Magnetoelectric Effect in Garnet Mn₃Al₂Ge₃O₁₂

Searching for novel magnetoelectric (ME) materials has been one of the major issues of multiferroics. In this work, we present a systematic research study on garnet Mn₃Al₂Ge₃O₁₂, including structural, magnetic, heat capacity, and ME characterizations. Below the Néel temperature T_N ∼ 6.8 K, Mn²⁺ spins form a long-range antiferromagnetic order, and a magnetic field H-driven electric polarization P is identified simultaneously. The relationship between P and H is nonlinear under low H and becomes linear under high H. Such transition is believed to originate from the H-induced variation of the magnetic structure. In addition, the P reaches 0.6 μC/m² under μ₀H = 9 T, corresponding to an ME coupling coefficient of α_ME ∼ 0.08 ps/m under high H. The small α_ME is attributed to the weak spin-orbit coupling and weak magnetic interactions in Mn₃Al₂Ge₃O₁₂. Furthermore, we realize the stable control of P by periodically varying H, which is crucial for potential application. We provide a rare case that a garnet material shows a first-order ME effect.