Multiferroicity and Semi‐Cylindrical Alignment in Janus Nanofiber Aggregates

1D multiferroic fibers are known to exhibit attractive characteristics, including enhanced magnetoelectric (ME) coupling compared to thin film and bulk architectures. A comprehensive understanding of composite fibers, however, has been hindered by the complexity of their structure, leading to limited reports. Here, clear and strong ME coupling is experimentally detected in a composite Janus nanofiber aggregate using second harmonic generation (SHG) polarimetry under different magnetic field orientations. The observation of such a pronounced effect using an all‐optical method has not been previously reported in multiferroic fibers. A series of global fits is performed to the SHG polarimetry results to investigate the behavior of nanofibers within an aggregate. We find the magnetically assembled fibers exhibit semi‐cylindrical alignment as well as the expected lengthwise alignment despite variations in size and composition from fiber to fiber. The ME coupling and the semi‐cylindrical alignment seen in SHG are further corroborated via X‐ray diffraction under similar magnetic field conditions. These findings contribute to the development of complex composite and multifunctional devices using multiferroic nanostructures as building blocks, even those with inhomogeneous shapes and geometries. Multiferroic nanofibers exhibit significantly higher magnetoelectric coupling than thin film and bulk materials, making them promising candidates for advanced multifunctional devices. However, experimental studies are limited due to their complex geometry. A contactless optical method is demonstrated using second harmonic generation and X‐ray diffraction to reveal unambiguous magnetoelectric coupling and semi‐cylindrical alignment in these nanofiber aggregates.