摆线螺旋中近纯电子性质的反常磁性铁电性
Improper magnetic ferroelectricity of nearly pure electronic nature in cycloidal spiral CaMn$_{7}$O$_{12}$
摘要：
非共线的摆线磁序打破了CaMn $_{7}$ O $_{12}$的反转对称性，产生了迄今为止测得的最大的自旋轨道驱动铁电极化之一。在这封信中，微观起源的极化，包括它的方向，电荷密度重新分布，磁交换相互作用，以及它的耦合到自旋螺旋，通过第一性原理计算进行了探索。Berry相位计算的极化显示出几乎纯电子的行为，因为Mn位移可以忽略不计，$\approx$ ~0.7~m \textrm Ω。极化的大小和方向都由Mn自旋电流决定，其中\emph{p} - \emph{d}轨道混合由\emph{B} -site Mn摆线螺旋链内沿每个笛卡尔方向的不等价交换相互作用驱动。我们采用广义的自旋电流模型与海森堡交换Dzyaloshinskiii-Moriya相互作用的能量提供洞察到这种自旋驱动的极化的基本物理。在近反转对称的离子晶格中，由螺旋自旋序引起的持续电子极化暗示了超快磁电响应的机会。
英文摘要：
The noncollinear cycloidal magnetic order breaks the inversion symmetry in CaMn$_{7}$O$_{12}$, generating one of the largest spin-orbit driven ferroelectric polarizations measured to date. In this Letter, the microscopic origin of the polarization, including its direction, charge density redistribution, magnetic exchange interactions, and its coupling to the spin helicity, is explored via first principles calculations. The Berry phase computed polarization exhibits almost pure electronic behavior, as the Mn displacements are negligible, $\approx$~0.7~m\textrmÅ. The polarization magnitude and direction are both determined by the Mn spin current, where the \emph{p}-\emph{d} orbital mixing is driven by the inequivalent exchange interactions within the \emph{B}-site Mn cycloidal spiral chains along each Cartesian direction. We employ the generalized spin-current model with Heisenberg-exchange Dzyaloshinskii-Moriya interaction energetics to provide insight into the underlying physics of this spin-driven polarization. Persistent electronic polarization induced by helical spin order in nearly inversion-symmetric ionic crystal lattices suggests opportunities for ultrafast magnetoelectric response.
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