In a process for producing organic substrate particles bonded to switchable ferromagnetic nanoparticles with a mean particle diameter in the range from 10 to 1000 nm, the ferromagnetic nanoparticles used are those nanoparticles which are nonferromagnetic at first, but become ferromagnetic when the temperature is lowered, these at first nonferromagnetic nanoparticles in dispersed form are bonded to the organic substance particles, and then the nanoparticles bonded to the substrate particles are made ferromagnetic as a result of the temperature being lowered.
