Magnetic interactions and magnon gap in the ferromagnetic superconductor RuSr2GdCu2O8

By means of X- and Q-band magnetic-resonance measurements we have investiga ted the magnetic interactions and derived the magnon gap in the ferromagnet ic superconductor RuSr2GdCu2O8. Two microwave absorptions are observed: fir st, a paramagnetic resonance of the Gd3+ ions, and second, appearing below the Curie temperature (T-Curie), a ferromagnetic resonance due to the ruthe nium lattice. Located between the superconducting CuO2 planes, the Gd ions serve as intrinsic probes of the internal magnetic fields near the Cu sites . Below T-Curie the Gd3+ signal shifts, evidencing the appearance of a homo geneous internal field, HRu-Gd similar to 600 Oe. We show that HRu-Gd is du e to a Ru-Gd ferromagnetic exchange interaction. The Ru ferromagnetic reson ance indicates the existence of a magnon anisotropy gap w/gamma greater tha n or similar to 4600 Oe. We estimate the out-of-plane and in-plane anisotro py fields to be, respectively, H-z similar to 110 kOe and H-x similar to 20 0 Oe. [S0163-1829(99)50242-7].