Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba_1−xNa_xFe₂As₂ that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state.

自旋共振模式（SRM）被用作铁基超导体中磁驱动配对的证据，但对它们的特性仍知之甚少。SRM的广度，分裂和自旋空间各向异性与人们普遍接受的自旋激子的解释形成对比。我们研究了空穴掺杂的Ba _{1− x} Na _x Fe ₂ As ₂显示自旋重新定向过渡。这种重新定向对具有高能量各向同性和低能量各向异性模式的共振激励的整体外观影响很小。但是，各向异性低能模式的强度在最高掺杂时急剧达到峰值，但仍表现出磁有序性，导致迄今为止在任何铁基超导体中观察到最强的SRM。进入SC相时，这种非常强的SRM伴随着约一半的布拉格磁强度损失。各向异性SRM因此可以允许系统补偿由于SC状态内反铁磁相关性降低而引起的交换能量的损失。