Magnetism and electron spin resonance in single crystalline beta-AgNpO2(SeO3)

摘要

We report magnetization, susceptibility, electrical transport, and electronspin resonance (ESR) studies of single crystals of beta-AgNpO2(SeO3). Here thevalence of the Np sites is expected to be Np(V). We observe a magnetictransition below 8 K, where the transition temperature is dependent on theeffective magnetic moment. Although the transition appears to be ferromagnetic,no hysteresis is seen in the magnetization, and the saturation moment above 0.1T is found to be about 60% of the free NpO2 ion moment. The decrease in the Npmoments determined experimentally is thought to arise from crystal field andspin-orbit effects. Although Np(V) is expected to be ESR silent, we observetemperature dependent ESR spectra at ~44 GHz (for fields above the saturationfield) that show slight shifts in the g-factor and line width at lowtemperatures. Our results provide evidence that both Np(V) and Np(IV) valencesare 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.