High temperature ferromagnetism of Li-doped vanadium oxide nanotubes

TL;DR

This study investigates the high temperature ferromagnetism in Li-doped vanadium oxide nanotubes, revealing that Li intercalation induces superparamagnetic nanoscale clusters responsible for room-temperature ferromagnetic behavior.

Contribution

It provides the first comprehensive spectroscopic evidence that Li doping creates superparamagnetic clusters, confirming the bulk nature of high temperature ferromagnetism in these nanotubes.

Findings

Li doping increases magnetic vanadium sites

Ferromagnetic response persists up to room temperature

Superparamagnetic nanosize spin clusters are responsible

Abstract

The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes.