Pressure and doping control of magnetic order and metallization in Ruddlesden-Popper La2NiO4

Abstract

The discovery of superconductivity in multilayer nickelates under pressure has intensified interest in understanding the magnetic and electronic properties of Ruddlesden-Popper nickelates. Using density functional theory with Hubbard corrections, we investigate the magnetic ground state, electronic structure evolution under pressure, and Sr-doping effects in La$_2$NiO$_4$. We find that at ambient pressure, tetragonal La$_2$NiO$_4$ exhibits G-type antiferromagnetic order with negligible interlayer magnetic coupling. Under hydrostatic pressure, the system undergoes a continuous insulator-metal transition at ~50 GPa while maintaining robust magnetic order up to 75 GPa, contrasting sharply with the rapid magnetic suppression in La$_3$Ni$_2$O$_7$. Sr doping induces a systematic evolution from G-type to A-type, to striped antiferromagnetic orders, and eventually to ferromagnetic order, accompanied by metallization. Furthermore, LaSrNiO$_4$ displays weak charge and orbital orders. These results reveal the unique pressure and doping effects of single-layer nickelates and provide insights into the magnetic mechanisms underlying nickelate superconductivity.

Figures (5)

• Figure 1: Crystal structure and schematic of candidate magnetic configurations for La$_2$NiO$_4$. Red and blue arrows denote Ni-site spin-up and spin-down orientations, respectively. The considered magnetic orders include ferromagnetic (FM), A-type antiferromagnetic (A-AFM), G-type antiferromagnetic (G-AFM), and double spin stripe (DSS) configurations.
• Figure 2: Magnetic stability and exchange interactions in La$_2$NiO$_4$. (a) Total energy differences of various magnetic configurations relative to the NM state as a function of the Hubbard $U$ parameter. (b) Calculated local magnetic moments of Ni ions across the same $U$ range. (c) Schematic illustration of the nearest-neighbor $J_{1}$ and next-nearest-neighbor $J_{2}$ exchange coupling paths within the NiO$_2$ planes.
• Figure 3: Pressure-induced evolution of the electronic structure in G-AFM La$_2$NiO$_4$. Band structures are shown for hydrostatic pressures of (a) 0 GPa, (b) 50 GPa, and (c) 75 GPa. The progression illustrates a continuous insulator-to-metal transition with increasing pressure.
• Figure 4: Doping-dependent electronic properties of La$_{2-x}$Sr$_x$NiO$_4$. Band structures are presented for (a) $x$=0.5 and (b) $x$=1.0. Panels (c) and (d) show the corresponding spin-resolved and orbital-projected partial density of states (PDOS) for the two crystallographically inequivalent Ni sites, highlighting the impact of hole doping on orbital occupancy and site symmetry.
• Figure 5: Predicted ground-state magnetic configuration for Sr-doped La$_{2-x}$Sr$_x$NiO$_4$ at $x=1$.