Low field microwave absorption studies of normal and superconducting states in Iron pnictides

Abstract

Low field microwave absorption (LFMA) or non-resonant microwave absorption (NRMA) phenomenon has turned out to be a powerful and exciting technique in characterizing superconductors and magnetic materials. The technique uses an electron spin resonance (ESR) spectrometer. Importantly, an ESR spectrometer is designed to study materials consisting of unpaired electrons (spins). Consequently, these materials show a resonance signal when the energy between two spin levels matches the radiation energy. In LFMA technique, a few magnetic materials show two absorption modes i.e a resonance spectrum and a LFMA signal centered around zero magnetic field. Interestingly, the mechanisms of LFMA signal in these magnetic materials are yet to be fully understood. High Tc cuprate superconductors have shown LFMA signal in the superconducting state. However, in their normal state, both the LFMA signal and the resonance signal are conspicuously not registered despite the fact that they contain paramagnetic species of Cu2+. The mechanisms behind the quenching of Cu2+ localized moment in these materials is unresolved too. Now, with the advent of iron-based superconductors (FeSc) which were discovered in 2008, a variety of materials have been discovered with diverse properties thus providing an excellent platform to study and address some of the aforementioned debatable topics. Therefore, this piece of work involves LFMA studies of normal and superconducting states of FeSc. We are motivated by the fact that (1) few reports have been presented on LFMA in superconducting state of these materials (2) no normal state LFMA studies have been reported thus far in any superconducting material (3) incomplete understanding of LFMA phenomenon in general (4) a possible normal state application of these materials in spintronics i.e. room temperature low field magnetic sensor and microwave absorbers. In this study, we have observed both LFMA and resonance spectrum in the normal state of SmFeAsO1-xFx. The resonance spectrum which simply marks a saturation state is observed to evolve as a function of temperature and becomes ‘ESR silent’ in the superconducting state. More importantly, we observed a novel LFMA signal in the normal state. This signal is reported here for the first time in any superconducting material. In the superconducting state, LFMA signal is detected. Here the LFMA signal evolves as a function of temperature magnetic field and microwave power. Furthermore, a structure (a broad peak and narrow peak) is exhibited in all sample forms (powder and pellet) and in all doping used. This suggests that this structure is a generic feature in SmFeAsO1-xFx. We have precisely differentiated the normal state LFMA signal with its counterpart in the superconducting state on three accounts namely: (1) a resonance signal is only observed in the normal state but is ‘ESR silent’ in superconducting state (2) the peak to peak LFMA signal intensity in the normal state increases as a function of temperature up to 180 K and then decreases onwards whereas in the superconducting state a decrease of intensity with increase in temperature is witnessed; and (3) the line width of LFMA signal in the superconducting region is ~ 100 G compared to ~ 650 G in normal state. Key words: low field microwave absorption (LFMA), high temperature superconductors (HTSC), iron-pnictide superconductors, Josephson junctions, fluxons and normal state.