A novel magnetic data storage system and a sensing system of magnetic characteristics are disclosed ; the systems have a magnetization direction that is irreversible in an external magnetic field. A method of manufacturing, a method of resetting or changing or repairing and a method of operating such systems are also disclosed. The systems can include a set of magnetic devices in a balancing configuration ; essentially each of said devices comprises a structure of layers including at least a first ferromagnetic layer and a second ferromagnetic layer with at least a separation layer of a non-magnetic material therebetween, said structure having at least a magneto resistance effect. The magnetization direction of the first ferromagnetic layer of at least one of said devices is irreversible in an external magnetic field higher than about 35 kA/m .