Photocontrol of Magnetism above 77~K in Nanoscaled Heterostructures of Cyanometallate Coordination Networks: Mechanism and Limits

Using nanometer-sized heterostructures of cyanometallate coordination networks, specifially core@shell nanoparticles of CoFe@CrCr-PBA (PBA = Prussian blue analogues), irradiation by white light at 80~K modifies the magnetic response, and these changes remain intact and persist without continued irradiation to nominally 125~K.\footnote{O.~N.~Risset, T.~V.~Brinzari, M.~W.~Meisel, D.~R.~Talham, submitted.} Preliminary pressure studies indicate the photoinduced changes can be maintained up to 200~K, the transition temperature of the ferromagnetic CrCr-PBA component. The effect, which we first reported up to 70~K,\footnote{D.~M.~Pajerowski, M.~J.~Andrus, J.~E.~Gardner, E.~S.~Knowles, M.~W.~Meisel, D.~R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 132} (2010) 4058.}$^,$\footnote{M.~F.~Dumont, E.~S.~Knowles, A.~Guiet, D.~M.~Pajerowski, A.~Gomez, S.~W.~Kycia, M.~W.~Meisel, D.~R.~Talham, \emph{\mbox{Inorg.}~Chem.~} {\bf 50} (2011) 4295.} arises from thermally induced interface strain, which is relaxed by irradiation of the photactive consituent, CoFe-PBA. The ferromagnetic domains in the strained interface region are affected and generate the persistent changes of the magnetism. Our understanding of this photo-magnetostructural mechanism enabled us to extend the phenomenon to include photoactive spin-crossover systems\footnote{C.~R.~Gros, M.~K.~Peprah, B.~D.~Hosterman, T.~V.~Brinzari, P.~A.~Quintero, M.~Sendova, M.~W.~Meisel, D.~R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 136} (2014) 9846.} and other ferromagnetic PBAs.\footnote{O.~N.~Risset, P.~A.~Quintero, T.~V.~Brinzari, M.~J.~Andrus, M.~W.~Lufaso, M.~W.~Meisel, D.~R.~Talham, \emph{J.~Am.~Chem.~Soc.~}{\bf 136} (2014) 15660.} The potential path to higher temperatures will be sketched.