The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.

中文翻译：

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沮丧的开罗五角反铁磁体Bi2Fe4O9中的二聚体物理

磁性挫折的研究领域主要是基于三角形的晶格，但在五边形网络中也可以观察到奇异现象。确实已知一种特殊的非共线磁阶在${双}_2{铁}_4{\mathrm{Ø}}_9$体现开罗五角形格子。我们介绍了通过非弹性中子散射获得的磁有序状态的自旋波激发。他们揭示了与自旋对的局部进动有关的非常规激发态。然后对磁激励进行建模以确定超交换相互作用，对于该超交换相互作用，挫折感实际上是在自旋布置的起点处。该分析揭示了相互作用中的层次结构，导致顺磁状态（接近Néel温度）由强耦合的二聚体构成，二聚体之间的相干关系少得多。如使用极化中子获得的磁化分布中所观察到的，这对与两个非等价的Fe位置相关的施加的磁场产生两种类型的响应。