Effect of Ni doping on vortex pinning in CaK(Fe1−xNix)

Abstract

We study the correlation between chemical composition and vortex dynamics in Ni-doped CaK(Fe1−xNix)4As4 (x=0, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities Jc and the flux creep rates S. The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy (U0<100K). The undoped crystals display modest Jc values at low temperatures and high magnetic fields applied along the c axis. Jc(T) dependences at high fields display an unusual peak. The enhancement in Jc(T) matches with an increase in U0 and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in Tc there is a nonmonotonic change in Jc. Initially Jc increases, reaching a maximum value for x=0.015, and then Jc decreases for x≥0.025. This change in Jc(x) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of Jc(H) also manifests a change in behavior between these x values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, x. This implies that there is no appreciable change in the mechanism that determines the vortex relaxation.We study the correlation between chemical composition and vortex dynamics in Ni-doped CaK(Fe 1−x Ni x ) 4 As 4 (x = 0, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities J c and the flux creep rates S. The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy (U 0 < 100 K). The undoped crystals display modest J c values at low temperatures and high magnetic fields applied along the c axis. J c (T ) dependences at high fields display an unusual peak. The enhancement in J c (T ) matches with an increase in U 0 and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in T c there is a nonmonotonic change in J c . Initially J c increases, reaching a maximum value for x = 0.015, and then J c decreases for x 0.025. This change in J c (x) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of J c (H ) also manifests a change in behavior between these x values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, x. This implies that there is no appreciable change in the mechanism that determines the vortex relaxation. EFFECT OF Ni DOPING ON VORTEX PINNING IN … PHYSICAL REVIEW B 100, 064524 (2019) FIG. 2. (a)-(e) Magnetic field dependence of the critical current densities J c a...

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

“…The J c can be increased significantly by introducing artificial pinning centers. It depends on a complex interplay of individual pinning centers, the interaction between vortices, and thermal fluctuations [5][6][7]. One of the most relevant parameters in producing highly functional superconducting materials is the current-carrying capacity.…”

“…One of the most relevant parameters in producing highly functional superconducting materials is the current-carrying capacity. The J c of the iron pnictides usually display several regimes as a consequence of a pinning landscape with random point disorder and a low density of large defects [5,8]. J c , depending on the disorder, either decreases monotonically or displays a second peak (SP) in the magnetization, also called a "fishtail" [9,10].…”

Disordered hyperuniformity in superconducting vortex lattices

“…In LiFeAs pinning is by point defects [26]. In CaKFe 4 As 4 pinning is very strong [27,[39][40][41][42]. The exponent β of the variance σ 2 (R) is obtained from the data in Fig.…”

“…In Co-doped NbSe 2 and in the W-film-1 thin film, pinning is strong but structured, leading to hexagonal vortex clusters observed at all magnetic fields. On the other hand, in CaKFe 4 As 4 pinning centers are so strong and randomly distributed that the vortex lattice is essentially randomly disordered in the whole range of magnetic fields studied [27,[39][40][41][42]. In W-film-2, where the disorder potential is very weak, the vortex lattice is only disordered when it is very soft, very close to H c2 , but then the vortex distribution shows a strong tendency to disorder randomly, because intervortex interactions are very weak.…”

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

“…This system lacks long-ranged magnetic order or a nematic electronic state at low temperatures [5][6][7][8][9][10][11] but upon doping with Ni a hedgehog magnetic structure is stabilized [9,12,13]. CaKFe 4 As 4 has an exceptionally large critical current density due to the strong point-like defects caused by local structural site effects as well as surface pinning [14][15][16]. Due to reduced symmetry compared with the 122 family of iron-based superconductors, CaKFe 4 As 4 is predicted to have up to ten different bands ( Fig.…”