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Giant gap in the magnon excitations of the quasi-1D chain compound Sr3NiIrO6

Inelastic neutron scattering on the spin-chain compound Sr$_3$NiIrO$_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of two dispersive modes. The spin wave spectrum could be well fitted with an antiferromagnetic anisotropic exchange model and single ion anisotropy on the Ni site. The extracted dominant anisotropic antiferromagnetic intra-chain exchange interaction between Ir and Ni ions are $J_z=19.5$ meV and $J_{xy}=12.1$ meV. These values justify previous electronic structure calculations, showing the importance of Ir spin orbit coupling on the electron correlations. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of $T_N \sim 70$ K, also indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiIrO$_6$. Our results not only support the idea of the existence of a new temperature scale well above $T_N$, but also emphasize the need to consider new exchange paths complicated by the SOC, resulting in additional characteristic temperatures in such spin-chain systems.