Crystalline electric field excitations in CeAgSb₂

Neutron scattering experiments have been carried out that reveal the magnetic excitation in the ferromagnet CeAgSb₂. Crystalline electric field excitations were observed at 5.2 and 12.5 meV in the paramagnetic state, indicative of the localized nature of the Ce 4f electron. Our analysis concluded that |J_z = ±1/2〉 is the ground state, while the first and second excited levels are mainly due to |±3/2〉 and |±5/2〉, respectively. The observed ferromagnetic moment is in good agreement with the ground-state saturation moment, g_Jμ_B J_z ∼ 0.43 μ_B. The spin wave dispersion is interpreted using the anisotropic Heisenberg model with ferromagnetic exchange interaction up to seventh-nearest neighbours. The spontaneous moment along the c-axis decreases with increasing field perpendicular to the c-axis, and disappears at the critical field H_c, which corresponds to the anomaly which appeared in the magnetization curve.