Decoupling size from magnetism: A length-scale boundary for curvature control in micrometer FePt Janus particles
Curvature reshapes magnetization when a structure's dimensions approach intrinsic magnetic length scales, but functional magnetic colloids and microrobots are often micrometers in size, where the radius of curvature exceeds these scales by several orders of magnitude. Whether particle diameter remains an effective parameter for tuning magnetic response in this regime is therefore unclear. We synthesized partially ordered FePt Janus caps on spherical SiO$_2$ particles with diameters of 3--10~$\mu$m, characterized their structure and magnetic response, and extended the investigated range to 1--20~$\mu$m using micromagnetic simulations. Across this range, coercivity, remanence, and hysteresis-loop shape showed no systematic dependence on particle diameter in either experiment or simulation. The ratio between exchange length and radius of curvature ($\ell_{\mathrm{ex}}/R \sim 10^{-3}$--$10^{-4}$) places these particles in a locally planar regime where diameter-dependent curvature effects are weak. Size and magnetic response are therefore effectively decoupled within the investigated regime: particle diameter can be selected according to transport, payload, and biocompatibility requirements without introducing a measurable magnetic penalty, but it does not provide an effective route for tuning magnetization reversal. Instead, the magnetic response is governed primarily by material state, including the balance between magnetically hard L1$_0$ and soft A1 FePt, with additional modulation by processing-induced morphology. The resulting length-scale map identifies the regime in which this decoupling is expected to hold and where diameter-dependent curvature effects may become significant.