The enhancement of vortex pinning in ferromagnet/superconductor bilayers

Abstract

The magnetic-domain induced vortex pinning is studied in the ferromagnet/superconductor bilayers (FSB's), in which the F layers are Co/Pt multilayers with perpendicular magnetic anisotropy, and the S layers are either niobium or high temperature superconductor YBa 2 Cu 3 O 7 (YBCO). The magnetization measurements reveal the enhancement of the flux pinning in both types of FSB's during the reversal of the magnetization of the F layer, but the details of the behavior depend on the type of the S layer. In the case of niobium FSB the maximum of pinning appears when the F layer is in the final stage of the magnetic reversal process, while the FSB with YBCO shows the maximum when the F layer is saturated. The possible origins of these differences are discussed. IntroductionThe vortex pinning in superconductors determines the critical current density and therefore has a direct impact on the possible applications of superconducting materials. A large effort is directed towards the development of methods of the enhancement of flux pinning. Recently, a novel method, with the use of ferromagnet/superconductor bilayers (FSB), has been suggested [1]. It is based on the idea that the magnetic stripe domains in the F layer with a perpendicular magnetic anisotropy pin the vortex core in the S layer. The relative ease of the realignment of the stripe domains offers the possibility of the adjustment of vortex pinning using small magnetic fields, provided that the S and F layers are sufficiently separated by the buffer layer which eliminates the proximity effect. To date, several attempts have been made to study various FSB's, and the evidence has been accumulated that some pinning enhancement indeed occurs [2][3][4][5]. However, only one study, of the FSB's with lead S layers (weak type II superconductor in thin film form) addressed directly the question of the origins of the pinning enhancement, and found it related not to stripe domains, but to the isolated magnetic domains nucleated during the reversal of magnetization of the F layer [4].To shed more light on the behavior of the magnetic domain induced pinning, we compare here two classes of FSB's, containing either conventional type II superconductor, niobium, or the high temperature superconductor, YBCO. The intrinsic pinning is much stronger in YBCO, and one may expect that this has some impact on the behavior of FSB's. The preliminary results on the behavior of the niobium FSB's have been described elsewhere [6].

Flux Penetration in a Ferromagnetic/Superconducting Bilayer

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

“…The interaction of the magnetic dipole and the vortex introduces the contribution to the energy of the dipole-vortex system, E = −mB, where m is the dipole moment, and B is magnetic field of a vortex [2]. It follows that the pinning is sensitive to the relative orientation between m and the external magnetic field H. The sensitivity of the pinning in the FSBs to the H direction has been demonstrated in several recent experiments [3][4][5][6]. The details of the behavior of the magnetic domain-induced pinning are not yet studied.…”

“…Depending on the amount of repeats of the Co/Pt superlattice, the MRP may proceed by domain wall motion, or by the nucleation of many inverted centers [7]. So far, we have studied the magnetic pinning induced by the first type of MRP, observed in the Co/Pt superlattice with small amount of repeats [4][5][6]. In the present study we compare it with the second type of the MRP.…”

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].…”