Magnetic field sensor utilizing magneto impedance in thin film multi-layers
Since the discovery of the Magneto Impedance (MI) effect in 1994 there has been a global increase in the research devoted to understanding the effect. In certain magnetic materials, the impedance change, often referred to as the MI ratio, is in the range of 50 to 100% for an excitation current in the MHz frequency range for external magnetic fields of a few Oe. The use of thin film multilayer structures allows the increase of sensitivity and the reduction of size for MI effect to be integrated with micro magnetic sensor technologies. In the present work, we explain the origins of the MI effect and its versatile nature for the development of sub nano Tesla magnetic field sensors. The matrix like nature of the MI effect allows a variety of MI characteristics to be implemented in a thin film, which allows the structure to be tailored for maximum sensitivity in the chosen field sensing application. In the case of a simple transverse magnetic anisotropy, the diagonal components of the MI matrix are symmetric and the off diagonal components are anti-symmetric with respect to the dc longitudinal field. The asymmetry in the MI behaviour can be related to either a certain asymmetric arrangement of the dc magnetization (crossed an isotropy), or a contribution to the measured voltage due to the ac cross-magnetization process, which is represented as an off-diagonal component. These asymmetrical characteristics are useful in producing linear bi-directional field sensors without DC biasing. In attempt to find optimal film systems with respect to relative impedance change, sensitivity, linearity, operational frequency range, and dimensions, thin film multi-layers, consisting of a magnetic / conductor / magnetic material configuration were fabricated. Variations in magnetic compositions, geometries, structures and magnetic configurations (transverse, longitudinal or cross anisotropy) and additional insulations layers were produced. Planar coil thin film multi-layers were constructed to utilize the more magnetic complex asymmetric characteristics of the MI effect. An experimental configuration was developed in order to measure all components of the MI matrix within the thin films and standardise their sensitivity using the MI ratio. Two sub nano Tesla magnetic field sensors were developed during the course of this work based on the fabricated thin films. The first sensor concentrates on utilizing two asymmetrical Magneto Impedance (AMI) elements combined differentially. The sensor is driven by a sinusoidal current of 90 MHz biased with a dc bias current. For AMI film element of 5mm long, 40µm wide and having anisotropy angle of 45° the field detection resolution is in the magnitude of 1µ Oe for both ac and dc for fields of ~ 20e magnitude. The maximum response speed is in the order of 1MHz. The use of MI to the measurement low frequency fields such as bio-medical signals drove the design of the second sensor. Extensive research was undertaken to improve the phase noise of the oscillator and sensitivity of the detection mechanism using novel RF techniques to improve the sensitivity at high frequencies, and secondly a method to improve the low frequency sensitivity by AC biasing the MI element with a magnetic field. A thin film multilayer MI sensor was produced based on the measurement of the modulation of the incident reflected power due to an external AC magnetic field. Direct field measurement performance at 1kHz produced a resolution of 3.73 x 10-7 Oe. AC biased performance at 5kHz of a 20Hz field was a resolution of 5.27 x 10-6 Oe, and at 10Hz of 9.33 x 10-6 Oe. With continued improvement of the electronic components utilized in this novel method of Magneto Impedance sensor presented in this work, the possibility of measuring bio magnetic signals of the human body at room temperature becomes a distinct reality.