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- Title
Flux Dynamics, ac Losses, and Activation Energies in (Ba $$_{0.6}$$ K $$_{0.4})$$ Fe $$_{2}$$ As $$_{2}$$ Bulk Superconductor.
- Authors
Nikolo, M.; Shi, X.; Choi, E.; Jiang, J.; Weiss, J.; Hellstrom, E.
- Abstract
Flux pinning and thermally assisted flux flow are studied in a (Ba $$_{0.6}$$ K $$_{0.4})$$ Fe $$_{2}$$ As $$_{2}(T_\mathrm{c}$$ =38.3 K) bulk samples in magnetic fields up to 18 T via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1,997 Hz in all fields. The frequency ( $$f)$$ shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law $$f = f_{0}\mathrm{exp}(-E_\mathrm{a}{ /kT})$$ to determine flux activation energy $$E_\mathrm{a}/k$$ as a function of ac field $$H_\mathrm{ac}$$ and dc magnetic flux density $$\mu $$ $$_\mathrm{0} H_\mathrm{dc}$$ . $$E_\mathrm{a}/k$$ ranges from 8,822 K (761 meV) at $$\mu $$ $$_{0} H_{dc}$$ = 0 T to 1,100 K (95 meV) at 18 T for $$H_\mathrm{ac}=$$ 80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases from 0 to 1 T, and more gradually, in a linear-like manner, as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases further to 18 T, suggesting some kind of vortex transition. For ac fields of 400 A/m (5 Oe) and higher, the Arrhenius model starts breaking down, at around $$\mu $$ $$_{0} H_{ \mathrm dc}$$ = 2 T. As the dc magnetic flux density increases further, this breakdown becomes significant for $$\mu _{0} H_\mathrm{dc}$$ = 15 and 18 T at ac fields of 400 A/m and higher. Extensive mapping of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the ac field, frequency, in addition to the dc magnetic flux density.
- Subjects
THERMAL properties of superconductors; FLUX pinning; ARRHENIUS equation; ACTIVATION energy; MAGNETIC flux density
- Publication
Journal of Low Temperature Physics, 2015, Vol 178, Issue 3/4, p188
- ISSN
0022-2291
- Publication type
Article
- DOI
10.1007/s10909-014-1237-y