ИСТИНА

There were several attempts to observe so-called spontaneous vortex phase (SVP) in the weak ferromagnetic (WFM) state with TWFM = 2.3 K and TSC = 10.5 K of ErNi2B2C single crystals (see refs. [1, 2, 3] but the effects were small and the interpretation controversial. In spite of a large number of theoretical works there exist only a few hints on the possibility of formation of spontaneous zero field flux line lattice in ErNi2B2C. The possibility of formation and visualization of SVP in the FM/SC phase of EuFe2(As1-xPx)2 superconductor with the magnetic ordering of the Eu+2 sub-lattice below TFM ~ 18 K <TSC~25 K (optimally doped), remain as open problems so far, see refs. [4, 5]. Here we study the coexistence of superconductivity and ferromagnetism in spatially uniform bulk crystals EuFe2(As1-xPx)2 with different doping levels, x, by mapping local magnetic field distribution with Bitter decoration technique and MFM imaging. By combining the capabilities of both methods one can get more information about the shape and dynamics of magnetic flux structures in these remarkable superconductors in a wide range of magnetic fields and temperatures, which is important both from theoretical and practical points of view. Magnetic flux structure FC 20 Oe at 6K on the (001) surface of underdoped x=0.13, optimally doped x=0.2 and overderdoped x=0.25 EuFe2(As1-xPx)2 single crystals revealed by decoration. [This work was supported by top-5-100 grant to MIPT] [1] H. Kawano-Furukawa, et al., Physica C 470, 716 - 718 (2010). [2] I. S. Veschunov, et al., Phys. Rev. B 76, 174506 (2007). [3] E. M. Chia, et al., Europhys. Lett., 73 (5), 772 - 778 (2006). [4] H. S. Jeevan, et al., Phys. Rev. B 83, 054511 (2011). [5] S. Nandi, et al., Phys. Rev. B 89 014512 (2014).