The regulation of magnetic anisotropy in two-dimensional (2D) van der Waals (vdW) ferromagnets is critical for enabling spintronic and solid-state cooling applications, yet the microscopic doping mechanism remains poorly understood, as conventional ideal-lattice models fail to reconcile the moderate structural and magnetic evolution observed in experiments. In this work, we investigate Ni-doped ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$, a room-temperature 2D ferromagnet, combining systematic magnetization measurements and first-principles calculations based on the realistic defective lattice structure of ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$. We demonstrate an intrinsic defect-assisted occupation mechanism: Ni dopants preferentially substitute the preexisting interstitial Fe atoms in the vdW gap rather than inserting into empty interlayer sites. This mechanism accounts well for all our experimental observations, including the minor $c$-axis contraction, the 80% reduction of magnetocrystalline anisotropy energy with preserved out-of-plane easy axis, and the monotonic decrease of Curie temperature ($T_{\text{C}}$) and saturation magnetization. Furthermore, we demonstrate that Ni doping effectively tunes the anisotropic magnetocaloric effect: The anisotropy of magnetic entropy change decreases from 0.68 to $0.29\text{J}{\text{kg}}^{-1}{\text{K}}^{-1}$, while the relative cooling power exhibits a nonmonotonic evolution, reaching an optimal value of $156\text{J}{\text{kg}}^{-1}$ at 4.6% Ni doping. The evolution of critical exponents $n$ and $m$ confirms that Ni doping introduces magnetic disorder, broadening the magnetic phase transition. Our results resolve the discrepancy between ideal-lattice theoretical predictions and experimental observations, providing a clear microscopic picture for anisotropy engineering in 2D magnets.

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• Entropy
• Magnetic anisotropy
• Magnetism
• Magnetocaloric effect
• Ferromagnets
• Single crystal materials
• van der Waals systems
• First-principles calculations