Polaronic Correlations in Magnetic Behavior of Oxygen Deficient V2O5

Abstract

In this paper, we investigate the magnetic behaviour of oxygen deficient vanadium pentoxide and its correlation with DC electrical transport. The undoped V 2 O 5 sample is synthesized by thermal decomposition resulting in oxygen deficient polycrystalline nature. It has a crystallite size of about 80 nm and a chemical stoichiometry of V 2 O 4.65 with a significant V 4+ states in addition to V 5+ as seen from X-ray diffraction and X-ray photoelectron spectroscopy respectively. The optical studies reveal a bandgap of 2.2 eV and a significant emission due to defects (1.8 eV) as well as Moss-Burstein effect(3.1 eV) which arises due to gap states and split in conduction band respectively. The temperature dependant electrical conductivity and thermopower measurements clearly demonstrate a polaronic conduction mechanism of small polaron hopping with hopping energy of 0.112 eV. The V 2 O 5 sample is found to show a strong field as well as temperature dependent magnetic behaviour when measured using sensitive SQUID magnetometer. It shows a positive magnetic susceptibility at all temperatures (5-300 K) at low field (µ 0 H= 0.1 T). The observed behavior is found to be a mix of ferromagnetism (FM) and antiferromagnetism (AFM, θ = −2.1 K in CW fit). On the other hand, for high field (µ 0 H= 1 T), it showed FM only at low temperatures and abruptly becomes diamagnetic at 31.2 K. This varied magnetic behavior has been attributed to small polaron hopping which arises because of oxygen vacancy defects compensated by charge defects of V 4+ and transition from bound to free polarons seen from transport. This is further revealed as strong hybridization of V bonded to oxygen vacancy V O and neighboring V 5+ ions, resulting in net magnetic moment per vacancy (3.959 µ B ) as predicted from first principle calculations. The insignificantly low energy difference between FM and AFM interactions observed from calculations and their critical dependence on oxygen vacancy concentrations justify the varied magnetic behaviour of V 2 O 5 reported in the literature.