Pressure-induced magnetic collapse and hydrogen segregation in the frustrated Laves hydridesTb(Mn0.88Al0.12)2Hx

We have studied magnetic and crystal properties of the frustrated Laves hydrides Tb(Mn 0 . 8 8 Al 0 . 1 2 ) 2 H x (x = 1-3) and HoFe 2 H 1 . 5 by x-ray diffraction up to 45 GPa and neutron diffraction up to 16 GPa. We found magnetostructural transitions. When the lattice constant approaches some critical value, the hydrogen atoms diffuse in the crystal lattice and form hydrogen-rich and hydrogen-poor domains. The chemical segregation drastically affects the magnetic properties. In Tb(Mn 0 . 8 8 Al 0 . 1 2 ) 2 H 1 , the hydrogen-rich domains show short-range antiferromagnetic correlations, whereas hydrogen-poor domains show long-range ferromagnetic order. We discuss the coupling between magnetic and structural phenomena and the possibility of a magnetically induced chemical segregation.