A conductive epitaxial underlayer for voltage control in magnetic garnet heterostructures

Magnetic iron garnets hold great interest for magnonic, spintronic, and magnetooptical devices, but utilizing voltage-driven phenomena in insulating garnets has been prevented by the lack of a conductive epitaxial garnet underlayer. Beyond providing sufficient electrical conduction, the underlayer should exhibit low saturation magnetization to minimize magnetic interaction with the magnetic layers of the heterostructure, while maintaining high crystalline quality to enable epitaxial growth. Here, defect engineering via site-selective doping is employed to enhance the electrical conductivity of Y₃Fe₅O₁₂ (YIG) while suppressing its magnetization. Epitaxial Fe-deficient Ca-doped YIG (Y_2.7Ca_0.3Fe_4.7O_12-δ, CaYIG) films, where Ca²⁺ substitutes Y³⁺ in the dodecahedral sites, exhibit electrical resistivities of 5.8 × 10³ Ω cm and above and show saturation magnetization of 145 kA/m comparable to that of YIG, with perpendicular magnetic anisotropy or an in-plane easy axis depending on the substrate. Substitution of Al³⁺ into tetrahedral Fe³⁺ sites in Ca_0.3Y_2.7Fe_3.5Al₁O_12-δ (CaAlYIG) reduces the saturation magnetization to below 25 kA/m and increases the resistivity to 1.65 × 10⁴ Ω cm or higher. The functionality of CaAlYIG was demonstrated in a heterostructure of Bi_1.2Y_1.8Fe₅O₁₂ (BiYIG)/CaAlYIG/Gd₃Sc₂Ga₃O₁₂, where the CaAlYIG layer enables a voltage to be applied through the thickness of the BiYIG layer.