A clear understanding of the physics of complex magnetic oxides is crucial for unlocking new pathways to design novel materials for technological applications. We present here a detailed investigation of the temperature- and magnetic-field-driven magnetic evolution of $\mathrm{BaErFe}{\mathrm{O}}_4$ using a suite of complementary techniques, including neutron diffraction, Mössbauer spectroscopy, $\text{dc}$ magnetization, and transverse susceptibility measurements. Below $T_{\mathrm{N}1}\approx 50\mathrm{K}$, the Fe sublattice orders into an incommensurate collinear spin-density-wave state with propagation vector $k_1=\left(0,0,\sim 0.4\right)$. The $3d-4f$ exchange coupling drives a transition at $T_{\mathrm{N}2}\approx 32\mathrm{K}$ to a commensurate noncollinear antiferromagnetic phase characterized by $k_2=\left(0.5,0,0.5\right)$ involving ordering of both $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{E}{\mathrm{r}}^{3+}$ ions, followed by a further modification of the Er magnetic configuration below $T_{\mathrm{N}3}\approx 5\mathrm{K}$ governed by dominant $4f-4f$ interactions. Strong geometrical frustration prevents complete long-range magnetic ordering, leaving a substantial fraction of spins in a short-range correlated state that serves as a metastable template for phase separation. Under an applied magnetic field of ${\mu }_{0}H\approx 0.2\mathrm{T}$, the $\mathrm{E}{\mathrm{r}}^{3+}$ spins within these regions undergo a metamagnetic spin-flip transition, nucleating ferromagnetic clusters. This metamagnetic response remains fully reversible down to 8 K, while a crossover to an irreversible ferromagneticlike regime occurs at lower temperatures, attributed to kinetic arrest of the magnetic clusters. A complex field-induced phase separation is established, in which ferromagnetic clusters of distinct populations coexist with the long-range antiferromagnetic order of the Er sublattice. These findings underscore the complex interplay among magnetic anisotropy, magnetic frustration, and quenched disorder in stabilizing the magnetic ground state of $\mathrm{BaErFe}{\mathrm{O}}_4$.

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• Frustrated magnetism
• Magnetic phase transitions
• Metamagnetism
• Magnetic systems
• Magnetization measurements
• Mössbauer spectroscopy
• Neutron scattering