A rich variety of physical effects in spin dynamics arise at the interface between different magnetic materials¹. Engineered systems with interlaced magnetic structures have been used to implement spin transistors, memories and other spintronic devices^2,3. However, experiments in solid-state systems can be difficult to interpret because of disorder and losses. Here we realize analogues of magnetic junctions using a coherently coupled mixture of ultracold bosonic gases. The spatial inhomogeneity of the atomic gas makes the system change its behaviour from regions with oscillating magnetization—resembling a magnetic material in the presence of an external transverse field—to regions with a defined magnetization, similar to magnetic materials with ferromagnetic anisotropy stronger than external fields. Starting from a far-from-equilibrium fully polarized state, magnetic interfaces rapidly form. At the interfaces, we observe the formation of short-wavelength magnetic waves. They are generated by a quantum torque contribution to the spin current and produce strong spatial anticorrelations in the magnetization. Our results establish ultracold gases as a platform for the study of far-from-equilibrium spin dynamics in regimes that are not easily accessible in solid-state systems.

不同磁性材料¹之间的界面处会出现自旋动力学中丰富多样的物理效应。具有交错磁结构的工程系统已被用于实现自旋晶体管、存储器和其他自旋电子器件^2,3. 然而，由于无序和损失，固态系统中的实验可能难以解释。在这里，我们使用超冷玻色子气体的相干耦合混合物实现了磁结的类似物。原子气体的空间不均匀性使得系统的行为从具有振荡磁化的区域（类似于存在外部横向场的磁性材料）变为具有确定磁化的区域，类似于具有强于外场的铁磁各向异性的磁性材料. 从远离平衡的完全极化状态开始，磁性界面迅速形成。在界面处，我们观察到短波长磁波的形成。它们是由对自旋电流的量子扭矩贡献产生的，并在磁化中产生强烈的空间反相关。我们的结果建立了超冷气体作为研究在固态系统中不易获得的状态下的远离平衡自旋动力学的平台。