Rare-Earth Iron Garnet Superlattices with Sub-unit Cell Composition Modulation

Oxide superlattices reveal a rich array of emergent properties derived from the composition modulation and the resulting lattice distortion, charge transfer, and symmetry reduction that occur at the interfaces between the layers. The great majority of studies have focused on perovskite oxide superlattices, revealing, for example, the appearance of an interfacial 2D electron gas, magnetic moment, or improper ferroelectric polarization that is not present in the parent phases. Garnets possess greater structural complexity than perovskites: the cubic garnet unit cell contains 160 atoms with the cations distributed between three different coordination sites, and garnets exhibit a wide range of useful properties, including ferrimagnetism and ion transport. However, there have been few reports of the synthesis or properties of garnet superlattices, with layer thicknesses approaching the unit cell dimension of 1.2 nm. Here, we describe superlattices made from Bi and rare earth (RE = Tm, Tb, Eu, Lu) iron garnets (IGs) grown by pulsed laser deposition. Atom probe tomography and transmission electron microscopy reveal the composition modulation without dislocations and layer thicknesses as low as 0.45 nm, less than half a unit cell. TmIG/TbIG superlattices exhibit perpendicular magnetic anisotropy that is qualitatively different from the in-plane anisotropy of the solid solution, and BiIG/LuIG superlattices exhibit ferromagnetic resonance linewidth characteristics of the end-members rather than the solid solution. Garnet superlattices provide a playground for exploring interface physics within the vast parameter space of cation coordination and substitution.