Zhang Qihan, Liang Jinghua, Bi Kaiqi, Zhao Le, Bai He, Cui Qirui, Zhou Heng-An, Bai Hao, Feng Hongmei, Song Wenjie, Chai Guozhi, Gladii O, Schultheiss H, Zhu Tao, Zhang Junwei, Peng Yong, Yang Hongxin, Jiang Wanjun
State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China.
Frontier Science Center for Quantum Information, Tsinghua University, Beijing 100084, China.
Phys Rev Lett. 2022 Apr 22;128(16):167202. doi: 10.1103/PhysRevLett.128.167202.
A broken interfacial inversion symmetry in ultrathin ferromagnet/heavy metal (FM/HM) bilayers is generally believed to be a prerequisite for accommodating the Dzyaloshinskii-Moriya interaction (DMI) and for stabilizing chiral spin textures. In these bilayers, the strength of the DMI decays as the thickness of the FM layer increases and vanishes around a few nanometers. In the present study, through synthesizing relatively thick films of compositions CoPt or FePt, CoCu or FeCu, FeGd and FeNi, contributions to DMI from the composition gradient-induced bulk magnetic asymmetry (BMA) and spin-orbit coupling (SOC) are systematically examined. Using Brillouin light scattering spectroscopy, both the sign and amplitude of DMI in films with controllable direction and strength of BMA, in the presence and absence of SOC, are experimentally studied. In particular, we show that a sizable amplitude of DMI (±0.15 mJ/m^{2}) can be realized in CoPt or FePt films with BMA and strong SOC, whereas negligible DMI strengths are observed in other thick films with BMA but without significant SOC. The pivotal roles of BMA and SOC are further examined based on the three-site Fert-Lévy model and first-principles calculations. It is expected that our findings may help to further understand the origin of chiral magnetism and to design novel noncollinear spin textures.
超薄铁磁体/重金属（FM/HM）双层膜中界面反转对称性的破坏通常被认为是容纳Dzyaloshinskii-Moriya相互作用（DMI）和稳定手性自旋纹理的先决条件。在这些双层膜中，DMI的强度随着FM层厚度的增加而衰减，并在几纳米左右消失。在本研究中，通过合成CoPt或FePt、CoCu或FeCu、FeGd和FeNi成分的相对较厚的薄膜，系统地研究了成分梯度诱导的体磁不对称（BMA）和自旋轨道耦合（SOC）对DMI的贡献。使用布里渊光散射光谱，实验研究了在存在和不存在SOC的情况下，具有可控BMA方向和强度的薄膜中DMI的符号和幅度。特别是，我们表明，在具有BMA和强SOC的CoPt或FePt薄膜中可以实现相当大的DMI幅度（±0.15 mJ/m²），而在其他具有BMA但没有显著SOC的厚薄膜中观察到可忽略不计的DMI强度。基于三格点Fert-Lévy模型和第一性原理计算，进一步研究了BMA和SOC的关键作用。预计我们的发现可能有助于进一步理解手性磁性的起源，并设计新型非共线自旋纹理。
