Single layer magnetoresistive sensors were designed in a Wheatstone bridge configuration using La 2/3 Sr 1/3 MnO3 ferromagnetic oxide thin film. Uniaxial anisotropy was induced by performing epitaxial deposition of the films on top of vicinal SrTiO3 substrate. X-ray scan confirms high crystalline quality of the films and the magnetic anisotropy was checked by Magneto-optical Kerr Effect measurements. Thanks to the anisotropic magnetoresistive effect and the very low noise measured in the devices, sub-nT resolution was achieved above 100 Hz at 310 K.
We present the design, fabrication, and characterization of an implantable neural interface based on anisotropic magnetoresistive (AMR) magnetic-field sensors that combine reduced size and high performance at body temperature. The sensors are based on La 0.67 Sr 0.33 MnO 3 (LSMO) as a ferromagnetic material, whose epitaxial growth has been suitably engineered to get uniaxial anisotropy and large AMR output together with low noise even at low frequencies. The performance of LSMO sensors of different film thickness and at different temperatures close to 37 °C has to be explored to find an optimum sensitivity of ∼400%/T (with typical detectivity values of 2 nT•Hz −1/2 at a frequency of 1 Hz and 0.3 nT•Hz −1/2 at 1 kHz), fitted for the detection of low magnetic signals coming from neural activity. Biocompatibility tests of devices consisting of submillimeter-size LSMO sensors coated by a thin poly(dimethyl siloxane) polymeric layer, both in vitro and in vivo, support their high suitability as implantable detectors of low-frequency biological magnetic signals emerging from heterogeneous electrically active tissues.
The current trend in magnetoresistive sensors development
is to
increase the sensitivity of single sensing elements by using multilayer
structures and to design them into arrays. Such arrays are designed
to compensate the excess low frequency noise of individual elements,
which limits their magnetic resolution. Here, we report the modeling,
design, and fabrication of single layer anisotropic magnetoresistive
(AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation
of the sensor is based on a step-induced uniaxial magnetic anisotropy,
described using the Stoner–Wohlfarth model. A coherent magnetization
reversal process is observed by magneto-optical Kerr effect imaging.
A good agreement between experimental data and the expected sensor
response confirms the correct operation of the device. Three main
fabrication parameters, namely the vicinal angle of the substrate,
the deposition temperature, the thin film thickness, and their effects
on film anisotropy field and device detectivity have been studied.
Detectivity levels as low as 1.4 nT Hz–1/2 at 1
Hz and 240 pT Hz–1/2 in the white noise region are
achieved with a single Wheatstone bridge element operating at 310
K. Compared to GMR and AMR sensors, these results are promising for
further development and for their use as single layer LSMO low field
AMR sensors, including applications as implantable biomedical devices.
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