Relation between the weak itinerant magnetism inA₂Ni₇compounds (A  =  Y, La) and their stacked crystal structures

Abstract

The weak itinerant magnetic properties of A2Ni7 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 [A2Ni4 + 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 [A2Ni4] and [ANi5] units: they are minimum in the [A2Ni4] 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.

Weak itinerant magnetic phases of La2Ni7

“…In addition, we find the intensity distribution of several magnetic Bragg peaks in the A-phase are consistent with a triangular wave modulated SDW with moments along the c direction shown in Fig. 2(c), and as theoretically predicted recently [44].…”

“…The La 2 Ni 7 structure can be described as a structure made up of blocks of La 2 Ni 4 and LaNi 5 , respectively. For the La 2 Ni 7 structure these form block layers of [La 2 Ni 4 + 2LaNi 5 ] which can have an AB or ABC stacking arrangement resulting in a hexagonal (2H) or rhombohedral (3R) structure, respectively, and as shown in detail in several studies [44,48]. La 2 Ni 7 used in this study and several others crystallizes with the 2H structure, but with the presence of both intra-block and inter-block stacking faults.…”

“…To agree with our observations we also require a magnetic moment component of µ c, so the possible MSG can be reduced to 4: P 6 3 /m ′ m ′ c ′ , P 6 3 /mm ′ c ′ , P 6 ′ 3 /m ′ m ′ c, and P 6 ′ 3 /mm ′ c. In addition, recent DFT calculations by Crivello and Paul-Boncour have predicted that for AFM order La 2 Ni 7 would preferentially order along c as a triangle-wave modulated spin-density wave as shown in Fig. 2(c) [44]. This magnetic structure is consistent with the possible MSG P 6 ′ 3 /mm ′ c. In addition, from Fig.…”

“…La 2 Ni 7 is an example of a small-moment itinerant magnetic system which has had its magnetic properties studied for many years [36][37][38][39][40][41][42][43][44][45], but until recently only in its polycrystalline form. Recently, La 2 Ni 7 has been successfully synthesised as large single crystals [45].…”

Catalogue of Flat-Band Stoichiometric Materials

“…• The electronic structure properties of Y 2 Ni 7 [ICSD 647060, SG 166 (R 3m)] have been theoretically studied in Ref. [87], pointing toward the presence the flat bands to explain the magnetic properties of these materials.…”