The spontaneous magnetostrictive effect induces anomalous negative thermal expansion in certain magnetic materials due to spin-spin correlations. Traditionally, this phenomenon is identified by lattice constant measurements, which do not directly reveal the dominant spin interactions. Using ultrafast, site-selective crystal-field excitations in polar antiferromagnetic ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, we uncover an excitation site–dependent strain pulse generation. This enables the realization of negative thermal expansion and the construction of spin-spin correlation functions. We find that strain pulses generated from $\mathrm{Fe}$ octahedral sites diminish near spin-ordering onset, while those from $\mathrm{Fe}$ tetrahedral sites decreases in intensity near the Néel temperature, independent of soft-mode behavior. This excitation site–dependent strain pulse emerges as a highly sensitive tool for measuring anomalous thermal expansion through phase shifts in probe interference and for identifying dominant spin-spin correlations via temperature-dependent strain intensity variations. Supported by magnetic field–dependent experiments and theoretical analysis, we confirm the absence of forced magnetostriction in the generation mechanism. This innovative approach bridges mechanical properties and spin-spin interactions, advancing the study of acoustic strain pulses and magnetic materials.

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• Electric polarization
• Magnetic phase transitions
• Magnetostriction
• Mechanical & acoustical properties
• Antiferromagnets
• Complex materials
• Magnetic insulators
• Multiferroics