Core and shell size dependences on strain in core@shell Prussian blue analogue (PBA) nanoparticles and the effect on photomagnetism.*

Rb$_x$Co[Fe(CN)$_6$]$_y$@K$_a$Ni[Cr(CN)$_6$]$_b$ core@shell heterostructures have been shown to exhibit a photoinduced decrease in magnetization that persists up to the $T_c = 70$~K of the KNiCr-PBA component, which is not photoactive as a single-phase material. A magnetomechanical effect can explain how the strain in the shell evolves from thermal and photoinduced changes in the volume of the core. Moreover, a simple model has been used to estimate the depth of the strained region of the shell, but only one size of core ($347 \pm 35$~nm) has been studied. Since the strain depth in the shell is expected to be dependent on the size of the core, three distinct RbCoFe-PBA core sizes were synthesized, and on each, three different KNiCr-PBA shell thicknesses were grown. The magnetization of each core-shell combination was measured before and after irradiation with white light. Our results suggest the strain depth, as expected, increases from $\approx 56$~nm in heterostructures with a core size of $328 \pm 29$~nm to more than 90~nm in heterostructures with a core size of $575 \pm 113$~nm. The data from the smallest core size also shows features indicating the model may be too simple.