Magnetic resonance in RuSr2RECu2O8 (RE=Eu, Gd) ferromagnetic superconductor

Magnetic resonance techniques at frequencies of 9.4, 34, 110, and 219 GHz w ere used to investigate the magnetism of the recently discovered ferromagne tic superconducting system RuSr2RECu2O8 (RE = Eu, Gd). One absorption is ob served in both compounds below the magnetic ordering temperature, T-M simil ar to 135 K, and can be ascribed to the ferromagnetic resonance of the orde red Ru ions. The resonance field of this absorption is frequency dependent and is well described assuming an in-plane ferromagnetic component with str ong out-of-plane (H-z) and weak in-plane (H-x) anisotropy fields. We have d erived almost the same anisotropy fields for both compounds at 70 K ([H-z] similar to 250 kOe and [H-x] similar to 165 Oe). Another absorption is obse rved only in the Gd compound and arises from the paramagnetic resonance of the Gd3+ ions. Below T-M the Gd3+ signal shifts from g = 2 evidencing the a ppearance of a homogeneous internal field, H-i,H-Gd, that is frequency (and hence field) dependent. This variation is probably due to the nonsaturated nature of the Ru lattice. Below 10 K there is an additional shift of the G d3+ line arising from the Gd dipole-dipole interaction. (C) 2001 American I nstitute of Physics.