Reliable detection of hazardous gases has become a key issue due to the more stringent environmental and safety regulations worldwide. Solid-state conductometric gas sensors present a high potential for applications where the use of conventional analytical systems such as gas chromatography or optical detection is prohibitively expensive or impossible. The operation of such sensors is based on a change of electric conductivity when exposed to an atmosphere containing specific reagents, due to charge transfer between the sensor material and the adsorbed species.  An important disadvantage of such sensors is the lack of chemical selectivity, and sensitivity to humidity. Sensing materials are typically sensitive to more than one chemical species and show cross-sensitivity when different reactive gases are present simultaneously in the surrounding atmosphere. When the only parameter measured by the sensor is the change of resistance one cannot discriminate between different gases and their concentrations that can generate the measured signal.
   We develop a new type of gas sensor that solves the problem of cross-sensitivity by monitoring two independent parameters sensitive to the target gases: resistance and magnetization. The sensor concept is based on the so-called Extraordinary Hall effect (EHE) by which magnetization of ferromagnetic films can be detected by a simple electronic transport measurement. The procedure is technically similar to the four-probe measurement of resistance, with two modifications: 1) Hall voltage is measured in the direction perpendicular to electric current flow, and 2) the measurement is generally done under a bias magnetic field. Thus, two independent gas-sensitive parameters are measured in the same magnetotransport setup. In the test-case of hydrogen detection, the EHE sensitivity of CoPd sensors in the ppm H2 concentration range is two orders of magnitude higher than the conductometric on
Schematic illustration of the sensor principle and examples of the conductometric and magnetic response of a CoPd  sensor to hydrogen.