Integrated Magnetic imAgery
based on spIntronics Components
The Giant MagnetoResistance (GMR) effect has been discovered in Fe-Cr multilayer. In absence of external field, antiferromagnetic coupling creates an alternation of moments. In this case, both electron channels are strongly difused in the layers and the resistance of the device is high. If a magnetic field is applied that tends to align the moments in the same direction, one electron channel is only slightly
difused and the resistance is low. Though historic this first device has a low sensitivity. More sensitive devices used nowadays in hard-drive read heads are Spin Valves which consist in a nanometric stack of Ferromagnetic-Nonmagnetic-Ferromagnetic layers.
Spin valve GMR sensors
The response of a spin valve sensor will depends on several parameters. The first one is the composition of the stack and the quality of growth. this will determine the amount of magnetoresistance and the temperature variation of this effect. The second parameter is the shape of the sensor which will determine the linearity of the response and the hysteresis of the response. The third is the creation of a specific anisotropy for the free layer by for example and external bias field. Playing on all these parameters, it is possible to optimize the response for a specific application and also the signal to noise ratio. Figure 1 gives an example of stack used for magnetic sensing and figure 2 gives a response of an optimised magnetic sensor. The MR variation is typically between 6 to 10% but values up to 15% have been reported