Proximity-Induced Magnetization in SrIrO₃ Thin Films

To reduce the increasing power consumption in information technology, energy-efficient devices are the need of the hour. In contrast to conventional electronic devices, which rely on charge transport, spintronic devices offer energy-saving alternatives and could pave the way for next-generation information processing. The magneto-electric coupling, essential for these devices, is usually provided by spin-orbit coupling (SOC). In 3d-transition metal oxides (TMOs), electron-electron correlation is strong; however, SOC, which evolves with increasing atomic numbers, is only small. On the other hand, in 5d-TMOs, which display strong SOC, the electron correlation is rather weak. Therefore, combining both 3d and 5d- TMOs in artificially grown heterostructures (HSs) provides a promising platform to search for new quantum materials showing exotic effects and potential for next-generation spintronic devices.
In this thesis, perovskite SrIrO3 (SIO) HSs, in which 5d-TMO SIO is combined with magnetic 3d-TMO perovskite, are analyzed in detail with respect to proximity induced changes of the magnetization and electronic transport in the presence of a neighboured magnetic layer, e.g., LaXO3 or NdNiO3 (LXO, NNO) with X = Mn, Fe and Co. ... mehrTo this purpose, high-quality LXO(NNO)/SIO HSs were produced by pulsed laser deposition (PLD). PLD is a commonly used thin film deposition technique to produce epitaxial TMO thin films and HSs with high epitaxial quality and thickness resolution of one unit cell.
In contrast to the SIO single layer, the LCO/SIO HSs, where LCO is a ferromagnetic (FM) insulator, display a strong intrinsic anomalous Hall effect (AHE) and anisotropic magnetoresistance indicating in-plane antiferromagnetic (AFM) ordering similar to that of layered Sr2IrO4 with effective moment (meff) along the pseudo-cubic (110) direction. Separating LCO and SIO by a 4 monolayer thick, insulating SrTiO3 layer and increasing the SIO thickness in LCO/SIO HSs suppress the effects strongly and document proximity-induced AHE, magnetic moment and magnetocrystalline anisotropy in SIO.
Electrostatic doping by electric field effect in back gate geometry enhances AHE by a factor of 7 but leaves the magnetic moment of meff ≈0.02 μB/Ir nearly unaffected. Therefore, the origin of the strong AHE is very likely caused by enhanced Berry curvature (BC) due to the topological band structure of SIO (also supported by recently published DFT results).
Substitution of the LCO layer by other magnetic 3d-TMO layers allows for more detailed analysis of the proximity effect, magnetic exchange, and coupling between SIO and the 3d magnetic layer. With increasing 3d-electron number, interfacial charge transfer is observed from 5d-SIO to 3d-TMO which increases systematically from Mn to Co (Ni). However, the induced spin and orbital magnetic moment in SIO is very similar in all HSs. In contrast, the AHE shows significant enhancement when a FM layer (LCO, LMO) is interfaced with SIO, in comparison to SIO/AFM HSs; where the AHE is one order of magnitude smaller, indicating the strong influence of the effective magnetic moment and effective local field associated with the magnetic layer at the interface on the BC and hence AHE.
The obtained results provide important information on the interfacial properties of 3d/5d HSs and demonstrate effective coupling between perovskite-related 3d-TMO and SIO. On the other side, proximity-induced SOC in the 3d-TMO layer is also very likely and could be useful for magnetization manipulation in 3d-TMOs by electric field via the Rashba effect. This issue will be addressed in future works.