Using the nanoscale to have large-scale impact.

designed to be altered.

The objective of our research is to design and manipulate magnetic, electronic, and optical properties of thin films on demand. My comprehensive research approach combines epitaxial, atomic-scale thin film synthesis and state-of-the-art in-situ materials characterization, where I am fascinated by solving fundamental nanoscale problems to tackle technological large-scale challenges. Our medium to study these scientific and applied questions is complex oxide materials. Oxides are as rich in their physical phenomena and potential functionalities as they are ubiquitous in nature; however, designing, synthesizing, and exploiting correlated spin, charge, and lattice behaviors in these systems remains nontrivial. Specifically, my interests focus on the intersection of the fundamental understanding of oxide materials physics and novel device applications. The quantum materials we design, the materials physics we discover, and the devices we develop have broad impact in energy-efficient and high-performance memory and logic, quantum information science, advanced sensing, and energy conversion.

Currently, our efforts are broadly focused in three areas:

1. A materials science approach to spintronics. Using complex oxides (and their novel functionalities) as a platform to uncover new materials physics and enhance spintronic device performance.

2. Emergent phenomena in epitaxial ferroelectric and magnetic heterostructures and superlattices.

3. The development of new thin film synthesis and in-situ characterization tools that allow us to manipulate correlated materials and their properties with external stimuli, such as electric fields, magnetic fields, light, and chemical potentials, and even mechanical strain.

Bonus – We are always interested in exploring new applications of our materials, devices, and their functionalities in creative and imaginative ways, including potential application is biomedical engineering, batteries, mechanics, and more.

Key words: quantum materials, spintronics and magnetism, ferroelectricity, multiferroics, spin-orbit coupling, interface engineering, oxide thin films, epitaxial thin films, emergent phenomena, atomically-precise growth, device design and lithography, magneto-optics, magnetotransport

Latest Discoveries and Innovations

Fast meets small with multisublattice magnetic materials

Despite more than a decade of research, current-driven spin textures, such as domain walls and skyrmions, continue to face a speed limit of a few hundred m/s, and room-temperature-stable skyrmions are an order of magnitude too large to be useful in competitive technologies (100’s of nanometers).

These problems are rooted in two fundamental characteristics of ferromagnets: stray fields, which limit spin texture size and packing density, and precessional dynamics, which limits spin texture speed. These limits explain why racetrack-based spintronic devices have, so far, failed to live up to their promise. In this work, we demonstrate that the key metrics for a competitive magnetic solid state drive – 10 nm bit size and 1 km/s data access speed – can be readily met at room temperature in a novel class of materials: compensated ferrimagnetic thin films with broken inversion symmetry.

L. Caretta, M. Mann, F. Büttner, et al. “Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet,” Nat. Nanotechnol. 3 (2018). Cover Issue

L. Caretta, E. Rosenberg, F. Büttner, et al. "Interfacial Dzyaloshinskii-Moriya interaction arising from rare-earth orbital magnetism in insulating magnetic oxides," Nature Commun., vol. 11, p. 1090 (2020).

C. O. Avci, E. Rosenberg, L. Caretta, et al. “Interface-driven chiral magnetism and current-driven domain walls in insulating magnetic garnets,” Nat. Nanotechnol., 2019

Materials with Chameleon-like properties

Crystal symmetry in condensed-matter materials largely dictates their micro- and macroscopic properties, and new material functionalities can arise when inversion symmetry is broken and when symmetry changes. Thus, the ability to control crystal inversion symmetry on demand is of both fundamental and technological importance. Perhaps the most pervasive example of the manifestation of broken inversion symmetry is in ferroelectrics, where crystal structures that inherently break inversion symmetry directly lead to a switchable spontaneous electrical polarization. Recently, the ability to synthesize heteroepitaxial ferroelectric superlattice systems has enabled new methods to control ferroic order and even crystal symmetry. Here, by using BiFeO3/TbScO3 superlattices as our model system, we take advantage of (i) the discontinuity of the spontaneous polarization; (ii) the lattice mismatch; and (iii) the octahedral tilt frustration between the two layers to engineer crystal inversion symmetry. We stabilize both a non-centrosymmetric polar and a centrosymmetric antipolar phase mediated by a first order phase transition at room temperature. Both phases are identified and characterized using a combination of high-resolution and four-dimensional (4D) scanning transmission electron microscopy (STEM), piezoforce microscopy (PFM), and confocal second harmonic generation (SHG) over atomic and mesoscopic length scales. Moreover, applying orthogonal in-plane electric fields to this mixed-phase system results in the deterministic, nonvolatile interconversion of the centrosymmetric and non-centrosymmetric phases, resulting in a three order of magnitude change in the non-linear optical (SHG) response of the system. Demonstration of such an optically addressable, multi-state memory device presents new avenues for cross-functional devices which take advantage of the interconversion between ferro- and anti-ferroelectric states.

L. Caretta, Y.T. Shao, et al. “Electric field control of Inversion Symmetry,” Nature Materials (2022)

Reaching the relativistic magnonic speed limit

We report the first experimental demonstration of relativistic motion of a magnetic soliton, made possible by harnessing new spin physics and materials that allow for ultralow dissipation dynamics resulting in the fastest current-driven domain walls ever reported. The entire framework of magnetic domain wall dynamics is premised on the idea that the “stiffer” a domain wall is, the faster it can move. Here, we show that this behavior finally breaks down in a most remarkable way at high speeds, for exactly the same reason, mathematically, that no particle can exceed the speed of light.

L. Caretta, S.-H. OH, T. Fakhrul, et al. “Relativistic kinematics of a magnetic soliton,” Science, vol. 18, p. 1438-1442 (2020)