AVS 53rd International Symposium
    Magnetic Interfaces and Nanostructures Tuesday Sessions
       Session MI-TuP

Paper MI-TuP8
Low Noise, High Sensitivity Anisotropic Magnetoresistive Sensors with Second Harmonic Readout

Tuesday, November 14, 2006, 6:00 pm, Room 3rd Floor Lobby

Session: Magnetic Interfaces and Nanostructures Poster Session
Presenter: S.T. Halloran, University of Colorado
Authors: S.T. Halloran, University of Colorado
H. Fardi, University of Colorado
F.C.S. da Silva, University of Colorado
D.P. Pappas, National Institute of Standards and Technology
R.R. Owings, National Institute of Standards and Technology
E.W. Hill, University of Manchester
Correspondent: Click to Email
Low noise magnetic field sensors have a wide range of applications. At present, the best signal-to-noise ratio (SNR) sensors on the market are based on anisotropic magneto-resistance (AMR). In this work we demonstrate low noise sensors that operate by modulating the bias on soft-adjacent-layer (SAL)-biased AMR devices and sensing the second harmonic signal. The 2f technique allows us to move the signal above the 1/f noise regime and into a less noisy bandwidth. We have measured the noise response of these sensors and demonstrated the scalability of the noise with the volume of the sensor. Using the Johnson noise limit, we show that a sensitivity of 1 pT/@sr@Hz is achievable in these devices at reasonable power levels. In addition, we show that the 2f detection is capable of providing high-contrast magnetic field images that reject thermal asperities. This is due to the fact that the interaction of the bias current and magnetic moment of the sensor is independent of the sign of the bias current. Sensors are fabricated by DC sputtering (80)Ni(20)Fe onto a Ta seed layer in a needle pattern. The SAL bias is achieved with NiFe as well, separated from the MR layer by a thin SiN insulating layer. The needle is 20 µm wide by 200 µm long with an overall thickness of 0.450 µm. The needle aspect ratio of 4:1 (or a needle angle of 30 degrees) is chosen to optimize the single-domain formation by preventing vortices from forming. The magneto-resistor (MR) layer was sputtered in a 200 Oe field along the easy axis of the sensor. Full bias is provided by the SAL. Initial findings show a preferred bias at 15 mA which corresponds to a RMS voltage (peak to peak) of 5.2 at 1 KHz. We used a lock-in technique to generate the 2f signal from the differential output of the bridge. A custom MR looper was built to characterize these sensors. Applications include arrays of bridges for high-resolution scanned MR microscopy including room temperature magnetocardiograms.