Surface-induced spin state locking of the [Fe(H₂B(pz)₂)₂(bipy)] spin crossover complex

Abstract

Temperature- and coverage-dependent studies of the Au(1 1 1)-supported spin crossover Fe(II) complex (SCO) of the type [Fe(H2B(pz)2)2(bipy)] with a suite of surface-sensitive spectroscopy and microscopy tools show that the substrate inhibits thermally induced transitions of the molecular spin state, so that both high-spin and low-spin states are preserved far beyond the spin transition temperature of free molecules. Scanning tunneling microscopy confirms that [Fe(H2B(pz)2)2(bipy)] grows as ordered, molecular bilayer islands at sub-monolayer coverage and as disordered film at higher coverage. The temperature dependence of the electronic structure suggest that the SCO films exhibit a mixture of spin states at room temperature, but upon cooling below the spin crossover transition the film spin state is best described as a mix of high-spin and low-spin state molecules of a ratio that is constant. This locking of the spin state is most likely the result of a substrate-induced conformational change of the interfacial molecules, but it is estimated that also the intra-atomic electron-electron Coulomb correlation energy, or Hubbard correlation energy U, could be an additional contributing factor.

Indications of magnetic coupling effects in spin cross-over molecular thin films

“…In the LS state of [Fe{H 2 B(pz) 2 } 2 (bipy)], the 3d electrons occupy the t 2g orbitals in pairs leaving the e g empty. 4,7,8,10,23,24 This is observable in the 2p 3/2 (Fe L 3 ) X-ray absorption spectra as a major e g peak at about 708 eV ( Fig. 2 and 3).…”

“…Such a suppression of the spin state transition has been observed for [Fe(1,10-phenanthroline) 2 (NCS) 2 ], 5 and [Fe{H 2 B(pz) 2 } 2 (bipy)] (H 2 B(pz) 2 = bis(hydrido)bis(1H-pyrazol-1-yl)borate, bipy = 2,20-bipyridine), 4,6-10 molecules or ultra-thin films interacting with a metallic substrate, with a resulting mixed spin state dominated by the LS state at high temperatures. [4][5][6]8 This implies a substrate-induced ''locking'' of the spin state.…”

“…They have a coercivity of about 200 Oe at room temperature along the out-of-plane direction and a Curie temperature that is about 350 K, which is suitable for our purpose of switching magnetic moment direction around room temperature or even slightly above. The [Fe(H 2 B(pz) 2 ) 2 (bipy)] SCO molecule was deposited by thermal evaporation (as done elsewhere 4,[6][7][8][9][10]20,24 ), in a UHV growth chamber, on the NiCo 2 O 4 surface to a 10 nm thickness.…”

Pressure-tuning of the photomagnetic response of heterostructured CoFe@CrCr-PBA core@shell nanoparticles

“…On the other hand, the application of pressure does increase this magnetic transition temperature, at least for the CrCr-PBA shell used in this work. Ultimately, the next step to realizing persistent photocontrolled magnetic switching at higher temperatures may be to tune the chemical composition [17] or to explore options afforded by spin-crossover complexes [40][41][42][43][44]. Externally applied pressure applied to CoFe@CrCr Prussian blue analogue core@shell nanoparticles shifts the magnetic ordering temperatures of the CoFe-PBA core, the CrCr-PBA shell, and the photo-CTIST of the CoFe-PBA core.…”

Enhanced Cooperativity in Supported Spin-Crossover Metal–Organic Frameworks

“…It is only recently that supported examples have emerged, starting from 3-dimensional (3D) materials with thin-films grown on gold. 11,16,17 0D systems followed based on single SCO molecules, [18][19][20][21][22][23][24] or molecular junctions. 25 However, 2D monolayer organized networks exhibiting SCO behavior have not been reported so far.…”