The peak effect in Fe-doped Bi-2212 single crystals

The peak effect (PE) in Fe-doped Bi2.1Sr1.9Ca1.0(Cu1−yFey)2Ox crystals with different nominal y values of 0, 0.005, 0.013 and 0.022 grown by the floating zone method was studied by measuring M–H loops. The peak effect appears over a narrow temperature range between 20 and 40 K for y up to 2.2%, but with a small peak field of about 200 Oe. The appearance of the PE in all the Fe-doped Bi-2212 can be explained by the existence of Bi5+ cluster regions in the Bi3+ 2212 matrix. ΔMmax/ΔM0 versus T/Tc and (Hmax − Hmin)/Hmax versus T/Tc were used to characterize the peak effect. ΔMmax and ΔM0 represent the width of the hysteresis loop at the secondary peak (Hmax) and the central peak H0 (H = 0). Hmin represents the field at which the magnetization starts to increase, giving the anomalous peak. The secondary peak moves away from the central peak in a linear way as the temperature increases as indicated by (Hmax − Hmin)/Hmax versus T/Tc, in agreement with what is found in pure Bi-2212 single crystals.