Weak Ferromagnetic Interaction at the Surface of the Ferrimagnetic RbCo[Fe(CN)]·11HO Photoexcited State.

CoFe Prussian blue analogues (PBAs) are well-known for their magnetic bistability tuned by external stimuli. The photoswitching properties are due to the electron transfer from Co-NC-Fe to Co-NC-Fe linkage, accompanied by the spin change of the Co ions (HS stands for high spin and LS for low spin). In this work, we investigated 100 nm particles of the RbCo[Fe(CN)]·11HO PBA (named RbCoFe). The photoexcited state of the PBA was reached by red laser excitation (λ = 635 nm) and observed by X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) that are element-specific probes. The XMCD measurements at the Co and Fe L edges, probing the magnetic 3d orbitals, have provided a direct evidence of the antiferromagnetic interaction between the Co and the Fe ions belonging to the core of the particles, thus confirming the previously published, though indirect XMCD measurements at K edges. Because of the surface sensitivity of XMCD at the L edges, the magnetic properties of the particle surface were also revealed. Surface Co-Fe pairs exhibit a weak ferromagnetic interaction. Thus, the magnetic structure of the photomagnetic RbCoFe 100 nm particles can be described as a ferrimagnetic core surrounded by a ferromagnetic shell. This finding brings new insights into the understanding of the complex magnetic properties of photoexcited RbCoFe and shows that the surface can have different magnetic behavior than the core. This should impact the nature of magnetic coupling in nanoparticles of CoFe PBA, where surface effect will dominate.