Direct visualization of magnetic vortex pinning in superconductors

TL;DR

This study visualizes how magnetic vortex pinning occurs in superconductors with ferromagnetic square microrings, revealing that the magnetic state of the rings influences vortex behavior and pinning effectiveness.

Contribution

It demonstrates direct imaging of vortex pinning in superconductors influenced by ferromagnetic microrings and shows how magnetic history controls the pinning potential.

Findings

Corners of square rings act as effective vortex pinning centers.

Magnetic stray fields dominate over vortex fields in the onion state.

Magnetic history controls the pinning potential strength.

Abstract

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high resolution imaging techniques: Bitter decoration and scanning Hall probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field in agreement with the results of simulations. Real space vortex imaging has revealed that, in the onion state, the corners of the square rings act as effective pinning centers for vortices.