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 L 3 -edge revealed well-resolved structures that had fairly different relative intensity depending on whether we probed the tetrahedral (S 4 ) sites of iron or the dodecahedral (D 2 ) 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 > T B = 69 K), the Gd L 2 -edge XDMR

Origin of the spin Seebeck effect probed by temperature dependent measurements in Gd$_{3}$Fe$_{5}$O$_{12}$

“…Each unit cell consists of 12 trivalent Fe atoms that are tetrahedrally coordinated with oxygen atoms (d sites, point group S 4 ), 8 Fe atoms that are octahedrally coordinated (a sites, point group S 6 ), and 12 dodecahedrally coordinated Gd atoms (c sites). The two Fe sub-lattices are strongly coupled via antiferromagnetic superexchange (exchange constant J ad 32 cm −1 ), [27] with a Néel temperature of about T N = 550 K. Since the Gd moments are more weakly exchange coupled to the Fe a sub-lattices (J ac 7 cm −1 ) [27] leading to an ordering temperature of the Gd spins at around 69 K, [28] they experience an effective magnetic field below T N and can be treated as an 'exchange-enhanced' paramagnetic moment. [29] As a consequence the Gd magnetization can be described by a strongly temperature-dependent Brillouin function for spin 7/2.…”

“…In other words, the magnons in the Fe sub-lattices will substantially contribute to the SSE for T < T C ≈ 550 K, while the Gd sub-lattice exhibits a qualitatively different thermomagnetic behavior and significantly contributes to the SSE only for T T Gd ≈ 65 − 85 K. As mentioned above, the latter temperature is often also referred to as the Gd ordering temperature in the literature. [28] This assumption is motivated by the existing literature investigating finite temperature effects on ferromagnets via the self consistent renormalization of magnon dispersions. [33] In particular, it has been shown that the magnons cease to be the admissible eigenstates above a temperature around the ordering temperature.…”