Imprinted labyrinths and percolation in Nd-Co/Nb magnetic/superconducting hybrids

Abstract

Magnetization reversal processes have been studied in hybrid magnetic/superconducting Nd-Co/Nb bilayers by the comparison of out-of-plane magnetic hysteresis loops and superconducting phase diagrams as a function of magnetic layer thickness and of disorder in the magnetic layer induced by a nanostructured copolymer template. A good correlation is found between the regimes corresponding to percolation effects in the superconductor and to the transition from extended to confined superconductivity with the characteristic fields for reverse domain nucleation and fast domain expansion in the magnetic layer, indicating that superconductivity nucleates on the disordered network imprinted on the superconducting layer by the labyrinth domain structure of the magnetic layer. As disorder increases in the magnetic layer, percolation effects disappear from the superconducting transitions in agreement with a more homogeneous magnetization reversal process.

Block copolymer self-assembly–directed synthesis of mesoporous gyroidal superconductors

“…In principle, this structural control provides an ideal platform for tuning and studying mesostructural effects on the superconducting properties of inorganic materials, but has not been exploited to date. To the best of our knowledge, the sole example in peer-reviewed literature involving block copolymers and superconductors describes the deposition of ferromagnetic/superconducting bilayers on a diblock copolymer substrate ( 14 ). In contrast, here, we demonstrate the use of block copolymer self-assembly to structure direct a superconductor, niobium nitride (NbN), into films of periodically ordered mesoporous materials, with the alternating gyroid (G A ) morphology exhibiting a superconducting transition at 7 to 8 K. We expect this study to pave the way toward investigations of superconductors with a plethora of block copolymer self-assembly–directed mesostructures.…”

Magnetic order and disorder in nanomagnets probed by superconducting vortices

“…In the demagnetized state, stripe domains become branched and curved in a labyrinth configuration 37 but they retain its characteristic width given by the competition between magnetostatic and anisotropy energies. At low temperatures (LT) (10 K), the anisotropy in the material increases up to K N (LT) = 1.8 × 10 6 J/m 3 at 10 K 37 , 38 , with M S (LT) = 1.4 × 10 6 A/m. Thus, at low temperature Q increases up to Q = 1.4 which corresponds to a material with moderate anisotropy.…”

“…Q(RT) = 0.3 corresponding to a weak PMA material. Equilibrium domain configuration consists of parallel stripes with alternating up-down magnetization and width in the 50-60 nm range [37][38][39] depending on sample thickness [39,57]. In the demagnetized state, stripe domains become branched and curved in a labyrinth configuration [37] but they retain its characteristic width given by the competition between magnetostatic and anisotropy energies.…”

“…Thus, at low temperature Q increases up to Q = 1.4 which corresponds to a material with moderate anisotropy. In NdCo films of similar composition, out-of-plane remanence at low temperature presents relatively low values, of the order of 0.1 M S [37,38], characteristic of strong disorder that favors reversed domain nucleation at positive fields and a disordered multidomain state at remanence. Superconducting vortex motion is the proposed "experimental technique" to detect the magnetic states of these systems.…”

“…High Q values (of the order 2 to 10) can be found, for example, in Co/Pd multilayers [32][33][34][35] with high out-of-plane remanence (M R M S ) and large domain sizes during magnetization reversal [36]. A smaller domain size regime can be accessed in lower Q materials such as NdCo amorphous alloys (Q in the 0.1-1.5 range, depending on temperature and composition) [37,38]. These films display reduced remanent magnetization (M R below 0.1M S ) characteristic of stripe domain patterns, with very small domain sizes [39].…”