# Pinning effects on flux flow instability in epitaxial Nb thin films

## TL;DR

This study investigates how different pinning strengths in epitaxial niobium thin films affect flux flow instability, revealing how pinning modifies vortex dynamics and critical velocities under various conditions.

## Contribution

It provides a comprehensive analysis of flux flow instability in Nb films with different pinning types, combining experimental measurements with theoretical modeling.

## Key findings

- Weak pinning leads to monotonic decrease of $v^*(B)$
- Irradiated films show non-monotonic $v^*(B)$ with a maximum
- Nanogroove patterning causes sharp maxima due to commensurability effects

## Abstract

The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. Investigated are the cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by focused ion beam. The generic feature of the current-voltage curves of the films consists in instability jumps to the normal state at some instability current density $j^\ast$ as the vortex lattice reaches its critical velocity $v^\ast$. While $v^\ast(B)$ monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence $v^\ast(B)$ attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both, $v^\ast(B)$ and $j^\ast(B)$, which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of $v^\ast(B)$ can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.