Giant Magnetoresistance Imaging for NDE of Conductive Materials
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There is another fundamental relationship that will affect coil responsivity. The flux through a coil can only generate a current in the coil if the magnetic field changes as a function of time. Specifically, the curl of the electric field vector, which generates the current flow in the coil, is defined by the change in the magnetic field orthogonal to the plane of the coil as described by Maxwell's equation:
vx- aB
VxE= __.
at
This equation demonstrates the fundamental problem associated with using coil-based sensors to detect low-frequency magnetic signals. As the frequency of the applied signal is reduced, so is the rate of change of the applied magnetic field with respect to time. This, in turn, reduces the coil current. AN INTRODUCTION TO MAGNETORESISTIVE MATERIALS The magnetoresistance phenomenon has been known for about 100 years. Magnetoresistance is a galvanomagnetic phenomenon caused by the force exerted on electronics by a magnetic field. This force is described by the Lorentz force equation: 151 MaL Res. Soc. Symp. Proc. Vol. 591 @2000 Materials Research Society
F = q(E + v x B)
where q is the charge on an electron, E is the electric field, v is the drift velocity of the electron, and B is the magnetic field intensity. The qE term is the familiar force created by applying an electric field to a material and causes current flow from device input to output. The q(v x B) term is the source of magnetoresistance effects and causes electrons to be pushed at right angles to the drift caused by the qE term. Magnetic materials organize into microscopic "domains" that have a consistent magnetic orientation. When electrons are moving in the magnetic material, they are redirected or "scattered" as they attempt to traverse these domains. The amount of scattering is a function of the electron drift direction and the orientation of the magnetic domain. When the drift direction lies along the magnetic domain there is no scattering, but when the drift and domain orientations are different the electron will experience increasing scattering. SPIN VALVE GMR DEVICES Spin valves are multilayer devices composed of a freely orientable magnetic layer, a conductive spacer, a pinned magnetic layer, and a pinning magnetic layer as shown in Figure 1. The pinning layer is a magnetic material (FeMn) that self-configures into an anti-ferromagnetic state and the pinned layer is a standard magnetic material such as Permalloy (NiFe). These two layers are arranged in such a manner that the pinned layer is forced into a ferromagnetic state. The conductive layer is a thin layer of a conductor such as copper (Cu). The free layer is an easily magnetized material such as NiFe. Electrons flowing through the conductive layer will interact with the upper and lower magnetic layers. If no external field is applied the free lower magnetic layer not will be aligned with the fixed upper magnetic layer. Electrons with spins that match the magnetic layer orientation are minimally scattered. Electrons with the opposite spin are strongly scattered.
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