Re-entrant Resistance and Transport Anomalies in P-doped Eu Pnictide: EuFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$, with coexisting superconducting and ferromagnetic phases*

The doping of Eu 122 pnictides by Phosphorus has been shown to result in superconductivity which coexists with ferromagnetism of spins ordered in Eu sites. Recently, in the course of exploring Fe-based superconductors, we observed both SC associated with Fe-3d electrons and ferromagnetism due to the long-range ordering of Eu-4f moments in EuFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ and in similar compound with lower P-doping, at 0.27 the $R(T)$ peak appears at $T \sim $ 19 K [1]. This $R(T)$ peak is associated with suppression of superconductivity by ferromagnetism and a re-entrance of resistance at low temperature. This presentation further investigates the pnictides at different P doping levels with transport measurements, correlated with magnetic and ESR data. Pnictides were made with a Solid state reaction method including the flux method to grow the single crystals. The dependence of the transition temperature and the behavior of the re-entrant $R(T)$ peak at 18-19 K on magnetic field and amount of dopant are addressed. [1]Aamir Ahmed, M. Itou, Shenggao Xu, Zhu'an Xu, Guanghan Cao, Y. Sakurai, James Penner-Hahn, Aniruddha Deb, Phys. Rev. Letters 105, 207003 (2010)