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In this letter, we present a study of optimized TMR magnetic field sensors as a function of voltage bias. The 1/f low-frequency noise is quantified by the Hooge-like parameter {\alpha} which allows to compare the low-frequency behavior of various TMR sensors. The sensitivity as well as the detectivity of the sensor are characterized in the parallel state and at 0 mT. We observe that the sensitivity shows a strong voltage dependence and the noise presents an unexpected decrease, not anticipated by the Hooge's law. Moreover, surprisingly, an almost stable detectivity (140-200 nT/sqrt(Hz) at 10 Hz and 15-20 nT/sqrt(Hz) at 1 kHz) as a function of the bias voltage is observed, tending to highlight that the variation of sensitivity and noise are correlated. Even if the I-V curves are strongly non-linear and reflect the different symmetries of the conduction bands channels, the variations in sensitivity and noise seems to depend mainly on the distortion of the MgO barrier due to bias voltage. With a simple model where the normal noise and sensitivity of the TMR sensors are modified by an element having no noise and a parabolic conductance with voltage, we describe the behavior of noise and sensitivity from mV to V.

In this manuscript we propose a theoretical model where the magneto-resistive elements are modelled as fluctuating resistances to correct the output voltage noise of tunnel magnetic junction (MTJ) from standard electronic circuits. This model is validated on single elements, partial and full Wheatstone bridge circuits, giving rise to a correction factor affecting the output noise voltage as well as sensitivity values. Combining the correction factor and a normalization by the number of MTJs pillars and the pillar surface, we show that the performances extracted by this method allow universal comparison between any results from literature.

Vertical packaging of multiple Giant Magnetoresistance (multi-GMR) stacks is a very interesting noise reduction strategy for local magnetic sensor measurements, which has not been reported experimentally so far. Here, we have fabricated multi-GMR sensors (up to 12 repetitions) keeping good GMR ratio, linearity and low roughness. From magnetotransport measurements, two different resistance responses have been observed with a crossover around 5 GMR repetitions: step-like (N<5) and linear (N>5) behavior, respectively. With the help of micromagnetic simulations, we have analyzed in detail the two main magnetic mechanisms: the Neel coupling distribution induced by the roughness propagation and the additive dipolar coupling between the N free layers. Furthermore we have correlated the dipolar coupling mechanism, controlled by the number of GMRs (N) and lateral dimensions (width), to the sensor performance (sensitivity, noise and detectivity) in good agreement with analytical theory. The noise roughly decreases in multi-GMRs as 1/\sqrt{N} in both regimes (low frequency 1/f and thermal noise). The sensitivity is even stronger reduced, scaling as 1/N, in the strong dipolar regime (narrow devices) while converges to a constant value in the weak dipolar regime (wide devices). Very interestingly, they are more robust against undesirable RTN noise than single GMRs at high voltages and the linearity can be extended towards much larger magnetic field range without dealing with the size and the reduction of GMR ratio. Finally, we have identified the optimal conditions for which multi-GMRs exhibit lower magnetic field detectivity than single GMRs: wide devices operating in the thermal regime where much higher voltage can be applied without generating remarkable magnetic noise.

The presence of magnetic noise in magnetoresistive-based magnetic sensors degrades their detection limit at low frequencies. In this paper, different ways of stabilizing the magnetic sensing layer to suppress magnetic noise are investigated by applying a pinning field, either by an external field, internally in the stack or by shape anisotropy. We show that these three methods are equivalent, could be combined and that there is a competition between noise suppression and sensitivity reduction, which results in an optimum total pinning field, for which the detection limit of the sensor is improved up to a factor of ten., Comment: 5 pages, 3 figures

A design study of a horizontal neutron reflectometer adapted to the general baseline of the long pulse European Spallation Source (ESS) is presented. The instrument layout comprises solutions for the neutron guide, high-resolution pulse shaping and beam bending onto a sample surface being so far unique in the field of reflectometry. The length of this instrument is roughly 55 m, enabling $\delta \lambda / \lambda$ resolutions from 0.5% to 10%. The incident beam is focussed in horizontal plane to boost measurements of sample sizes of 1*1 cm{^2} and smaller with potential beam deflection in both downward and upward direction. The range of neutron wavelengths untilized by the instrument is 2 to 7.1 (12.2, ...) {\AA}, if every (second, ...) neutron source ulse is used. Angles of incidence can be set between 0{\deg} and 9{\deg} with a total accessible q-range from 4*10^{-3} {\AA}^{-1} up to 1 {\AA}^{-1}. The instrument operates both in {\theta}/{\theta} (free liquid surfaces) and {\theta}/2{\theta} (solid/liquid, air/solid interfaces) geometry. The experimental setup will in particular enable direct studies on ultrathin films (d ~ 10 {\AA}) and buried monolayers to multilayered structures of up to 3000 {\AA} total thickness. The horizontal reflectometer will further foster investigations of hierarchical systems from nanometer to micrometer length scale, as well as their kinetics and dynamical properties, in particular under load (shear, pressure, external fields). Polarization and polarization analysis as well as the GISANS option are designed as potential modules to be implemented separately in the generic instrument layout. The instrument is highly flexible and offers a variety of different measurement modes. With respect to its mechanical components the instrument is exclusively based on current technology. Risks of failure for the chosen setup are minimum., Comment: Matched to the version submitted to Nuclear Instruments and Methods A

Fitting a data set with a parametrized model can be seen geometrically as finding the global minimum of the chi^2 hypersurface, depending on a set of parameters {P_i}. This is usually done using the Levenberg-Marquardt algorithm. The main drawback of this algorithm is that despite of its fast convergence, it can get stuck if the parameters are not initialized close to the final solution. We propose a modification of the Metropolis algorithm introducing a parameter step tuning that optimizes the sampling of parameter space. The ability of the parameter tuning algorithm together with simulated annealing to find the global chi^2 hypersurface minimum, jumping across chi^2{P_i} barriers when necessary, is demonstrated with synthetic functions and with real data.

Recent advances in microtechnology allow realization of planar microcoils. These components are integrated in MEMS as magnetic sensor or actuator. In the latter case, it is necessary to maximize the effective magnetic field which is proportional to the current passing through the copper track and depends on the distance to the generation microcoil. The aim of this work was to determine the optimal microcoil design configuration for magnetic field generation. The results were applied to magnetic actuation, taking into account technological constraints. In particular, we have considered different realistic configurations that involve a magnetically actuated device coupled to a microcoil. Calculations by a semi-analytical method using Matlab software were validated by experimental measurements. The copper planar microcoils are fabricated by U.V. micromoulding on different substrates: flexible polymer (Kapton) and silicate on silicon. They are constituted by a spiral-like continuous track. Their total surface is about 1 mm2., Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/EDA-Publishing)