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

We report magnetic torque measurements on iron-pnictide superconductors Ba1-xKxFe2As2 (x approximate to 0.69 and 0.76) to an applied field of B-a = 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 (B-a) 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.