Magnetic study of pentavalent neptunium phthalate hexahydrate (NpO2)2(O2C)2C6H4·6H2O

Magnetic properties of the polycrystalline neptunium(V) phthalate complex hexahydrate (NpO2)2(O2C)2C6H4·6H2O were investigated in the temperature range of 2–300 K and in the applied fields between −5.5 and +5.5 T. This compound exhibits a magnetic ordering below 4.5 K. At 2 K, the initial magnetization curve exhibits a two-step saturation process; a kink around ∼0.4 T (∼0.68 μB/Np) and a further increase to a second plateau with a saturation moment, μsat=1.23 μB/Np at 5.5 T. The former makes a small hysteresis loop (−0.4 T≤H≤+0.4 T) with a coercive field HC∼0.076 T. Such behavior most likely indicates the existence of the two kinds of Np sublattices, one is ferromagnetic and the other is antiferromagnetic (metamagnetic).