Relation between the weak itinerant magnetism in A ₂Ni₇ compounds (A  =  Y, La) and their stacked crystal structures

The weak itinerant magnetic properties of A ₂Ni₇ compounds with A  =  {Y, La} have been investigated using electronic band structure calculations in the relation with their polymorphic crystal structures. These compounds crystallizes in two structures resulting from the stacking of two and three blocks of [A ₂Ni₄  +  2 ANi₅] units for hexagonal 2H-La₂Ni₇ (Ce₂Ni₇ type) and rhombohedral 3R-Y₂Ni₇ (Gd₂Co₇ type) respectively. Experimentally, 2H-La₂Ni₇ is a weak itinerant antiferromagnet whereas 3R-Y₂Ni₇ is a weak itinerant ferromagnet. From the present first principles calculation within non-spin polarized state, both compounds present an electronic density of state with a sharp and narrow peak centered at the Fermi level corresponding to flat bands from 3d-Ni. This induces a magnetic instability and both compounds are more stable in a ferromagnetic (FM) order compared to a paramagnetic state (ΔE  ≈  -35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with their positions relative to the [A ₂Ni₄] and [ANi₅] units: they are minimum in the [A ₂Ni₄] unit and maximum at the interface between two [ANi₅] units. For 2H-La₂Ni₇, an antiferromagnetic (AFM) structure has been proposed and found with an energy comparable to that of the FM state. This AFM structure is described by two FM unit blocks of opposite Ni spin sign separated by a non-magnetic layer at z  =  0 and ½. The Ni (2a) atoms belonging to this intermediate layer are located in the [La₂Ni₄] unit and are at a center of symmetry of the hexagonal cell (P6₃/mmc) where the resultant molecular field is cancelled. Further non-collinear spin calculations have been performed to determine the Ni moment orientations which are found preferentially parallel to the c axis for both FM and AFM structures.