Establishing Magneto-Elastic Coupling in a Spin-Crossover Molecular Network Grown on 2D Surfaces

Spin-crossover (SCO) complexes show bistability of spin states under various actuation parameters including temperature. This bistability along with hysteresis loop has potentiality of using them as binary units for information storage application. However, despite of cooperativity in SCO complexes, they remain non-magnetic. This is probably due to the reason that the microscopic origin of cooperativity has been primarily described by means of elastic interactions in pristine SCO. Apart from elastic coupling, long range magnetic interaction must be given equivalent importance. The spin-crossover phenomenon originates from the competition between the mean spin pairing energy (Δ), which favors the high-spin (HS) paramagnetic state and the ligand field parameter (10Dq), which favors the low-spin (LS) diamagnetic state. In addition to non-magnetic spin state transitions, the magnetic interaction can be alternatively introduced via substrate incorporation. In comparison to molecular crystals with isolated units, 3D molecular network are better candidates for extending the interaction through chemical bridges. Hybrid heterostructure based on SCO/2D surfaces remain a sensitive tool for detecting spin transitions. The fragility of SCO materials upon interfacing them on 2D surfaces is an unsolved issue. Hence, determining the ultimate scale limit with suitable 2D substrate at which cooperativity becomes effective is of major interest. Here, we showcase an easily processable, robust SCO/2D hybrid heterostructures that show long range magnetic ordering through exchange interactions. Fe-Triazole based SCO complex has been chosen for this types and chemically synthesized reduced graphene oxide (rGO) was used as the 2D substrate. Due to presence of large number of functional groups on the surface of rGO, it is a promising candidate for anchoring SCO molecular network epitaxially on rGO. Theoretically, it has been found that energy stabilization of SCO/rGO is better than Graphene/SCO. After formation of the heterostructure, interfacial charge transfer (CT) between SCO and rGO has been evidenced by means of various characterization techniques. The CT changes the high spin (HS) and low spin (LS) transition states of the molecular network which is of fundamental importance. With enough number of HS states and via superexchange interaction between the Fe-centers, a long-range magnetic ordering has been established during the spin state transition which favors like spin pairings (HS-HS or LS-LS). We have found that hysteresis width (ΔK) and transition temperatures (T_1/2^up, T_1/2^down) can also be controlled in a vast way by tuning the thickness of the SCO molecular network on rGO surface. Depending on layer number, ferromagnetic (cooperative) to antiferromagnetic (anti-cooperative) ordering has been evolved. This makes a magnetically ordered SCO/2D hybrid which could perform plethora of applications in nanomagnetics. Apart from magnetic characterization, the spin state dependence of electrical conductivity in SCO is also arrested through transport measurement due to the presence of highly delocalized π-surface electrons of 2D rGO. While pristine SCO remain insulating (~10^-11S) due to weak intermolecular interaction of van der waals nature, after attachment to highly conducting rGO surface, the conductance enhances in many folds (~10^-5S). While the overall transport is mediated via rGO surface, the epitaxial SCO network acts as scattering potential with the itinerant conduction electrons. Since the LS/HS states of SCO also depend on magnetic field in this case, the SCO/2D heterostructure shows magnetoresistance which is the main prototype for magnetoresistive- switching application. The conductance changing along with enhanced magnetic coupling could be utilized for switching application in spintronics using SCO/2D material.