Low-temperature magnetic sensors are based on Hall effect in thin metallic films. Their sensitivity is more than one order of magnitude smaller than silicon-based Hall sensors operating at room-temperature, because so is the Hall effect in metals. Besides, in order to avoid significant Joule heating, only very small bias currents can be injected. Here we show that a sensor based on photo-induced Hall effect, in which charge is photo-generated in a semiconductor and injected into an adjacent metallic layer, can be used as a bias-free, cryogenic sensor. The system consists of a platinum thin-film deposited on intrinsic-silicon substrate. The film forms a Schottky interface with the semiconductor. At room temperature, carriers photo-generated in the semiconductor are injected into the metal because of rounding-off of the Schottky barrier and deflected by a magnetic field applied in the film-plane. At cryogenic temperature and well-below the freeze-out temperature for silicon, the photo-generated electrons can tunnel through the barrier and the sensor recovers the same sensitivity as that observed at room temperature.