Synthesis, crystal structure and magnetism of beta-Fe1.00(2)Se1.00(3)   single crystals
  Understanding iron based superconductors requires high quality impurity free single crystals. So far they have been elusive for beta-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic impurity phases. Herein we report synchrotron - clean beta-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect induced weak ferromagnetism that coexists with superconductivity below Tc. In contrast to Fe1+yTe - based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1). 
Rongwei Hu, Hechang Lei, Milinda Abeykoon, Emil S. Bozin, Simon J. L.   Billinge, J. B. Warren, Theo Siegrist, and C. Petrovic
cond-mat.supr-con cond-mat.mtrl-sci