Transition metal-based layered organic-inorganic hybrid perovskites (OIHPs) exhibit diverse magnetic phenomena, yet the atomistic origin of magnetism in this class of materials remains elusive after decades of study. Here we demonstrate a notable ligand tunability of magnetism in two new quasi-two-dimensional OIHPs (${\mathrm{C}}_7{\mathrm{H}}_9\mathrm{NBr}$)${}_2{\mathrm{CuX}}_4$ (${\mathrm{C}}_7{\mathrm{H}}_9\mathrm{NBr}$ = 4-Bromobenzylammonium = A, X = Cl, Br) from comprehensive magnetic measurements and provide its microscopic understanding supported by ab initio DFT calculations. Despite being isostructural and having easy plane magnetocrystalline anisotropy (MCA), the Cl and Br analogs exhibit contrasting magnetic response. While ${\mathrm{A}}_2{\mathrm{CuCl}}_4$ shows an in-plane ferromagnetic and out-of-plane antiferromagnetic-like response, ${\mathrm{A}}_2{\mathrm{CuBr}}_4$ follows the reverse trend. The origin of this intriguing behavior is argued to stem from Dzyaloshinskii-Moriya interaction (DMI) present in these layered systems. We quantify DMI by employing constrained noncollinear DFT calculations in the presence of SOC which otherwise is challenging in the domain of OIHPs due to their large system size. Based on the competition between DMI and MCA, we propose a canted and transverse conical spiral spin structure for ${\mathrm{A}}_2{\mathrm{CuCl}}_4$ and ${\mathrm{A}}_2{\mathrm{CuBr}}_4$, respectively, which captures the observed magnetic response. Our study provides an effective way of tailoring and understanding the occurrence of nontrivial spin textures in this class of OIHPs.

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• Magnetism
• Hybrid perovskites
• Layered crystals
• Noncollinear magnets
• Density functional theory
• Magnetization measurements
• X-ray diffraction