In recent years, studies on cyclic molecular nanomagnets have captivated the attention of researchers. These magnets are finite in size and contain very large spins. They are interesting because they possess macroscopic quantum tunneling of Néel vectors. For antiferromagnetic molecular nanomagnets with finite number of even-numbered coupled spins, tunneling involves two classical localized Néel ground states separated by a magnetic energy barrier. The question is: can such phenomena be observed in nano magnets with odd number of magnetic ions? The answer is not directly obvious because cyclic chains with odd-numbered coupled spins are frustrated as one cannot obtain a perfect Néel order. These frustrated spins can indeed be observed experimentally, so they are of interest. In this Letter, we theoretically investigate macroscopic quantum tunneling in these odd spin systems with arbitrary spins $s$, in the presence of a magnetic field applied along the plane of the magnet. In contrast to systems with an even-numbered coupled spins, the ground state of the cyclic odd-spin system contains a topological soliton due to spin frustration. Thus the classical ground state is $2N$-fold degenerate as the soliton can be placed anywhere along the ring with total $S_z=\pm s$. Small quantum fluctuations delocalize the soliton with a formation of an energy band. We obtain this energy band using degenerate perturbation theory at order $2s$. We show that the soliton ground state is chiral for half-odd integer spins and non-chiral for integer spins. From the structure of the energy band we infer that as the value of the spin increases the inelastic polarized neutron-scattering intensity may increase or decrease depending on the strengths of the parameters of the Hamiltonian.
Nanomagnetism of magnetoelectric granular thin-film antiferromagnets
反铁磁体最近成为自旋电子学应用的有吸引力的平台，与铁磁体相比，提供了全新的功能。虽然纳米薄膜材料是未来反铁磁自旋电子学技术发展的关键，但在纳米尺度上探索这种薄膜的实验工具仍然很少。在这里，我们提供了一个解决这个技术瓶颈，通过解决无处不在的表面磁化的磁电反铁磁体的颗粒状薄膜样品在纳米尺度上使用单自旋磁强计结合自旋敏感的运输实验。具体来说，我们定量成像了200 nm Cr $_2$ O $_3$薄膜中单个纳米级反铁磁畴在真实空间中以及顺磁到反铁磁相变中的演变。这些实验使我们能够辨别的Cr $_2$ O $_3$薄膜的关键性能，包括域形成的机制和组成膜的各个晶粒之间的交换耦合的强度。我们的工作为Cr $_2$ O $_3$的磁有序机制提供了新的见解，并建立了单自旋磁测量作为一种新的，广泛适用的反铁磁薄膜纳米级寻址的工具。
Multi-rotational switching in a noncollinear antiferromagnet by spin-orbit torque
Tohoku University（东北大学）；National Institute for Materials Science（物质材料研究机构）；National Institutes for Quantum Science and Technology（量子科学技术研究开发机构）；Inamori Research Institute for Science（稻盛科学研究所）
