Vortex phase diagram in 12442-type RbCa2Fe4As4F2single crystal revealed by magneto-transport and magnetization measurements

The recently discovered 12442-type ACa(2)Fe(4)As(4)F(2)(A = K, Rb, Cs) compounds are the only iron-based superconductors (IBSs) with double FeAs layers between neighboring insulating layers, analogous to the double CuO(2)layers in some high-T(c)cuprates. Here, we report the study of vortex phase diagram of RbCa(2)Fe(4)As(4)F(2)single crystal via magneto-transport and magnetization measurements. The resistive transition under magnetic fields shows a foot-like kink at a characteristic temperature,T-s, followed by a resistive tail in nearly zero resistivity region. Such behavior is ascribed to a vortex slush transition atT(s), below which the vortex state has short-range vortex lattice correlation, and then a second-order transition into the vortex glass phase occurs with further decreasing temperature. AboveT(s), the Arrhenius plot of resistivity shows two linear regions that are separated by a crossover lineT(cr)(B), which is associated with a crossover from collective to plastic pinning or different flux pinning behaviors resulted from different types of defect. In addition, the magnetic hysteresis loops reveal a second magnetization peak (SMP), which is shifted to lower fields with increasing temperature forT< 12 K. However, the SMP unexpectedly moves back to a higher field atT= 12 K, and then gradually turns into a shoulder or kink that moves to higher fields at high temperatures, such anomalous behavior has never been observed in IBSs. According to the magneto-transport and magnetization data, the vortex phase diagram of RbCa(2)Fe(4)As(4)F(2)is finally constructed. Details on the different vortex phase transitions and relevant physical scenarios are given and discussed.