Site-selective couplings in x-ray-detected magnetic resonance spectra of rare-earth-substituted yttrium iron garnets

Abstract

Site-selective x-ray detected magnetic resonance (XDMR) spectra were recorded in transverse detection geometry on two iron garnet thin films grown by liquid phase epitaxy (LPE) on oriented gadolinium gallium garnet (GGG) substrates: whereas the stoichiometry of the first film corresponded to pure yttrium iron garnet (1 = YIG) used as reference, yttrium was partly substituted with lanthanum and lutetium in the second film (2 = La–Lu–YIG). Surprisingly, the XDMR spectra of film 2 recorded at either the Fe K-edge or the La L3-edge revealed well-resolved structures that had fairly different relative intensity depending on whether we probed the tetrahedral () sites of iron or the dodecahedral () sites of lanthanum. The narrow XDMR lines measured at the Fe K-edge also contrast with the broad, foldover distorted lineshapes of the ferrimagnetic resonance spectra measured in the same scan. Further XDMR experiments were carried out with a thin, disc-shaped, single crystal of gadolinium iron garnet (3 = GdIG). At temperatures slightly above the gadolinium ordering temperature (T > TB = 69 K), the Gd L2-edge XDMR spectra were dominated by two well-resolved lines of nearly equal intensities. Similarly, the Fe K-edge XDMR spectra recorded under identical conditions did also split into several narrow lines but of strongly unequal intensity. These results suggest that, in the exchange-enhanced paramagnetic regime, spins precessing at the dodecahedral () sites of gadolinium do not couple in the same way with spins precessing at either the tetrahedral () or octahedral () sites of iron. On the other hand, destructive interferences between modes of opposite helicities were also observed in Fe K-edge XDMR spectra recorded far above the compensation temperature (T ≫ Tcp = 290 K). This looks like a typical signature of nonlinear four-magnon scattering processes at a very high pumping power.