Structural Evolution of Printed Ternary Magnetic Hybrid Thin Films Containing Soft and Hard Magnetic Nanoparticles for Coupled Composites

Diblock copolymer thin films templating two types of magnetic nanoparticles are ternary nanocomposites that can show tunable magnetic behavior depending on the size and magnetic properties of the nanoparticles. In the case of utilizing both soft and hard magnetic nanoparticles, the ternary films become interesting for applications in permanent magnets and microwave devices. In this work, ternary hybrid thin films composed of the diblock copolymer polystyrene-<i>block</i>-poly(methyl methacrylate) (PS<i>-b</i>-PMMA), cobalt ferrite (CoFe₂O₄, <i>d</i> = 21.7 ± 12.2 nm) nanoparticles, and nickel (Ni, <i>d</i> = 46 ± 10 nm) nanoparticles are fabricated from solution in a slot-die printing process. The film morphology evolution is tracked <i>in situ</i> by grazing-incidence small-angle X-ray scattering (GISAXS). For comparison, a binary hybrid film with only PS-<i>b</i>-PMMA and CoFe₂O₄ nanoparticles and a pure PS-<i>b</i>-PMMA film are also investigated. All films show similar kinetics during film formation, where the wet film undergoes solvent evaporation followed by rapid microphase separation and coalescence into the final dry film. Complementary atomic force microscopy (AFM) measurements reveal the as-printed surface morphology of the polymer nanocomposites. To probe the magnetic behavior of the hybrid thin films, a superconducting quantum interference device (SQUID) magnetometer is used to measure the magnetic response in both the in-plane direction and out-of-plane direction. The ternary film shows a single-phase hysteresis loop at 300 K that evolves into a two-phase hysteresis as the temperature is decreased, as the soft and hard magnetic phases switch individually. Compared to the binary film, the ternary film shows increased coercivity over the measured temperature range due to dipolar coupling between the NPs in the system. Thus, the ternary film demonstrates the potential for utilizing dipolar interactions in the fabrication of coupled composites, allowing for the tuning of magnetic behavior without the need for complex material synthesis.