Instability of Magnons in Two-dimensional Antiferromagnet at High Magnetic Fields

Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is studied by a combination of bulk measurements, neutron diffraction, and inelastic neutron scattering techniques. Easy plane type antiferromagnetic order is identified at $T\le 4.0$ K. The exchange interactions are estimated as $J_1$ = 27.8(3)$\mu$eV and $J_2$ = 1.0(1) $\mu$eV, and the saturation field $H_{\mathrm{C}}$ is 9.75 T. Magnetic excitation measurements with high experimental resolution setup by triple axis neutron spectrometer reveals the instability of one magnon excitation in the field range of $0.7H_{\rm C} \lesssim H \lesssim 0.85H_{\rm C}$.