Crystal-Field Excitations, Magnon Gap, and Berry-Curvature Effects in Ferromagnetic NdGaSi Single Crystals

We report the synthesis, magnetic structure, and transport properties of NdGaSi single crystals grown using a high-temperature Ga flux method. Single-crystal X-ray diffraction confirms a tetragonal centrosymmetric structure (space group I4₁/amd). Magnetization and heat-capacity measurements show that NdGaSi orders ferromagnetically below T_C ≈ 11 K, with a pronounced easy axis along the crystallographic c direction. The λ-type anomaly in the heat capacity corresponds to the ordering of Nd³⁺ (4f³, J = 9/2) moments. A 10-level Schottky fit to the heat capacity captures the crystal-electric-field (CEF) scheme, indicating a well-isolated ground-state doublet responsible for the strong uniaxial anisotropy. The magnon gap (Δ ≈ 5.8 K), estimated from low-temperature heat capacity, highlights robust single-ion anisotropy. Neutron diffraction confirms long-range ferromagnetic alignment of Nd³⁺ moments along the c-axis. Hall-effect and magnetotransport measurements reveal a large anomalous Hall effect (AHE) below T_C, attributed to intrinsic Berry curvature originating from Weyl points near the Fermi surface induced by ferromagnetic order.