Resolving the atomic and electronic structures of bismuth iron garnet thin films
Authors: Bocher Laura , Teurtrie Adrien , Popova Elena , Warot‐Fonrose Bénédicte , Keller Niels , Stéphan Odile , Gloter Alexandre 
Published: December 2016

Bismuth iron garnet (Bi
3
Fe
5
O
12
‐ BIG) presents large interests in both fundamental studies and applications thanks to a plethora of fascintating physical properties. Among them, BIG is ferrimagnetic with a relatively high magnetization of 1600 G at 300 K and presents magnetic ordering temperature from 650 K to 700 K [1]. One of its most useful properties is the giant Faraday rotation (~105 deg/cm at 550 nm) making it potential for magneto‐optical recording and the fabrication of non‐reciprocal magneto‐optical devices based on magnetophotonic crystals [2]. More recently, a strong magneto‐electric coupling at room temperature has been reported in BIG thin films opening new perspectives for an electric control of the magnetization [3].

Contrary to its parent structures, e.g. the well‐known yttrium iron garnet (YIG), this material can only be elaborated in thin film form using non‐equilibrium growth techniques. BIG films were grown onto distinct isostructural Y
3
Al
5
O
12
(YAG) and substituted‐Gd
3
Ga
5
O
12
(SGGG) substrates; each is supposed to induce a compressive epitaxial strain depending on the lattice mismatch, i.e. approx. of 5% and 1%, respectively. The interplay between strain engineering and control of its promising magnetic and transport properties is of main interest in these systems, however no bulk reference is available yet for detailed structural and magnetic studies. Hence resolving the atomic and electronic structures of BIG thin films remains a key challenge to understand better their structure‐property relationship. Here we use advanced spectromicroscopy studies combining a Cs‐corrected scanning transmission electron microscope (STEM), the NION UltraSTEM200, with high‐energy resolved electron energy‐loss spectroscopy (EELS) experiments to characterize these BIG thin films, from their structural aspect to their elemental and spectroscopic properties.

Peculiar relaxation mechanisms are highlighted in both BIG/substrate systems by Geometric Phase Analysis (GPA) based on low‐magnified high‐resolution HAADF‐STEM images. All the films are epitaxially grown and relaxed from thicknesses above a few tens of nanometers. Combined GPA studies and atomically‐resolved HAADF‐STEM images at the interfaces reveal how BIG can accommodate different lattice mismatches through a variety of growth mechanisms. Fig.1 gathers the dominant scenarii observed in BIG thin films with the typical “cube‐on‐cube” growth at the BIG/SGGG interface or the more complexed Vernier misfit alternating with tilted grains presenting a network of dislocations in the BIG/YAG system. The evolution of the local in‐plane and out‐of‐plane lattice parameters confirms a rather strong compressive strain of the BIG film grown onto SSGG occurring at the first few interfacial unit cells. Whereas an immediate relaxation is observed at the BIG/YAG interface with a relatively uniform evolution of both local in‐plane and out‐of‐plane parameters revealing a quasi‐absence of strain thanks to the particular coexistence of tilted grains and well‐relaxed Vernier misfit.

Finally, all BIG films seem to preserve the garnet cubic structure (Fig. 2 a). From a spectroscopic study, no cation interdiffusion is evidenced when probing the film/substrate interface down to the scale of the atomic columns, and only the signature of Fe
3+
valence state is detected confirming the absence of oxygen deficiency. Besides, the evolution of the O‐K near‐edge fine‐structures through the BIG/YAG interface enables us to figure out directly possible electronic reconstruction at the interface by probing the Fe 3d ‐ O 2p hybridization (Fig. 2b).