Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa$$_2$$Fe$$_4$$As$$_4$$F$$_2$$ bilayer pnictide superconductor

Abstract

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa$$_2$$ 2 Fe$$_4$$ 4 As$$_4$$ 4 F$$_2$$ 2 , with $$T_c\;$$ T c $$\simeq$$ ≃ 34 K, for $$H$$ H $$\parallel$$ ‖ $$c$$ c -axis and $$H$$ H $$\parallel$$ ‖ $$ab$$ ab -planes. A second magnetization peak (SMP) was observed in the isothermal M(H) curves measured below 16 K for $$H$$ H $$\parallel$$ ‖ $$ab$$ ab -planes. A peak in the temperature variation of the critical current density, $$J_{c}$$ J c (T), at 16 K, strongly suggests the emergence of Josephson vortices at lower temperatures, which leads to the SMP in the sample. In addition, it is noticed that the appearance of Josephson vortices below 16 K renders easy magnetic flux penetration. A detailed vortex dynamics study suggests that the SMP can be explained in terms of elastic pinning to plastic pinning crossover. Furthermore, contrary to the common understanding, the temperature variation of the first peak field, $$H_1$$ H 1 , below and above 16 K, behaves non-monotonically. A highly disordered vortex phase, governed by plastic pinning, has been observed between 17 and 23 K, within a field region around an extremely large first peak field. Pinning force scaling suggests that the point defects are the dominant source of pinning for H$$\parallel$$ ‖ $$ab$$ ab -planes, whereas, for H $$\parallel$$ ‖ $$c$$ c -axis, point defects in addition to surface defects are at play. Such disorder contributes to the pinning due to the variation in charge carrier mean free path, $$\delta l$$ δ l -pinning. Moreover, the large $$J_c$$ J c observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.

Order–disorder phase transition and elastic-to-plastic vortex creep crossover in a triclinic iron pnictide superconductor (Ca$$_{0.85}$$La$$_{0.15}$$)$$_{10}$$(Pt$$_3$$As$$_8$$)(Fe$$_2$$As$$_2$$)$$_5$$

“…Such high anisotropy was also observed in another iron pnictide KCa Fe As F , which has a bilayer structure and a possible pseudogap behavior similar to BSCCO 29 – 31 . Moreover, interesting vortex dynamics behaviour has also been observed in a recent study on anisotropic KCa Fe As F compound 32 . These salient features, in addition to the triclinic crystal structure (contrary to the tetragonal crystal structure in cuprates and other iron pnictide superconductors), make the 10-3-8 phase a unique compound to explore interesting and possibly new vortex phases.…”

Influences of quenching treatment and cobalt-doping on the thermally activated flux-flow behavior in KCa2(Fe 1−x Co x )4As4F2

“…In our previous studies, a very steep slope of the upper critical field near T c and a strong pauli-paramagnetic effect in KCa 2 Fe 4 As 4 F 2 have been reported [10,11]. The second magnetization peak were revealed both in the RbCa 2 Fe 4 As 4 F 2 and KCa 2 Fe 4 As 4 F 2 systems by the magnetization measurements [22][23][24]. Recent scanning tunneling microscopy measurements have revealed the Friedel oscillations of vortex bound states and a new type of vortex pattern with bamboo-like chains in confined canals in the KCa 2 Fe 4 As 4 F 2 compound [25,26].…”

The Depairing Current Density of a Fe(Se,Te) Crystal Evaluated in Presence of Demagnetizing Factors

“…Moreover, they have lower anisotropy values than cuprates, with associated high pinning energy values [8][9][10][11]. Among these properties, there are also non-monotonic responses to magnetization with applied magnetic fields; this has led to particular phenomena that have been deeply studied in recent years [12][13][14][15][16]. These phenomena are strictly correlated 2 of 11 with vortex dynamics [17][18][19][20][21].…”