Controlled switching between paramagnetic and diamagnetic Meissner effects in superconductor-ferromagnet Pb-Co nanocomposites

A hybrid system which consists of a superconducting (SC) Pb film (100 nm thickness) containing $\sim 1\text{vol}\%$ single domain ferromagnetic (FM) Co particles of mean size $\sim 4.5\text{nm}$ reveal unusual magnetic properties: (i) a controlled switching between the usual diamagnetic and the unusual paramagnetic Meissner effect in field cooling as well as in zero-field-cooling experiments (ii) amplification of the positive magnetization when the sample enters the SC state below $T_c$. These experimental findings can be explained by the formation of spontaneous vortices and the possible alignment of these vortices due to the alignment of the Co particle FM moments by an external magnetic field.

Figure 1

(Color online) Hysteresis loops of Pb/Co nanocomposites at 8 and 5 K, which are above and below $T_c$, respectively. The upper inset shows the hysteresis loop at 5 K with the ferromagnetic signal subtracted, i.e., it is only the superconducting signal. The lower inset shows the center part of the curve at 8 K.

Figure 2

(Color online) dc magnetization as a function of temperature of the Pb/Co nanocomposites (a) ZFC and FC in positive external magnetic field from 300 to 4 K, (b) enlargement of the FC curves, and (c) FC in a negative magnetic field from 300 to 8K and then FC in positive external field to 4.5 K.

Figure 3

(Color online) The virgin magnetization curves of Pb/Co nanocomposites at 5 K with the moments of the Co particles (a) aligned to the direction of the external field, (b) randomly oriented, and (c) aligned to the opposite direction of the external field. The insets show the direction of the internal moments in the sample during the measurements.