Spin electronic magnetic sensor based on functional oxides for medical imaging

8th Spintronics Symposium , AUG 09-13, 2015 , San Diego, CAInternational audienceTo detect magnetic signals coming from the body, in particular those produced by the electrical activity of the heart or of the brain, the development of ultrasensitive sensors is required. In this regard, magnetoresistive sensors, stemming from spin electronics, are very promising devices. For example, tunnel magnetoresistance (TMR) junctions based on MgO tunnel barrier have a high sensitivity. Nevertheless, TMR also often have high level of noise. Full spin polarized materials like manganite La0.67Sr0.33MnO3 (LSMO) are attractive alternative candidates to develop such sensors because LSMO exhibits a very low 1/f noise when grown on single crystals, and a TMR response has been observed with values up to 2000%. This kind of tunnel junctions, when combined with a high Tc superconductor loop, opens up possibilities to develop full oxide structures working at liquid nitrogen temperature and suitable for medical imaging. In this work, we investigated on LSMO-based tunnel junctions the parameters controlling the overall system performances, including not only the TMR ratio, but also the pinning of the reference layer and the noise floor. We especially focused on studying the effects of the quality of the barrier, the interface and the electrode, by playing with materials and growth condition

Magnetic field microscopy of rock samples using a giant magnetoresistance–based scanning magnetometer

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95412/1/ggge1634.pd

Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements indicate that the Rashba coupling constant increases linearly with electrostatic doping. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates

Enhancing the sensitivity of magnetic sensors by 3D metamaterial shells

Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is non-uniform, as for dipolar magnetic field sources, field gradient is increased as well. A proof-of-concept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate time-dependent magnetic fields upto frequencies of 100 kHz

Superconducting-magnetoresistive sensor: Reaching the femtotesla at 77 K

In order to measure extremely weak magnetic fields, such as those produced by the neuronal activity during cognitive tasks in the brain, we have proposed and realized a femtotesla (10-15T) sensor based on the association of spin electronics and superconductivity which offers an alternative in thin film technology and at 77K to the most sensitive devices which are low-TC SQUIDs (Superconducting Quantum Interference Devices). The principle of these mixed sensors is to combine an efficient flux-to-field transformer, realized by a large superconducting loop containing a constriction, and a magnetoresistive sensor with very good sensitivity (GMR or TMR). Field levels of few fT/√Hz in the thermal noise have been reached at liquid nitrogen temperature, which is comparable to performances of SQUIDs in liquid helium. Performances are nevertheless reduced in the low frequency (below 1kHz) range due to 1/f noise present because of the small volume of the magnetoresistive element. Cancellation techniques based on switching on and off the sensor to reference points have been developed and already allow reducing the low frequency noise of more than one order of magnitude, leading to sensitivity in field of 0.1pT/√Hz at 1Hz. First measurements of the magnetic component of the cardiac signal (few pT/√Hz at 1Hz) have been acquired with mixed sensors. The very low thermal noise level has also allowed realizing nuclear quadrupolar resonance measurements on nitrogen compounds, which is a non invasive detection technique for solid explosives. We have also achieved first proton Low-Field Nuclear Magnetic Resonance experiments with such sensors, which has led to develop and build a Magnetic Resonance Imaging setup to realize 3D images at low field (<20mT), which is of great interest for low cost and portable equipment development.Afin de permettre la mesure de champs magnétiques extrêmement faibles, comme ceux produits par l'activité neuronale lors des taches cognitives, nous avons proposé et réalisé un capteur femtotesla (10-15T) associant l'électronique de spin et la supraconductivité offrant une alternative en technologie couches minces et à 77K aux capteurs les plus sensibles actuellement que sont les SQUIDs (Superconducting Quantum Interference Devices) à basse température critique (4K). Le principe de base de ces capteurs, appelés capteurs mixtes, repose sur l'association d'un transformateur flux-champ très performant, obtenu grâce à une large boucle supraconductrice comprenant une constriction, et d'un capteur magnétique de type GMR ou TMR métallique de grande sensibilité. Des niveaux de détectivité de quelques fT/√Hz dans le bruit thermique ont pu ainsi être atteints à l'azote liquide, ce qui est comparable aux performances des SQUIDs à l'hélium liquide. Il existe cependant un bruit en 1/f à basse fréquence, dû au faible volume de l'élément magnétorésistif, qui limite les performances du capteur en dessous de 1kHz. Des techniques de suppression de ce bruit basse fréquence par bascule sur des points de référence du capteur sont étudiées et permettent déjà de réduire ce bruit de plus d'un ordre de grandeur, ce qui permet d'atteindre 0.1pT/√Hz à 1Hz. Grâce à ces capteurs, il a été possible de réaliser des premières mesures de la composante magnétique du signal cardiaque (signal de quelques pT/√Hz). Le très faible niveau de bruit thermique a permis également de réaliser des mesures de résonance quadrupolaire nucléaire sur des composés azotés qui est une technique de détection sélective d'explosifs. Nous avons aussi montré la possibilité de mesurer la résonance magnétique nucléaire du proton à très faible champ. Cela nous a conduit à développer un dispositif d'imagerie par résonance magnétique pour réaliser des images 3D à très bas champ (<20mT), ce qui présente un très grand intérêt pour la réalisation de systèmes d'IRM portables, faible coût, qu'il serait aussi possible d'intégrer dans un système de magnétoencéphalographie

Électronique de

Les capteurs de champ magnétique ont de nombreuses applications : lecture de disques durs, contrôle de positionnement dans l’espace, codage de position et d’angle, mesure de courant électrique sans contact, imagerie magnétique, contrôle non destructif, imagerie médicale… Parmi les différents types de capteurs magnétiques, ceux issus de l’électronique de spin offrent la possibilité d’une grande sensibilité sur une petite échelle spatiale. Utilisés systématiquement dans le domaine des têtes de lecture des disques durs (voir l’article de J.P. Nozières, p. 12), leur production en grande série pour d’autres applications vient de commencer

Noise in GMR and TMR sensors

International audienceGiant Magnetoresistances (GMR) and Tunnel Magnetoresistances (TMR) take an increasing part in many applications like current sensing, magnetometry or position sensing, thanks to their high magnetoresistance at room temperature, which leads to a large output signal variation. But the real performances of such sensors can only be estimated with respect to the sources of noise. In this chapter, we give first some bases on noise theory and data treatment. Fluctuations, ergodicity and volume considerations will be discussed. A second part will detail noise measurement techniques and data analysis of typical noise power spectra. Sources of noise will be discussed in a third part. In the end of the chapter, specific cases of GMR and TMR magnetic noise and non magnetic noise will be discussed with their physical origin and their analytical or phenomenological expression. We will then present ways to design GMR and TMR sensors for noise reduction, depending on the applications targeted

Optimised GMR sensors for low and high frequencies applications

International audienc