Tuning magnetoresistance in molybdenum disulphide and graphene using a molecular spin transition

Coupling spins of molecular magnets to two-dimensional (2D) materials provides a framework to manipulate the magneto-conductance of 2D materials. However, with most molecules, the spin coupling is usually weak and devices fabricated from these require operation at low temperatures, which prevents practical applications. Here, we demonstrate field-effect transistors based on the coupling of a magnetic molecule quinoidal dithienyl perylenequinodimethane (QDTP) to 2D materials. Uniquely, QDTP switches from a spin-singlet state at low temperature to a spin-triplet state above 370 K, and the spin transition can be electrically transduced by both graphene and molybdenum disulphide. Graphene-QDTP shows hole-doping and a large positive magnetoresistance ( ~ 50%), while molybdenum disulphide-QDTP demonstrates electron-doping and a switch to large negative magnetoresistance ( ~ 100%) above the magnetic transition. Our work shows the promise of spin detection at high temperature by coupling 2D materials and molecular magnets.

Graphene hybrid shows higher magnetoresistance at higher temperature (400 K). b MoS 2 -QDTP shows a jump in carrier density above spin transition temperature (360 K).

Significance of A(T). A(T)
shows similar behaviour as the temperature dependent magnetic signal (χT) of the QDTP molecule above spin transition temperature. Considering the exchange interaction between the spin triplet of the molecule and the carriers of MoS 2 , one can deduce the spin-disorder resistivity as discussed in previous figure [2]. In that equation, A(T) contains the square of the thermal average of the spin, <S>, of QDTP molecule. <S> can be calculated from the SQUID data (χT vs T plot in red) of the molecule at high temperature [3]. The qualitative agreement of A(T) and χT, with good fitting of the resistivity data, confirms the presence of magnetic interaction mediated by carriers in MoS 2 -QDTP hybrid.
Supplementary Figure 17. Hysteresis loop as a function of field. Both hybrids (graphene-QDTP, MoS2-QDTP) show no hysteresis in resistance as magnetic field is swept back and forth for two different temperatures, 300 K (singlet state of QDTP), and 400 K (triplet state of QDTP).
This suggests that there is no anisotropy in the QDTP molecule which can cause hysteresis.
Supplementary Figure 18. Angle dependent magnetic field effect on conductance. Out-ofplane magnetic field is 0°, while in-plane is 90°. a graphene-QDTP hybrid has cosine θ dependence with magnetic field similar to graphene. b MoS2-QDTP has no orientation dependence with the direction of the magnetic field, for both singlet and triplet states of the molecule.
Supplementary Figure 19. The coupling constant J can be estimated from DFT calculations using the energy difference between the spin triplet and singlet states (ΔE ~ 0.36 eV). The theoretical values of J based on the current structural model is estimated to be around 131 eV Å 2 . It is expected that this value is sensitive to the packing configurations of the adsorbing QDTP molecules above the 2D sheets.
We have also estimated J to be 200 eV A 2 from the experiment, following equation (3) Here, we have used S = 1 (as triplet state), n is the electron density in MoS 2 which is estimated to be 0.6x10 21 e per cm 2 , ns is the doping concentration of the molecule which is estimated to be around 0.27x10 18 m -2 , m is the effective mass ~ 0.4m e , and <S> calculated from the SQUID measurement. It is noted that the n s can vary up to orders depending on the doping concentration and the packing density of the molecules. Thermal average of <S> can be found from the SQUID measurement.
We have also estimated J (considering intermolecular spin interaction) based on experimental