Mohammad Saghayezhian

Interface-induced multiferroism by design in complex oxide superlattices

Significance Developments in synthesis and characterizing artificially structured materials have greatly advanced the possibility to explore new states of matter in material science. Recent discoveries show that new quantum states can be achieved at heterointerfaces with various electric and mechanical boundary conditions. It remains an open question of how to design ultrathin layers with properties inaccessible in bulk phases that are amenable to technological applications. In this work, we grow heterostructures with extremely high-quality interfaces shown by state-of-the-art atomically resolved electron microscopy and spectroscopy. This combination allows us to identify an interface-induced structure that stabilizes ferromagnetism. Coupled with theory, we provide a conceptually useful recipe to design low-dimensional materials with unique functionalities, in line with the loop “make, measure, model.” Interfaces between materials present unique opportunities for the discovery of intriguing quantum phenomena. Here, we explore the possibility that, in the case of superlattices, if one of the layers is made ultrathin, unexpected properties can be induced between the two bracketing interfaces. We pursue this objective by combining advanced growth and characterization techniques with theoretical calculations. Using prototype La2/3Sr1/3MnO3 (LSMO)/BaTiO3 (BTO) superlattices, we observe a structural evolution in the LSMO layers as a function of thickness. Atomic-resolution EM and spectroscopy reveal an unusual polar structure phase in ultrathin LSMO at a critical thickness caused by interfacing with the adjacent BTO layers, which is confirmed by first principles calculations. Most important is the fact that this polar phase is accompanied by reemergent ferromagnetism, making this system a potential candidate for ultrathin ferroelectrics with ferromagnetic ordering. Monte Carlo simulations illustrate the important role of spin–lattice coupling in LSMO. These results open up a conceptually intriguing recipe for developing functional ultrathin materials via interface-induced spin–lattice coupling.

Manipulating the polar mismatch at the LaNiO3/SrTiO3 (111) interface

Heteroepitaxial growth of transition-metal oxide films on the open (111) surface of SrTiO3 results in significant restructuring due to the polar mismatch. Monitoring the structural and composition on an atomic scale of LaNiO3/SrTiO3 (111) interface as a function of processing conditions has enabled the avoidance of the expected polar catastrophe. Using atomically resolved transmission electron microscopy and spectroscopy as well as Low energy electron diffraction, the structure of the thin film, from interface to the surface, has been studied. In this paper, we show that the proper processing can lead to a structure that is ordered, coherent with the substrate without intermediate structural phase. Angle-resolved X-ray photoemission spectroscopy shows that the oxygen content of thin films increases with the film thickness, indicating that the polar mismatch is avoided by the presence of oxygen vacancies.

Designing antiphase boundaries by atomic control of heterointerfaces

Significance The mechancial and transport properties of every material are intimately coupled to the generation and formation of defects. In many systems, the nucleation process of defects is highly unpredictable due to the multiple coexisting nucleation mechanisms. A strategy to design and manipulate defect nucleation and formation can improve our understanding and in principle lead to the ability to control performance. Here we present a simple approach of designing 2D defects—named antiphase boundaries—using an atomic-controlled high-quality interface. These defects have a well-defined origin, location, and nucleation mechanism. Via advanced synthesis technology, atomically resolved electron microscopy, and theory, we reveal that these defects display distinctive properties such as physical merging and antipolar structural phases to potentially achieve memory devices. Extended defects are known to have critical influences in achieving desired material performance. However, the nature of extended defect generation is highly elusive due to the presence of multiple nucleation mechanisms with close energetics. A strategy to design extended defects in a simple and clean way is thus highly desirable to advance the understanding of their role, improve material quality, and serve as a unique playground to discover new phenomena. In this work, we report an approach to create planar extended defects—antiphase boundaries (APB) —with well-defined origins via the combination of advanced growth, atomic-resolved electron microscopy, first-principals calculations, and defect theory. In La2/3Sr1/3MnO3 thin film grown on Sr2RuO4 substrate, APBs in the film naturally nucleate at the step on the substrate/film interface. For a single step, the generated APBs tend to be nearly perpendicular to the interface and propragate toward the film surface. Interestingly, when two steps are close to each other, two corresponding APBs communicate and merge together, forming a unique triangle-shaped defect domain boundary. Such behavior has been ascribed, in general, to the minimization of the surface energy of the APB. Atomic-resolved electron microscopy shows that these APBs have an intriguing antipolar structure phase, thus having the potential as a general recipe to achieve ferroelectric-like domain walls for high-density nonvolatile memory.

Polar compensation at the surface of SrTi O 3 (111)

We have systematically investigated the annealing effect on the structure and composition of the polar surface of SrTiO3 (111), starting with an ex-situ chemical etch. The relative surface concentration between Ti and Sr strongly depends on both the annealing temperature and the oxygen processing. There is a critical annealing temperature at which the maximum concentration ratio of Ti to Sr is achieved, while still maintaining a (1 x 1) surface structure. We demonstrate that with proper processing it is possible to avoid surface reconstruction over a broad temperature range. Our results provide an optimal temperature window for epitaxial film growth.

Observation of large exchange bias and topological Hall effect in manganese nitride films

Magnetic and magneto-transport properties of manganese nitride films grown by molecular beam epitaxy have been investigated. Due to the mixed ferrimagnetic (FI) phase (

Unusual Fe–H bonding associated with oxygen vacancies at the (001) surface of Fe 3 O 4

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Ultrafast Carrier Dynamics in Self-Assembled Lai- xSrxMnO3/SrTiÖ3 Heterostructures

-phase with TFI ~ 738 K) and the antiferromagnetic phase (

Unusual Thermal Properties in Magnetic MnFe 2 O 4 and FeMn 2 O 4 Single Crystals

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Role of dimensionality in SrIrO 3 /BaTiO 3 superlattices

-phase with TN ~ 273 K), we observe magnetization hysteresis loops with non-zero exchange bias below TN, reaching ~ 0.22 T at 5 K. This indicates that noncollinear spins exist at the interfaces between two phases, creating a competition between interfacial Dzyaloshinskii-Moriya (DM) and exchange interactions. Strikingly, in addition to the normal Hall effect by Lorentz force and anomalous Hall effect by magnetization, we observe new contribution namely topological Hall effect below 75 K. This verifies the existence of topological spin texture, which is the consequence of competing interactions controlled by both applied field and temperature. Our work demonstrates that spintronic devices may be fabricated exploiting rich magnetic properties of different phases.