Bi-substituted iron garnet films are extensively used in the fabrication of nonreciprocal devices in optical telecommunications. The miniaturization of these devices for on-chip integration requires the development of more efficient magneto-optic materials than presently available. Recent evidence has emerged of large near-surface enhancements in the magneto-optic response in these materials. However, their operative mechanisms at the atomic and electronic levels are not as yet understood. We report significant differences in the ionic structure between surface and bulk in bismuth-substituted iron garnet materials. It is found that the unit cell is elongated normal to the surface, thus enlarging the separation between ${{\rm Fe}^{3 +}}$ ions. These ions play a central role in the magneto-optic response of this material. A marked displacement of Fe ions creates gaps at the surface that are populated in the bulk. Concomitantly, surface- and bulk-sensitive measurements of spin-polarized 3d ${{\rm Fe}^{3 +}}$ states show significant differences in the magnitude of ${{\rm L}_2}$ edge x-ray magnetic circular dichroism, as well as differences in ${{\rm L}_3}$ edge dichroism, which, in the presence of spin-orbit coupling in 3d states, can be assigned to high-energy states. An increase in magnetic circular dichroism correlates with larger Faraday rotation. These findings provide a deeper understanding of the role of the surface in the electronic transitions to excited ${{\rm Fe}^{3 +}}$ 3d states, responsible for these nonreciprocal phenomena. Together with the surface reconstruction underlying these effects reported here, they provide a useful tool for the further development of improved materials technologies to advance the integration of nonreciprocal devices in optical circuits.

中文翻译：

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增强光学不可逆性：铁石榴石的表面重建

双取代铁石榴石薄膜被广泛用于光通信中不可逆器件的制造中。这些器件用于芯片上集成的小型化要求开发比目前可用的更有效的磁光材料。最近的证据表明，这些材料在磁光响应方面有很大的近表面增强作用。但是，它们在原子和电子水平上的作用机理尚不清楚。我们报道了铋取代铁石榴石材料的表面和体积之间的离子结构上的显着差异。发现单位晶胞垂直于表面拉长，从而扩大了$ {{\ rm Fe} ^ {3 +}} $之间的间隔离子。这些离子在这种材料的磁光响应中起着核心作用。Fe离子的显着位移会在堆积在主体中的表面上产生间隙。同时，对自旋极化的3d $ {{\ rm Fe} ^ {3 +}} $状态的表面和体积敏感的测量结果显示$ {{\ rm L} _2} $边缘x射线的大小存在显着差异磁性圆二色性以及$ {{\ rm L} _3} $边缘二色性的差异，在存在3d态自旋轨道耦合的情况下，可以将其分配给高能态。磁性圆二色性的增加与法拉第旋转更大有关。这些发现提供了对表面在电子跃迁到激发$ {{\ rm Fe} ^ {3 +}} $中的作用的更深刻的理解。3d状态，对这些不可逆的现象负责。结合此处报道的这些效应所依据的表面重建，它们为进一步开发改良的材料技术提供了有用的工具，以推进光学电路中不可逆器件的集成。