MTL Annual Research Report 2011

We maintain a thin-film laboratory that includes a pulsed-laser deposition (PLD) system and an ultra-high vacuum sputter system. The PLD is particularly useful for making complex materials such as oxides because it can preserve the stoichiometry of the target material.

We have been using PLD to deposit a variety of oxide films for magneto-optical devices such as isolators. The ideal material for an isolator combines high Faraday rotation with high optical transparency. Garnets, such as bismuth iron garnet (BIG, Bi₃Fe₅O₁₂), have excellent properties but do not grow well on silicon substrates, making it difficult to integrate these materials. One way to solve this problem is to develop new magneto-optical active materials, which can grow epitaxially on Si by using buffer layers. Through being doped with transitional metal ions, these materials can exhibit strong Faraday rotation as well as low optical loss. We have examined Fe and Co-doped SrTiO₃ thin films (Figure 1) ^{[1] [2]} , which show high magneto-elastic anisotropy ^[3] , strong magneto-optical properties, and lower optical absorption compared with iron oxide. We also demonstrated epitaxial integration of these films on silicon using a CeO₂/YSZ buffer layer. When Sr(Ti_0.6Fe_0.4)O₃ films are doped with Ga ions on the Ti site up to 50 at.%, the optical absorption of the material decreases by more than one order of magnitude at 1550 nm (Figure 2), while it still shows a Faraday rotation of ~400 deg/cm ^[4] . A high material figure of merit of 3~4 dB/cm was achieved in Sr(Ti_0.6Ga_0.4Fe_0.2)O₃. These films could be useful for waveguide isolators and other magnetoelectronic devices in which optical absorption losses are critical.