Structure and magnetic properties of the Ho2 Ge2 O7 pyrogermanate

We report the anisotropic magnetic properties of Ho2 Ge2 O7 determined from dc and ac magnetization, specific heat and powder neutron-diffraction experiments. The magnetic lanthanide sublattice, seen in our refinement of the tetragonal pyrogermanate crystal structure, is a right-handed spiral of edge-sharing and corner-sharing triangles; the local Ho-O coordination indicates that the crystal field is anisotropic. Susceptibility and magnetization data indeed show that the magnetism is highly anisotropic, and the magnetic structure has the Ho moments confined to the plane perpendicular to the structural spiral. The ordered moment of Ho3+, as determined from refinement of the neutron-diffraction data, is 9.0 μB. Magnetic ordering occurs around 1.6 K. Temperature and field dependent ac susceptibility measurements show that this compound displays spin relaxation phenomena analogous to what is seen in the spin ice pyrochlore system Ho2 Ti2 O7.