Thin-Film Magneto-Impedance Sensors

Abstract

We review the state-of-the-art of thin films displaying the magneto-impedance (MI) effect, with focus on the aspects that are relevant to the successful design and operation of thin film-based magnetic sensors. After a brief introduction of the MI effect, the materials and geometries that maximize the performance, together with the measurement procedure for their characterization, are exposed. A nonexhaustive survey of applications is included, mostly with the aim of displaying the capabilities of thin film structures in the field of magnetic sensing, and the variety of topics covered by them. A special emphasis is made on some concepts that are not commonly treated in the literature, such as the influence of the measuring circuit on the magneto-impedance ratio, the geometry optimization by means of numerical simulation by finite element methods, or noise measurements on thin films. Additionally, a brief description of the patterning procedure by photolithography is included, since the major advantage of thin film sensors over other types of magneto-impedance materials as ribbons or wires is the possibility of patterning the sensible element in micrometric shapes, and most of all, their easy integration with the interface microelectronic circuitry.

Ultrasensitive Magnetic Field Sensors for Biomedical Applications

“…However, due to the fact of technical problems and unstable behavior, it did not receive proper development until its rediscovery as the giant magnetoimpedance effect (GMI) in 1992-1994 in amorphous wires and ribbons [98][99][100]. In an attempt to decrease the GMI element size, thin films and multilayered films were proposed [101,102].…”

Magneto-Impedance Biosensor Sensitivity: Effect and Enhancement

“…( b ) Open path magnetic flux ϕ as conductor is not enclosed by magnetic layers resulting in low MI effect. Reprinted with permission from [ 9 ], under the Creative Commons Attribution (CC BY 3.0) license.…”

“…For the longitudinal anisotropy configuration, at H = 0, the transverse permeability is maximum and indicated by Z(H) curve that poses a single peak. For transverse anisotropy, the transverse permeability is maximum at H k , given by Equation (4) [8,9]:…”

“…In this layout, the magnetic flux produced by the AC faces a closed path resulting in high permeability. The configuration shown in Figure 3b, magnetic and conductive layers have the same width, the magnetic open-flux created by the AC field crosses a low permeability path diminishing the resultant MI effect [9,69]. The open-flux sandwich configuration is easily fabricated in one step as compared to the complex fabrication required for the closed-flux configuration.…”

“…The Z(H) curve is influenced by anisotropy where longitudinal anisotropy occurs when the magnetized sample is orientated parallel to the applied field and the current flow as shown in Figure 1 c. Whilst, transverse anisotropy occurs when the magnetized sample is orientated perpendicular to the applied field and the current flow as shown in Figure 1 d. For the longitudinal anisotropy configuration, at H = 0, the transverse permeability is maximum and indicated by Z ( H ) curve that poses a single peak. For transverse anisotropy, the transverse permeability is maximum at H k , given by Equation (4) [ 8 , 9 ]: where K denotes the anisotropy constant and M s describes saturation magnetization. The maximum sensitivity is achieved when MI is maximum and H k is at its minimum.…”