  We report magnetization, susceptibility, electrical transport, and electron
spin resonance (ESR) studies of single crystals of beta-AgNpO2(SeO3). Here the
valence of the Np sites is expected to be Np(V). We observe a magnetic
transition below 8 K, where the transition temperature is dependent on the
effective magnetic moment. Although the transition appears to be ferromagnetic,
no hysteresis is seen in the magnetization, and the saturation moment above 0.1
T is found to be about 60% of the free NpO2 ion moment. The decrease in the Np
moments determined experimentally is thought to arise from crystal field and
spin-orbit effects. Although Np(V) is expected to be ESR silent, we observe
temperature dependent ESR spectra at ~44 GHz (for fields above the saturation
field) that show slight shifts in the g-factor and line width at low
temperatures. Our results provide evidence that both Np(V) and Np(IV) valences
are present, where the latter may be a minority population. The crystals,
although dark in appearance, are electrically insulating (rho > 10^10 Ohm-cm)
at room temperature.
