Visualization of the vortex-mediated pinning of ferromagnetic domains in superconductor-ferromagnet hybrids

A thin superconducting (S) film reduces the mobility of magnetic domains in an underlying ferromagnet (F). The magnetic domain pattern of a barium hexaferrite single crystal was imaged as a function of temperature and external magnetic field, in both presence and absence of a covering niobium film. Comparative analysis of these two cases reveals that the relaxation of the domains is hampered, if the external field is changed while the Nb-film is superconducting. This effect in the S/F hybrids is interpreted in terms of a vortex-mediated pinning of magnetic domains by pinning centers in the superconductor.

Figure 1

The phase boundary between normal and superconducting state of the transport bridge, defined by 50% of the resistance in the normal state. A hatched area indicates the region where SHPM imaging was performed. The inset illustrates the processed sample with the two locations $\mathcal{P}$ and $\mathcal{Q}$, and a transport bridge on top of the ferromagnet. (Note that the image of the magnetic domain pattern, obtained by polarized light microscopy, is not in scale with the sketched structure.)

Figure 2

(Color online) SHPM images of the magnetic domains in the ${\text{BaFe}}_{12}{\text{O}}_{19}$ substrate at ${\mu }_0{H}_{\text{ext}}=420\text{mT}$, obtained successively at 5 K (left), 11 K (middle), and again 5 K (right).

Figure 3

(Color online) Illustration of a systematic series of SHPM images. Top and middle row: images of the magnetic domains in the substrate during the first and second temperature sequence, respectively $\left({\mu }_0{H}_{\text{ext}}=170\text{mT}\right)$. Bottom row: differences between the domain images of the first and second sequence.

Figure 4

(Color online) The distribution of $B_{\text{stray}}$ found in the SHPM images, obtained at 5 K during the first imaging sequences. The solid line shows the bulk magnetization $M\left(H_{\text{ext}}\right)$ of the substrate at 5 K. Markers indicate the differences ${\Delta }_{1,2}$ and ${\Delta }_{2,1}$ between the averages of the 5K images obtained during the first and second sequence at the same $H_{\text{ext}}$ (diamonds) and during the second and first sequence at subsequent fields (circles).

Figure 5

Statistics of the instability of magnetic domains to temperature rise. For all investigated values of $T$ and $H_{\text{ext}}$, the measure $\mathcal{S}$ [Eq. (1)] is shown for (a), location $\mathcal{Q}$ (without S layer) and (b), location $\mathcal{P}$ (with S layer).