Fabrication and characterization of exchange-biased magnetic Janus particles

Magnetic Janus particles (MJPs), or particles designed with one side being magnetic and the other nonmagnetic, have promising applications for lab-on-a-chip devices as stirrers, cargo transporters, and sensors. MJPs for such tasks require specific magnetic properties that allow for precise motion control.

To this end, Tomita et al. have developed MJPs with extremely defined magnetic textures by depositing an exchange bias layer system on top of micron-sized silica spheres. The layer system contained a buffer layer of copper, an antiferromagnetic layer (Ir₁₇Mn₈₃), a ferromagnetic layer (Co₇₀Fe₃₀), and a capping layer of silicon.

To create a strong exchange bias field, thus a strong unidirectional anisotropy, the layer thicknesses were specifically adjusted. During the fabrication via magnetron sputtering, an external magnetic field was applied parallel to the tangential plane of the particles’ pole. The resulting MJPs had magnetic caps, where the magnetic moments aligned tangentially to the surface in a macroscopic remanent onion configuration.

The researchers aimed to understand and characterize the micromagnetic behavior of this imposed texture on the curved surfaces of the particles. They used longitudinal magneto-optical Kerr effect magnetometry and magnetic force microscopy, which revealed an onion state in the magnetic caps of the ensemble. The magnetic system was modeled with micromagnetic simulations, which further confirmed the conclusions drawn from their experimental measurements.

Tomita et al. plan to explore more transport properties with the particles being included in microfluidic systems used for biomedical applications. In addition, they hope a fundamental understanding of the micromagnetic behavior of exchange-biased materials on curved surfaces will help the future development of flexible magnetic systems.

Source: “Magnetic textures in hemispherical thin film caps with in-plane exchange bias,” by Andreea Tomita, Meike Reginka, Rico Huhnstock, Maximilian Merkel, Dennis Holzinger, and Arno Ehresmann, Journal of Applied Physics (2020). The article can be accessed at http://doi.org/10.1063/5.0033772 .