Observation of long-range magnetic ordering in pyrohafnate Nd2Hf2O7: A neutron diffraction study

We have investigated the physical properties of a pyrochlore hafnate Nd2Hf2O7 using ac magnetic susceptibility chi(ac)(T), dc magnetic susceptibility chi(T), isothermal magnetization M(H), and heat capacity C-p(T) measurements, and determined the magnetic ground state by neutron powder diffraction. An upturn is observed below 6 K in C-p(T)/T, however both C-p(T) and chi(T) do not show any clear anomaly down to 2 K. The chi(ac)(T) shows a well-pronounced anomaly indicating an antiferromagnetic transition at T-N = 0.55 K. The long-range antiferromagnetic ordering is confirmed by neutron diffraction. The refinement of the neutron diffraction pattern reveals an allin/all- out antiferromagnetic structure, where for successive tetrahedra the four Nd3+ magnetic moments point alternatively all-into or all-out-of the tetrahedron, with an ordering wave vector k = (0, 0, 0) and an ordered state magnetic moment of m = 0.62(1) mu(B)/Nd at 0.1 K. The ordered moment is strongly reduced, reflecting strong quantum fluctuations in ordered state.