayto the more targeted development of magnetic filmsfor RF applications using more conventional depo-sition techniques for microelectronics (Figure 3),in partnership with STMicroelectronics at Crolles.

Materials for high-frequencyapplications

FeN and FeCoN-based nanocrystal filmsThe growing need for materials with high per-meability at increasingly high operating frequen-cies has prompted studies into new laminated mate-rials guaranteeing very-high-frequency performancewithout any major skin-effect limitations. The mainfactor driving the development of Fe and FeCo-based soft ferromagnetic films is related to the factthat they offer a 20% to 40% higher saturationmagnetisation, which can generate higher intrin-sic permeability at higher frequencies. FeXN filmsor more recently FeCoXN films (where X is prefe-rably tantalum or hafnium) have been shown topresent outstanding dynamic properties with excep-tionally low damping constants at several giga-hertz. Their highly unusual crystal microstructurecomposed of grains (< 5 nm) very finely disper-sed in an amorphous matrix enables these mate-rials to combine high resistivity (typically100 µΩ.cm) with high saturation magnetisation atover 1 tesla. This remarkable combination makesit technologically possible to integrate magneticmaterial right up close to the inductive elementwhile minimising the risk of stray capacitance thatcould undermine the high-frequency performan-ces of spiral inductors. Digital modelling of thegain in surface inductance density under theseconditions indicates record values of over 100%.

Ferromagnetic/antiferromagnetic multilayersInterface exchange coupling with an antiferro-magnetic material results in a shift in the hyste-resis loop of the ferromagnetic material towardsfield values that can be exceptionally high. This isthe property upon which modern magnetoresis-tive devices (spin valves, tunnel junctions, etc.)are built. The CEA has been able to show that thesematerials, which are built of successive layers ofNiMn, IrMn and FeCo films, present unmatcheddynamic behaviour up to frequencies in excess ofseveral tens of gigahertz. This makes them strongcandidates as alternative solutions to the materialsdescribed above. They offer unrivalled perspecti-ves for radiofrequency applications within a range

Figure 2. Static magnetic

configuration of an in-band domain layer (a),

showing the associatedmagnetic susceptibilities(b) and resonance peaks

(c). δhrf stands for the'microwave excitation

magnetic field' (where 'h'is the magnetic field, δ is

'weak', and rf is for'alternative').

Figure 3. Illustrations of AF/F/AF

(antiferromagnetic/ferromagnetic/antiferromagne

tic) stacking that can bereproduced in multiple

iterations, plus theassociated theoretical

and experimentalpermeability spectra.

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that has hitherto remained outside of the grasp offerromagnetic materials, i.e. at over ten gigahertz.Using these materials, it has been possible to pro-duce radiofrequency inductors in simpler formatthan spirals yet registering exceptional linear induc-tance density(2) and record-breaking operating fre-quencies. In these structures, the coupling betweenthe conductor strand and the magnetic material ismaximised. This layout format is currently beinginvestigated as a solution for producing miniatureresonating or radiating wires for integrated anten-nae and filters.

Paving the way to multiferroicmultilayers

This multilayer approach (ferromagnetic/anti-ferromagnetic), which can be described as hetero-structured, opens up a new and relatively unex-plored path towards other thin-layered materialsfor microwave applications. The goal is to artifi-cially combine different types of properties andwork with materials such as ferroelectric materials,piezoelectric materials, etc. This takes us into thefield of thin-layer heterostructured multiferroicmaterials, or more broadly speaking, microwave-tunable materials. Furthermore, the current trendtowards convergence between materials, compo-nents and systems-specific research disciplines hasconsiderably shortened the distance between phy-sicists and applications engineers, so much so thatdynamic physical material models can now be fac-tored into complex RF architectures early on in thedesign phase. A good example of this landmarkchange in our approach to research is the field ofopportunistic radio communication(3), where toughchallenges in terms of multifunctionality in trans-ceiver units and signal processing could only befaced through this type of revolution in researchculture. The net result today is that the field ofapplication of microwave magnetic layers has beenextended radically from RF inductors towards fre-quency-agile solutions, such as the coupling ofthin-layer magnetism and piezoelectricity. Anotherexample is the down-scaling of antennae followingthe down-scaling of inductors where ferromagne-tism was coupled with ferroelectricity at thin-layerlevels to make the breakthrough towards truly novelsolutions.

Application to frequency-tunable devicesThere has been a flurry of applications using wire-less technologies that operate with their own com-munications frequency standards. Current trendsin cost-reduction tend to cut down on the num-ber of components and move towards frequency-tunable systems. A new solution has been put for-ward, using a composite built from stackedferromagnetic films on polymers integrated into a

transmission line (Figure 4). The first step was tovalidate the concept by tuning the frequency, whichwas achieved by applying a static magnetic fieldwhich shifted the resonance frequency of the magne-tic layer. Nevertheless, the integration of a coil tocreate a control field was still incompatible withintegration into miniaturised circuits. A new solu-tion was therefore explored: to exploit a propertyknown as magnetostriction, i.e. how far a mate-rial's magnetic properties can cause it to changeshape. Deposits of ferromagnetic materials havebeen produced that present high magnetostrictionon piezoelectric substrates, whose change in phy-sical dimensions can be controlled simply byapplying an electric voltage. The first system hasgiven promising results, and its design will be opti-mised to increase its performance levels.Investigations are also being conducted into MEMStechnologies with the aim of implementing theprinciple at microsystem scale. A specially-desi-gned piezoelectric micro-actuator can be used tobetter control the constraints (high amplitude anduniaxiality). This makes it possible to fine-tune thedynamic properties of very soft (and therefore

Materials – the three familiesThe ferromagnetic thin layers used for microwave applications are gene-rally metal alloys that mainly incorporate nickel, cobalt and iron. Theyare generally given the 'soft magnetic' property required in order to obtainhigh permeabilities by 'knocking out' the effect of the magnetocrystal-line anisotropy constant(1), which is high in these heavy elements. Thereare currently three categories of ferromagnetic thin layers for microwaveapplications. The first group includes polycrystalline alloys like permalloy (FeNi), wherethe alloy is made soft by adjusting the content ratio of the two transitionmetals. Since the all-round performance of a layer is related to its per-meance (the product of its permeability and the film thickness), low-resis-tive crystalline layers present thin skin depths that curb the performanceof these materials. The second category is the amorphous alloys, which combine ferroma-gnetic transition metals with non-magnetic transition metals (Zr, Pt, Nb,Ta, and others) or metalloids (B, Si, etc.)(2) which guarantee that the layerwill maintain its amorphous structure when being fabricated by cathodesputtering(3).The third group features the nanocrystalline alloys often produced byannealing and growing grains of amorphous alloy, or by reactive depo-sition in order to produce nitrogenous or carbon FeN (Ta, Hf, etc.) or FeCcomposites. These materials are manufactured via a vacuum coating process, magne-tron sputtering(4). This flexible technique can be used to coat a wide rangeof polycrystalline, amorphous or nanocrystalline ferromagnetic work-pieces. The deposition parameters can be tailored to adjust the micro-wave magnetic properties. The magnetic layers can be deposited not onlyon silicon but also on glass or plastic substrates.

(1) Magnetocrystalline anisotropy constant: term used when talking about the energydensity of magnetocrystalline energy. Quantifies how magnetisation tends to alignitself in preferred crystallographic directions.

(2) Metalloids: elements with properties that are in between those of metals and non-metals. Most are semiconductors (boron, silicon, germanium, arsenic, antimony,tellurium and polonium).

(3) Cathode sputtering: forming thin layers by ejecting atoms from a target materialwhile bombarding it with ions from inert gases accelerated under high electric fields.

(4) Magnetron (cathode) sputtering: cathode sputtering employing a magnetron (a set of permanent magnets placed underneath the target) to increase the ion densitysurrounding the target. The magnetron effect makes it possible to keep the dischargegoing at lower pressure, thereby also giving better-quality sputtering.

(2) Linear inductance density: the value of the induction coils(expressed in nanohenries, nH) converted into unit length ofthe inductive element (in mm), i.e., in this case, nH⋅mm-1.

(3) Opportunistic radio: radio transmission system, whereinthe simplest possible radiocommunication hardware is able todynamically reconfigure itself digitally in order to process anykind of signal.



modestly magnetostrictive) magnetic layers withina broad tunability bracket and at low actuation vol-tages (several GHz for just a handful of volts). Theperformances offered by varactors(4) (CMOS orMEMS) have now been bettered by variable MEMs-based inductors(5), offering new perspectives forengineering agile 'band-pass' filters and tunableVCOs(6). The CEA's main partners on these researchthrusts are the University of Western Brittany

(Université de Bretagne occidentale, via the LEST,laboratory of electronics and telecommunicationssystems), the University of Limoges (XLIM researchcluster, combining mathematics, optics, electro-magnetism and electronics departments) and theUniversity of Rennes (IETR, institute of electro-nics and telecommunications).

> Sébastien DubourgMilitary Applications Division

CEA Le Ripault Centre> Bernard Viala

LETI InstituteTechnology Research Division

CEA Grenoble Centre

(4) CMOS varactors: variable capacitors using live circuits.

(5) Variable MEMS-based inductors: variable inductors thatemploy a mechanical actuator.

(6) VCO: voltage-controlled oscillators, using both inductorsand capacitors.

In the field of micron-scale-diameter fibers andwires, silicon fiber (or optical fiber), with the

ultrafast information transmission speeds it offers,has revolutionised our daily lives. Their metal-based counterparts do not share the same lime-light, but their properties, notably their magneticproperties, open up novel applications, especiallyfor detectors, as highlighted with the 'magneticbarcodes'. The CEA, through its Magnetic and OpticalMaterials laboratory (the 'MMO') at the Le Ripaultfacilities in the Indre-et-Loire, has since 1997 beenusing a novel process (box 1) to produce glass-shea-thed metal wire with a micron-scale diameter.

Wires 'quenched' to obtain an amorphousmaterial

The working principle is to take a material in liquidstate and spin out a wire through a glass sheath.This gives metallic wire of a diameter ranging from1 to 15 micrometers (µm), but whose length onthe reel can reach around 20 kilometres! Most appli-cations keep the glass cladding, often because itprovides additional functions (such as electricalinsulation or mechanical strength), but this glasscan be removed if need be, by processes like che-mical etching.

A wide range of different alloys can be transfor-med using this process. The laboratory's flagshipapplications run research on ferromagnetic alloys.Ingots of these ferromagnetic alloys are producedat the laboratory via a process called cold cruciblemelting (box 1). Since the wires are drawn out ata rate of around 10 m/s, the alloy undergoes rela-tively quick quenching, i.e. around 105 K/s. Forsome cobalt or iron-based alloy mixtures incor-porating between 15% and 25% metalloids (likeboron or silicon), the quenching process is suffi-

Ferromagnetic metal wires possess special properties that open up novel applications,especially for detectors. 'Magnetic barcodes' are just one example.

Ferromagnetic microwires

Scanning electron microscope (SEM) image showing a crosscutof a ferromagnetic wire in its glass sheathing.

CEA

Figure 4. Illustrations

of RF ferromagneticCMOS-compatible spiral

inductors operating at 0.9 to 2.4 GHz (a),

a co-planar RF conductorstrand fully encapsulated

in an AF/F/AF typematerial (b) a meander

inductor operating at 5 GHz and working on

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ciently violent to kinetically prevent the alloy frombecoming crystalline. The metal therefore stays inamorphous state, i.e. a metastable state characte-rised by an absence of crystal grains, which puri-fies the magnetic behaviour of structure ordering-related artefacts like grain boundaries, grain-sizedispersion, orientation, etc. The hysteresis loopthus observed can get phenomenally close to theo-retical hysteresis loops.

A 'magnetic barcode'

Under these ideal conditions, magnetic propertiesare controlled by an independent source – the geo-metric characteristics of the wires. This is wherethe glass sheathing comes in, to play a crucial and

highly remarkable role. It actually exerts mecha-nical stresses in the metal that are dependent onthe metal-to-glass surface ratio, resulting in magne-tic anisotropy. In other words, the magnetic energyrequired in order to magnetise the wire in a par-ticular direction (parallel to its axis, for example)is dependent on the intensity of the magnetoelas-tic coupling coefficient (sometimes called themagnetostriction coefficient) and the degree ofstresses being exerted in the metal. The figure showsthe hysteresis loop observed for a positive or nega-tive magnetostriction coefficient when a magne-tic field is applied parallel to the wire axis. In thefirst case, the hysteresis loop is rectangular, i.e. thewire is magnetised in its natural axis. This propertyis extremely advantageous for deploying detection

From the Taylor-Ulitovsky method to the cold crucible melting process

Far from the technological prowess usually required for micron-scale materials development, the Le Ripault centre has since 1997been using a simple yet novel metallurgical process for manu-facturing micron-scale metallic wires: the Taylor-Ulitovskymethod. Although the method was invented by Taylor back in 1925,it was comprehensively reworked and perfected in the 1960s byUlitovsky, who introduced induction heating into the process.The high surface tension of metals means that, like glass orpolymers, they cannot be drawn out from a liquid metal bathinto a narrow-diameter wire. In order to get round this stum-bling block, the metal melt bath is placed in a glass tube thatsoftens, and it is the glass-metal mixture that is drawn out toyield a perfectly cylindrical glass-coated metal wire that can runto several kilometres in length. The viscosity of glass, whichvaries strongly depending on the temperature of the melt metal,is a determinant factor in controlling the diameter of the metal-lic wire. The other determinant factor in the method is the speedat which the wire is drawn, which will determine the total dia-meter (metal plus glass sheath) of the wire. The glass sheath isbetween 1 and 10 μm thick. This method, which was regularlyused in the former Soviet Bloc to produce ultra-thin copper wirefor microcoils, was squeezed out from this application as it couldnot compete with the microelectronics technologies developedin the West. In 2001, the CEA's Le Ripault centre was equipped with facili-ties for producing metal alloys by cold crucible melting. The pro-cess essentially involves melting gauged proportions of pureelements to obtain a liquid alloy with the target composition.

The liquid metal is then cast into a cooled mould under a resi-dual argon atmosphere. The cold crucible resembles a bowlbuilt of 17 individual compartments made of cooled copper. Thereis an orifice at the bottom that is blocked by a mobile rod calledthe "cold finger". The crucible is set inside a solenoid inductorpowered by a HF aperiodic generator. Once the elements (Co, Fe, Si, B, or others) are placed in thecrucible, the induction causes the ferromagnetic metals to melt;the metalloids melt in turn as they come into contact with themolten ferromagnetic material. When a fully melted mixture isobtained, a ball of liquid alloy of no more than 30 cm3 is blen-ded inside the crucible. Magnetic levitation makes the ball float,keeping it away from any contact with the walls which, in conven-tional processes, are a significant source of pollution. Pullingthe cold finger away breaks the magnetic field lines, and theliquid metal runs into a cooled ingot mould, which comes in avariety of shapes. This simple, time-efficient method that gua-rantees excellent alloy purity has been used to research themagnetic properties of a whole range of compositions for ferro-magnetic microwires.

Alloy beingmanufactured in the coldcrucible.C

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Wire drawing using the Taylor-Ulitovsky method. The glass tube, inductorand hot filament (small red vertical line in the middle of the image) are clearly visible.

CEA

Amorphous ferromagnetic alloys have tradi-tionally been spun as ribbons via a techniquecalled melt spinning, which can produce a conti-nuous 20 μm-thick, 20 mm-wide ribbon.However, these alloys have only limited appli-cations (such as in power transformers) as theirproperties degrade sharply at temperaturebecause the alloy crystallises. However, in the late 1980s, Yoshizawa et al.(Journal of Applied Physics, 1988, 64, 6044)injected new impetus by presenting the 'FINE-MET' family - Fe-based, nanocrystallised metal-lic alloys with an Fe73.5Cu1Nb3Si13.5B9 compo-sition. Achieving the nanocrystallised staterequires heat treating the amorphous metal-lic ribbon at 600°C for 1 hour, which precipi-tates the crystalline phase α-(Fe-Si). Afterannealing, the microstructure of the materialexhibits two phases: a phase comprising crys-tallised centred cubic Fe-Si grains, embeddedin a ferromagnetic amorphous 'matrix' phasewith high Fe, Nb and B content. What makesthe composition so novel is the addition of cop-per and niobium. These elements will keep thegrain diameter down to 15-20 nm by promo-ting nucleation(1) while curbing the growth ofthe crystalline phase. A grain size of around ananometre is critical to obtaining the targetedmagnetic properties. This nanocrystallisation

increases the thermal stability of the magne-tic properties without losing the characteris-tics of conventional alloys. Building on theconcept pioneered by Yoshizawa, new familiesof nanocrystalline alloys have been developed,all of which are pushing towards higher satu-ration magnetisation combined with tempera-ture-resistant magnetic properties. One exam-ple is the FeMBCu family, where M = Zr, Nb orHf, the most widely-known alloy being NANO-PERM, which has an Fe88Zr7B4Cu1 composition.The choice of transition metals 'M' will hingeon their capacity to curb grain growth. The HIT-PERM family is a derivative of the NEOPERMsin which iron is replaced with cobalt, which hasthe effect of increasing both saturation magne-tisation and the Curie temperature of the amor-phous phase.Over the last few years, these families of nano-crystalline alloys have been fully integratedinto industrial spheres (Hitachi, Imphy,Magnatec, and others), where the materialsare employed in the manufacture of transfor-mers or magnetic sensors, or in magneticcoding systems.

(1) nucleation: clustering step where objects correctlyreorder as they grow.



application, such as product ID applications. Oneexample made possible is the 'magnetic barcode',an application that has in fact been patented by theCEA, which has gone on to license further deve-lopment to a start-up called Cryptic. The second type of hysteresis loop stems from amore complex spatial configuration of magnetisa-tion with circumferential magnetic domains. Thisset-up stimulates a magnetic permeability paral-

lel to the wires. This generates a wide range of appli-cations, particularly in radiofrequency compo-nents.

Temperature-driven high-frequencymagnetic properties

The MMO lab is a leading specialist in permeabi-lity in the microwave regime (i.e. at around a GHz).Their research is focused on gaining as much com-mand as possible over this permeability and impro-ving the performances rendered. One of the stum-bling blocks for example is the use of CoFeSiBalloys, since its permeability lacks in temperatureresistance (evolution of the metastable state, it hasa low Curie temperature, etc. These alloys are cur-rently under pressure from a new brand called"nanocrystalline" alloys (box 2). The CEA teamshave recently managed to apply this family of alloysto high-permeability microwave wires. The per-meability loss for these composite materials bet-ween room temperature and 350°C was only 30%,whereas the permeability of comparable amor-phous-material wires fell by over 80%. These recentdevelopments can therefore extend the potentialscope of application of the wires.

> Anne-Lise Adenot-Engelvin, Frédéric Bertin and Vincent Dubuget

Military Applications DivisionCEA Le Ripault Centre

Nanocrystalline alloys 2

Figure. Typical hysteresis loops (magnetisation according to the magnetic field applied) exhibited by ferromagnetic wires. Top: hysteresis loop of a wire with positive magnetostriction and associated domain structure. Bottom: hysteresis loop of a wire with negativemagnetostriction and associated domain structure.

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The origins of magnetism lie in theproperties of electrons as explained

by the laws of quantum physics. Part ofan electron's magnetic properties (spinmagnetism) results from its quantum-mechanical spin state, while another partresults from the orbital motion of elec-trons around an atom's nucleus (orbitalmagnetism) and from the magnetism ofthe nucleus itself (nuclear magnetism).This is put to use, in particular, for nuclearmagnetic resonance imaging in the medi-cal field. Magnetism is therefore produ-ced by electric charges in motion. Theforce acting on these charges, called theLorentz force, demonstrates the pre-sence of a magnetic field.Electrons have an intrinsic magneticdipole moment (the magnetic quantumstate being the Bohr magneton), whichcan be pictured as an electron's rotatio-nal motion of spin around itself in onedirection or another, oriented eitherupwards or downwards. The spin quan-tum number (one of the four numbers that'quantifies' the properties of an electron)equals 1/2 (+ 1/2 or - 1/2). A pair of elec-trons can only occupy the same orbital ifthey have opposite magnetic dipolemoments.Each atom acts like a tiny magnet car-rying an intrinsic magnetic dipolemoment. A nucleus (the neutron andproton individually have a half-integerspin) will have a half-integer spin if it hasan odd atomic mass number; zero spinif the atomic mass number and chargeare even, and an integer spin if the ato-mic mass number is even and the chargeodd.On a larger scale, several magneticmoments can together form magnetic

domains in which all these moments arealigned in the same direction. These spa-tial regions are separated by domainwalls. When grouped together, thesedomains can themselves form a macro-scopic-scale magnet (Figure E1). The type of magnetism that comes intoplay is determined by how these ele-mentary constituents are ordered, and isgenerally associated with three maincategories of material: ferromagnetic,paramagnetic and diamagnetic. Any material that is not diamagnetic isby definition paramagnetic provided thatits magnetic susceptibility is positive.However, ferromagnetic materials haveparticularly high magnetic susceptibilityand therefore form a separate category.1. Ferromagnetic materials are formedof tiny domains inside which atoms exhi-biting parallel magnetisation tend to alignthemselves in the direction of an exter-nal magnetic field like elementary dipo-les. In fact, the magnetic moments ofeach atom can align themselves sponta-neously within these domains, even inthe absence of an external magnetic field.Applying an external field triggers domainwall movement that tends to strengthenthe applied field. If this field exceeds acertain value, the domain most closelyoriented with the direction of the appliedfield will tend to grow at the expense ofthe other domains, eventually occupyingthe material's whole volume. If the fielddiminishes, the domain walls will move,but not symmetrically as the walls can-not fully reverse back to their originalpositions. This results in remanentmagnetisation, which is an important fea-ture of naturally occurring magnetite, orof magnets themselves.

The whole process forms a hysteresisloop, i.e. when the induced field is plot-ted against the applied field it traces outa hysteresis curve or loop where the sur-face area represents the amount ofenergy lost during the irreversible partof the process (Figure E2). In order tocancel out the induced field, a coercivefield has to be applied: the materials usedto make artificial permanent magnetshave a high coercivity. Ferromagnetic materials generally havea zero total magnetic moment as thedomains are all oriented in different direc-tions. This ferromagnetism disappearsabove a certain temperature, which isknown as the Curie Temperature or Curiepoint.The magnetic properties of a given mate-rial stem from the way the electrons inthe metallic cores of a material or of atransition metal complex collectively cou-ple their spins as this results in all theirspin moments being aligned in the samedirection. Materials whose atoms are widely dis-tributed throughout their crystal struc-ture tend to better align these elemen-tary magnets via a coupling effect. Thiscategory of materials, which is charac-terised by a very high positive magnetic

The different types of magnetismAFOCUS

Figure E2. The induction B of a magnetic material by a coilis not proportional to its magnetic excitation(field H). While the initial magnetisation formsan OsS-type curve, shown in blue in the figure,it reaches saturation at point s. Only a partialinduction is retained if the field approacheszero; this remanent induction can only becancelled out by reversing the magnetic field to a "coercive" field value. This hysteresis loopillustrates the losses due to "friction" betweenthe magnetic domains shown on the areabounded by the magnetisation anddemagnetisation curves.

Figure E1.Intrinsic magnetic dipole moments have parallel alignment in ferromagnetic materials (a), anti-parallel alignment but zero magnetisation in antiferromagnetic materials (b), and anti-parallelalignment with unequal moments in ferrimagnetic materials (c).

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susceptibility, includes iron, cobalt andnickel and their alloys, steels in particu-lar, and some of their compounds, and, toa lesser extent, some rare earth metalsand alloys with large crystal lattices, andcertain combinations of elements that donot themselves belong to this category. Inferrimagnetic materials, the magneticdomains group into an anti-parallel align-ment but retain a non-zero magneticmoment even in the absence of an exter-nal field. Examples include magnetite,ilmenite and iron oxides. Ferrimagnetismis a feature of materials containing twotypes of atoms that behave as tiny magnetswith magnetic moments of unequal magni-tude and anti-parallel alignment. Anti-ferromagnetism occurs when the sum ofa material's parallel and anti-parallelmoments is zero (e.g. chromium or hae-matite). In fact, when atoms are in a closeconfiguration, the most stable magneticarrangement is an anti-parallel alignmentas each magnet balances out its neigh-bour so to speak (Figure E1). 2. Paramagnetic materials behave in asimilar way to ferromagnetic materials,although to a far lesser degree (they havea positive but very weak magnetic sus-ceptibility of around 10- 3). Each atom in aparamagnetic material has a non-zeromagnetic moment. In the presence of anexternal magnetic field, the magneticmoments align up, thus amplifying thisfield. However, this effect decreases astemperature rises since the thermal agi-tation disrupts the alignment of the ele-mentary dipoles. Paramagnetic materialslose their magnetisation as soon as theyare released from the magnetic field. Mostmetals, including alloys comprising ferro-magnetic elements are paramagnetic, as

are certain minerals such as pegmatite. 3. Diamagnetic materials exhibit a nega-tive and an extremely weak magnetic sus-ceptibility of around 10- 5. The magnetisa-tion induced by a magnetic field acts in theopposite direction to this field and tendsto head away from field lines towards areasof lower field strengths. A perfect diama-gnetic material would offer maximumresistance to an external magnetic fieldand exhibit zero permeability. Metals suchas silver, gold, copper, mercury or lead,plus quartz, graphite, the noble gases andthe majority of organic compounds are alldiamagnetic materials. In fact, all materials exhibit diamagneticproperties to a greater or lesser extent,resulting from changes in the orbitalmotion of electrons around atoms inresponse to an external magnetic field, aneffect that disappears once the externalfield is removed. As Michael Faraday sho-wed all that time ago, all substances canbe "magnetised" to a greater or lesserdegree provided that they are placed withina sufficiently intense magnetic field.

ElectromagnetismIt was the Danish physicist Hans ChristianØrsted, professor at the University ofCopenhagen, who, in 1820, was first to dis-cover the relationship between the hithertoseparate fields of electricity and magne-tism. Ørsted showed that a compass needlewas deflected when an electric currentpassed through a wire, before Faraday hadformulated the physical law that carrieshis name: the magnetic field produced isproportional to the intensity of the current.Magnetostatics is the study of staticmagnetic fields, i.e. fields which do notvary with time.

Magnetic and electric fields together formthe two components of electromagnetism.Electromagnetic waves can move freelythrough space, and also through mostmaterials at pretty much every frequencyband (radio waves, microwaves, infrared,visible light, ultraviolet light, X-rays andgamma rays). Electromagnetic fields the-refore combine electric and magnetic forcefields that may be natural (the Earth'smagnetic field) or man-made (low fre-quencies such as electric power trans-mission lines and cables, or higher fre-quencies such as radio waves (includingcell phones) or television.Mathematically speaking, the basic lawsof electromagnetism can be summarisedin the four Maxwell equations (or Maxwell-Lorentz equations) which can be used toprovide a coherent description of all elec-tromagnetic phenomena from electrosta-tics and magnetostatics to electromagne-tic wave propagation. James Clerk Maxwellset out these laws in 1873, thirty-two yearsbefore Albert Einstein incorporated thetheory of electromagnetism in his specialtheory of relativity, which explained theincompatibilities with the laws of classi-cal physics.

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A Transrapid train using magnetic levitation arriving at the Long Yang bus station in Shanghai (China).This German-built high-speed, monorail train was commissioned in 2004 to service the rail link

to Pudong international airport.

Close-up of the magnets used to guide and power the train.

microwave magnetic materials: from ferrites to metamaterials · two other systems – thin layers...

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