Efforts on healthcare everywhere have led magnetoresistive biosensors to a fascinating field. One of its precious advantages is the creation of a cheap handheld point-of-care tool which in a few minutes can easily perform multiplexed diagnose the life-threatening diseases or monitoring chronically ill patients. In this biosensor, the magnetic nanoparticles are coated with chemical groups that will bind to the biomolecules, and then could be detected by magnetic sensor. The nanoparticle-biomolecule complexes react with 'probe' molecules that are immobilized onto the surface of the magnetoresistive sensors. Finally, the presence of the magnetic nanoparticles on the sensor surface produces a signal change of the magnetoresistive sensor by applying an external magnetic field [1]. Consequently, both the high magnetic moment nanoparticles such as α-Fe, CoFe, α-Fe@Fe3O4 nanoparticles, and high field-sensitivity magnetoresistive sensors are never-ending desired for enhancement of the biosensor resolution in detecting the bioanalytes. Among various magnetoresistive biosensors, planar Hall effect (PHE) sensor has received vast interest since it meets the needs for high sensing biosensor technology such as: high resolution owing to its small noise, small offset and linear response. To enhance the sensitivity of a PHE sensor, we have optimized both the sensor materials, and the sensor designs. In continuity development of the sensor sensitivity, we report here, the improved measurement configuration to enhance the sensitivity of a PHE sensor. The voltage change of the sensor causing by paramagnetic particles is calculated for a wide range of applied field. Thus, the critical field for maximum change of the voltage is deduced. In addition, the sensitivity of the sensor has been investigated as a function of the external magnetic field direction. By optimizing the measurement setup, the sensitivity of the sensor could be enhanced by one order in magnitude. The experimental setup for a hybrid alternative and continuous field is introduced for enhancing the sensitivity of the sensor. This optimizing method could be a paradigm easily adapted for enhancement of the sensitivity of other magnetoresitive biosensors. References [1] Adarsh Sandhu: New probes offer much faster results. Nature Nanotechnology, 2, (2007) 746.

The work is driven by two reasons: (i) Spin-crossover (SCO) materials are of interest since they are one of the best-known forms of an inorganic electronic switching. Variation thermal energy at the crossover leads to both an electronic and structural change, often observed as a change of colour and/or a magnetic moment. A number of research have been reported concerning the synthesis of nanoscale of SCO nanoparticles [1]. The development of these new, tailored architectures is central for the exploration of the physical properties of these materials, at the mesoscopic scale the SCO is a prototype example of mixed-valence crystals and many efforts have been done to understand charge transfer process between the iron sites. Indeed, metal-to-metal electron transfer phenomena in the complexes play an important role in the intense colors of these compounds, in their electrical conductivity, in their electrochemical properties and influences also their magnetic properties. Not surprisingly, electron transfer processes are involved in nearly all practical applications of these new functional materials, such as electrochromic windows, electrocatalytic and electroanalytical devices, molecular based magnets, sensor, display, information storage and nanophotonic devices. (ii) Up to now, the observations are essentially reduced to the simple investigation of the temperature dependence of the magnetisation or the optical absorption in a huge ensemble of nanoparticles with different degrees of size dispersion by using high sensitivity conventional measurement systems such as superconducting quantum interface device (SQUID), or by transmission electron microscopy and/or dynamical light scattering measurements. Magnetoresistive magnetometer [2] can overcome theses drawbacks. Compared to the SQUID based ultrasensitive magnetic detection systems, the magnetoresistive technology has advantages of room temperature operation, less complex instruments, and hence more portable and flexible implementation. They could be microscopic in size and sensitive to the presence of small magnetic particles when they are in close proximity. They can detect extremely weak magnetic fields, for example, the magnetic field at nano or picotesla generated by the magnetization of a single micro/nano size object. Such a magnetometer will enable and well adapt for studying the magnetic properties of a single object of SCO molecular based materials. The talk deals with optimization of novel magnetoresistive sensor to detect the spin-switching of SCO materials down to nano world and to understand of size-related spin-crossover properties. Several measurement methods to enhance the sensitivity of the magnetometer are also discussed. The final target to detect a single SCO particle will be done in collaboration among three groups by combining the forces from France and Korea researchers. [1] A. Bousseksou et al., Chem. Soc. Rev., 40, 3313 (2011) [2] C. Kim et al., Book chapter, INTECH (2012) http://dx.doi.org/10.5772/52820

A micromagnetometry system for detecting the presence of very small quantities of magnetic particles comprises a first magnetic hybrid AMR/PHR multi-ring sensor (4) using a Wheastone bridge electrical configuration, a first current source (6), a first voltage measurement device (8), a set of at least one magnetic particles (12) deposited on the first magnetic sensor (4) and a processing unit (22) for detecting from a set of different measured differential voltages a magnetic flux shift representative of the presence of a least one deposited magnetic particle (12). The micromagnetometry system comprises means (20) for creating a magnetic excitation field HAC to make produce by each motionless magnetic particle (12) a stray magnetic field oscillating along the time at a constant frequency ranging from 10 Hz to 3 KHz.

Recently, nanoscale spin-crossover (SCO) particles have been the subject of great interest. The change in the 3d electronic configuration of the metal ion results in significant changes in the metal-ligand bond length and geometry, as well as in the molecular volume. Hence the spin switching process is accompanied by a remarkable change in the color, mechanical properties, dielectric properties, and magnetic susceptibility. The synthesis and investigation of these materials at reduced length scales is central not only to the exploration of fundamental effects of size reduction in these systems, but also for the development of new functional materials with applications, including guest molecule sensing, memory devices, and molecular switches

We explore the magnetic anisotropy of GaMnAs ferromagnetic semiconductor by Planar Hall Effect (PHE) measurements. Using low magnitude of applied magnetic field (i.e., when the magnitude H is smaller than both cubic H-c and uniaxial H-u anisotropy field), we have observed various shapes of applied magnetic field direction dependence of Planar Hall Resistance (PHR). In particular, in two regions of temperature. At T < T-C/2, the "square-shape" signal and at T > T-C/2 the "zigzag-shape" signal of PHR. They reflect different magnetic anisotropy and provide information about magnetization reversal process in GaMnAs ferromagnetic semiconductor. The theoretical model calculation of PHR based on the free energy density reproduces well the experimental data. We report also the temperature dependence of anisotropy constants and magnetization orientations. The transition of easy axis from biaxial to uniaxiale axes has been observed and confirmed by SQUID measurements.

During the last decade, considerable experimental and theoretical efforts have been devoted tothe study of GaMnAs semiconductors due to their high ferromagnetic transition; "Curie temperature (Tc)" and their interesting magnetic properties. The potential implementation of structures based on GaMnAs diluted magnetic semiconductors as spintronic devices needs solid understanding of magnetic anisotropy and magnetization reversal processes. Nowadays, it is well established that the ferromagnetic interaction between magnetic ions is mediated by charge carriers, holes in this case. However, defects such as As antisites and Mn interstitials play a crucial role in determining the magnetic properties of GaMnAs. The objective of this study is to investigate the magnetic properties and anisotropy using magnetotransport measurements as well as the dynamics domain wall by high resolution magneto-optical imaging. Predicted theoretical models and developed ones by our team are in good agreement with experimental data. Finally, in this work, we emphasize the first evidence by transport measurements of two in-plane uniaxial axis reorientations. Another part of the talk will focus on new potential applications using magnetic multilayers structures to detect very small field, down to nanotesla, to investigate magnetic properties of spin-crossover materials

Recent acheivement on diluted magnetic semiconductor and new perspective of magnetoresistive sensor