Magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) nanoparticles exhibit size-dependent magnetic and thermal properties crucial for spintronics, thermoelectrics, and hyperthermia applications, yet the nanoscale behavior of magnons and phonons remains poorly understood. Combining time-of-flight inelastic neutron scattering and numerical simulations, we reveal a universal mechanism of confinement-driven quasiparticle renormalization in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles. In 8-nm particles, optical phonons soften by 0.5 meV and significantly broaden, while acoustic magnons exhibit suppressed group velocities (from $14000\mathrm{m}{\mathrm{s}}^{-1}$ to $10000\mathrm{m}{\mathrm{s}}^{-1}$) due to surface-induced exchange disorder. Heterogeneous spin environments, combining bulklike ($J$) and surfacelike ($J^{\prime }<J$) interactions, quantitatively explain spectral broadening and magnon softening. These findings establish finite-size confinement as a general paradigm for engineering spin and thermal transport in nanoscale systems, with direct implications for enhancing spin-wave lifetimes in spintronic devices and phonon engineering in thermoelectrics.

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• Confinement
• Magnons
• Phonons
• Quasiparticles & collective excitations
• Ferrites
• Magnetic nanoparticles
• Ab initio molecular dynamics
• Time-of-flight neutron spectroscopy