Interplay of spin crossover molecules plus polyaniline plus magnetic nanoparticles

Fe(II)-based spin crossover molecules ([Fe(Htrz)₂(trz)](BF₄) or Fe(phen)₂(NCS)₂, are molecules in which the strength of the octahedral ligand-field surrounding the Fe atom is in the realm where even slight external perturbations, like change of temperature or magnetic field, can trigger a spin-state transition in these molecules. Depending on the spin-pairing energy, the octahedral ligand-field splitting (D_oct) between the t_2g and e_g orbitals of Fe(II) complexes dictates the diamagnetic (S = 0) of low-spin state (LS) or paramagnetic (S = 2) of high-spin state (HS). This gives rise to SCO has a bi-stable spin states transition.



However, despite their bi-stable spin-states at room temperature, SCO naturally has a very high on-state resistivity (~10³ Ω.cm) acts as an obstacle when it comes to their applications in spintronics and in nonvolatile memory devices. Therefore, to lower their on-state resistivity, semiconducting polymers like polyaniline (PANI) are added to these materials. The incorporation of PANI has a deleterious effect which tends to nullify the cooperative effects in the SCO complexes. Hence, magnetic nanoparticles are introduced to the bi-composite where it is proven to restore the cooperative effect.



The magnetic coupling between the magnetic nanoparticles and SCO in PANI matrix has been shown both ferromagnetic and antiferromagnetic for [Fe(Htrz)₂(trz)](BF₄) with Fe₃O₄ and Fe(phen)₂(NCS)₂ plus Fe₃O₄. Furthermore, from the concentration of magnetic nanoparticle added into the bi-composite, it has been showed that there exists a long paramagnetic correlation length of SCO in the tri-composite.