Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1−xKx)F

Abstract

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of (Ba 1-x K x )Fe 2 As 2 (0.18≤x≤1). The critical current density J c reaches maximum in the underdoped sample x=0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, magnetic relaxation rate S rapidly increases and the flux creep activation barrier U 0 sharply decreases in the overdoped sample x=0.70. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x=0.38, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in T c become small in the samples x=0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the (Ba 1- * Corresponding author. Email address: yliu@ameslab.gov 2 x K x )Fe 2 As 2 crystals: (i) Strong δl pinning resulted from the fluctuations in the mean free path l and δT c pinning from the spatial variations in T c in the underdoped regime; (ii) Weak δT c pinning in the optimally doped and overdoped regime. PACS number(s): 74.70.Xa, 74.25.Wx, 74.25.Sv 3 I. INTRODUCTIONThe studies of vortex dynamics revealed many interesting phenomena in the iron pnictide superconductors. Because of the availability of large and high-quality single crystals, most of researches have been done on 122-type superconductors with ThCr 2 Si 2 structure. Similar to high T c cuprate superconductors, a pronounced second magnetization peak (SMP) was observed in magnetization hysteresis loops (MHLs) of (Ba 1−x K x )Fe 2 As 2 [1-6], Ba(Fe 1−x Co x ) 2 As 2 [3,6,7-13], BaFe 2 (As 1−x P x ) 2 [6,14-15]. One of striking features that that distinguish iron pnictide superconductors from high T c cuprate superconductors is the observation of strongly disordered vortex structure in iron pnictide superconductors by Bitter decoration [16][17][18][19][20][21][22], scanning tunneling microscopy (STM) [23][24], magnetic force microscopy (MFM) [25], small-angle neutron scattering (SANS) measurements [17][18][26][27]. Although long-range ordered vortex lattice (VL) was not observed yet, the so-called Bragg glass may exist, which is a glass but nearly as ordered as a perfect crystal. An ordered vortex structure was observed in an area of 130 50 nm 2 in optimally doped Ba 0.6 K 0.4 Fe 2 As 2 by STM measurement [28]. With a large Ba 0.64 K 0.36 Fe 2 As 2 single crystal, SANS measurement observed Bragg peaks corresponding to a long-range ordered triangular lattice below H=0.75 T. With increasing magnetic field above 0.75 T, diffraction spots smeared and gave the characteristic pattern of circles from a polycrystalline structure, which was interpreted as a vortex order-disorder transition associated with the appearance of SMP in MHLs [29]. In optimally doped BaF...

Second magnetization peak, rhombic-to-square Bragg vortex glass transition, and intersecting magnetic hysteresis curves in overdoped BaFe2(As1−xPx)2 single crystals

“…The improved quality of iron-based superconducting single crystals 49,50 made possible the observation of ordered vortex phases 34,50,51 . It is now established that the overdoped 122-type specimens exhibit a relatively weak, pointlike δT c pinning 52 and a pronounced SMP develops 31 . The P-Ba122 single crystals analysed in this work have been grown by the Ba 2 As 3 /Ba 2 P 3 -flux method 53 at the Institute of Physics, Chinese Academy of Sciences.…”

Sharp peak of the critical current density in BaFe2−xNixAs2 at optimal composition

“…J c , depending on the disorder, either decreases monotonically or displays a second peak (SP) in the magnetization, also called a "fishtail" [9,10]. Interestingly, the existence of the SP is found to be dopingdependent [11]. Therefore understanding the superconducting J c is a central issues pertaining to both fundamental science as well as technological applications.…”

Doping Dependence of the Second Magnetization Peak, Critical Current Density, and Pinning Mechanism in BaFe2–xNixAs2 Pnictide Superconductors

“…However, the mechanism responsible for SMP in Ni-doped BaFe 2 As 2 pnictide superconductors is as yet unresolved [29,36,37]. Interestingly, in iron-pnictide superconductors, it has been observed that the existence of the SMP is doping dependent [31,38].…”

“…Besides, pnictides are known as multiband superconductors, which may play a role on the pinning of vortices through the inter-band and intra-band electron scatterings [23]. Since then, vortex dynamics studies were performed on different pnictides compounds discovered over the years [13,[24][25][26][27][28][29][30][31], and, most of them are devoted to the study of the mechanism responsible for the appearance of SMP in isothermal magnetization curves. Contrary to the cuprates, where the SMP is mostly observed only for H c-axis, in Fe-pnictides, due to the low anisotropy, it is observed for both, H c-axis and H ab planes.…”