Magnetic impurities in HNb2O5

Cu, Fe, Gd and Nd dopants were added to HNb2O5 in a range of concentrations to investigate the influence on the magnetic properties. The dopants fill the vacant sites in the channels along the b axis of the structure. Magnetic moments tend to remain localized in the dopants. AC susceptibility measurements display low temperature undulations associated with short range magnetic correlations within small clusters (of two or three ions) of dopants. Glauber spin-flip kinetics for Ising spin rings can be applied to these data. Although the magnetic behaviour is mostly Curie-Weiss above 20 K, the susceptibility data display a temperature-independent residual paramagnetic signal. We attribute this signal to Van Vleck orbital paramagnetism associated with bonding-antibonding transitions within Nb4+ or Cu2+ spin-paired dimers. (author)