Recently, the group of Jiangfeng Du and Development Shi at the Key Laboratory of Microscopic Magnetic Resonance, Chinese Academy of Sciences, University of Science and Technology of China (USTC), together with Yuefeng Nie and Yurong Yang at Nanjing University, has made progress in the experimental study of scanning magnetic imaging of antiferromagnetic thin films by using diamond nitrogen-vacancy chromatography (NV chromatography for short) to perform in situ stress-tuned scanning imaging of self-supported films of antiferromagnetic BiFeO3. The research results were published as "Observation of uniaxial strain tuned spin cycloid in a freestanding BiFeO3 film" in Advanced Functional Materials [Adv. Funct. Mater. 2023, 2213725].
BiFeO3 (BFO) is an antiferromagnetic material with cycloidal order due to Dzyalonshinskii-Moriya interaction, and the mechanism of interaction between cycloidal order and stress within BFO is a major research focus in this field. Current studies have used epitaxial methods to regulate stress in BFO materials, which are difficult to modulate in situ and continuously. This makes it difficult to experimentally investigate some important issues in the magnetic-stress interaction, such as the change of magnetic order under arbitrary orientation stress and the evolution process near the phase transition of the magnetic order.
In this work, the researchers prepared a self-supported BFO film by a process of molecular beam epitaxy and soluble sacrificial layer, and performed scanning magnetic imaging of the film under stress modulation with a scanning NV microscope. The imaging results show that the cycloidal sequence twists about 12.6° at a strain of 1.5%. First principles calculations show that the experimentally observed inverse magnetic sequence twist has the lowest energy at the corresponding stress.
Figure 1. (a), (b) Real-space scanning magnetic imaging results of the BFO in the free state and at 1.5% strain. (c), (d) Fourier transform results of the scanned imaging data. (e) Statistical results of the angular distribution of the Fourier transform results in the free state and 1.5% strain state showing 12.6° of torsion.
This work is the first study of the magnetic order of BFO self-supported thin films, and the in situ modulation and high spatial resolution of the scanning imaging technique provide a new way of thinking for the study of magnetic-stress interactions. This result is valuable for the theoretical study of antiferromagnetic thin films and the application of new magnetic memory devices.
Fig. 2. The energy-pendulum line sequence period relationship curve calculated by the first nature principle. The calculated results for the pendulum line sequence direction parallel to the crystal direction are shown in blue curves, and the energy curves for the angles of 7°, 14°, 18° and 27° with the crystal direction are shown in different colors, respectively, see the legend. The calculation results show that the pendulum line order deviating from 14~18° is more stable.
Postdoctoral student Zhe Ding, PhD student Yumeng Sun and collaborating group PhD students Ningchong Zheng and Xingyue Ma are co-first authors of this work, and Academician Jiangfeng Du, Professor Yufeng Nie and Professor Yurong Yang are co-corresponding authors of this work. The research was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences and Anhui Province.
Link to the paper:
CIQTEK-Quantum Diamond Atomic Force Microscopy
In the Acknowledgements, the authors mention that the NV scanning probe was provided by CIQTEK.
The NV scanning probe is a quantum precision measurement instrument based on NV color-centered spin magnetic resonance and AFM scanning probe technology, which enables quantitative non-destructive imaging of the magnetic properties of samples with high spatial resolution at the nanometer level and ultra-high detection sensitivity of individual spins. It has been widely used in the fields of magnetic domain imaging, two-dimensional materials, topological magnetic structures, superconducting magnetism, and cell imaging.