Neutron diffraction determination of magnetic order in holmium trifluoride, HoF3

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Abstract
The Ho3+ ion (4f10, 5I8) in holmium trifluoride has a singlet ground state and an excited state, also a singlet, at 6.59 cm-1. In HoF3 the nuclear moments (165Ho, I=7/2) are strongly enhanced through the hyperfine interaction, and previous results concluded that at TN=0.53 K the crystal undergoes a transition to an ordered antiferromagnetic state with both nuclear and electronic moments parallel and antiparallel to the orthorhombic a axis. Neutron diffraction studies at temperatures well below TN have revealed that the ordered magnetic state is at variance with earlier conclusions and is more complex. A refinement of the neutron diffraction data shows that the ordered state may be described as Pnm'a' (FxCz) with Ho3+ moments of 5.7(2) mu B at 66 degrees to (001). The temperature dependence of the intensities of the (100), (140) and (420) reflections have been determined for 0.07<T<0.6 K and are compared with the predictions of molecular field theory.