Magnetic structure and metamagnetism in single crystals of NpCoGa5 -: art. no. 014460

The magnetic structure and H-T phase diagram of NpCoGa5 single crystals (space group P4/mmm) have been investigated by neutron diffraction, magnetization, and specific heat. The antiferromagnetic ordering at T-N=47 K has a wave vector q=(0 0 1/2), corresponding to a simple +- arrangement of the Np moments stacked along the tetragonal c axis, with the Np moments pointing along c. The Np magnetic moment 0.8(1) mu(B)/Np, deduced from the present neutron experiments, is consistent with the results of the magnetization measurements 0.70(2) mu(B)/Np in the high-field ferromagnetic state and previous Mossbauer studies on polycrystalline sample 0.84(5) mu(B)/Np. The magnetic scattering intensity is well explained without any moment on the Co atom and the magnetic form factor is consistent with the presence of a significant orbital moment on the Np site. The susceptibility shows Curie-Weiss behavior with strong anisotropy in the antiferromagnetically ordered state. The Sommerfeld coefficient is similar to 65 mJ mol(-1) K-2 in both the antiferromagnetic and field-induced ferromagnetic states. With an applied magnetic field along the c axis a first-order metamagnetic transition occurs with rather low critical field [mu H-0(c) (T=0)=4.5 T]. No metamagnetic transition has been observed for fields up to 9 T applied perpendicularly to c. The small metamagnetic field compared to the high magnetic ordering temperature indicates that the ferromagnetic coupling within each ferromagnetic Np plane is strong, but the antiferromagnetic interplanar coupling is relatively weak.