Vortex Pinning in Ferromagnet/Superconductor Bilayers - the Dependence on the Ferromagnetic Layer and Temperature

Abstract

The behavior of vortex pinning induced by the magnetic domain reversal is studied in the ferromagnet/superconductor bilayers in which superconducting film is niobium and the ferromagnetic layers are the Co/Pt superlattices with perpendicular magnetic anisotropy. The local magnetic field across narrow ferromagnet/superconductor bilayer stripe is measured using a line of miniature Hall sensors. The pinning is studied for samples with different amount of repeats of Co/Pt superlattice, and as a function of temperature.

Flux Penetration in a Ferromagnetic/Superconducting Bilayer

“…An F layer is the [Co(0.4 nm)/Pt(1 nm)]×8 multilayer, covered by an amorphous, 10 nm thick Si film to avoid the proximity effect, and Nb film, 78 nm thick, as an S layer. The parameters of the S film are as follows: T c = 8.8 K, penetration depth λ(0) = 95 nm, coherence length ξ(0) = 35 nm [5]. The hysteresis loop for the F layer, measured at T = 10 K is typical of the F film with perpendicular magnetic anisotropy, with a sudden drop of magnetization when the inverted domains nucleate and with the long "tails" caused by the residual uninverted (RU) domains.…”

“…The density of the RU domains is the largest when the magnetic reversal process is in the final stage. These RU domains provide MP in the sample, as described previously [4,5]. To study the flux entry the sample is cut into 200 µm wide strip, and the set of 10 collinear miniature Hall sensors, of the area 5 × 5 µm 2 each, is placed across the strip 20 µm apart.…”

“…In the present work we study the flux profiles in the presence of magnetic pinning (MP) induced by the domain pattern predefined in the ferromagnetic (F) layer. The MP dominates at high temperatures (close to the superconducting transition temperature, T c ) when the thermomagnetic instabilities are absent [4,5]. This allows us to observe the terraced flux profile.…”

Vortex Dynamics in Ferromagnetic/Superconducting Bilayers

“…The domain structure in FSB's may be reversibly defined and erased allowing easy modulation of pinning. While there are many experiments which confirm the existence of the MP, only few of them attempt to correlate the vortex dynamics in the FSB's with the geometry of the magnetic domain patterns [3][4][5][6][7][8][9][10].…”