Field-induced spin cycloidal modulation to antiferromagnetic transition and possible flexomagnetic effect in BiFeO3 nanoparticles

Beyond its various properties, the model multiferroic BiFeO3 (BFO) displays a rich magnetic structure illustrated in the bulk by its long period (similar to 62 nm) spin cycloidal modulation. Here, BFO nanoparticles are produced by a facile hydrothermal method and show average size of 8 nm and a narrow size distribution, as determined using x-ray diffraction analysis and transmission electron microscopy images. Mossbauer spectrometry (MS) unambiguously reveals that a cycloidal modulation does still exists with particles about 5 times smaller than the bulk cycloid. Combining macroscopic magnetic measurements and in situ Mossbauer spectrometry, we demonstrate that a critical magnetic field of similar to 0.2 T destabilizes the cycloidal modulation to lead to a homogenous antiferromagnetic state, as the result of magnetic anisotropy due to magnetoelastic and surface-confinement effects. More interestingly, further increasing of the external magnetic field up to 8 T does not change the average magnetic hyperfine field and results into multiple Mossbauer sextets we propose to explain by a flexomagnetic effect i.e. magnetic anisotropies resulting from strain gradients due to a continuous variation of the coupling between magnetization and the structural distortion from the surface to the particle core. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

The multiferroic BiFeO3 (BFO) nanoparticles with an average size of 8 nm and a narrow size distribution were successfully synthesized by a hydrothermal method. The nanoparticles exhibit a cycloidal modulation with a period about 5 times smaller than that in the bulk material. The cycloidal modulation can be destabilized by a critical magnetic field of approximately 0.2 T, leading to a homogeneous antiferromagnetic state. Moreover, increasing the external magnetic field up to 8 T does not affect the average magnetic hyperfine field, but results in multiple Mossbauer sextets due to the flexomagnetic effect caused by strain gradients.