Anomalous peak effect in iron-based superconductors Ba1−xKxFe2As2 (x ≈ 0.69 and 0.76) for magnetic-field directions close to the ab plane and its possible relation to the spin paramagnetic effect

Abstract

We report magnetic torque measurements on iron-pnictide superconductors Ba1−xKxFe2As2 (x≈ 0.69 and 0.76) to an applied field of Ba=45 T. The peak effect is observed in torque-vs-field curves below the irreversibility field. It is enhanced and becomes asymmetric as the field is tilted from the c axis. For field directions close to the ab plane, increasing- and decreasing-field curves peak at markedly different fields and exhibit a sharp jump, suggestive of a first-order phase transition, on the high- and the low-field sides of the peak, respectively. Complicated history dependence of the torque is observed in the peak-effect region. We construct and discuss the temperature– (T) applied-magnetic-field (Ba) phase diagram. Since the upper critical field for the ab-plane direction is comparable to the Pauli limit, we also consider possible influence of the spin paramagnetic effect on the anomalous peak effect.

Source-reported materials — not catalogue approval

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