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 electromagnetism 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 related 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 certain number of applications. The more recently developed metamaterials represent a novel approach for obtaining microwave magnetic properties, and they are opening up extraordinary perspectives. Although they cannot fully replace conventional magnetic materials, 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 inductors for mobile phones, or anti-theft marking systems. Thin layer engineering has employed newly- developed homogenous or composite materials to tailor the properties of the layers to individual applications. 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 frequencies below the frequencies of the main peak. This Overcoming dimensional limitations has been observed in polycrystalline alloy micro- The ferromagnetic layers being built are designed structures like NiFe alloys for the polar parts of to be incorporated into very small devices where read heads or micro-inductor cores where the walls the material close to the film edges no longer has that form side-closing domains (at 90° to the axis a negligible effect in relation to the material at the of magnetisation) enter into resonance well before core of the system. This makes it important to cha- the main gyromagnetic phenomenon occurs. racterise the response of the film in these poten- Amorphous alloys like CoZr or CoFeSiB also adopt tially disturbed zones whose magnetic properties a strongly non-homogenous "needle-shaped" struc- deviate from the gyromagnetic behaviour of a thin ture formed of interlaced alternating triangular- layer of supposedly infinite dimensions. Thus, in shaped domains with antiparallel magnetisation order to keep its self-energy to a minimum, micro- (Figure 1). In both cases, as CEA research has shown, metric layers adopt magnetic domain-based struc- the number and position of these 'secondary peaks' tures that, depending on the conditions of excita- is a function of the thickness deposited and the tion, will induce an overall drop in energy levels main magnetic characteristics of the layer (magnetisation, anisotropy and exchange constant). (1) Band-pass filter: a device that only allows electromagnetic waves within a certain range bracket to pass through, based on A further dimensional limitation is the thickness their wavelength. deposited. At below the skin depth, modifications

0.2 μm 0.5 μm

0 1000 2000 0 1000 2000 200 μm 200 μm

0.95 μm 1.9 μm permeability μ Figure 1. Kerr microscope images of needle-shaped domains and associated permeability spectra for CoZr layers of increasing 0 1,000 2,000 0 1,000 2,000 thickness, running from 200 μm frequency (MHz) 200 μm 0.2 μm to 1.9 μm.

CLEFS CEA - No.56 - WINTER 2007-2008 21 Magnets and magnetic materials

1 y 20 a c x 10 z ␦h rf 0 1 4 y ␦h rf b Figure 2. 2 Static magnetic x configuration of an 1 2 2 z in-band domain layer (a), 0 y showing the associated 4 3 magnetic susceptibilities ␦h rf (b) and resonance peaks x δ imaginary permeability 2 (c). hrf stands for the 3 4 z 5 'microwave excitation 4 magnetic field' (where 'h' 0 is the magnetic field, δ is 0102030 'weak', and rf is for frequency (GHz) 'alternative').

in the layer's magnetic structure can completely Materials for high-frequency lose the uniform gyromagnetic state being targe- applications ted. In fact, during the deposition process, porous- ness gradient or constraint effects can drive the FeN and FeCoN-based nanocrystal films development of anisotropy perpendicular to the The growing need for materials with high per- layer and lead the layer to take on a magnetic struc- meability at increasingly high operating frequen- ture called 'in-band domains' where a layer magne- cies has prompted studies into new laminated mate- tisation component is out of plane. The microwave rials guaranteeing very-high-frequency performance response of this structure is highly chaotic, com- without any major skin-effect limitations. The main prising a series of extremely narrow peaks. This is factor driving the development of Fe and FeCo- a phenomenon studied under a project led in tan- based soft ferromagnetic films is related to the fact dem with Dassault Aviation. A digital approach that they offer a 20% to 40% higher saturation was employed to account for these spectra, with magnetisation, which can generate higher intrin- excellent agreement between theoretical and expe- sic permeability at higher frequencies. FeXN films rimental results (Figure 2). or more recently FeCoXN films (where X is prefe- rably tantalum or hafnium) have been shown to Application to planar inductors present outstanding dynamic properties with excep- One of the heaviest constraints to miniaturising tionally low damping constants at several giga- RF circuits (CMOS or BiCMOS) is the integration hertz. Their highly unusual crystal microstructure of the inductors which are one of the least space- composed of grains (< 5 nm) very finely disper- efficient of the passive components. A highly pro- sed in an amorphous matrix enables these mate- mising way forward is to integrate high-permea- rials to combine high resistivity (typically bility magnetic films on silicon. The CEA-Leti has 100 µΩ.cm) with high saturation magnetisation at integrated amorphous CoZr and FeCoSiB films over 1 tesla. This remarkable combination makes deposited at the Le Ripault centre into a high-qua- it technologically possible to integrate magnetic lity-factor RF inductor system. This research made material right up close to the inductive element the breakthrough discovery of a potential effective while minimising the risk of stray capacitance that Figure 3. reduction of around 10% to 15% in the footprint could undermine the high-frequency performan- Illustrations of AF/F/AF of the induction coils without losing electrical pro- ces of spiral inductors. Digital modelling of the (antiferromagnetic/ferro magnetic/antiferromagne perties. This pioneering research has paved the way gain in surface inductance density under these tic) stacking that can be to the more targeted development of magnetic films conditions indicates record values of over 100%. reproduced in multiple for RF applications using more conventional depo- iterations, plus the associated theoretical sition techniques for microelectronics (Figure 3), Ferromagnetic/antiferromagnetic multilayers and experimental in partnership with STMicroelectronics at Crolles. Interface exchange coupling with an antiferro- permeability spectra. magnetic material results in a shift in the hysteresis loop of the ferromagnetic material towards field values that can be exceptionally high. This is 150 μ’ measured the property upon which modern magnetoresis- 100 μ’ theoretical tive devices (spin valves, tunnel junctions, etc.) NiCr μ’’measured NiMn 50 μ’’theoretical are built. The CEA has been able to show that these CoFe materials, which are built of successive layers of 0 NiMn, IrMn and FeCo films, present unmatched NiMn NiCr -50 dynamic behaviour up to frequencies in excess of 20 nm Si02 permeability relative -100 several tens of gigahertz. This makes them strong 0 51015 20 25 30

(real part μ’ and imaginary μ’’) (real candidates as alternative solutions to the materials frequency (GHz) described above. They offer unrivalled perspectives for radiofrequency applications within a range

22 CLEFS CEA - No.56 - WINTER 2007-2008 that has hitherto remained outside of the grasp of ferromagnetic materials, i.e. at over ten gigahertz. Materials – the three families Using these materials, it has been possible to produce radiofrequency inductors in simpler format The ferromagnetic thin layers used for microwave applications are gene- than spirals yet registering exceptional linear induc- rally metal alloys that mainly incorporate nickel, cobalt and iron. They tance density(2) and record-breaking operating fre- are generally given the 'soft magnetic' property required in order to obtain quencies. In these structures, the coupling between high permeabilities by 'knocking out' the effect of the magnetocrystal- the conductor strand and the magnetic material is line anisotropy constant(1), which is high in these heavy elements. There maximised. This layout format is currently being are currently three categories of ferromagnetic thin layers for microwave investigated as a solution for producing miniature applications. The first group includes polycrystalline alloys like permalloy (FeNi), where resonating or radiating wires for integrated anten- the alloy is made soft by adjusting the content ratio of the two transition nae and filters. metals. 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- Paving the way to multiferroic tive crystalline layers present thin skin depths that curb the performance multilayers of these materials. The second category is the amorphous alloys, which combine ferroma- This multilayer approach (ferromagnetic/anti- gnetic transition metals with non-magnetic transition metals (Zr, Pt, Nb, ferromagnetic), which can be described as hetero- Ta, and others) or metalloids (B, Si, etc.)(2) which guarantee that the layer structured, opens up a new and relatively unex- will maintain its amorphous structure when being fabricated by cathode (3) plored path towards other thin-layered materials sputtering . The third group features the nanocrystalline alloys often produced by for microwave applications. The goal is to artifi- annealing and growing grains of amorphous alloy, or by reactive depo- cially combine different types of properties and sition in order to produce nitrogenous or carbon FeN (Ta, Hf, etc.) or FeC work with materials such as ferroelectric materials, composites. piezoelectric materials, etc. This takes us into the These materials are manufactured via a vacuum coating process, magne- field of thin-layer heterostructured multiferroic tron sputtering(4). This flexible technique can be used to coat a wide range materials, or more broadly speaking, microwave- of polycrystalline, amorphous or nanocrystalline ferromagnetic work- tunable materials. Furthermore, the current trend pieces. The deposition parameters can be tailored to adjust the micro- towards convergence between materials, compo- wave magnetic properties. The magnetic layers can be deposited not only nents and systems-specific research disciplines has on silicon but also on glass or plastic substrates. considerably shortened the distance between physicists and applications engineers, so much so that (1) Magnetocrystalline anisotropy constant: term used when talking about the energy density of magnetocrystalline energy. Quantifies how magnetisation tends to align dynamic physical material models can now be fac- itself in preferred crystallographic directions. tored into complex RF architectures early on in the (2) Metalloids: elements with properties that are in between those of metals and design phase. A good example of this landmark non-metals. Most are semiconductors (boron, silicon, germanium, arsenic, antimony, change in our approach to research is the field of tellurium and polonium). opportunistic radio communication(3), where tough (3) Cathode sputtering: forming thin layers by ejecting atoms from a target material challenges in terms of multifunctionality in trans- while bombarding it with ions from inert gases accelerated under high electric fields. ceiver units and signal processing could only be (4) Magnetron (cathode) sputtering: cathode sputtering employing a magnetron (a set of permanent magnets placed underneath the target) to increase the ion density faced through this type of revolution in research surrounding the target. The magnetron effect makes it possible to keep the discharge culture. The net result today is that the field of going at lower pressure, thereby also giving better-quality sputtering. application of microwave magnetic layers has been extended radically from RF inductors towards frequency-agile solutions, such as the coupling of transmission line (Figure 4). The first step was to thin-layer magnetism and piezoelectricity. Another validate the concept by tuning the frequency, which example is the down-scaling of antennae following was achieved by applying a static magnetic field the down-scaling of inductors where ferromagne- which shifted the resonance frequency of the magnetism was coupled with ferroelectricity at thin-layer tic layer. Nevertheless, the integration of a coil to levels to make the breakthrough towards truly novel create a control field was still incompatible with solutions. integration into miniaturised circuits. A new solution was therefore explored: to exploit a property Application to frequency-tunable devices known as magnetostriction, i.e. how far a mate- There has been a flurry of applications using wire- rial's magnetic properties can cause it to change less technologies that operate with their own com- shape. Deposits of ferromagnetic materials have munications frequency standards. Current trends been produced that present high magnetostriction in cost-reduction tend to cut down on the num- on piezoelectric substrates, whose change in phy- ber of components and move towards frequency- sical dimensions can be controlled simply by tunable systems. A new solution has been put for- applying an electric voltage. The first system has ward, using a composite built from stacked given promising results, and its design will be opti- ferromagnetic films on polymers integrated into a mised to increase its performance levels. Investigations are also being conducted into MEMS (2) Linear inductance density: the value of the induction coils technologies with the aim of implementing the (expressed in nanohenries, nH) converted into unit length of principle at microsystem scale. A specially-desi- ⋅ -1 the inductive element (in mm), i.e., in this case, nH mm . gned piezoelectric micro-actuator can be used to (3) Opportunistic radio: radio transmission system, wherein better control the constraints (high amplitude and the simplest possible radiocommunication hardware is able to dynamically reconfigure itself digitally in order to process any uniaxiality). This makes it possible to fine-tune the kind of signal. dynamic properties of very soft (and therefore

CLEFS CEA - No.56 - WINTER 2007-2008 23 Magnets and magnetic materials

Figure 4. Illustrations of RF ferromagnetic CMOS-compatible spiral inductors operating at 0.9 to 2.4 GHz (a), b c a co-planar RF conductor strand fully encapsulated in an AF/F/AF type material (b) a meander inductor operating 1 mm at 5 GHz and working on this principle (c).

modestly magnetostrictive) magnetic layers within (Université de Bretagne occidentale, via the LEST, a broad tunability bracket and at low actuation vol- laboratory of electronics and telecommunications tages (several GHz for just a handful of volts). The systems), the University of Limoges (XLIM research performances offered by varactors(4) (CMOS or cluster, combining mathematics, optics, electro- MEMS) have now been bettered by variable MEMs- magnetism and electronics departments) and the based inductors(5), offering new perspectives for University of Rennes (IETR, institute of electro- engineering agile 'band-pass' filters and tunable nics and telecommunications). VCOs(6). The CEA's main partners on these research thrusts are the University of Western Brittany > Sébastien Dubourg Military Applications Division CEA Le Ripault Centre (4) CMOS varactors: variable capacitors using live circuits. > Bernard Viala (5) Variable MEMS-based inductors: variable inductors that employ a mechanical actuator. LETI Institute (6) VCO: voltage-controlled oscillators, using both inductors Technology Research Division and capacitors. CEA Grenoble Centre

Ferromagnetic microwires Ferromagnetic metal wires possess special properties that open up novel applications, especially for detectors. 'Magnetic barcodes' are just one example. n the field of micron-scale-diameter fibers and Iwires, 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 magnetic properties, open up novel applications, especially for detectors, as highlighted with the 'magnetic barcodes'. The CEA, through its Magnetic and Optical Materials laboratory (the 'MMO') at the Le Ripault facilities in the Indre-et-Loire, has since 1997 been using a novel process (box 1) to produce glass-shea- 5 μm

thed metal wire with a micron-scale diameter. CEA Scanning electron microscope (SEM) image showing a crosscut Wires 'quenched' to obtain an amorphous of a ferromagnetic wire in its glass sheathing. material A wide range of different alloys can be transfor- The working principle is to take a material in liquid med using this process. The laboratory's flagship state and spin out a wire through a glass sheath. applications run research on ferromagnetic alloys. This gives metallic wire of a diameter ranging from Ingots of these ferromagnetic alloys are produced 1 to 15 micrometers (µm), but whose length on at the laboratory via a process called cold crucible the reel can reach around 20 kilometres! Most appli- melting (box 1). Since the wires are drawn out at cations keep the glass cladding, often because it a rate of around 10 m/s, the alloy undergoes rela- provides additional functions (such as electrical tively quick quenching, i.e. around 105 K/s. For insulation or mechanical strength), but this glass some cobalt or iron-based alloy mixtures incor- can be removed if need be, by processes like che- porating between 15% and 25% metalloids (like mical etching. boron or silicon), the quenching process is suffi-

24 CLEFS CEA - No.56 - WINTER 2007-2008 From the Taylor-Ulitovsky method to the cold crucible melting process 1 CEA

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

Far from the technological prowess usually required for micron- scale materials development, the Le Ripault centre has since 1997 been using a simple yet novel metallurgical process for manu- facturing micron-scale metallic wires: the Taylor-Ulitovsky method. Although the method was invented by Taylor back in 1925, Alloy being manufactured it was comprehensively reworked and perfected in the 1960s by in the cold

Ulitovsky, who introduced induction heating into the process. CEA crucible. The high surface tension of metals means that, like glass or polymers, they cannot be drawn out from a liquid metal bath The liquid metal is then cast into a cooled mould under a resi- into a narrow-diameter wire. In order to get round this stum- dual argon atmosphere. The cold crucible resembles a bowl bling block, the metal melt bath is placed in a glass tube that built of 17 individual compartments made of cooled copper. There softens, and it is the glass-metal mixture that is drawn out to is an orifice at the bottom that is blocked by a mobile rod called yield a perfectly cylindrical glass-coated metal wire that can run the "cold finger". The crucible is set inside a solenoid inductor to several kilometres in length. The viscosity of glass, which powered by a HF aperiodic generator. varies strongly depending on the temperature of the melt metal, Once the elements (Co, Fe, Si, B, or others) are placed in the is a determinant factor in controlling the diameter of the metal- crucible, the induction causes the ferromagnetic metals to melt; lic wire. The other determinant factor in the method is the speed the metalloids melt in turn as they come into contact with the at which the wire is drawn, which will determine the total dia- molten ferromagnetic material. When a fully melted mixture is meter (metal plus glass sheath) of the wire. The glass sheath is obtained, a ball of liquid alloy of no more than 30 cm3 is blen- between 1 and 10 μm thick. This method, which was regularly ded inside the crucible. Magnetic levitation makes the ball float, used in the former Soviet Bloc to produce ultra-thin copper wire keeping it away from any contact with the walls which, in conven- for microcoils, was squeezed out from this application as it could tional processes, are a significant source of pollution. Pulling not compete with the microelectronics technologies developed the cold finger away breaks the magnetic field lines, and the in the West. liquid metal runs into a cooled ingot mould, which comes in a In 2001, the CEA's Le Ripault centre was equipped with facili- variety of shapes. This simple, time-efficient method that gua- ties for producing metal alloys by cold crucible melting. The pro- rantees excellent alloy purity has been used to research the cess essentially involves melting gauged proportions of pure magnetic properties of a whole range of compositions for ferro- elements to obtain a liquid alloy with the target composition. magnetic microwires.

ciently violent to kinetically prevent the alloy from highly remarkable role. It actually exerts mecha- becoming crystalline. The metal therefore stays in nical stresses in the metal that are dependent on amorphous state, i.e. a metastable state characte- the metal-to-glass surface ratio, resulting in magne- rised by an absence of crystal grains, which puri- tic anisotropy. In other words, the magnetic energy fies the magnetic behaviour of structure ordering- required in order to magnetise the wire in a par- related artefacts like grain boundaries, grain-size ticular direction (parallel to its axis, for example) dispersion, orientation, etc. The hysteresis loop is dependent on the intensity of the magnetoelas- thus observed can get phenomenally close to theo- tic coupling coefficient (sometimes called the retical hysteresis loops. magnetostriction coefficient) and the degree of stresses being exerted in the metal. The figure shows A 'magnetic barcode' the hysteresis loop observed for a positive or negative magnetostriction coefficient when a magne- Under these ideal conditions, magnetic properties tic field is applied parallel to the wire axis. In the are controlled by an independent source – the geo- first case, the hysteresis loop is rectangular, i.e. the metric characteristics of the wires. This is where wire is magnetised in its natural axis. This property the glass sheathing comes in, to play a crucial and is extremely advantageous for deploying detection

CLEFS CEA - No.56 - WINTER 2007-2008 25 Magnets and magnetic materials

Nanocrystalline alloys 2

Amorphous ferromagnetic alloys have tradi- increases the thermal stability of the magne- tionally been spun as ribbons via a technique tic properties without losing the characteris- called melt spinning, which can produce a conti- tics of conventional alloys. Building on the nuous 20 μm-thick, 20 mm-wide ribbon. concept pioneered by Yoshizawa, new families However, these alloys have only limited appli- of nanocrystalline alloys have been developed, cations (such as in power transformers) as their all of which are pushing towards higher satu- properties degrade sharply at temperature ration magnetisation combined with tempera- because the alloy crystallises. ture-resistant magnetic properties. One exam- However, in the late 1980s, Yoshizawa et al. ple is the FeMBCu family, where M = Zr, Nb or (Journal of Applied Physics, 1988, 64, 6044) Hf, the most widely-known alloy being NANO- injected new impetus by presenting the 'FINE- PERM, which has an Fe88Zr7B4Cu1 composition. MET' family - Fe-based, nanocrystallised metal- The choice of transition metals 'M' will hinge lic alloys with an Fe73.5Cu1Nb3Si13.5B9 compo- on their capacity to curb grain growth. The HIT- sition. Achieving the nanocrystallised state PERM family is a derivative of the NEOPERMs requires heat treating the amorphous metal- in which iron is replaced with cobalt, which has lic ribbon at 600°C for 1 hour, which precipi- the effect of increasing both saturation magne- tates the crystalline phase α-(Fe-Si). After tisation and the Curie temperature of the amor- annealing, the microstructure of the material phous phase. exhibits two phases: a phase comprising crys- Over the last few years, these families of nano- tallised centred cubic Fe-Si grains, embedded crystalline alloys have been fully integrated in a ferromagnetic amorphous 'matrix' phase into industrial spheres (Hitachi, Imphy, with high Fe, Nb and B content. What makes Magnatec, and others), where the materials the composition so novel is the addition of cop- are employed in the manufacture of transfor- per and niobium. These elements will keep the mers or magnetic sensors, or in magnetic grain diameter down to 15-20 nm by promo- coding systems. ting nucleation(1) while curbing the growth of the crystalline phase. A grain size of around a nanometre is critical to obtaining the targeted (1) nucleation: clustering step where objects correctly magnetic properties. This nanocrystallisation reorder as they grow.

1.5 lel to the wires. This generates a wide range of applications, particularly in radiofrequency compo- 1 nents. 0.5 Temperature-driven high-frequency 0 -10 -8 -6 -4 -2 0 246810 magnetic properties -0.5 The MMO lab is a leading specialist in permeabi- -1 1.5 lity in the microwave regime (i.e. at around a GHz). -1.5 Their research is focused on gaining as much com- 1 mand as possible over this permeability and impro-

0.5 ving the performances rendered. One of the stum- bling blocks for example is the use of CoFeSiB 0 -10 -8 -6 -4 -2 0 2 4 6 8 10 alloys, since its permeability lacks in temperature -0.5 resistance (evolution of the metastable state, it has a low Curie temperature, etc. These alloys are cur- -1 rently under pressure from a new brand called -1.5 "nanocrystalline" alloys (box 2). The CEA teams have recently managed to apply this family of alloys to high-permeability microwave wires. The per- Figure. Typical hysteresis loops (magnetisation according to the magnetic field applied) exhibited meability loss for these composite materials bet- by ferromagnetic wires. Top: hysteresis loop of a wire with positive magnetostriction ween room temperature and 350°C was only 30%, and associated domain structure. Bottom: hysteresis loop of a wire with negative whereas the permeability of comparable amor- magnetostriction and associated domain structure. phous-material wires fell by over 80%. These recent developments can therefore extend the potential application, such as product ID applications. One scope of application of the wires. example made possible is the 'magnetic barcode', an application that has in fact been patented by the > Anne-Lise Adenot-Engelvin, CEA, which has gone on to license further deve- Frédéric Bertin and Vincent Dubuget lopment to a start-up called Cryptic. Military Applications Division The second type of hysteresis loop stems from a CEA Le Ripault Centre more complex spatial configuration of magnetisation with circumferential magnetic domains. This set-up stimulates a magnetic permeability paral-

26 CLEFS CEA - No.56 - WINTER 2007-2008 FOCUS A The different types of magnetism he origins of magnetism lie in the domains in which all these moments are Tproperties of electrons as explained aligned in the same direction. These spa- B by the laws of quantum physics. Part of tial regions are separated by domain S an electron's magnetic properties (spin walls. When grouped together, these s magnetism) results from its quantum- domains can themselves form a macro- +BR mechanical spin state, while another part scopic-scale magnet (Figure E1). results from the orbital motion of elec- The type of magnetism that comes into trons around an atom's nucleus (orbital play is determined by how these ele- -HS -HC magnetism) and from the magnetism of mentary constituents are ordered, and is O +HC +HS H the nucleus itself (nuclear magnetism). generally associated with three main This is put to use, in particular, for nuclear categories of material: ferromagnetic, -BR magnetic resonance imaging in the medi- paramagnetic and diamagnetic. cal field. Magnetism is therefore produ- Any material that is not diamagnetic is S’ ced by electric charges in motion. The by definition paramagnetic provided that force acting on these charges, called the its magnetic susceptibility is positive. Lorentz force, demonstrates the pre- However, ferromagnetic materials have Figure E2. sence of a magnetic field. particularly high magnetic susceptibility The induction B of a magnetic material by a coil is not proportional to its magnetic excitation Electrons have an intrinsic magnetic and therefore form a separate category. (field H). While the initial magnetisation forms dipole moment (the magnetic quantum 1. Ferromagnetic materials are formed an OsS-type curve, shown in blue in the figure, state being the Bohr magneton), which of tiny domains inside which atoms exhi- it reaches saturation at point s. Only a partial induction is retained if the field approaches can be pictured as an electron's rotatio- biting parallel magnetisation tend to align zero; this remanent induction can only be nal motion of spin around itself in one themselves in the direction of an exter- cancelled out by reversing the magnetic field direction or another, oriented either nal magnetic field like elementary dipo- to a "coercive" field value. This hysteresis loop upwards or downwards. The spin quan- les. In fact, the magnetic moments of illustrates the losses due to "friction" between the magnetic domains shown on the area tum number (one of the four numbers that each atom can align themselves sponta- bounded by the magnetisation and 'quantifies' the properties of an electron) neously within these domains, even in demagnetisation curves. equals 1/2 (+ 1/2 or - 1/2). A pair of elec- the absence of an external magnetic field. trons can only occupy the same orbital if Applying an external field triggers domain The whole process forms a hysteresis they have opposite magnetic dipole wall movement that tends to strengthen loop, i.e. when the induced field is plot- moments. the applied field. If this field exceeds a ted against the applied field it traces out Each atom acts like a tiny magnet car- certain value, the domain most closely a hysteresis curve or loop where the sur- rying an intrinsic magnetic dipole oriented with the direction of the applied face area represents the amount of moment. A nucleus (the neutron and field will tend to grow at the expense of energy lost during the irreversible part proton individually have a half-integer the other domains, eventually occupying of the process (Figure E2). In order to spin) will have a half-integer spin if it has the material's whole volume. If the field cancel out the induced field, a coercive an odd atomic mass number; zero spin diminishes, the domain walls will move, field has to be applied: the materials used if the atomic mass number and charge but not symmetrically as the walls can- to make artificial permanent magnets are even, and an integer spin if the ato- not fully reverse back to their original have a high coercivity. mic mass number is even and the charge positions. This results in remanent Ferromagnetic materials generally have odd. magnetisation, which is an important fea- a zero total magnetic moment as the On a larger scale, several magnetic ture of naturally occurring magnetite, or domains are all oriented in different direc- moments can together form magnetic of magnets themselves. tions. This ferromagnetism disappears above a certain temperature, which is known as the Curie Temperature or Curie a b c point. The magnetic properties of a given material stem from the way the electrons in the metallic cores of a material or of a transition metal complex collectively cou- ple their spins as this results in all their spin moments being aligned in the same direction. Materials whose atoms are widely dis- tributed throughout their crystal structure tend to better align these elemen- Figure E1. tary magnets via a coupling effect. This Intrinsic magnetic dipole moments have parallel alignment in ferromagnetic materials (a), anti-parallel alignment but zero magnetisation in antiferromagnetic materials (b), and anti-parallel category of materials, which is charac- alignment with unequal moments in ferrimagnetic materials (c). terised by a very high positive magnetic Stoiber Productions, München Productions, Stoiber

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. susceptibility, includes iron, cobalt and are certain minerals such as pegmatite. nickel and their alloys, steels in particu- 3. Diamagnetic materials exhibit a nega- lar, and some of their compounds, and, to tive and an extremely weak magnetic sus- a lesser extent, some rare earth metals ceptibility of around 10- 5. The magnetisa- and alloys with large crystal lattices, and tion induced by a magnetic field acts in the certain combinations of elements that do opposite direction to this field and tends not themselves belong to this category. In to head away from field lines towards areas ferrimagnetic materials, the magnetic of lower field strengths. A perfect diama- domains group into an anti-parallel align- gnetic material would offer maximum ment but retain a non-zero magnetic resistance to an external magnetic field moment even in the absence of an exter- and exhibit zero permeability. Metals such Close-up of the magnets used to guide nal field. Examples include magnetite, as silver, gold, copper, mercury or lead, and power the train. ilmenite and iron oxides. Ferrimagnetism plus quartz, graphite, the noble gases and is a feature of materials containing two the majority of organic compounds are all Magnetic and electric fields together form types of atoms that behave as tiny magnets diamagnetic materials. the two components of electromagnetism. with magnetic moments of unequal magni- In fact, all materials exhibit diamagnetic Electromagnetic waves can move freely tude and anti-parallel alignment. Anti- properties to a greater or lesser extent, through space, and also through most ferromagnetism occurs when the sum of resulting from changes in the orbital materials at pretty much every frequency a material's parallel and anti-parallel motion of electrons around atoms in band (radio waves, microwaves, infrared, moments is zero (e.g. chromium or hae- response to an external magnetic field, an visible light, ultraviolet light, X-rays and matite). In fact, when atoms are in a close effect that disappears once the external gamma rays). Electromagnetic fields the- configuration, the most stable magnetic field is removed. As Michael Faraday sho- refore combine electric and magnetic force arrangement is an anti-parallel alignment wed all that time ago, all substances can fields that may be natural (the Earth's as each magnet balances out its neigh- be "magnetised" to a greater or lesser magnetic field) or man-made (low fre- bour so to speak (Figure E1). degree provided that they are placed within quencies such as electric power trans- 2. Paramagnetic materials behave in a a sufficiently intense magnetic field. mission lines and cables, or higher fre- similar way to ferromagnetic materials, quencies such as radio waves (including although to a far lesser degree (they have Electromagnetism cell phones) or television. a positive but very weak magnetic sus- It was the Danish physicist Hans Christian Mathematically speaking, the basic laws ceptibility of around 10- 3). Each atom in a Ørsted, professor at the University of of electromagnetism can be summarised paramagnetic material has a non-zero Copenhagen, who, in 1820, was first to dis- in the four Maxwell equations (or Maxwell- magnetic moment. In the presence of an cover the relationship between the hitherto Lorentz equations) which can be used to external magnetic field, the magnetic separate fields of electricity and magne- provide a coherent description of all elec- moments align up, thus amplifying this tism. Ørsted showed that a compass needle tromagnetic phenomena from electrosta- field. However, this effect decreases as was deflected when an electric current tics and magnetostatics to electromagne- temperature rises since the thermal agi- passed through a wire, before Faraday had tic wave propagation. James Clerk Maxwell tation disrupts the alignment of the ele- formulated the physical law that carries set out these laws in 1873, thirty-two years mentary dipoles. Paramagnetic materials his name: the magnetic field produced is before Albert Einstein incorporated the lose their magnetisation as soon as they proportional to the intensity of the current. theory of electromagnetism in his special are released from the magnetic field. Most Magnetostatics is the study of static theory of relativity, which explained the metals, including alloys comprising ferromagnetic fields, i.e. fields which do not incompatibilities with the laws of classi- magnetic elements are paramagnetic, as vary with time. cal physics.