A method for modifying the value of a electric resistor (30), comprising a ferromagnetic material (31) having a first direction of magnetisation, comprising the following steps: a) illuminating a first area (32) of the ferromagnetic material (31) with a first LASER beam (14), such that this area is heated to a temperature greater than or equal to the Curie temperature of the ferromagnetic material (31); b) applying, to the first area (32), a magnetic field of which the direction is the opposite to the first direction of magnetisation of the ferromagnetic material (31) ; c) reducing the energy provided by the first LASER beam (14) at the first area (32) in order to allow the first area to cool to form a first controlled magnetic domain (36C).