Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA.
Nature. 2023 Aug;620(7976):988-993. doi: 10.1038/s41586-023-06279-y. Epub 2023 Aug 2.
Understanding how microscopic spin configuration gives rise to exotic properties at the macroscopic length scale has long been pursued in magnetic materials. One seminal example is the Einstein-de Haas effect in ferromagnets, in which angular momentum of spins can be converted into mechanical rotation of an entire object. However, for antiferromagnets without net magnetic moment, how spin ordering couples to macroscopic movement remains elusive. Here we observed a seesaw-like rotation of reciprocal lattice peaks of an antiferromagnetic nanolayer film, whose gigahertz structural resonance exhibits more than an order-of-magnitude amplification after cooling below the Néel temperature. Using a suite of ultrafast diffraction and microscopy techniques, we directly visualize this spin-driven rotation in reciprocal space at the nanoscale. This motion corresponds to interlayer shear in real space, in which individual micro-patches of the film behave as coherent oscillators that are phase-locked and shear along the same in-plane axis. Using time-resolved optical polarimetry, we further show that the enhanced mechanical response strongly correlates with ultrafast demagnetization, which releases elastic energy stored in local strain gradients to drive the oscillators. Our work not only offers the first microscopic view of spin-mediated mechanical motion of an antiferromagnet but it also identifies a new route towards realizing high-frequency resonators up to the millimetre band, so the capability of controlling magnetic states on the ultrafast timescale can be readily transferred to engineering the mechanical properties of nanodevices.
长期以来，人们一直致力于研究微观自旋结构如何在宏观尺度上产生奇异性质，这在磁性材料中表现得尤为明显。一个典型的例子是铁磁体中的爱因斯坦-德哈斯效应，其中自旋的角动量可以转化为整个物体的机械旋转。然而，对于没有净磁矩的反铁磁体，自旋有序如何与宏观运动耦合仍然难以捉摸。在这里，我们观察到反铁磁纳米层膜的倒易晶格峰的跷跷板式旋转，其千兆赫结构共振在冷却到奈尔温度以下后表现出超过一个数量级的放大。使用一系列超快衍射和显微镜技术，我们直接在纳米尺度的倒易空间中可视化这种自旋驱动的旋转。这种运动对应于实空间中的层间剪切，其中膜的单个微区作为相干振荡器行为，它们相位锁定并沿相同的面内轴剪切。使用时间分辨光学极化测量，我们进一步表明，增强的机械响应与超快退磁强烈相关，后者释放存储在局部应变梯度中的弹性能量，以驱动振荡器。我们的工作不仅提供了反铁磁体中自旋介导的机械运动的第一个微观视角，而且还确定了实现高达毫米波段的高频谐振器的新途径，因此控制磁态的超快时间尺度的能力可以很容易地转移到纳米器件的机械性能工程中。
