Volume 32 Issue 2 - February 7, 2020 PDF
Control of Multiferroic Orderings by Breaking the Spatial Symmetry
Yi-Chun Chen, Yi-De Liou, Yuan-Chih Wu, and Jan-Chi Yang
Department of Physics, National Cheng Kung University
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Multiferroic materials possess both magnetic and electrical long-range orderings, and the ordered states depend on the history of external fields applied, so they are motivated for the application of nonvolatile multi-bit memory. Moreover, the magnetoeletric couplings inside the materials make it possible to control the magnetism with an electric field, or vice versa, which also provides more freedom for the design of novel electronic devices. Taking electrical ordering as an example, electrical dipoles oriented in the same direction form an electrical domain. In the initial state, electrical domains are distributed in random directions to reduce electrostatic energy. The ability to control the electrical (magnetic) domains of nanometer sizes in an ordered structure will be a key step for the future device applications. To make a particular direction unique for the memory unit, it is necessary to destroy the spatial isotropy. For realization, one can utilize the multiferroic memory of the historical process under external fields. In our previous study, we have used a scanning microscope probe to move on the nanometer scale and change the trajectory of voltage applying positions on a multiferroic mixed-phase BiFeO3, and it successfully results in different ordered domain structures (Fig. 1). The phase distributions and physical properties of the mixed-phase BiFeO3 can thus be further controlled.
Fig. 1. Topography images (left) and domain structures measured by in-plane PFM (middle, right) after poled by a positive-biased tip moving with different combinations of fast and slow scanning axis.

Recently, our team further control the multiferroic orderings by light. Unlike DC electric field with specific directivity, light is a high-frequency AC electromagnetic wave, so other factors need to be introduced to disrupt spatial symmetry. What plays a key role in the optical control mechanism is the strain caused by light on the nanometer scale. This strain causes extremely local high gradients, which results in a non-negligible flexoelectric fields and causes the tendency of electric polarization along the radial direction (Fig. 2). This photo-induced strain effects bring new directions to the manipulation of nanometer orderings.
Fig. 2. a. Schematic of strain-gradient-induced flexoelectric polarization under illumination. b. Ferroelectric, c. antiferromagnetic, and d. ferromagnetic orderings of light induced structures.
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