Quantitative Imaging of Exotic Antiferromagnetic Spin Cycloids in BiFeO3 Thin Films

Abstract

BiFeO 3 is a rich room-temperature multiferroic material in which noncollinear antiferromagnetic spin cycloids can be deterministically controlled by an electric field through the magnetoelectric interaction, opening perspectives for low-power reconfigurable antiferromagnetic spintronics. Using a commercial scanning nitrogen-vacancy (N-V) magnetometer, we are able to image two different types of spin cycloids stabilized in strain-engineered BiFeO 3 epitaxial thin films. We show that, in these samples harboring two ferroelectric variants, each ferroelectric domain is coupled to a single spin cycloid, giving rise to a zigzag magnetic pattern. These ferroelectric domains can be manipulated at the local scale by piezoresponse force microscopy, allowing the design of micron-sized single domains. Thanks to its coupled optical microscope and fast-imaging capabilities, the scanning N-V magnetometer enables a quick repositioning in such areas of interest. Finally, quantitative imaging on single ferroelectric domains provides insights into the physical parameters of each spin-cycloid type and their impact on the magnetic-stray-field measurements.

Voltage-based magnetization switching and reading in magnetoelectric spin-orbit nanodevices

“…Here, the periodic variation of the magnetic stray field comes from the spin-density wave that is locked to the cycloid and perpendicular to the cycloidal plane defined by Q and P (Refs. 27,29). As exemplified in Fig.…”

Ptychographic Nanoscale Imaging of the Magnetoelectric Coupling in Freestanding BiFeO3

“…Previously, the spin cycloid in BFO has only been imaged by scanning nitrogen-vacancy (NV) magnetometry. [12][13][14][15][16][17] This surface sensitive technique was used to map the weak stray field that results from uncompensated spins due to a second DMI interaction. [18] Recently, ferroelectric domains have been observed by measuring the Stark shift of the NV spin with a resolution of ≈100 nm.…”

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

“…Despite the single ferroelectric domain state, a complex magnetic landscape appears (Figure c, left), consisting of periodic magnetic stray fields with propagation directions varying in space, over typical length scales of a few hundred nanometers. This periodic magnetic stray field is the signature of the antiferromagnetic spin cycloid of BiFeO 3 . ,− Considering the (111) orientation of BiFeO 3 with polarization along the growth axis , we expect all of the antiferromagnetic spin cycloids to propagate in the (111) film plane. For the bulk, the three propagation vectors, k 1 – k 3 , are degenerate along the high-symmetry crystallographic ⟨−110⟩ directions of the (111) plane .…”

“…In thin-film form, while the influence of epitaxial strain on ferroelectric domain engineering has attracted a great deal of attention, − the investigation of the antiferromagnetic order only recently boomed with the development of ultrasensitive neutron − or synchrotron − experiments as well as scanning magnetometry − techniques. The substrate-induced epitaxial strain can trigger either a noncollinear cycloidal spin texture or a pseudocollinear canted G-type antiferromagnetic ordering for low or high strain values, respectively. , Such polar and magnetic complexity challenges the quest for deterministic magnetoelectric switching in BiFeO 3 and hence hinders the integration of multiferroics into CMOS competitive technologies.…”

“…Here, the ferroelectric landscape is reduced to a striped ferroelectric domain pattern with only two ferroelastic variants separated by 71° domain walls . In this configuration, the magnetoelectric coupling results in a zigzag pattern of antiferromagnetic spin cycloids with two in-plane propagation vectors. − On one hand, an electric-field controllable net polarization emerges in the films. , On the other hand, the accompanying antiferromagnetic order remains nontrivial due to the different long-range periodic cycloids (64 nm) resulting from the ferroelectric domains . Thus, despite tremendous progress, a deterministic magnetoelectric coupling remains to be demonstrated in such nanoscale MESO devices …”