A Viewpoint on : Giant Improper Ferroelectricity in the Ferroaxial Magnet CaMn 7 O 12
A Viewpoint on : Giant Improper Ferroelectricity in the Ferroaxial Magnet CaMn 7 O 12
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发表时间：
2012
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通讯作者：
R. D. Johnson;L. Chapon;D. Khalyavin;P. Manuel;P. Radaelli
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作者：
R. D. Johnson;L. Chapon;D. Khalyavin;P. Manuel;P. Radaelli
利用外加电场（也称为磁电效应）控制磁绝缘体中的自旋顺序可以显著降低存储设备的功耗，但在没有移动电荷的情况下，这似乎是一项不可能完成的任务。令人鼓舞的是，最近发现一些磁序诱导电极化，其自旋与电场耦合。到目前为止，这种磁性铁电体（也称为多铁电体）的电极化倾向于很小，并且nsamel磁转变温度通常远低于液氮温度。然而，现在，英国牛津大学的罗杰·约翰逊和他的同事，以及法国的合作者，在《物理评论快报》上报道了在camn7012中实现巨大极化的情况。测量到的极化是测量到的最高磁致极化，持续到90k的nsamel温度。值得注意的是，这种极化似乎是由长周期螺旋（或固有螺旋）自旋螺旋引起的[见图1(a)]，其中自旋围绕螺旋波矢量[1]旋转。这一发现代表了磁性铁电体领域的一个重要发展，因为大极化对于自旋的电操纵至关重要。它证实了早先对多晶样品[2]的极化估计。引起铁电性的最普遍的自旋顺序是摆线螺旋，其中自旋围绕与螺旋波矢量垂直的轴旋转。摆线——由车轮在平面上滚动的边缘上的一点所描绘的曲线——在与运动方向和车轮轴垂直的方向上是不对称的，这也是自旋摆线引起电极化的方向[见图1(b)]。相比之下，螺旋形铁电体非常罕见，迄今为止研究的所有材料都只是弱铁电体[3,4]。螺旋螺旋中自旋的顺时针或逆时针方向用一个称为螺旋度的量来描述。这个量在inverFIG下改变符号。1:(a)螺旋自旋螺旋，其中自旋旋转轴和诱导极化平行于螺旋波矢量。(b)摆线自旋螺旋，其中自旋（红色箭头）围绕与螺旋波矢量q垂直的轴旋转，感应电极化P与波矢量和自旋旋转轴都垂直。绿色的曲线是椭球体。(c) CaMn7O12中Mn-O八面体的螺旋桨状结构，当样品转动时改变了旋转方向。(APS / Carin凯恩)
Control of spin ordering in magnetic insulators with an applied electric field (also known as the magnetoelectric effect) can significantly reduce the power consumption of memory devices, but with no mobile charges present, it would seem to be an impossible task. Encouragingly, it was recently discovered that some magnetic orders induce an electric polarization, which couples spins to electric field. So far, the electrical polarization in such magnetic ferroelectrics (also called multiferroics) tends to be small and the Néel magnetic transition temperature is usually well below liquid nitrogen temperature. Now, however, Roger Johnson and co-workers at the University of Oxford, UK, with collaborators in France, report in Physical Review Letters on achieving giant polarization in CaMn7O12. The measured polarization is the highest measured magnetically induced polarization, persisting up to a Néel temperature of 90 K. Remarkably, this polarization appears to be induced by a long-period helicoidal (or proper-screw) spin spiral [see Fig. 1(a)], in which spins rotate around the spiral wave vector [1]. This discovery represents an important development for the field of magnetic ferroelectrics, as large polarization is crucial for electric manipulation of spins. It confirms earlier estimates of polarization from studies of polycrystalline samples [2]. The most ubiquitous spin ordering that gives rise to ferroelectricity is the cycloidal spiral, in which spins rotate around an axis normal to the spiral wave vector. A cycloid—a curve traced by a point on the rim of a wheel rolling over a flat surface—is asymmetric along the direction normal to both the direction of motion and the wheel axis, and this is also the direction of the electric polarization induced by a spin cycloid [see Fig. 1(b)]. By contrast, helicoidal ferroelectrics are rare, and all materials studied so far are only weakly ferroelectric [3, 4]. The clockwise or counterclockwise direction of spin rotation in the helicoidal spiral is described by a quantity called helicity. This quantity changes sign under inverFIG. 1: (a) Helicoidal spin spiral in which the spin rotation axis and the induced polarization are parallel to the spiral wave vector. (b) Cycloidal spin spiral in which spins (red arrows) rotate around an axis normal to the spiral wave vector Q. The induced electric polarization P is normal to both the wave vector and the spin rotation axis. Green curve is the cyloid. (c) Propellerlike structure of Mn-O octahedra in CaMn7O12, which changes the rotation direction when the sample is turned around. (APS/Carin Cain)