UC Berkeley

The wider application of spintronic devices requires the development of several new material platforms that can address the following challenges: 1) efficient conversion between spin and charge currents, 2) storage of magnetic information that can be easily manipulated, and 3) transmission of spin information that can be easily detected. With respect to the first challenge, Bismuthate-based superconductors are systems that are generally thought to offer weak spin-orbit coupling, despite the heavy elements that make up such compounds. Here we use spin-torque ferromagnetic resonance to measure a large spin-orbit torque efficiency driven by spin polarization generated in heterostructures based on BaPb$_{1-x}$Bi$_{x}$O$_3$ in a non-superconducting state. We suggest that the unexpectedly large current-induced torques could stem from an orbital Rashba effect associated with local inversion symmetry breaking in BaPb$_{1-x}$Bi$_x$O$_3$.

In response to the second challenge, Bismuth Ferrite has garnered considerable attention as a promising candidate for magnetoelectric spin-orbit coupled logic-in memory. While this model system offers a magnetic texture controllable by electric fields, epitaxial BiFeO$_3$ films have typically been deposited at temperatures higher than allowed for direction integration with silicon-CMOS platforms. Here, we solve this engineering problem by growing La-doped BiFeO$_3$ at temperatures reasonably compatible with silicon-CMOS integration on BaPb$_{1-x}$Bi$_x$O$_3$ electrodes. Despite the large lattice mismatch between the two materials, all layers are well-ordered with a [001] texture, and the La-doped BiFeO$_3$ exhibits desirable ferroelectric properties. These results provide a possible route for realizing epitaxial multiferroics on complex-oxide buffer layers at low temperatures and opens the door for potential silicon-CMOS integration. Furthermore, the incorporation of a material with efficient conversion between spin and charge with a multiferroic will drive innovation and application of new spintronic devices, such as all-oxide multiferroic magnonic memory architectures.

In response to the second and third challenges, spin waves in magnetic materials, or magnons, are promising information carriers due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials for magnetic information storage due in part to their stability to external fields and larger group velocities. Multiferroic antiferromagnets such as BiFeO$_3$ have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus magnons, with an electric field. Unfortunately, spin-wave propagation in BiFeO$_3$ is not well-understood due to the complexity of the magnetic structure. In this work, we discover an anisotropy in spin transport within the spin cycloid magnetic structure of BiFeO$_3$. We also show that through Lanthanum substitution, a single ferroelectric domain can be engineered with a stable, single-variant spin cycloid controllable by electric field. The strong anisotropy discussed is an important development in the understanding of magnon-spin currents in the spin cycloid magnetic texture.

Finally, understanding the anisotropies and other characteristics of magnon-spin currents in a multiferroic requires an understanding of the symmetries inherent to the magnetic and polar orders of the multiferroic. In this work, we present a phenomenological model to elucidate the existence of magnon spin currents in generalized multiferroics. This model takes inspiration from the symmetries of multiferroics such as BiFeO$_3$, and is grounded in experimental data obtained from BiFeO$_3$ and its derivatives. By introducing this model, we address the issue of symmetry-allowed, switchable magnon spin transport in multiferroics, thereby establishing a critical framework for comprehending magnon transport within complex magnetic textures.

Our responses to the three challenges posed above all point to a magnon-based multiferroic memory founded in epitaxial oxide heterostructures. In researching a path towards this application, we make new discoveries of the physics of spin-charge interconversion, heterostructure growth, and magnon dynamics in complicated magnetic textures. We hope that the following research will prove applicable not only towards the specific goal of multiferroic magnonic memory, but also to the wider field of epitaxial oxide spintronics.