RESUMO
Flux quantization has been widely regarded as the hallmark of the macroscopic quantum state of superconductivity. However, practical design of superconductor devices exploiting finite size confinement effects may induce exotic phenomena, including nonquantized vortices. In our research, the magnetic flux of vortices has been studied in a series of superconducting strips as a function of the strip width and the penetration depth. In both circumstances, the observation using scanning Hall probe microscope (SHPM) displays a controlled evolution from singly quantized vortices to nonquantized ones. It is also found that the magnetic flux is immune to the flowing supercurrent. The simulations based on Ginzburg-Landau theory agree well with experimental results. The observed behavior of the vortex flux may open new perspectives for fundamental research and applications based on vortex matter, such as vortex-memory devices and magnetic field traps for ultracold atoms.
RESUMO
The Z-type hexaferrites Ba3(Zn1-xCox)2Fe24O41 (x = 0.2, 0.4, 0.6, 0.8, defined as Z1-Z4) were synthesized by a sol-gel method. With increasing cobalt concentration, the origin of magnetoelectric (ME) coupling and the effects of crystal parameters, occupation of ions, and magnetocrystalline anisotropy (MCA) on ME current were studied systematically. The mechanism of magnetic phase transition, revealing the evolution of the magnetic order in the temperature range of 10-400 K, was discussed in detail. Our results suggest that the ferroelectricity of Z1-Z4 originates from both inverse Dzyaloshinskii Moriya (DM) interaction and p-d hybridization mechanism. In particular the ME coupling property is only dominated by p-d hybridization with spin-orbit coupling. This study provides an effective way to improve the ME coupling property of hexaferrites, which have potential applications in the design of new electronic devices.