Correlations between stacked structures and weak itinerant magnetic properties of La${}_{2-x}$ Y${}_x$ Ni${}_7$ compounds

Abstract

Hexagonal La${}_2$Ni${}_7$ and rhombohedral Y${}_2$Ni${}_7$ are weak itinerant antiferromagnet (wAFM) and ferromagnet (wFM), respectively. The crystal structure and magnetic properties of $A_2{B}_7$ intermetallic compounds ($A$ = La, Y, $B$ = Ni) have been investigated combining X-ray powder diffraction and magnetic measurements. The La${}_{2-x}$Y${}_x$Ni${}_7$ intermetallic compounds with $0\le x\le 1$ crystallize in the Ce${}_2$Ni${}_7$-type hexagonal structure with Y preferentially located in the [$A{B}_2$] units. The compounds with larger Y content ($1.2\le x<2$) crystallize in both hexagonal and rhombohedral (Gd${}_2$Co${}_7$-type) structures with a progressive substitution of Y for La in the $A$ sites belonging to the [$A{B}_5$] units. Y${}_2$Ni${}_7$ crystallizes in the rhombohedral structure only. The average cell volume decreases linearly versus Y content, whereas the c/a ratio presents a minimum at $x=1$ due to geometric constrains. The magnetic properties are strongly dependent on the structure type and the Y content. La${}_2$Ni${}_7$ displays a complex metamagnetic behavior with split AFM peaks. Compounds with x = 0.25 and 0.5 display a wAFM ground state and two metamagnetic transitions, the first one towards an intermediate wAFM state and the second one towards a FM state.T${}_N$ and the second critical field increase with the Y content, indicating a stabilization of the AFM state. LaYNi${}_7$, 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 with $x>1$ and containing a rhombohedral phase are wFM with $T_C$ = 53(2) K. In addition to the experimental studies, first principles calculations using spin polarization have been performed to interpret the evolution of both structural phase stability and magnetic ordering for $0\le x<2$.

26 pages (7 for supplementary material), 4 tables, 9 main figures and 8 figures in supplementary material