The microscopic magnetic model of the low-dimensional quantum magnet α-${\mathrm{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ has remained controversial. We present a comprehensive study of its magnetic ground state and excitation spectrum using temperature-dependent inelastic neutron scattering, neutron diffraction, magnetization measurements, and comprehensive spin-wave modeling. Our results unambiguously establish α-${\mathrm{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ as a two-dimensionally coupled spin-dimerized antiferromagnetic (AF) lattice within the $bc$ plane, with a dominant AF exchange $J_2=7.73\pm 0.02$ meV (hereafter referred to as the ‘‘intradimer exchange’’) and weaker exchange couplings $J_1$, $J_3$, and $J_4$ in the two-dimensional lattice (hereafter referred as ‘‘interdimer exchange’’), in agreement with LDA-based density functional theory and in contrast to previous GGA+U predictions. The dominant intradimer AF exchange is found between the seventh-nearest-neighbor Cu–Cu ion pairs $[d_{\mathrm{Cu}-\mathrm{Cu}}=5.125\left(3\right)\AA ]$ rather than the nearest-neighbor Cu–Cu ion pairs $[d_{\mathrm{Cu}-\mathrm{Cu}}=3.014\left(1\right)\AA ]$ of the structural dimers. Weak interlayer coupling $(J_5=0.03\pm 0.01 \mathrm{meV})$ stabilizes long-range antiferromagnetic order below $T_N\approx 25$ K. We further identify a weak single-ion anisotropy, associated with the distorted ${\mathrm{CuO}}_5$ polyhedra that opens a gap in the spin-excitation spectrum and drives a field-induced metamagnetic transition. Systematic spin-wave calculations elucidate the distinct roles of interlayer coupling $J_5$ and the anisotropy term $D$ in producing two distinct energy gaps at different antiferromagnetic zone centers. Complementary neutron diffraction and magnetization measurements as a function of applied magnetic field uncover a previously overlooked metamagnetic transition near 13 kOe and allow the construction of the magnetic phase diagram in the H-T plane. Our results unambiguously establish α-${\mathrm{Cu}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ as a realization of a two-dimensionally coupled spin-dimerized AF lattice with weak interlayer coupling and magnetic anisotropy, and thus resolve the discrepancy in the microscopic magnetic model. The present findings also highlight the importance of extended exchange pathways and subtle anisotropic exchange interactions in governing the ground state and excitation spectrum of low-dimensional quantum magnets.

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• Magnetic interactions
• Magnetic order
• Magnetism
• Antiferromagnets
• Inelastic neutron scattering
• Neutron diffraction
• Spin wave theory