  A rich variety of physical effects in spin dynamics arises at the interface
between different magnetic materials. Engineered systems with interlaced
magnetic structures have been used to implement spin transistors, memories and
other spintronic devices. 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 behavior from regions with oscillating magnetization -- resembling a
magnetic material in the presence of an external transverse field -- to regions
with a defined magnetization, as in magnetic materials with a 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.
