Abstract
AC susceptibility chi (T, H)= chi '-i chi and static magnetisation measurements M(T, H) up to 5T were performed on theta -MxV2O5 vanadium oxide bronzes containing Al and Fe donor atoms between 1.4 and 300K. The susceptibility of Fe/sub /xV2O5 (0.33<or=x<or=0.37) shows two magnetic transitions indicating that two magnetic subsystems are present. In the case of AlxV2O5 (0.33<or=x<or=0.50) as well as in the mixed samples (Fe, Al)0.33V2O5 only one transition is observable. For AlxV2O5 and (Fe, Al)0.33V2O5. The authors attribute the magnetic ordering to antiferromagnetically coupled spin pairs of V4+ ions. The results can be described by a simple model containing V4+ dimers with S=0 spin and paramagnetic V4+ ions with S=1/2. The second transition appearing only in FexV2O5 is discussed in terms of a spin glass phase formed in the iron sublattice.
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