TMR-sensors from Crocus are based on a new technology for magnetic field measurement. The main advantage of the TMR-sensors compared to Hall-sensors are low current consumption and high sensitivity.
The basic unit of TMR-sensors is the so called magnetic logic unit (MLU) which is a stack of several layers of different materials. The resistance of this stack perpendicular to the layers is dependent on the orientation of an external magnetic field compared to a fixed axis.
The substrate for this stack is an antiferromagnetic layer. Above this layer there is the so called pinned layer which is a ferromagnetic layer that has a fixed magnetization in a fixed direction. The direction of this magnetization defines the reference direction and it cannot be changed by an external field. Above this pinned layer there is a very thin, insulating layer with a thickness of only a few Nanometers. Above this insulaing layer another magnetic layer is deposited. The orientation of the magnetization of this so called free layer can be influenced by a external field. The resistance of the whole stack of layers is dependent on angle between the magnetizations of the free and the fixed layer. It can be measured by applying a voltage at conducting electrodes at the top and bottom side of the stack. The resistance of the stack varies between 10 kW and 60 kW. This resistance range of the MLU is much higher compared to other magnetoresistive technologies like AMR (Anisotropic Magnetic Resistance) und GMR (Giant Magneto Resistance). The field necessary to rotate the magnetization of the free layer is quite small. Most of the TMR-sensors operate below 10 mT, some even below 1 mT.
Basically this technology can be used to build magnetic switches or magnetic sensors with analog output. For magnetic switches the current through the MLU is measured and a comparator drives the output stage of the sensor (either open collector or push-pull) high or low, depending on fixed internal threshold values.