Electric and antiferromagnetic chiral textures at multiferroic domain walls

Abstract

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls 1 and in related objects such as skyrmions 2. However, chiral textures should also exist in other types of ferroics such as antiferromagnets for which theory predicts that they should move faster for lower power 3 , and ferroelectrics where they should be extremely small and possess unusual topologies 4,5. Here we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO 3. Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavorable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics. Metallic ferromagnets have been the elemental bricks of spintronics for the last three decades and continue to hold promises on the basis of non-collinear chiral spin textures such as skyrmions. These topologically protected objects are envisioned to be the future of magnetic data storage thanks to their specific stability, dynamics, and scalability 2. In parallel, antiferromagnets (AFs) are emerging as a new paradigm for spintronics 6. They are intrinsically stable (being insensitive to spurious magnetic fields), scalable (no cross talk between neighbouring memory cells), and fast (switching frequencies in the THz regime). The opportunity of gathering the best of these two worlds and realize "antiferromagnetic skyrmions" is then tremendously appealing but faces at least two major challenges. The first one is to achieve antiferromagnetic chirality and the second one is to identify appropriate control stimuli to create, annihilate and move these chiral objects. On one hand, chirality may naturally emerge at domain walls. The antiferromagnetic domain wall structure is a virtually uncharted territory but this is where translational symmetry is broken and spin rotation favoured. On the other hand, AF manipulation is hampered by the intrinsic lack of net magnetization, which prevents a straightforward magnetic actuation. This fundamental issue may be

Antiferromagnetic textures in BiFeO3 controlled by strain and electric field

“…4a) with identical periods gives rise to two orthogonal lines of diffracted spots, thus defining a square diffracted pattern. The fine structure of this pattern is rendered more complex by additional spots that arise from the modulation of the magnetic periodicity by the ferroelectric domain structure; 23 however, here our focus is on the cycloid propagation direction and periodicity. The spacing between the +q and −q spots corresponds to a cycloid period of 72 ± 5 nm for both spin cycloids with k 1 propagation vector.…”

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…This innovative use of resonant X‐ray scattering has provided unprecedented insight to the characteristics of the “stitching” of the cycloid that occurs at domain walls, and was used alongside real‐space nitrogen vacancy magnetometry to provide complementary information to help support their conclusion about stray fields occurring at the chiral ferroelectric domain wall. [ 121 ] Finally, we note that REXS has also been pivotal in confirming the presence of exotic types of cycloids (the so‐called type‐2 cycloid) under various levels of epitaxial strain, which is further discussed in Section 5.3. [ 89 ]…”

“…The cross coupling between the chiral characteristics of the ferroelectric domain walls with the magnetic order will prove to be useful tools with which to design novel logic architectures. Indeed, the chiral structures that occur at the confluence of cycloids at domain walls in BFO films [ 121 ] (discussed in Section 4.4.2) can be thought of the precursor to the nucleation of skyrmions. These topologically protected entities show great promise in the quest for future high density, low energy electronics technologies.…”

“…The most comprehensive picture of the magnetic order of a given sample would be ascertained by applying various techniques given in Table 1; indeed, various combinations thereof have been carried out in studies over the years. [ 79,84,89,120,121 ] In an attempt to be as instructive as possible, for each technique we include a “tutorial style” description of the physics, then outline key results on bulk BFO, before finishing with relevant thin film experiments.…”

“…in BFO epitaxial thin films. [ 121 ] Using circularly polarized X‐rays, they detected dichroic signals relating to the domain structure of characteristic 71° and 109° domain walls. [ 197 ] First tuning the X‐rays to the oxygen K‐edge (near 530 eV), the resonant scattering experiments were sensitive to chiral characteristics of the local polarization in the domain walls.…”