High-temperature ferromagnetism of Li-doped vanadium oxide nanotubes

¹  Institute for Solid State Research, IFW Dresden - 01171 Dresden, Germany, EU
²  Zavoisky Physical-Technical Institute of RAS - 420029 Kazan, Russia
³  IFP, TU Dresden - D-01069 Dresden, Germany, EU
⁴  Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut - CH-5232 Villigen PSI, Switzerland

Corresponding author: v.kataev@ifw-dresden.de

Received: 25 July 2009
Accepted: 9 November 2009

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.