Coupled domain structures in superconductor/ferromagnet Nb-Fe/garnet bilayers

We investigate the magnetic structure in a superconducting Nb and ferromagnetic garnet hybrid film. Direct magneto-optical observations reveal a strong interaction between the superconducting vortices in Nb and the magnetic domains in the garnet, resulting in a unique coupled domain structure. The cooperative motion of vortices and domain walls results in an enhanced pinning and a modified electromagnetic response of the hybrid similar to that in type I superconductors. Application of ac field, routinely used for equilibration of domains, leads to their significant contraction. Our calculations explain this effect by a domain shrinkage instability induced by the vortex annihilation around oscillating domain walls.

Figure 1

(Color online) Temperature dependence of the domain width in bare garnet and Nb covered areas. Low-$T$ data collected at several cooling/heating runs around $T_{\text{c}}$ are shown in the insert.

Figure 2

(a) Pictures of the domain structure in a Fe-garnet film half covered with Nb layer (on the right from white line) after application and gradual decrease in a perpendicular ac field $h_{\perp }\left(17\text{Hz}\right)=50\text{Oe}$ at shown temperatures. Bright and dark contrast corresponds to opposite local normal fields in domains visualized using magneto-optical indicator on top of the hybrid. Dashed lines marks the boundary of the transition zone near the Nb edge (see text). (b) MO images at $+40$ and $-40\text{Oe}$. The sample is slightly shifted.

Figure 3

(Color online) Dependence of the CDS energy on the domain width, $E\left(D\right)$, for the equilibrium (EQ) and nonequilibrium (NE) states. Arrows illustrate the evolution of the system from the NE to the EQ state.