A thin-film device for detecting the variation of intensity of physical quantities, in particular a magnetic field, in a continuous way, comprises an electrical circuit including one or more sensitive elements, which are designed to vary their own electrical resistance as a function of the intensity of a physical quantity to be detected. One or more of the sensitive elements comprise at least one nanoconstriction, and the nanoconstriction comprises at least two pads made of magnetic material, associated to which are respective magnetizations oriented in directions substantially opposite to one another and connected through a nanochannel. The nanochannel is able to set up a domain wall that determines the electrical resistance of the nanoconstriction as a function of the position, with respect to the nanochannel, of the domain wall formed in the sensor device. At least one cross section of the nanochannel is configured so as to present a variable extension along one or more axes as a function of different values of the physical quantity to be detected.