Ordering governs magnetic tunability in FePt-based Janus particles independent of curvature

TL;DR

This study shows that in micrometer-sized FePt Janus particles, magnetic properties are primarily influenced by material ordering rather than curvature, challenging previous nanoscale assumptions.

Contribution

The paper introduces experimental and simulation evidence that curvature has minimal effect on magnetization reversal at micrometer scales, emphasizing the role of chemical ordering.

Findings

Magnetic hysteresis remains nearly constant across particle sizes from 3 to 20 microm.

Chemical ordering significantly impacts coercivity and hysteresis shape.

Curvature effects are negligible on magnetization reversal at micrometer scales.

Abstract

Magnetic Janus particles enable remote actuation in biomedical, microfluidic, and materials applications. While curvature-driven magnetic effects are well known at the nanoscale, their influence on magnetization reversal in micrometer-sized particles is still unclear. In this work, we combine experiments and micromagnetic simulations to study curvature-dependent magnetism in FePt-coated Janus particles with diameters ranging from 3-10 microm, and extend the analysis to 1-20 microm through simulations. Structural and crystallographic characterization confirms continuous FePt coatings with near-equiatomic composition and partial L1_0 ordering. Magnetometry measurements show nearly unchanged hysteresis behavior across particle sizes, with coercivity remaining approximately constant m_0Hc = 1.13 +/- 0.05 T, pooled n = 8). Statistical analysis reveals no significant dependence of coercivity or remanence on particle diameter (p = 0.61 for Hc and p = 0.85 for Mr/Ms). To explain these results, we introduce FunMaP, an open-source micromagnetic simulation framework that enables direct comparison between experiments and idealized FePt caps. Simulations confirm that curvature has little effect on magnetization reversal at micrometer scales, consistent with a locally planar magnetic limit where the exchange length is much smaller than the particle radius. In contrast, differences in chemical ordering strongly affect hysteresis shape and coercivity. These findings demonstrate that magnetic behavior in micrometer-scale FePt Janus particles is governed mainly by material ordering rather than curvature. This work establishes a quantitative boundary for curvature-dependent magnetism and provides design guidelines for programmable magnetic micro-systems.