Krycka Kathryn L, Rhyne James J, Oberdick Samuel D, Abdelgawad Ahmed M, Borchers Julie A, Ijiri Yumi, Majetich Sara A, Lynn Jeffrey W
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, United States of America.
Applied Physics, National Institute of Standards and Technology, Boulder, CO 80305, United States of America.
New J Phys. 2018;20(12). doi: 10.1088/1367-2630/aaef17.
Inelastic neutron scattering is utilized to directly measure inter-nanoparticle spin waves, or magnons, which arise from the magnetic coupling between 8.4 nm ferrite nanoparticles that are self-assembled into a close-packed lattice, yet are physically separated by oleic acid surfactant. The resulting dispersion curve yields a physically-reasonable, non-negative energy gap only when the effective is reduced by the inter-particle spacing. This renormalization strongly indicates that the dispersion is a collective excitation between the nanoparticles, rather than originating from within individual nanoparticles. Additionally, the observed magnons are dispersive, respond to an applied magnetic field, and display the expected temperature-dependent Bose population factor. The experimental results are well explained by a limited parameter model which treats the three-dimensional ordered, magnetic nanoparticles as dipolar-coupled superspins.
非弹性中子散射被用于直接测量纳米粒子间的自旋波，即磁振子，这些磁振子源于自组装成密排晶格的8.4纳米铁氧体纳米粒子之间的磁耦合，然而它们被油酸表面活性剂物理隔开。只有当有效长度因粒子间距而减小时，所得的色散曲线才会产生物理上合理的非负能隙。这种长度重整化强烈表明，色散是纳米粒子之间的集体激发，而不是源于单个纳米粒子内部。此外，观察到的磁振子是色散的，对施加的磁场有响应，并显示出预期的温度依赖玻色子布居因子。通过一个有限参数模型可以很好地解释实验结果，该模型将三维有序的磁性纳米粒子视为偶极耦合的超自旋。
