${\mathrm{Cr}}_{1/4-\delta }{\mathrm{NbSe}}_2$ $(\delta =0.065)$ is a triangular lattice magnet in which magnetic ${\mathrm{Cr}}^{3+}$ ions are intercalated to form $2a\times 2a$ triangular superlattices between ${\mathrm{NbSe}}_2$ van der Waals layers stacked along the $c$ axis. By unpolarized and polarized neutron scattering experiments, we have revealed that the magnetic ground state of this system is a ${120}^{\circ }$-type antiferromagnetic order characterized by the magnetic propagation wave vector of $q=(\frac{1}{3},\frac{1}{3},0)$. We also performed inelastic neutron scattering measurements using coaligned single crystals, and we determined dispersion relations of magnetic excitations at low temperatures. Comparing the observed spectra with calculations based on the linear spin-wave theory, we revealed that the out-of-plane ferromagnetic interaction is fairly strong as compared to the in-plane nearest-neighbor antiferromagnetic interaction. Although the crystal structure of this system is composed of two-dimensional van der Waals layers, the magnetic order has a three-dimensional character, which would be attributed to long-range magnetic interactions mediated by conduction electrons.

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• Antiferromagnetism
• Exchange interaction
• van der Waals systems
• Inelastic neutron scattering
• Neutron diffraction
• Neutron spin echo spectroscopy
• Spin wave spectroscopy