We combine single-crystal neutron diffraction studies and Monte Carlo simulations to determine the spin structures and finite-temperature phase diagram of the spin-5/2 triangular-lattice antiferromagnet ${\mathrm{Na}}_2\mathrm{BaMn}{\left({\mathrm{PO}}_4\right)}_2$ in magnetic field. With the application of a magnetic field in two different directions, namely along the $c$ axis and in the $ab$ plane of the trigonal symmetry, we track the evolution of the spin structure through changes of the magnetic propagation vector. We account for these results with a minimal Heisenberg Hamiltonian that includes easy-axis anisotropy and weak, frustrated interlayer couplings in addition to intralayer exchange. Guided by representation analysis, we refine symmetry-allowed modes to the measured intensities and obtain the spin structures for all field-induced phases, which we compare quantitatively with simulated configurations. Taken together, our measurements and simulations show that frustrated interlayer exchange—rather than purely two-dimensional physics—organizes the unexpectedly rich field-induced phases of ${\mathrm{Na}}_2\mathrm{BaMn}{\left({\mathrm{PO}}_4\right)}_2$.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic interactions
• Magnetism
• Phase diagrams
• Antiferromagnets
• Landau-Lifschitz-Gilbert equation
• Monte Carlo methods
• Neutron diffraction
• Specific heat measurements