Imaging Topological Defects in a Noncollinear Antiferromagnet

Morphogenesis of spin cycloids in a noncollinear antiferromagnet

Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging are challenging. Here, we employ scanning nitrogen vacancy (NV) magnetometry to unveil the morphogenesis of spin cycloids at both the local and global scales within a single ferroelectric domain of (111)-oriented BiFeO3, which is a noncollinear antiferromagnet, resulting in formation of a glassy labyrinthine pattern. We find that the domains of locally oriented cycloids are interconnected by an array of topological defects and exhibit isotropic energy landscape predicted by first-principles calculations. We propose that the CRS of spin-cycloid propagation directions within the (111) drives the formation of the labyrinthine pattern and the associated topological defects such as antiferromagnetic skyrmions. Unexpectedly, reversing the as-grown ferroelectric polarization from [[Formula: see text][Formula: see text][Formula: see text]] to [111] produces a noncycloidal NV image contrast which could be attributed to either the emergence of a uniformly magnetized state or a reversal of the cycloid polarity. These findings highlight that (111)-oriented BiFeO3 is not only important for studying the fascinating subject of pattern formation but could also be utilized as an ideal platform for integrating novel topological defects in the field of antiferromagnetic spintronics.

Characterizing G-type antiferromagnetism quantitatively with optical second harmonic generation

Antiferromagnetism has become a promising candidate for the next generation electronic devices due to its thermal stability, low energy consumption, and fast switching speed. However, the canceling of the net magnetic moment in antiferromagnetic order presents great challenge on quantitative characterization and modulation, hindering its investigation and application. In this work, utilizing the optical second harmonic generation (SHG) in a wide temperature range, the integrated differential phase contrast scanning transmission electron microscopy, and first-principles calculations, we performed a quantitative study on the evolution of non-collinear antiferromagnetic order in BiFeO3 films with a series of strains. We found that the antiferromagnetic coupling was significantly enhanced, featured by the increase of Néel temperature from 428 K to 646 K, and by one order of enhancement of SHG intensity contributed from the G-type antiferromagnetic order by strain manipulation from -2.4% to +0.6%. We attributed the enhancement of the antiferromagnetic coupling to the enhancement of the superexchange interaction as the Fe-O-Fe bond angle approaches 180° when the in-plane lattice constants increase, which might also result in a tendency from a non-collinear antiferromagnetic order to a collinear one. Our work not only bridges the antiferromagnetic order and the strain manipulation in epitaxial multiferroics, more importantly, also paves a way for quantitative characterization by SHG technology and the precise manipulation of antiferromagnetism.

Minimizing Sensor-Sample Distances in Scanning Nitrogen-Vacancy Magnetometry

Scanning magnetometry with nitrogen-vacancy (NV) centers in diamond has led to significant advances in the sensitive imaging of magnetic systems. The spatial resolution of the technique, however, remains limited to tens to hundreds of nanometers, even for probes where NV centers are engineered within 10 nm from the tip apex. Here, we present a correlated investigation of the crucial parameters that determine the spatial resolution: the mechanical and magnetic stand-off distances, as well as the subsurface NV center depth in diamond. We study their contributions using mechanical approach curves, photoluminescence measurements, magnetometry scans, and nuclear magnetic resonance (NMR) spectroscopy of surface adsorbates. We first show that the stand-off distance is mainly limited by features on the surface of the diamond tip, hindering mechanical access. Next, we demonstrate that frequency-modulated (FM) atomic force microscopy feedback partially overcomes this issue, leading to closer and more consistent magnetic stand-off distances (26-87 nm) compared with the more common amplitude-modulated feedback (43-128 nm). FM operation thus permits improved magnetic imaging of sub-100-nm spin textures, shown for the spin cycloid in BiFeO3 and domain walls in a CoFeB synthetic antiferromagnet. Finally, by examining 1H and 19F NMR signals in soft contact with a polytetrafluoroethylene surface, we demonstrate a minimum NV-to-sample distance of 7.9 ± 0.4 nm.

Electrical Quantum Coupling of Subsurface-Nanolayer Quasipolarons

We perform dielectric and impedance spectrums on the compressively-strained ceramics of multiferroic bismuth ferrite. The subsurface-nanolayer quasipolarons manifest the step-like characteristic of pressure-dependent transient frequency and, furthermore, pressure-dependency fails in the transformation between complex permittivity and electrical impedance, which is well-known in classic dielectric physics, as well as the bulk dipole chain at the end of the dissipation peak.

Electric-field-induced multiferroic topological solitons

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.

Switching the spin cycloid in BiFeO3 with an electric field

Bismuth ferrite (BiFeO3) is a multiferroic material that exhibits both ferroelectricity and canted antiferromagnetism at room temperature, making it a unique candidate in the development of electric-field controllable magnetic devices. The magnetic moments in BiFeO3 are arranged into a spin cycloid, resulting in unique magnetic properties which are tied to the ferroelectric order. Previous understanding of this coupling has relied on average, mesoscale measurements. Using nitrogen vacancy-based diamond magnetometry, we observe the magnetic spin cycloid structure of BiFeO3 in real space. This structure is magnetoelectrically coupled through symmetry to the ferroelectric polarization and this relationship is maintained through electric field switching. Through a combination of in-plane and out-of-plane electrical switching, coupled with ab initio studies, we have discovered that the epitaxy from the substrate imposes a magnetoelastic anisotropy on the spin cycloid, which establishes preferred cycloid propagation directions. The energy landscape of the cycloid is shaped by both the ferroelectric degree of freedom and strain-induced anisotropy, restricting the spin spiral propagation vector to changes to specific switching events.

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.

Revealing emergent magnetic charge in an antiferromagnet with diamond quantum magnetometry

Whirling topological textures play a key role in exotic phases of magnetic materials and are promising for logic and memory applications. In antiferromagnets, these textures exhibit enhanced stability and faster dynamics with respect to their ferromagnetic counterparts, but they are also difficult to study due to their vanishing net magnetic moment. One technique that meets the demand of highly sensitive vectorial magnetic field sensing with negligible backaction is diamond quantum magnetometry. Here we show that an archetypal antiferromagnet-haematite-hosts a rich tapestry of monopolar, dipolar and quadrupolar emergent magnetic charge distributions. The direct read-out of the previously inaccessible vorticity of an antiferromagnetic spin texture provides the crucial connection to its magnetic charge through a duality relation. Our work defines a paradigmatic class of magnetic systems to explore two-dimensional monopolar physics, and highlights the transformative role that diamond quantum magnetometry could play in exploring emergent phenomena in quantum materials.

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films

In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.