Imaging ferroelectric domains with a single-spin scanning quantum sensor

A scanning probe microscope compatible with quantum sensing at ambient conditions

We designed and built up a new type of ambient scanning probe microscope (SPM), which is fully compatible with state-of-the-art quantum sensing technology based on the nitrogen-vacancy (NV) centers in diamond. We chose a qPlus-type tuning fork (Q up to ∼4400) as the current/force sensor of SPM for its high stiffness and stability under various environments, which yields atomic resolution under scanning tunneling microscopy mode and 1.2-nm resolution under atomic force microscopy mode. The tip of SPM can be used to directly image the topography of nanoscale targets on diamond surfaces for quantum sensing and to manipulate the electrostatic environment of NV centers to enhance their sensitivity up to a single proton spin. In addition, we also demonstrated scanning magnetometry and electrometry with a spatial resolution of ∼20 nm. Our new system not only paves the way for integrating atomic/molecular-scale color-center qubits onto SPM tips to produce quantum tips but also provides the possibility of fabricating color-center qubits with nanoscale or atomic precision.

Ptychographic Nanoscale Imaging of the Magnetoelectric Coupling in Freestanding BiFeO3

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80 nm thin freestanding film of the room-temperature multiferroic BiFeO3 (BFO). The antiferromagnetic spin cycloid of period 64 nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L3 edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71° between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with soft X-rays.

Low-temperature grapho-epitaxial La-substituted BiFeO3 on metallic perovskite

Bismuth ferrite has garnered considerable attention as a promising candidate for magnetoelectric spin-orbit coupled logic-in-memory. As model systems, epitaxial BiFeO3 thin films have typically been deposited at relatively high temperatures (650-800 °C), higher than allowed for direct integration with silicon-CMOS platforms. Here, we circumvent this problem by growing lanthanum-substituted BiFeO3 at 450 °C (which is reasonably compatible with silicon-CMOS integration) on epitaxial BaPb0.75Bi0.25O3 electrodes. Notwithstanding the large lattice mismatch between the La-BiFeO3, BaPb0.75Bi0.25O3, and SrTiO3 (001) substrates, all the layers in the heterostructures are well ordered with a [001] texture. Polarization mapping using atomic resolution STEM imaging and vector mapping established the short-range polarization ordering in the low temperature grown La-BiFeO3. Current-voltage, pulsed-switching, fatigue, and retention measurements follow the characteristic behavior of high-temperature grown La-BiFeO3, where SrRuO3 typically serves as the metallic electrode. 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.