The potential advantage of some magnetic sensors having a large response is greatly decreased because of the 1 / f noise. We are developing a device, the microelectromechanical system ͑MEMS͒ flux concentrator, that will mitigate the effect of this 1 / f noise. It does this by placing flux concentrators on MEMS structures that oscillate at kilohertz frequencies. By shifting the operating frequency, the 1 / f noise will be reduced by one to three orders of magnitude depending upon the sensor and the desired operating frequency. We have succeeded in fabricating the necessary MEMS structures and observing the desired kilohertz normal-mode resonant frequencies. Only microwatts are required to drive the motion. We have used spin valves for our magnetic sensors. The measured field enhancement provided by the flux concentrators agrees to within 4% with the value estimated from finite element calculations. No difference was detected in noise measurements on spin valves with and without the flux concentrators. This result provides strong evidence for the validity of our device concept. Solutions to the sole remaining fabrication problem will be discussed.
The phase evolution during annealing of AlJNi multilayer samples prepared by ion-beam sputtering with composition modulation wavelengths A between 10 and 400 nm was determined using x-ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of &n,N10,6,, and Al,,,N&,~ were investigated. For the Al,,40Ni0.60 samples the following results were obtained. A measure of the degree of periodic@ and the sharpness of the interfaces in a sample with A=80 mn was the large number (over 20) of peaks observed in small-angle x-ray scattering measurements. A sample with A=10 nm was transformed by heat treatment directly to the AlNi phase. In the A=80 nm sample, the first phase formed after annealing was the metastable Y,I phase. The r] phase was identified as Al,Ni,. In the 400 nm wavelength sample, both the metastable 17 phase and the stable Al,Ni formed after the first exothermic reaction. For the Alr,75Ni0,25 samples two results were obtained. A A=11.4 nm sample transformed directly on annealing into Al,Ni. The 77 phase was the first phase formed on annealing a A=100 nm sample. The difference in the component diffusivities and the concentration gradient play an important role in controlling phase formation and evolution.
The mechanical properties of multilayered Cu-Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force-displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch-type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.
A device, a microelectromechanical system flux concentrator, is described that can minimize 1/f noise in magnetic sensors by modulating the magnetic field at the position of the sensor. This has the effect of shifting the operating frequency to higher frequencies where the 1/f noise can be 1 or 2 orders of magnitude smaller. Magnetic and mechanical modeling results on a design that will operate at 29 kHz are presented.
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