99,101Ru NMR study of EuSr2RuCu2O8: Coexistence of ferromagnetism and superconductivity

Abstract

The copper-ruthenium oxide EuSr2RuCu2O8 (EuRu1212) has been studied by means of nuclear-magnetic resonance (NMR) and nuclear spin-lattice relaxation rate (T1−1) experiments together with measurements of x-ray diffraction, electrical resistivity, and magnetization. This compound shows the ferromagneticlike transition at TM∼130K, and the superconducting transition at Tcon∼30K. The EuRu1212 does not show an Invar effect below TM in the temperature dependence of the volume. The 99,101Ru NMR signals under zero applied field at 4.2 K have been observed in the frequency ranges from 30 to 80 MHz and from 110 to 150 MHz, both of which could be identified as ferromagnetic resonances even below Tcon. These results clearly indicate that the Ru moments have a ferromagnetic component even below Tcon. These frequency-swept spectra around 60 MHz and around 130 MHz are ascribed to 99,101Ru nuclei of Ru4+(S=1) and to those of Ru5+ (S=32), respectively. These results reveal that the Ru ions are in a mixed valent state and that the Ru 4d electrons are not itinerant but localized. The 101Ru spin-lattice relaxation rate T1−1 decreases steeply just below Tcon, because of the formation of superconducting gap. The divergent behavior of (T1T)−1 below ∼10K indicates that the “spontaneous vortex phase” occurs in the superconducting state, in which the transferred field from the Ru sublattice to the CuO2 layer is present between the lower and the upper critical field.

Source-reported materials — not catalogue approval

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