High-temperature ferromagnetism of Li-doped vanadium oxide nanotubesA. I. Popa1, E. Vavilova1, 2, Y. C. Arango1, V. Kataev1, C. Täschner1, H.-H. Klauss3, H. Maeter3, H. Luetkens4, B. Büchner1 and R. Klingeler1
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
received 25 July 2009; accepted in final form 9 November 2009; published December 2009
published online 7 December 2009
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.
75.20.-g - Diamagnetism, paramagnetism, and superparamagnetism.
75.75.+a - Magnetic properties of nanostructures.
73.22.-f - Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals.
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