Oxide Nanotubes
A bottom-up approach to realize nanoscaled magnetic structures is to synthesize self-assembled nanoparticles which contain magnetic ions.
One example which is studied in our group are mixed valent vanadium oxide multiwall nanotubes (VOx-NTs). VOx-NTs (diameter ~ 50-100 nm, length ~ 0.5-15μm) consist of several walls which are seperated by organic spacer molecules. They exhibit diverse properties ranging from spin frustration and semiconductivity to ferromagnetism by doping with either electrons or holes [1]. In order to obtain insights into the electronic and magnetic properties of these novel nanosize magnets we have applied optical, photoemission, and electron energy-loss spectroscopy as well as static magnetization, ESR and NMR studies. Our data confirm the tubular structure of VOx-NTs and imply an averaged vanadium valency of about 4.4+ in the undoped case [2]. The structural low dimensionality and mixed valency of vanadium ions yield a complex temperature dependence of the static magnetization and the nuclear relaxation rates. We find two magnetically nonequivalent vanadium sites in the structure. These sites can be presumably attributed to V4+ (d1, S=1/2) ions in the octahedral and tetrahedral oxygen coordination, respectively. In particular, the data give strong indications that antiferromagnetic dimers and trimers occur in the structurally well-defined vanadium-spin chains in the walls owing to an inhomogeneous charge distribution. We find a spin gap of the order of 700 K [3]. Upon electron doping of VOx-NTs, our spectroscopic data confirm a higher number of magnetic V4+ sites. Interestingly, room temperature ferromagnetism evolves after electrochemical intercalation of Li making VOx-NTs a novel type of self-assembled nanoscaled ferromagnets.
We are investigating these nanocompounds in order to study the correlations between spin and charge in a low-dimensional tubular system. The possibility to tune the magnetic properties by intercalation or electrochemical doping also makes these tubes interesting for future applications, e.g. in order to realize novel nanoscale magneto-electronic devices. Especially the insertion of Lithium is also of importance for the battery research where VOx-NT could serve as cathode material in an electrochemical cell. We are adressing the following aspects of VOx-NT:
For further information please contact: Dr. V. Kataev
[1] L. Krusin-Elbaum et al., Nature 431, 672 (2004)
Own publications:
[2] X. Liu et al., Phys. Rev. B 72, 115407 (2005): cf. here
[3] E. Vavilova et al., Phys Rev. B 73, 144417 (2006): cf. here