RFID tags and devices disclosed herein use ferromagnetic films to store information. The tags include patches of ferromagnetic materials, each patch having a particular ferromagnetic resonance frequency determined by the composition of the ferromagnetic film. When stimulated, the patches emit microwave or RF signals at their resonance frequencies and at intensities proportional to the patch sizes. The signals are read and the frequency spectrum of the tag may be determined by using a FFT. Identity and other information may be provided by the spectrum.