Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu₂OSeO₃ by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state and should therefore be a universal property of skyrmion strings in chiral helimagnets.

磁性斯格明子是类似涡旋的自旋纹理，通常被视为二维物体。在它们的晶格状态下，它们形成有序的六边形结构，这些结构已经被详细研究过。为了获得三维 (3D) 斯格明子晶体，可以设想将这些平面堆叠起来，在三维空间中形成斯格明子串。在这里，我们报告了对 Cu ₂ OSeO _{3 中}3D 斯格明子相的观察通过在共振弹性 X 射线散射中进行倒易空间映射。我们在磁场冷却相图中观察到斯格明子相与锥形相明显共存的区域。然而，由于对称性论证，这种共存是被禁止的。相反，正如微磁模拟所证实的那样，斯格明子弦本身会沿着它们的轴周期性地调制。周期性调制实际上是斯格明子相脱离锥形状态的必然结果，因此应该是手性螺旋体中斯格明子弦的普遍特性。