Correlations between stacked structures and weak itinerant magnetic properties of La_2-xY_xNi₇compounds

Hexagonal La₂Ni₇and rhombohedral Y₂Ni₇are weak itinerant antiferromagnet (wAFM) and ferromagnet (wFM), respectively. To follow the evolution between these two compounds, the crystal structure and magnetic properties ofA₂B₇intermetallic compounds (A= La, Y,B= Ni) have been investigated combining x-ray powder diffraction and magnetic measurements. The La_2-xY_xNi₇intermetallic compounds with 0 ⩽x⩽ 1 crystallize in the hexagonal Ce₂Ni₇-type structure with Y preferentially located in the [A₂B₄] units. The compounds with larger Y content (1.2 ⩽x< 2) crystallize in both hexagonal and rhombohedral (Gd₂Co₇-type) structures with a substitution of Y for La in both [A₂B₄] and [AB₅] units. Y₂Ni₇crystallizes in the rhombohedral structure only. The average cell volume decreases linearly versus Y content, whereas thec/aratio presents a minimum atx= 1 due to geometric constrains. The magnetic properties are strongly dependent on the structure type and the Y content. La₂Ni₇displays a complex metamagnetic behavior with split AFM peaks. Compounds withx= 0.25 and 0.5 display a wAFM ground state and two metamagnetic transitions, the first one toward an intermediate wAFM state and the second one toward a FM state.T_Nand the second critical fieldμ₀H_c2increase with the Y content, indicating a stabilization of the AFM state. LaYNi₇, which is as the boundary between the two structure types, presents a very wFM state at low field and an AFM state as the applied field increases. All the compounds withx> 1, and which contains a rhombohedral phase are wFM withT_C= 53(2) K. In addition to the experimental studies, first principles calculations using spin polarization have been performed to interpret the evolution of structural phase stability for 0 ⩽x⩽ 2.