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1. Magneto‐Optical Bi‐Substituted Yttrium and Terbium Iron Garnets for On‐Chip Crystallization via Microheaters

   https://doi.org/10.1002/adom.202400708  

   Hayashi, Kensuke; Dao, Khoi Phuong; Gross, Miela J; Ranno, Luigi; Sia, Jia_Xu Brian; Fakhrul, Takian; Du, Qingyang; Chatterjee, Nilanjan; Hu, Juejun; Ross, Caroline A (August 2024, Advanced Optical Materials)

   Abstract Ferrimagnetic iron garnets enable magnetic and magneto‐optical functionality in silicon photonics and electronics. However, garnets require high‐temperature processing for crystallization which can degrade other devices on the wafer. Here bismuth‐substituted yttrium and terbium iron garnet (Bi‐YIG and Bi‐TbIG) films are demonstrated with good magneto‐optical performance and perpendicular magnetic anisotropy (PMA) crystallized by a microheater built on a Si chip or by rapid thermal annealing. The Bi‐TbIG film crystallizes on Si at 873 K without a seed layer and exhibits good magneto‐optical properties with Faraday rotation (FR) of −1700 deg cm⁻¹. The Bi‐YIG film also crystallizes on Si and fused SiO₂at 873 K without a seed layer. Rapidly cooled films exhibit PMA due to the tensile stress caused by the thermal expansion mismatch with the substrates, increasing the magnetoelastic anisotropy by 4 kJ m⁻³versus slow‐cooled films. Annealing in the air for 15 s using the microheater yields fully crystallized Bi‐TbIG on the Si chip. 

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2. Crystallization of high gyrotropy garnets with decreasing thermal processing budgets as analyzed by electron backscatter diffraction

   https://doi.org/10.1364/OME.476482  

   Srinivasan, Karthik; Seaton, Nicholas C.; Peng, Ruoming; Li, Mo; Stadler, Bethanie J. (January 2023, Optical Materials Express)

   Rare-earth iron garnets with large magnetic gyrotropy, made with reduced thermal budgets, are ideal magneto-optical materials for integrated isolators. However, reduced thermal budgets impact Faraday rotation by limiting crystallization, and characterization of crystallinity is limited by resolution or scannable area. Here, electron backscatter diffraction (EBSD) is used to measure crystallinity in cerium substituted yttrium- and terbium-iron garnets (CeYIG and CeTbIG) grown on planar Si, crystallized using one-step rapid thermal processes, leading to large Faraday rotations > −3500 °/cm at 1550 nm. Varying degrees of crystallinity are observed in planar Si and patterned Si waveguides, and specific dependences of crystallite size are attributed to the nucleation/growth processes of the garnets and the lateral dimensions of the waveguide. On the other hand, a low thermal budget alternative–exfoliated CeTbIG nanosheets–are fully crystalline and maintain high Faraday rotations of −3200 °/cm on par with monolithically integrated thin film garnets. 

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3. Growth of magnetooptical cerium-substituted yttrium iron garnet on yttrium aluminum garnet using ion beam sputtering

   https://doi.org/10.1063/5.0161296  

   Yoshihara, Yuki; Ishiyama, Kazushi; Watanabe, Toshiaki; Lim, Pang Boey; Inoue, Mitsuteru; Ross, Caroline A.; Goto, Taichi (September 2023, Applied Physics Letters)

   Cerium-substituted yttrium iron garnet (Ce:YIG, Ce0.9Y2.1Fe5O12) was epitaxially grown on a (111)-oriented yttrium aluminum garnet (YAG) substrate using radio frequency ion beam sputtering. Magnetic hysteresis loops, transmissivity spectra, and magnetooptical (MO) responses, including Faraday rotation and Faraday ellipticity, were measured. The structural properties of the grown Ce:YIG were characterized using the x-ray rocking curve, reciprocal space map, pole figure, and x-ray reflectivity. X-ray photoelectron spectrometry revealed a dominant Ce3+ state in the grown Ce:YIG, but the transmission electron microscopy images showed columnar growth of Ce:YIG. This study demonstrates integration of epitaxial Ce:YIG on YAG, marking a significant step toward the fusion of MO garnets and laser crystals. 

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4. Hydrogen Dissolution Mechanisms in Bridgmanite by First‐Principles Calculations and Infrared Spectroscopy

   https://doi.org/10.1029/2024JB030403  

   Zhang, Yanyao; Tsuchiya, Jun; Li, ChingChien; Ye, Zefang; Yan, Wei; Okuchi, Takuo; Karato, Shun‐ichiro; Kung, Jennifer; Lin, Jung‐Fu (January 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Understanding hydrogen dissolution mechanisms in bridgmanite (Bgm), the most abundant mineral in the lower mantle, is essential for understanding water storage and rheological and transport properties in the region. However, interpretations of O‐H bands in Fourier transform infrared spectroscopy (FTIR) spectra of Bgm crystals remain uncertain. We conducted density functional theory (DFT) calculations on vibrational characteristics of O‐H dipoles and performed polarized FTIR measurements to address this issue. DFT calculations for four substitution models—Mg vacancies, Si vacancies, Al³⁺ + H⁺substitution for Si⁴⁺, and Al substitution with Mg vacancies—reveal distinct O‐H bands with different polarizations. Deconvolution of polarized FTIR spectra on Mg_0.88Fe²⁺_0.035Fe³⁺_0.065Al_0.14Si_0.90O₃and Mg_0.95Fe²⁺_0.033Fe³⁺_0.027Al_0.04Si_0.96O₃crystals shows five major O‐H bands with distinct polarizations along principal crystallographic axes. These experimental and calculated results attribute O‐H bands centered at 3,463–3,480, 2,913–2,924, and 2,452–2,470 cm⁻¹to Mg vacancies, Si vacancies, and Al³⁺ + H⁺substitution for Si⁴⁺, respectively. The total absorbance coefficient of bridgmanite was calculated to be 82,702(6,217) L/mol/cm². Mg and Si vacancies account for 43%–74% of the total water content, making them dominant hydrogen dissolution mechanisms in Bgm. The band frequencies for the Mg and Si vacancies in Bgm are drastically different from those in olivine and ringwoodite, corresponding to the significant changes in O‐H bond strengths and in the Si and Mg coordination environments from upper‐mantle to lower‐mantle minerals. These results highlight the need to incorporate hydrogen dissolution mechanisms in Bgm for understanding electrical conductivity and rheology of the lower mantle. 

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5. Oxygen diffusion in garnet: Experimental calibration and implications for timescales of metamorphic processes and retention of primary O isotopic signatures

   https://doi.org/10.2138/am-2022-7970  

   Scicchitano, Maria Rosa; Jollands, Michael C.; Williams, Ian S.; Hermann, Jörg; Rubatto, Daniela; Kita, Noriko T.; Nachlas, William O.; Valley, John W.; Escrig, Stéphane; Meibom, Anders (July 2022, American Mineralogist)

   Abstract Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotope signatures in natural samples. A series of experiments was undertaken to determine the diffusivity of oxygen in garnet, which remains poorly constrained. The first suite included high-pressure (HP), nominally dry experiments performed in piston-cylinder apparatus at: (1) T = 1050–1600 °C and P = 1.5 GPa and (2) T = 1500 °C and P = 2.5 GPa using yttrium aluminum garnet (YAG; Y3Al5O12) cubes. Second, HP H2O-saturated experiments were conducted at T = 900 °C and P = 1.0–1.5 GPa, wherein YAG crystals were packed into a YAG + Corundum powder, along with 18O-enriched H2O. Third, 1 atm experiments with YAG cubes were performed in a gas-mixing furnace at T = 1500–1600 °C under Ar flux. Finally, an experiment at T = 900 °C and P = 1.0 GPa was done using a pyrope cube embedded into pyrope powder and 18O-enriched H2O. Experiments using grossular were not successful. Profiles of 18O/(18O+16O) in the experimental charges were analyzed with three different secondary ion mass spectrometers (SIMS): sensitive high-resolution ion microprobe (SHRIMP II and SI), CAMECA IMS-1280, and NanoSIMS. Considering only the measured length of 18O diffusion profiles, similar results were obtained for YAG and pyrope annealed at 900 °C, suggesting limited effects of chemical composition on oxygen diffusivity. However, in both garnet types, several profiles deviate from the error function geometry, suggesting that the behavior of O in garnet cannot be fully described as simple concentration-independent diffusion, certainly in YAG and likely in natural pyrope as well. The experimental results are better described by invoking O diffusion via two distinct pathways with an inter-site reaction allowing O to move between these pathways. Modeling this process yields two diffusion coefficients (D values) for O, one of which is approximately two orders of magnitude higher than the other. Taken together, Arrhenius relationships are:log⁡Dm2s-1=-7.2(±1.3)+(-321(±32)kJmol-12.303RT) for the slow pathway, andlog⁡Dm2s-1=-5.4(±0.7)+(-321(±20)kJmol-12.303RT) for the fast pathway. We interpret the two pathways as representing diffusion following vacancy and inter-stitial mechanisms, respectively. Regardless, our new data suggest that the slow mechanism is prevalent in garnet with natural compositions, and thus is likely to control the retentivity of oxygen isotopic signatures in natural samples. The diffusivity of oxygen is similar to Fe-Mn diffusivity in garnet at 1000–1100 °C and Ca diffusivity at 850 °C. However, the activation energy for O diffusion is larger, leading to lower diffusivities at P-T conditions characterizing crustal metamorphism. Therefore, original O isotopic signatures can be retained in garnets showing major element zoning partially re-equilibrated by diffusion, with the uncertainty caveat of extrapolating the experimental data to lower temperature conditions. 

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