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Hybrid sensors for fT field detection: biomedical imaging
Within the BIOMAGSENSE FP6 project, INESC MN together with Trinity College are developing MTJs for incorporation in flux guide structures (superconducting or ferromagnetic) aiming at hybrid sensors with a low frequency (1Hz) sensitivity near 1fT/sqrt(Hz). INESC MN is also placing one or both ferromagnetic flux guides in an AC MEMS structure, producing at the sensor, a modulated AC field (modulated at twice the MEMS gate frequency) , from the starting DC field..This will allow using the sensor at its thermal background, avoiding 1/f noise contributions.
a) spin valve sensors with CoZrNb flux guides
We have shown that down to 60 pT/sqrt(Hz), CoZrNb amorphous ferromagnetic flux guides do not introduce extra 1/f noise. Hybrid sensors incorporating spin valves, and CZN flux guides with gains up top 30 were fabricated, and noise measurements performed showing 2.2nT/sqrt(Hz) sensitivity at 10Hz , and 64pT/sqrt/Hz sensitivity at 10kHz ( thermal background, see Fig.1).
Fig. 1: Magnetic field detection in the 2 kHz–500 kHz range, for the different SV sensors. The applied external field at which the noise level was measured is indicated. The applied bias current was 0.4 mA.
b) AC MEMS + flux guides + spin valve sensors
Fig.2 shows the new AC-MEMS structure fabricated where one of the flux guides is on a vibrating MEMS cantilever. This flux guide has a gain of 3, and conveys the modulated DC signal field into the spin valve sensor. This device already allows the determination of the cantilever resonance frequency by the spin valve sensor, and demonstrates the capability of measuring DC fields down to tens of pT, using the standard spin valve sensor.
Figure 2: Optical microscope picture of the microfabricated AC MEMS device ( 2nd batch). Cantilever resonance frequency measured using the integrated spin valve sensor response.
c) MTJs with picture frame geometry: reduction of 1/f noise
MTJ sensors with picture frame geometry were fabricated in order to minimize 1/f noise in the sensor linear range. Fig.3 shows the absence of significant magnetic 1/f noise in the sensor linear region.,contrary to what was observed previously in rectangular high aspect ratio sensors in both MgO and AlOx MTJs. Noise measurements made in saturation have the same order of magnitude as in the sensitive region, indicating only a small contribution of magnetic 1/f noise.
Fig.3-Noise level at three different frequencies with Ibias = 1mA. The inset shows an optical microscope picture taken from the finished picture frame sensor with the following PF dimensions: length = 70mm, width = 30mm, height = 3mm; and junction area = 2´5mm2. The transfer curve shows the measured points. The magnetic tunnel junction was deposited by ion beam deposition and oxidation and has the structure: (thickness in nm): Al 70/ Ta 9/ NiFe 3/ CoFeB 5/ Al 0.5 (+ oxidation)/ CoFeB 4/ Ru 0.7/ CoFeB 3/ MnIr 25/ Ta 6/ TiW(N) 30. Here NiFe, MnIr, CoFeB, and TiW(N) stand for Ni80Fe20, Mn76Ir24, Co73Fe17B10 and Ti10W90(N) respectively (alloy compositions in at%).