Small Magnetic Sensors for Space Applications

Small magnetic sensors are widely used integrated in vehicles, mobile phones, medical devices, etc for navigation, speed, position and angular sensing. These magnetic sensors are potential candidates for space sector applications in which mass, volume and power savings are important issues. This work covers the magnetic technologies available in the marketplace and the steps towards their implementation in space applications, the actual trend of miniaturization the front-end technologies, and the convergence of the mature and miniaturized magnetic sensor to the space sector through the small satellite concept

Gamma Irradiation of Magnetoresistive Sensors for Planetary Exploration

A limited number of Anisotropic Magnetoresistive (AMR) commercial-off-the-shelf (COTS) magnetic sensors of the HMC series by Honeywell, with and without integrated front-end electronics, were irradiated with gamma rays up to a total irradiation dose of 200 krad (Si), following the ESCC Basic Specification No. 22900. Due to the magnetic cleanliness required for these tests a special set-up was designed and successfully employed. Several parameters of the sensors were monitored during testing and the results are reported in this paper. The authors conclude that AMR sensors without front-end electronics seem to be robust against radiation doses of up to 200 krad (Si) with a dose rate of 5 krad (Si)/hour and up to a resolution of tens of nT, but sensors with an integrated front-end seem to be more vulnerable to radiation

Entwicklung eines magnetoresistiven Biosensors zur Detektion von Biomolekülen

Schotter J. Development of a magnetoresistive biosensor for the detection of biomolecules. Bielefeld (Germany): Bielefeld University; 2004.Diese Arbeit präsentiert eine neue Nachweismöglichkeit für Biomoleküle, in deren Rahmen Sub-Mikrometer große magnetische Marker and magnetoresistive Sensoren in einen magnetischen Biochip integriert werden. Die interessierenden Moleküle werden an Oberflächen-immobilisierte Proben hybridisiert und spezifisch mit magnetischen Partikeln markiert. Im Folgenden werden die Streufelder der magnetischen Marker als Widerstandsänderung in einem eingebetteten magnetoresistiven Sensor nachgewiesen. Jedes einzelne Sensorelement deckt die Fläche eines typischen Proben-DNA Spots ab, und über 200 Sensorelemente sind in einen magnetischen Sensor-Prototypen integriert, wodurch er kompatibel zu DNA-Microarray Applikationen ist. Die Eigenschaften verschiedener kommerziell erhältlicher magnetischer Partikel werden verglichen und hinsichtlich ihrer Eignung als Marker für magnetische Biosensoren untersucht. Sensoren, welche entweder auf dem Riesen-Magnetowiderstand oder dem Tunnel-Magnetowiderstand basieren, werden präsentiert, und ihre Reaktion auf lokale Streufelder, welche von magnetischen Markern auf ihrer Oberfläche induziert werden, wird untersucht. DNA-Hybridisierungsexperimente werden präsentiert, die zeigen, dass unser Prototyp eines magnetischen Biosensors komplexe DNA-Sequenzen mit einer Länge von tausend Basen bis herab zu einer Konzentration von etwa 20 pM nachweisen kann. Ein direkter Vergleich unserer magnetoresistiven und einer Fluoreszenz-basierten Detektionsmethode zeigt, dass unser magnetischer Biosensor bei dem Nachweis geringer DNA-Konzentrationen überlegen ist. Außerdem weist der magnetische Biosensor eine kompakte Größe auf und übersetzt die vorhandene Menge einer bestimmten Sorte Biomoleküle direkt in ein elektronisches Signal, wodurch dies eine sehr vielversprechende Wahl für die Detektionseinheit eines zukünftigen Lab-on-a-Chip Gerätes darstellt.In this thesis, a new sensing scheme for biomolecules is presented that combines sub-micron sized magnetic markers and magnetoresistive sensors into a magnetic biochip. The molecules of interest are hybridized to surface-immobilized probes and get specifically labeled by magnetic markers. Afterwards, the stray fields of the magnetic markers are detected as a resistance change by an embedded magnetoresistive sensor. Each sensor element covers the area of a typical probe DNA spot, and over 200 sensor elements are integrated into a magnetic biosensor prototype, thus making it compatible to standard DNA microarray applications. The properties of different commercially available magnetic particles are investigated and compared with respect to their suitability for magnetic biosensor applications. Sensors based both on giant and tunneling magnetoresistance are presented, and their response to local stray fields induced by magnetic markers on their surface is studied. DNA hybridization experiments are presented that prove that our prototype magnetic biosensor can detect complex DNA with a length of one thousand bases down to a concentration of about 20 pM. A direct comparison of the magnetoresistive and a fluorescent detection methods shows that our magnetic biosensor is superior to standard fluorescent detection at low DNA concentrations. Furthermore, the magnetic biosensor has compact size and directly translates the abundance of desired biomolecules into an electronic signal, thus making it a very promising choice for the detection unit of future lab-on-a-chip devices

Ultrafast spin and heat transport after femtosecond pulsed laser excitation in metals

Ever since the discovery that the magnetization of a ferromagnetic thin film can be altered on sub-picosecond timescales, researchers have been putting a lot of effort into investigating this effect. However, many questions are still present on this subject. The main question addressed in this work is how transport of electrons influences these ultrafast magnetization dynamics.In this talk electron and heat transport are investigated on sub-picosecond timescales. This is done by measuring the influence of transport on the demagnetization of a magnetic material after heating by a short laser pulse.The main focus of this talk is the control of heat transport on ultrafast timescales using a so-called spin-valve geometry. The heat conduction of such a spin-valve can be altered using a magnetic field. The demagnetization of a magnetic layer placed on top of the spin-valve was then measured, and it has been unambiguously shown that the demagnetization is affected by this change in heat conduction. With this a proof of concept has been provided for the control of heat transport on these timescales, opening up new possibilities for future devices as a method to investigate the interplay between the charge and spin degrees of freedom on ultrafast timescales. Furthermore, simulations were done on the device using different models to describe heat transport, in search of the best way to describe transport on these short length- and timescales. Ever since the discovery that the magnetization of a ferromagnetic thin film can be altered on sub-picosecond timescales, researchers have been putting a lot of effort into investigating this effect. However, many questions are still present on this subject. The main question addressed in this work is how transport of electrons influences these ultrafast magnetization dynamics.In this talk electron and heat transport are investigated on sub-picosecond timescales. This is done by measuring the influence of transport on the demagnetization of a magnetic material after heating by a short laser pulse.The main focus of this talk is the control of heat transport on ultrafast timescales using a so-called spin-valve geometry. The heat conduction of such a spin-valve can be altered using a magnetic field. The demagnetization of a magnetic layer placed on top of the spin-valve was then measured, and it has been unambiguously shown that the demagnetization is affected by this change in heat conduction. With this a proof of concept has been provided for the control of heat transport on these timescales, opening up new possibilities for future devices as a method to investigate the interplay between the charge and spin degrees of freedom on ultrafast timescales. Furthermore, simulations were done on the device using different models to describe heat transport, in search of the best way to describe transport on these short length- and timescales

Combined magnetic, electric, ferroelectric and magnetoelectric characterization of novel multiferroic perovskites obtained by high pressure/temperature synthesis

The mass technological revolution, exploded in the mid XX century, has invaded the global market and completely changed the way of making scientific research. In this contest, the efforts of Materials Science, Physics and Engineering are today prevalently devoted to seek innovative solutions in the new generation devices scalability within different economical macro-areas (i.e. electronics, energy, transports etc.). A main issue is related to the increasing request of multi-functionality, or rather the possibility of joining different and independent “functions” in a sole physical component. To this wide category also belong multiferroic materials, which are principal actors of my Ph.D activity. Multiferroism is defined as the coexistence of two or more primary ferroic orders in the matter: namely (anti-)ferromagnetism, ferroelectricity, ferroelasticity. Particularly, if a material shows superposition and interplay between the magnetic and the electric order parameters, it is specifically called magnetoelectric multiferroic. Although magnetoelectric multiferroic-based devices are considered innovative solutions for different technological fields (namely data storage, spintronics, electric and magnetic field multi-sensing, electronics), nowadays a large-scale application is not started yet. This is due to three fundamental reasons: the lack of natural compounds combined with the difficulty to obtain them artificially; the low temperature occurrence of multiferroic behavior (well below room temperature); and the extremely weak magnetoelectric coupling. If the first drawback can be in principle overtaken by means of unconventional synthesis techniques, the last two are complementary criticalities, since an improvement of the former usually implies a worsening of the latter and vice versa. Moreover, another fundamental problem adds to the previous ones, i.e., the huge experimental complexity of performing specific combined electric and magnetic characterization. I engaged this intricate situation using a double approach. Since the first months of activity I worked to establish a self-standing laboratory committed to the experimental characterization of multiferroic magnetoelectric properties. In particular, standard magnetometric techniques and standard electrical measurements techniques have been assembled in a unique platform (based on a “simple” SQuID magnetometer), to properly perform combined electric and magnetic investigations. Moreover, a high-voltage setup for ferroelectric characterization, equipped with the AIXACCT TF-Analyzer 2000, has been optimized and tested, allowing to study also bulk samples with non-dielectric properties (just like many multiferroics). Beside this experimental work, an equal effort was devoted to the production and characterization of novel bulk systems with potential multiferroic magnetoelectric character, specifically by means of HP/HT solid state reactions In four years, I stabilized more than ten single-phase compounds belonging to the perovskite ABO3 class. Due to its large tolerance, perovskite lattice enables a variegate number of chemical substitutions and structural distortions. In these materials, magnetism and ferroelectricity derive from independent mechanisms; ferroelectricity is induced exploiting the stereochemical effect of Bi3+ or Pb2+ ions on the A-site of perovskite, while magnetism is promoted by the introduction of two different III-IV period metal cations (i.e. Cr, Mn, Fe, Co, Cu; Mo) on the B-site of perovskite. Systems obtained by these chemical substitutions, are usually called double perovskites, with general formula A2BB’O6. The choice of a double substitution on the B and B’-site can be explained considering that it may allow a lowering of the space and time-symmetry (operation that in some cases contributes to the coexistence of magnetic and electric order); on the other hand the presence of different magnetic interactions usually promotes high Curie temperatures despite an enhancement of the system complexity. BFMO in particular revealed intriguing, although unusual, properties requiring magnetometric, structural, ferroelectric and magnetoelectric characterization to investigate its overall physical behavior. BFMO displayed a highly distorted cell with a strong compositional inhomogeneity involving the spatial distribution of iron and manganese; it showed coexistence of a RT antiferromagnetic order (TN = 288 K) and ferroelectricity, which is irreversibly induced by an external DC electric bias (just below the semiconductor-to-insulator onset, occurring at TP = 140 K). In addition, some interesting evidences of magnetoelectric coupling were highlighted by means of our combined magnetic/electric techniques, such as the observation of magnetic ordering-induced changes of the transport properties, the occurrence of magnetocapacitance effects and the detection of a tuning of the magnetization thermal dependence under a DC electric bias. Especially the latter experimental outcome unequivocally promotes BFMO as a possible bulk multiferroic magnetoelectric compound. Despite such preeminent results, BFMO gave also the chance to study exotic phenomenologies subsidiary to multiferroism and magnetoelectricity but incredibly fascinating, specifically: - the thermal activated field-dependent spontaneous magnetization reversal process; - the Mott’s Variable Range Hopping transport mechanism characterized by 1D conductance. These two mechanisms were deeply investigated since nowadays a general consensus on their interpretation is still lacking. The presented data allowed to describe them as disorder-related phenomena, pointing out the crucial role played by composition inhomogeneity in the spatial distribution of iron and manganese ions. All these aspects, together with many others less relevant, are deeply treated in my Ph.D thesis, whose writing want to be a tribute to me and to my scientific effort, but mainly to all the people who collaborate with me during these years.La rivoluzione tecnologica di massa, esplosa a metà del XX secolo, ha invaso profondamente il mercato globale cambiando il modo di fare ricerca scientifica. In questo contesto, le attenzioni della Scienza dei Materiali, della Fisica e dell’ Ingegneria si sono intensamente rivolte alla scoperta di soluzioni alternative di mercato, attraverso lo sviluppo di dispositivi sempre più performanti da proporre in diverse macroaree economiche (elettronica, energia, ambiente, infrastrutture e trasporti). In particolare, un aspetto rilevante è rappresentato dalla continua e crescente richiesta di multifunzionalità tecnologica, ovvero la possibilità di eseguire differenti funzioni attraverso un unico componente fisico. A questa sterminata categoria appartengono di diritto i materiali multiferroici, che rappresentano il cuore della mia attività di ricerca di dottorato. Il multiferroismo è una proprietà fisica definita come la coesistenza di due o più ordini ferroici primari nella materia: ovvero (anti-)ferromagnetismo, ferroelettricità e ferroelasticità. In aggiunta alla coesistenza, un materiale multiferroico che mostri accoppiamento tra i parametri di ordine elettrico e magnetico prende il nome di multiferroico magnetoelettrico. Sebbene la possibilità di sfruttare dispositivi a base di componenti multiferroici magnetoelettrici rappresenti una straordinaria innovazione in diversi settori (memorie magnetiche di nuova generazione, spintronica, sensoristica di campo elettrico e magnetico ed elettronica) ad oggi non si è ancora registrato un loro sostanziale avvento nel mercato. Ciò è dipeso da tre motivi fondamentali: la scarsità di composti naturali che mostrino questa proprietà (ed, insieme, l’enorme difficoltà di produrli artificiale); le basse temperature di ordinamento multiferroico (ben al di sotto di temperatura ambiente) che ne limitano l’applicazione su larga scala; i debolissimi coefficienti di accoppiamento magnetoelettrico finora osservati. Se tuttavia il primo problema può essere in linea di principio sorpassato mediante l’utilizzo di tecniche di sintesi non convenzionali, le altre due criticità appaiono invece fortemente correlate. A tutto questo va aggiunto un’ulteriore problematica rappresentata dalla difficoltà di eseguire un corretto studio delle proprietà magnetoelettriche per vincoli essenzialmente di natura strumentale. Io ho affrontato tutti questi problemi sfruttando un duplice approccio a cavallo di due piani scientifici distinti. Sin da primo mese di attività di ricerca, ho lavorato per sviluppare un laboratorio completamente dedicato alla caratterizzazione di materiali multiferroici magnetoelettrici. La sfida è stata quella di unire le funzionalità di un laboratorio standard di magnetometria a quelle di un laboratorio classico di studio delle proprietà elettriche. Per quanto questa possa sembrare un’operazione semplice, essa non la è affatto, ne sia di prova il fatto che, nonostante il grande di numero di pubblicazioni scientifiche sui materiali multiferroici, sono rari gli articoli in cui sia riportata una convincente caratterizzazione magnetoelettrica. Nella maggior parte dei casi lo studio della magnetoelettricità non viene nemmeno affrontato. Dopo tre anni di lavoro, siamo riusciti nell’intento di assemblare una piattaforma multifunzionale, basata su un “semplice” magnetometro SQuID, che consente di effettuare la caratterizzazione combinata, magnetico, elettrica e magnetoelettrica dei materiali multiferroici. Parallelamente abbiamo implementato il sistema di misura AIXACCT TF-Analyzer 2000, equipaggiandolo con strumenti volti ad estenderne le potenzialità. Attraverso questa apparato è possibile studiare le proprietà ferroelettriche di materiali anche non idealmente dielettrici, quali sono spesso i multiferroici. Per superare le criticità legate alla deficienza di materiali multiferroici naturali, la scelta è stata quella di ottenere il multiferroismo all’interno della struttura cristallografica della perovksite ABO3, sfruttandone le straordinarie doti tolleranza a sostituzioni chimiche e distorsioni strutturali che questo reticolo cristallino. In questi materiali, il magnetismo e la ferroelettricità originano da diversi meccanismi fisici, indipendenti tra di loro. La ferroelettricità viene indotta mediante l’effetto stereochimico dello ione Bi3+ o Pb2+ .posizionato sul sito A della perovskite che attraverso un’ibridizzazione direzionale dei legami con gli ossigeni circostanti, può causare la rottura di simmetria per inversione spaziale. Parallelamente il magnetismo viene promosso dall’introduzione di due differenti cationi magnetici del III e del IV periodo (Cr, Mn, Fe, Co, Cu; Mo) sul sito B della perovskite. La classe di materiali così ottenuta prende il nome di classe delle perovskiti doppie (A2BB’O6). La decisione di operare una doppia sostituzione sul sito B si spiega proprio a fronte delle criticità fisiche menzionate prima: da un lato la presenza di disordine composizionale, causato dalla presenza di differenti ioni sul sito B della perovskite, può aiutare ad abbassare della simmetria strutturale; dall’altro lato uno schema di differenti interazioni magnetiche di superscambio sono di solito responsabili dell’innalzamento delle temperature critiche ma anche della complessità magnetica. Più di dieci composti a struttura perovskitica doppia sono stati sintetizzati in questi anni di tirocinio sulla base di queste sostituzioni chimiche. Fra tutti questi, BiFe0.5Mn0.5O3 (BFMO) si è rivelato quello di gran lunga più interessante. Lo studio delle proprietà magnetiche, strutturali, ferroelettriche e magnetoelettriche ha consentito di verificare la coesistenza di antiferromagnetismo ambientale (TN = 288 K) con una ferroelettricità irreversibile indotta da un bias di tensione elettrica in continua (al di sotto della transizione semiconduttore-isolante localizzata a TP = 140 K). Ulteriori studi sulle proprietà magnetoelettriche, effettuati attraverso l’utilizzo delle tecniche combinate di caratterizzazione magnetica ed elettrica, hanno permesso di osservare: (a) una trasformazione delle proprietà di trasporto a seguito dello spontaneo ordinamento magnetico del materiale; (b) la presenza di magneto-capacità indotta dalla struttura polare (c) un effetto di tuning della suscettività magnetica in funzione della temperatura per mezzo dell’applicazione di tensione elettrica in continua sul materiale. In particolare quest’ultimo risultato sperimentale ha dimostrato che BFMO, inequivocabilmente, è uno dei primi materiali bulk multiferroici magnetoelettrici. Oltre a questi risultati fondamentali, che rappresentavano l’obbiettivo principale della mia attività di ricerca, BFMO ha consentito di studiare una serie di proprietà esotiche, sussidiarie al multiferroismo e alla magnetoelettricità, ma al contempo incredibilmente affascinanti. Le principali sono: - il meccanismo di inversione spontanea della magnetizzazione termicamente attivato e dipendente dal campo magnetico, - il meccanismo di trasporto elettrico sulla base dell’inusuale modello di Mott’s Variable Range Hopping, caratterizzato da un tipo di conduzione monodimensionale Questi due fenomeni sono stati approfonditamente esplorati soprattutto perché, allo stato dell’arte, mancavano di una chiara e condivisa interpretazione scientifica. Lo studio sperimentale ha mostrato quanto entrambi i fenomeni risultino essere fortemente dipendenti dal disordine composizionale sul sito B della perovskite, individuando il ruolo chiave svolto dalla presenza di disomogeneità nella distribuzione spaziale degli ioni ferro e manganese. Ognuno di questi aspetti, qui soltanto introdotti, e tanti altri di minore rilevanza sono approfonditamente trattati all’interno della Tesi, la quale è stata scritta anche in tributo alle persone che in questi anni hanno collaborato con me

Study of Magnetization Switching in Coupled Magnetic Nanostructured Systems

A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored