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A dc to ac magnetic field transformer was developed using a magnetic tunnel junction (MTJ)/microelectromechanical system (MEMS) mixed device. A MEMS torsionator was fabricated with an incorporated magnetic flux guide, that when actuated by a gate electrode, modulates an external dc field into the same frequency of the micro-torsionator oscillation. Attached to it a MgO based MTJ was fabricated, performing the detection of the generated ac magnetic field. This dc to ac field transformation enables the detection of dc magnetic fields in the high frequency thermal noise regime, where the 1/f noise is typically two orders of magnitude lower. Measurements show the detection of a modulated field at 460 kHz, coming from the MEMS torsionator designed to exhibit high modulation efficiency. This efficiency was of 11%, i.e., 11% of the external dc field is modulated. As a result, from a MTJ exhibiting a noise of [S.sup.MTJ.sub.v] = 4.4 nT/[Hz.sup.1/2] at 460 kHz, this hybrid device enables a dc magnetic field detection of 40nT/[Hz.sup.1/2]. This work acts as a major improvement to a previous one [7], mainly through the optimization of the MEMS magnetic field modulation efficiency. Index Terms--Field modulation, magnetic flux guides, MEMS, MgO tunnel junctions, noise, 1/f noise.

Abstract: We developed spin vale tunneling magnetoresistance devices based on CoFe-MgO-CoFeB materials capable of sensing magnetic field in wide range. The devices offer magnetic field range up to ±100 mT and down to ±0.4 mT, nonlinearity of 1% of full scale and ultra low hysteresis below 0.025% of full scale. In the largest field range devices show sensitivity up to 0.06%/mT and hysteresis below 0.025% of full scale, while in the lowest field range exhibit sensitivity up to 12%/mT and hysteresis below 2% of FS. [Copyright &y& Elsevier]

We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel junctions. By taking advantage of the perpendicular magnetic anisotropy of the MgO/CoFeB interface, the magnetization of the sensing layer is tilted out-of-plane which results in a linear magnetoresistance response to in-plane magnetic fields. The application of a bias voltage across the MgO tunnel barrier of the sensor affects the magnetic anisotropy and thereby its sensing properties. We propose a voltage-tunable magnetic field sensor design that allows for active control of the sensitivity and the operating field range by the strength and polarity of the applied bias voltage. [ABSTRACT FROM AUTHOR]