${\mathrm{Cr}}_2{\mathrm{O}}_3$ is the archetypal magnetoelectric (ME) material, which has a linear coupling between electric and magnetic polarizations. Quadratic ME effects are forbidden for the magnetic point group of ${\mathrm{Cr}}_2{\mathrm{O}}_3$, due to space-time inversion symmetry. In ${\mathrm{Cr}}_2{\mathrm{O}}_3$ films grown by sputtering, we find a signature of a quadratic ME effect that is not found in bulk single crystals. We use Raman spectroscopy and magnetization measurements to deduce the removal of space-time symmetry and corroborate the emergence of the quadratic ME effect. We propose that metastable site-selective trace dopants remove the space, time, and space-time inversion symmetries from the original magnetic point group of bulk ${\mathrm{Cr}}_2{\mathrm{O}}_3$. We include the quadratic ME effect in a model describing the switching process during ME field cooling and estimate the effective quadratic susceptibility value. The quadratic magnetoelectric effect in a uniaxial antiferromagnet is promising for multifunctional antiferromagnetic and magnetoelectric devices that can incorporate optical, strain-induced, and multiferroic effects.

中文翻译：

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溅射生长的 Cr2O3 薄膜场冷却过程中的二次磁电效应

${铬}_2{\mathrm{哦}}_3$是典型的磁电 (ME) 材料，它在电极化和磁极化之间具有线性耦合。磁点群禁止二次 ME 效应${铬}_2{\mathrm{哦}}_3$，由于时空反演对称。在${铬}_2{\mathrm{哦}}_3$在通过溅射生长的薄膜中，我们发现了在块状单晶中没有发现的二次 ME 效应的特征。我们使用拉曼光谱和磁化测量来推断时空对称性的消除并证实二次 ME 效应的出现。我们建议亚稳态位点选择性痕量掺杂剂从体块的原始磁点群中去除空间、时间和时空反转对称性${铬}_2{\mathrm{哦}}_3$. 我们将二次 ME 效应包含在描述 ME 场冷却期间切换过程的模型中，并估计了有效的二次磁化率值。单轴反铁磁体中的二次磁电效应有望用于多功能反铁磁和磁电器件，这些器件可以结合光学、应变诱导和多铁性效应。