A FAMILY OF MICROINSTRUMENTS FOR SMART MATERIALS, ENERGY MANAGEMENT, AND BIOMEDICINE IN SPACE MISSIONS 1 M. Dudziak, PhD, Silicon Dominion Computing, Inc. (USA) and Dept. of Physics, Moscow State University (Adjunct Faculty) 2 A. Chervonenkis, PhD, MODIS Corporation (USA) ABSTRACTUsing a family of Fe-Ga based thin films fabricated epitaxially to have a range of magneto-optic(Faraday effect) properties, we have designed a class of instruments that can be used for a varietyof challenging tasks pertaining to the maintenance and performance of spacecraft as well asastronaut crews on long-term missions into deep space. The principle and the generic design ofthis class of instruments applies to four diverse areas of utility and interest to the spaceexploration community: Smart surface sensing and non-destructive testing for spacecraft components, particularly protective shield, hull, and structural assembly parts Detection and mapping of electromagnetic field activity in and around the spacecraft, including at substantial (> 10,000 km) distances, for use in safety management and also possible novel energy production and propulsion techniques Monitoring and mapping of biomagnetic fields detectable from human crew members as a remote-sensing / multi-spectral technique in diagnostic and preventative medicine High-speed spatial light modulators for optical signal processing and optical switching including application to neural-network-like pattern processing.Each device is based upon the operation of a MODE ™ sensor designed from the thin-filmmaterial with its uniaxial anisotropic properties of magnetic domain transfer and reorganization inresponse to external proximate magnetic fields of varying strength, intensity, and duration. Thesensor is coupled with a polarizing light source fed to the sensor via fiber optic channel, and aspatial light modulator switching element that operates in response to the changes effected in thesensor. The switching element provides input via fiber optic to an electro-optics element that isinterfaced with digital logic for activating a response based upon one or several of the outputconfigurations from the spatial light modulator.1 CEO and Director of Research & Development, Silicon Dominion Computing, Inc., 3413 HawthorneAvenue, Richmond, VA 23222-1821, 804-329-8704, 804-329-1454 fax, email@example.com Exec Vice President and Principal Scientist, MODIS Corporation, 1318 Pavilion Club Way, Reston, VA20194, (703) 281-2100, (703) 281-2131 fax, firstname.lastname@example.org
1. INTRODUCTIONThe goal of the MicroMAG project, conducted as a joint venture of Silicon DominionComputing, Inc. and MODIS Corporation, has been to develop a highly compact and modular setof devices for magnetic imaging and control functions based upon Faraday-effect magneto-optics,using a proprietary class of thin-film sensors with extremely sensitive (< 10-7 Oe) properties.The initial phase of the project has been to refine the sensor materials and to design a class ofinstrumentation that will meet the requirements of several functions for aerospace-basedapplications that benefit from magneto-optic methods in non-destructive defect detection andother EMF intensity measurements for structural, engineering, and bioengineering applications.Modularity, lightweight composition, compactness, and robustness for unmanned or low-maintenance applications have been goals of the project in order to provide for systems that areconsistent with earth-orbit and deep space missions.Surface fatigue, stress, crack, and other defect testing operations can be performed by introducingan array of sensor apparatus throughout the spacecraft structure, or by a member of the crew (or arobot) using a portable inspection unit. Generation of a “flash” eddy current in the metalstructure is required but can be effected by either a built-in or portable apparatus includingpermanent magnet units that can be incorporated into the sensor device.EMF activity can be observed and measured through modification of the MODE sensor thin filmfor increasing sensitivity at some cost in field effect intensity or duration (memory) within thesensor. The possible applications of this technology for energy generation have been discussed ina previous paper, “ Design of Magneto-Optic Wide-Area Arrays for Deep Space EMF Studies and Power System Control”[Dudziak, 1998a].For biomedical applications, the principle is similar to that which would be employed in detectionof EMF outside the ship or in deep space. The instrumentation is intended principally forexternal measurements, using a band-aid strip device that can be taped to different locations onthe body, with fiber-optic links to and from the sensor.Spatial light modulators (SLM) for optical switching and image processing have been designedusing magneto-optic media with typical switching times of 1 µs but with recent new methods it ispossible to attain reduction in size and increase of speed to approx. 0.1 µs and optical contrastexceeding 1000:1. Such characteristics enable SLMs to be considered as a competitive solutionfor massively parallel or widely-dispersed network applications onboard spacecraft includingembedded devices in smart structural materials.
All of these applications involve a fundamental common technology, using Fe-Ga basedmagneto-optically sensitive thin films of variable composition and sensitivity, and known asMODE ™ (Magneto-Optic Detection and Encoding).2. MODE™ THIN-FILM AND ITS PROPERTIESThe MODE ™ technology is based upon a field visualizing film (FVF). It consists of atransparent ferromagnetic layer of Bi-substituted iron-garnet grown by LPE technique on a non-magnetic substrate. The composition of the FVF is characterized by the formula (R Bi)3 (MFe)5012, where R is a rare-earth ion (Y, Lu, Tm, Gd, Ho, Dy, Tb, Eu for example) and M isgenerally Ga or Al. Magnetic and magneto-optic properties of the FVF are controlled bycomposition, growth conditions and post-epitaxial treatment [Randoshin, 1990]. The specificFaraday rotation of 10^4 deg/sm and absorption coefficient less then 10^3 cm-1 are available in ageneric composition (Tm Bi)3 (Fe Ga)5012. High contrast domain structures can be easilyobserved using a polarizing microscope. Figures 1 and 2 illustrate four sample images obtainedwith the MODE ™ technology, all laboratory images made in ambient environments usingsample materials (microprocessor chip circuitry (pads) and steel plates with defects) such as maybe encountered on space vehicles and satellite assemblies. Figure 1 MODE imaging of 16-bit microprocessor lead pads
Figure 2 Digital imaging of steel plate by MODE (left) and ordinary light (right)The magneto-optic layer or FVF is created by growing the epitaxial layer on the garnet substrate,deposited in a supercooled flux, containing a solvent of composition Bi203-PbO-B203 as well asgarnet-formed oxides at a temperature range of 940K to 1108K. By introducing a high level ofBi3+ ion substitution into the FVF a high MO figure of merit can be achieved, s.t. Ψ= 2ΘF / α >10 grad/dB. An important feature of the FVF of value for possible deep space magnetic anomalyand variation studies is the high domain wall velocity (> 1000m/s) obtained in four types of films:(i) high-anisotropic-oriented films with Y and Lu composition, in the presence only of in-planemagnetic fields, (ii) films with Gd and Tm, with angular momentum compensation (AMC), (iii)films with Y, Lu, and Pr (orthorhombical magnetic anisotropy (ORMA), and (iv) films with Gdand Eu (both AMC and ORMA).Figure 3 illustrates saturation magnetization properties of the MODE film [B(G)] and an ironplatelet [B(Fe)] - the ratio of the anisotropy field H / B(G) increases over the normal distance z.
Figure 3 MODE Saturation Magnetization LevelsFigure 4 provides a schematic of the basic operation of magneto-optic imaging using a MODE ™thin film crystal sensor. By incorporating the polarized light source into a fiberoptic deliverysystem, the packaging of a sensor unit can be sized down to a chip set incorporating CCD andcontrol logic in one device and optics in a second hybrid device. Figure 4 Basic operation of MODE Imaging3. MagVision PROTOTYPE SCANNERAn early laboratory workbench scanner has been produced which generates NTSC or PALcompatible video output from a magneto-optic imaging apparatus. The basic design is illustratedin Figure 5 below. The “rotatable analyzer” is replaceable with a micro videocam assembly andcan be adapted with an objective lens for a microscope. The solid housing can be a permanentmagnet of varying strength (typically 5G) for enhancing the magnetic field of the sample as in
imaging applications where the magnetic field of interest is affixed to a nonmagnetizable surfacesuch as plastic or some insulator. The minimal detection of the current scanner is @ 0.1 Oe butthe theoretical limit of the thin film extends to 10 -8 Oe. A yellow-orange halogen lamp is usedwith a thin-film polarizer for the light source. Figure 5 Schematic of MagVision Prototype Scanner4. MODE ™ DEFECT AND STRESS DETECTIONThe MODE ™ technology has been tested in several experiments for use in detecting microcracksand other defects in metallic surfaces. These tests have been performed using a variety ofmaterials, both magnetized and non-magnetized but ferromagnetic. The objective has been torefine methods for magnetic imaging that can be implemented without the use of an external eddycurrent applied to the sample, either as a steady low-amp current or using a “flash” technique(approx. 10-15 kA for 10-20 ms).Real-time images of defects are obtained in both magnetic and nonmagnetic metal parts as well asin magnetic ceramics (ferrites) [Randoshin, 1990]. Magnetic anomalies caused by these defectsare sensed by the MODE ™ device. In ferromagnetic metals such as steel, the instrument sensesand displays an image of the flux image leakage associated with the flaw in a previouslymagnetized test piece, while in nonferromagnetic metals, particularly in the case of Al, theinstrument excites in the test piece eddy currents in order to provide a direct image of defectsresulting from the magnetic fields with the flow of eddy currents.The whole FVF region spontaneously divides into a labyrinthine domain space (DS), themagnetization vector M being strictly normal to the FVF surface. Domain walls (DWs) are takento be infinitely thin and thus M direction changes sharply to the opposing vector coming acrossthe DW. The DS varies due to DW displacement when field H o normal to FVF plane is applied.If M in the domain and H o directions coincide, the domain increases in size; otherwise itdecreases and inevitably collapses at the saturation field H s. The primary FVF parameters (i.e.uniaxial anisotropy constant Ku, saturation magnetization M s, coercive field Hc and thickness h)characterizing the DS behavior under the influence of inhomogeneous field, are customized
through the composition process to satisfy the specific requirements. If a purely uniaxialanisotropy and zero Hc could ever be reached, the DS would be in unique conformity with theexternal field. It is extremely important that the FVF possess a very high uniaxial anisotropyfield Hk=2Ku/Ms At a fixed Ms the desired Hk > 103 kA/m is attained by increasing Bi content. Ifthe constants of non-uniaxial anisotropy and K u are of same order then in the presence of someinvariable inhomogeneous field the DS patterns would differ when rotating the film in its plane.The requirements on the quality of the MODE ™ FVF coincide with those for classical bubblememory devices. It is essential that the FVF possess a very low concentration of magnetic defects(i.e. single dislocations, inclusions, scratches) which impede a DW movement. All magneticparameters and thickness are to be held constant to 1% over FVF region. Under such conditionshomogeneously distributed coercivity always exist. Taking this into account, if the MODE defectdetection sensor should possess "memory" capability, than it is necessary to induce elevated anduniform Hc of the order of Ms. There are several technological methods to increase H c eitherduring the epitaxial process or by special post-epitaxial treatment of the film.4.1 Defect Detection Operating PrinciplesA magnetically soft object is magnetized up to saturation in its plane by an external field, createdfor example by a pair of permanent magnets such as are affixed to the sensing device. MODE ™film (FVF) grown on a transparent substrate is brought into contact with the object surface. Ifwithin the object thickness under the MO film a defect is present, the magnetic flux formed bythe permanent magnets will be distorted in such a way that a portion of the flux force lines willemerge from the object surface and penetrate into the sensitive element, thereby rearranging itsequilibrium domain structure. The changes in the DS may be visualized by the help of Faradayeffect. The light from a polarized light-source by the help of a light-dividing cube is directed onthe MODE ™ film, which is closely attached to the object surface. Passing twice through thesensing film (due to reflection from a mirror layer) the light after the objective comes onto thescreen where the contrast image of the domain structure is formed.In the absence of the defect a uniform ("gray") DS will be seen on the screen. This refers to thedemagnetized state of the film. However in the presence of the defect the DS in the film will bedistorted under the influence of the magnetic flux force lines going out of the object surface.These distortions will be seen on the screen and thereby identify the position of the defect.Computer modeling in agreement with the experiment has shown, that the visualized picture ofthe defect substantially depends on the defect dimensions and the deepness of its position.When the inspection subject is a nonmagnetic metal the principle scheme of the detector issimilar to the fore-mentioned (see for example [Fitzpatrick, 1993]) with the exception thatexcitation of stray magnetic fields by means of eddy currents are additionally introduced. Bothsinusoidal and pulse power sources may be used. The excitation of high magnetic fields by highpower unique current pulses has significant advantage over traditional excitation by highfrequency eddy currents/ In the former case, due to thermal limitations, high values of currentsare prohibited, hence the induced magnetic fields are small. As a result the sensitivity and spatialresolution of the sensor is limited. The use of unique current pulses eliminates these limitations,however, due to the short duration of the pulses (several ms), and dictates the introduction ofMODE ™ pattern "memory" capability.Two mechanisms are proposed for MicroMAG implementation. In one case the sensitive MOelement is produced with artificially induced and elevated DW coercivity. This provides image-
memory of the rearranged DS by the element. In the second case one uses an intermediateflexible magnetic carrier (a kind of magnetic tape) that “remembers” or “captures” the structureof the stray magnetic field above the object surface in the case of close attachment of the tape tothe object surface. After that step the visualization of the formed magnetic pattern is conductedby the use of a standard sensitive MODE ™ detecting element. The second mechanism providesan additional advantage connected with the opportunity to reveal hidden defects in the objectswith nonflat surface (welds, tubes etc.) since, unlike the fixed-element sensor unit (cf. Figure 5),magnetic resin tape can easily replicate the surface of any arbitrary object.4.2 Summary of AdvantagesMODE ™ defect detection has a number of advantages over existing eddy-current instrumentsand techniques. The system is capable of providing direct, real time images of cracks, corrosionand other anomalies in inspected areas, as well as high resolution, with low probabilities of falsecalls (indicating a defect when none is present). Direct visual images, which can be directlyrecorded on video-tape for complete documentation of an examination, are intuitivelyunderstandable and easy to interpret and require no preprocessing. In contrast, interpretation ofthe impedance plane displays typically provided by eddy-current instruments are often difficult tostabilize and tune and require a highly-trained operator. The MODE ™ approach lends itself tooperation by the non-NDT-specialist and to automated expert-system enhanced operation.Figures 6 through 9 below illustrate several representative images. Figure 6 MODE picture of nonmagnetic inclusion (round spot) in a platelet of conventional stainless steel. The black area to the right is the edge of the platelet.
Figure 7 MODEpicture of aninternal microcrackin a conventionalsteel alloyplatelet. Thedepth anddimensions of thecrack may beevaluatable throughtranslation of theFigure 8 MODE image(left) of steelplatelet0.8mm thick(upper surface)with defects onlower surfaceranging from 0.1mmto 0.2mm deep(shown in opticalimage (right)Figure 9 (Upper)Optical-only imageof a steel bladecovered by a thin(0.5 – 2 mm) layerof ice. (Lower)MODE image showingtransverse crack inthe steel andvisible through theice.
5. MODE™ APPLIED TO EMF DETECTION THROUGH NONMAGNETIC, NONCONDUCTIVE SURFACESEarly tests with MODE ™ thin-films using the laboratory sensor device illustrated in Figure 5have been performed with a number of materials including paper and plastic. Figure 10 belowillustrates some of the results. Imaging is performed with the assistance of a peripheralpermanent magnet, a hexaferrite composition (BaFe 12O19), measuring 20 x 15 x 7mm, with 6000G magnetic saturation and uniaxial anisotropy normal to the flat sample surface.Figure 10 (Left) Image (10X) of magnetic code layered on plastic;(Right) Image (10X) of magnetic ink printed onto paperThere are several possibilities for how MODE ™ can be employed in spacecraft operationsthrough the detection of EMF and for the measurement of anomalies that may be the result offerromagnetic artifacts or inclusions within asteroids, planets, and other natural bodiesencountered in space. While much of the following is hypothetical and speculative, it is basedupon experiments indicating that the sensitivity of the MODE ™ films can serve a wider purposethan the traditional non-destructive testing and evaluation for metallic components which itselfcan be useful for spacecraft and other man-made devices.An array of sensors equipped with fiber optic light sources and return channels could bedistributed throughout critical parts, sections, and components of a spacecraft, or in a deployedexternal array, for detection of changes in magnetic fields. Such field variations might beindicate systemic flaws within portions of the spacecraft structure, particularly in regions of acraft that may be powered by ionic or other nuclear-reaction based propulsion. Alternatively suchEMF fluxes might serve as predictable behaviors, a type of EMF barometer, indicative ofchanging conditions in the external environment, much as the MODE ™ sensor has been appliedto measurement of EMF leakage from high-voltage power lines. The deployment of such anexternal array could be extended for literally millions of kilometers provided that there wereminimal-energy triggered transmitters that could in turn relay useful data upon detection to areceiving array, either on a spacecraft or in an intermediate location such as a geo(planet)-stationary orbital platform.The possibility that such a wide-area array could be applied to novel “breakthrough physics”propulsion systems such as the extraction of useful energy from coherent vacuum currents hasbeen explored in [Dudziak, 1998b]. It would necessitate a similar type of simple, small, and
fault-tolerant (through parallelism) device such as would play a role in the non-destructive testingand evaluation applications, leading to the MicroMAG architecture (cf. Section 8.)6. MODE™ APPLIED TO BIOENGINEERINGA third application for MODE ™ is in the measurement of biomagnetic fields that may be emittedeither naturally by the human body or induced artificially through the introduction of magnetictags into pharmaceutical agents. The latter approach has been investigated by Davis et al[Wagreich, 1996a; Wagreich, 1996b] using Mach-Zehnder and Fabry-Perot interferometers withpromising results. It is possible that the increased sensitivity of the MODE films can accomplishin a smaller and simpler instrumentation the same level of accuracy (low pT/Hz 1/2) over severalcubic cm or more in spatial resolution.Figure 11 shows the results of very preliminary studies using the MODE sensor to reproduce themagnetic field structure above test samples of thin film permanent magnets (1 cm in diameter and100 micron thick). The top row has images made with the thin film directly on the surface, thelower row with the sensor positioned 0.5 mm higher. There are different levels of bias field andclearly visible asymmetry caused by imperfect technology of SmCo 5 sputtering. Figure 11 MODE imaging of permanent magnet thin filmsBy reducing the size of the MODE sensor element it is possible that an array of such sensors canbe positioned in a configuration that is analogous to an array of EEG or MEG sensors.Experiments are underway to develop an improved technique of affixing the thin film element toflexible materials which include embedded optical fibers woven into the fabric. In such manner amultitude of geometries can be handled for measurement tasks, with the optical outputs from thesensor array feeding into a spatial light modular device (SLM) as described briefly in the nextsection.
7. MODE™ SPATIAL LIGHT MODULATORSA fourth projected use for a modular microscale class of MODE elements is for spatial filteringand optical correlation and switching that can be employed not only in fast binary imagecomparison and recognition but in other pattern recognition and matching tasks. Such processingmay not be derived from images per se but can originate with data sets that are measured andstored as optical patterns. Applications in space of this nature could include measurement ofvibration data, for instance, where similar to atomic force microscopy principles, laser beamreflections are routed to a photodiode array. The foremost use may be in optical computingwhere operations such as Fourier and Gabor transforms could be rapidly executed through aMOSLM (magneto-optic spatial light modulator).What makes it possible to have a practical and small scale SLM is the typical switching time of ~0.1 µs while at the same time attaining optical contrast exceeding 1000:1 and optical efficiency of0.15 at λ = 0.63 µm, with a specific Faraday rotation at that λ of Θρ = 2.0 ± 0.1° / µm with opticalabsorption coefficient of α=0.35 – 0.4 dB/µm. These properties apply to (Rbi) 3(FeGa)5O12 filmsgrown on (GdCa)3(ZrMgGa)5O12 substrate with crystalline lattice parameter of a=12.495 Α. Inthe diffusion heat process, employing an Si mask allows the formation of a domain structure thatcan have considerable variation in geometry and the areas of decreased magnetization and singledomain structures provide for light shutters with typical cell structure @ 100 µm2.By switching from a (111) orientation to an in-plane field the domain wall velocity V s can beincreased to @ 1000 m/s and particularly by composition with high g-factor and highorthorhombic anisotropy to @ 3000 m/s as an upper limit. At V s = 3000 m/s and with MOSLMdimensions of 100 µm2 the switching time is approx. 0.03 µs.An experimental platform for a high-speed optical correlator based upon the MODE thin film wasdeveloped and is described in [Chervonenkis, 1992]. This was a large scale laboratory deviceusing two MOSLMs in series separated by a polarizer and Fourier processing element. Testimages consisted of 16 x 16 elements and comparisons were made between input and controlimages in order to filter out the differences between patterns.Reduction of the MOSLM could be attained to a degree limited by the construction of the Simask and the limits of domain wall distinguishability. Reducing the cell structure size is a topicof current research. The proposed approach is to create the cells in the form of pits orindentations in the background on the initial film thickness such that the thickness within the pitdoes not exceed 1 µm. Single-domain states for each cell can be maintained even in highsaturation magnetization (e.g., @ 300 G). Ion implantation can create a uniaxial anisotropicgradient that is normal to the plane of the film and thereby the remagnetization for cell switchingoccurs not as a result of vertical domain wall propagation inwards from the edges to the centers ofthe cells but instead by vertical propagation of the horizontal domain walls themselves. even at alower Vs = 1000 m/s the switching time can be estimated at 1 ns maximum. The object for doingso is to reduce not only the performance time for such a MOSLM but to enable, once again, thedeployment of a potentially massive and distributed number of simple and small sensing arraysacross arbitrarily large surfaces such as for smart materials applications.
8. MicroMAG MODULAR ARCHITECTURE: FUTURE DIRECTIONSA truly microscalar MicroMAG device has not yet been constructed, only simulated throughlarger apparatus with the theoretical limits and constraints of the MODE thin film providing thesupporting evidence that the scale can be miniaturized. Scale however is not the only issuegoverning the utility of this technology for the multiple applications indicated above. For near-term space applications most technical implementation will effectively rest upon fabricationperformed on Earth or in limited geo-stationary or lunar bases. Developing an apparatus that canbe easily reproduced and introduced into a number of different structures will be a step forwardfrom having very complex and unique instrumentation that cannot be interchangeable. TheMicroMAG project aims at creating the equivalent of the simple diode or transistor in terms offunctional simplicity and ubiquity. The goal is a basic magneto-optic component that can beincorporated into a variety of “circuits” so to speak just like a simple capacitor or resistor can beused in a variety of tasks and in a variety of physical configurations.One area of current attention by the MODIS research team is on the development of a commoninterface for optic channel devices to MODE sensors that may require variable lines for input andoutput depending upon the nature of how many individual sensors are employed. This is theequivalent of designing a crossbar switch to serve a variable number of line-in and line-outsignals and where the actual switching element can be replaced depending upon the task. In thisway a common hardware component can be designed which will enhance the production andfabrication of optical processing networks such as MONA (magneto-optic neural array), anextension of earlier work by one of the authors and others at using MOSLM technology forneural-like distorted image recognition and correction [Nikerov, 1991].This MODE element may never be reducible to the nanoscale level due to the constraints ofdomain size and domain wall behavior. However, some type of nanostructured material,embedded with ferromagnetic atoms in a prescribed geometry, perhaps through a controlledAFM nanofabrication process, could give be employed to enhance sensitivity to a MODE sensorembedded within or placed on the immediate surface of the material. The closest analogy seemsto be that of the use of magnetic ink or fibers within paper or plastic (cf. Figure 10 above). Thenonmagnetized, non-conductive material, perhaps part of the structure of a spacecraft or theprotective suit of an astronaut, would be difficult to test for cracks, leaks, and other micro-damage. Embedding a very thin and distributed layering of ferromagnetic atoms, either through aphysical spray process or as fibers, could enable a MODE apparatus to sense breaks indicative ofthe embedding material failure. What might be detected are breaks like those irregularities inFigure 10 (right) or Figure 11.No doubt structural testing and integrity measurement is the dominant and leading application ofMODE technology in space. SLM-based computing and biomedical applications, much less deepspace energy extraction, are quite futuristic and hypothetical. However it seems to be sensible tostart from the outset to address both that which is known and predictable and that which ishypothetical so that what emerges is something that can hopefully serve both should thehypothetical, as remote as it may seem today, turn out to be quite concrete and definite. Spacetravel and colonization itself seemed to be relegated by many to the domain of Jules Verne untilsome four decades ago, and despite the hopefully brief hiatus in major space-based undertakings,the next decade or two may yield more surprises and changes of thought than those brought on bythe likes of Gagarin and Glenn.
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[Wagreich, 1996b] Wagreich, R. B. & Davis, C. C, Performance Enhancement of a Fiber-OpticMagnetic Field Sensor Incorporating an Extrinsic Fabry-Perot Interferometer, Proc. Photonics,Beijing, China, November, 1996