Low Temperature Critical Fluctuations of the EPR Spectrum of Yb³⁺ Ions in the Yb_0.5Sm_0.5Ba₂Cu₃O_6+x Compound in the Tetragonal Phase

Abstract

Ceramics of the type REBa2CU306+, (RE123) in which RE is a rare earth element have been intensively investigated in recent years by various techniques, including the electron paramagnetic resonance (EPR). At low temperatures, an EPR signal arising from the exchange interaction of pairs of coupled divalent copper ions has been detected in the tetragonal phase of the RE123 compounds [l, 21. The effective g-value of that EPR signal exhibits a strong increase below z 20 K, that can be interpreted by considering the influence of an additional internal magnetic field. Additionally, when approaching the lowest temperature investigated from above the EPR linewidth shows a substantial increase. Magnetic measurements for the corresponding RE123 compounds have systematically shown a positive divergence of the inverse magnetic susceptibility indicating a ferromagneticlike ordering [l to 31 which according to fluctuation theory can justify the strong temperature dependence of the EPR parameters. Another possible manifestation of such an ordering may be the observed temperature variation of the non-resonant zero-field absorption line arising from the Josephson effect in the LaBaSrCu306+, system [4]. Upon lowering the temperature below 20 K the position of that line shifted at higher magnetic fields being eventually split at about 3 K. The aim of this note is to investigate the effect of the magnetic ordering process taking place at low temperature on the EPR spectrum of the Yb3+ ions, observed in the tetragonal phase of the Yb,,,Sm,~,Ba,Cu30,+x compound.The investigated sample was prepared by the standard solid state reaction method, reported previously in details [5]. After the initial X-ray diffraction (XRD) and EPR measurements the sample was reannealed at 450 "C for 12 h in oxygen flow and slowly cooled to room temperature (RT). Finally, in order to obtain the tetragonal phase the sample was submitted to further annealing at 450 "C in helium flow for 5 h followed by fast cooling to RT in a reducing atmosphere. EPR measurements were performed using a standard spectrometer (Bruker ER-200 D) operating at X-band frequency (v = 9.234 GHz) with 100 kHz field modulation. The static