Composites of magnetically hard and soft phases are present in multiple and diverse applications, ranging from bulk permanent magnets in motors and generators to state-of-the-art recording media devices. The nature of the magnetic coupling between the hard and soft phases is of great technological relevance, as the macroscopic properties of the functional composite material ultimately depend on the atomic-scale interactions between phases. In this work, the hard/soft bilayer system SrFe₁₂O₁₉/Co has been studied based on photoemission electron microscopy combined with x-ray absorption and magnetic circular dichroism. Our experiments show that the magnetization of the hard magnetic oxide has a direction perpendicular to the layer plane, whereas the magnetization of the soft metallic overlayer remains in-plane. As a consequence, the magnetic domain patterns observed for the hard and soft phases are very different and completely uncorrelated to one another, indicating that no soft spins align with the hard phase by pure magnetodipolar arguments. The results are understood as the consequence of an absence of exchange-coupling between phases, in a scenario in which the shape anisotropy of the soft layer overcomes the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite. Micromagnetic simulations of our system predict that low degrees of exchange-coupling effectively prevent substantial softening of the composite and lead to the alignment of soft and hard magnetic moments. A strategy thus emerges for the development of future hard-soft magnets, based on minimizing the degree of exchange-coupling while avoiding complete uncoupling.

硬磁性和软磁性相的复合材料存在于多种应用中，从电动机和发电机中的大块永磁体到最新的记录介质设备。硬相和软相之间的磁耦合性质在技术上具有重大意义，因为功能复合材料的宏观性能最终取决于相之间的原子尺度相互作用。在这项工作中，硬/软双层系统SrFe ₁₂ O ₁₉/ Co的研究基于光电子显微镜结合X射线吸收和磁性圆二色性。我们的实验表明，硬磁性氧化物的磁化方向垂直于层平面，而软金属覆盖层的磁化保持面内。结果，在硬相和软相中观察到的磁畴模式非常不同并且彼此完全不相关，这表明通过纯磁双极论证，没有任何软自旋与硬相对齐。在软层的形状各向异性克服了硬铁氧体产生的垂直磁场的塞曼能的情况下，将结果理解为相之间不存在交换耦合的结果。我们系统的微磁模拟预测，低程度的交换耦合有效地防止了复合材料的充分软化，并导致软磁矩和硬磁矩对齐。因此在最小化交换耦合程度的同时避免完全解耦的基础上，出现了一种开发未来软硬磁铁的策略。