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

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.