Molecular materials offer advantages for spin-based electronics due to long spin lifetimes and synthetically-tunable molecular spin states [1]. An exciting example of tunable spin-states is found in a class of so-called “spin crossover” compounds for which molecular spin can be externally-controlled by environmental parameters such as temperature, pressure, light irradiation, or magnetic or electric field [2]. The most common instance involves coordination of Fe²⁺ with intermediate field ligands so that the high-spin and low-spin electronic configurations on the ion are comparable in energy, resulting in magnetic bistability. These materials have recently been predicted to show an intimate coupling between spin-state and electronic structure that is ideal for spintronic device applications [3].

We have used scanning tunneling microscopy and spectrscopy to study an Fe²⁺ spin crossover compound adsorbed on Au(111) in a unique two-layer thick film first described by Gopakumar et al. [4]. Surprisingly coexistence of different molecular spin states can be observed by STM imaging and local tunneling spectroscopy even at room temperature, in dramatic contrast to the abrupt bulk spin crossover behavior in bulk solids [5]. This departure originates from unique packing constraints in the bilayer structure that apparently do not allow complete conversion to the lower density high-spin state even at room temperature. This effect is similar to the constraint imposed on isolated spin crossover adsorbates directly bound to a metal surface [6].

Different spin-states can be identified by different STM imaging appearance (Figure 1a) as well as significantly different electronic structures (Figure 1b). It is the later that motivates interest in these materials. The smaller HOMO-LUMO gap we [5] and others [4,6] have observed for high- spin compared to low-spin molecules give this spin state a larger conductance and suggest possibilities for spin-dependent charge transport [3,7].

Figure 1. a) STM image of a mixed high-spin and low-spin bilayer of Fe[(H₂Bpz₂)₂bpy] on Au(111) at 130 K; b) Local spectroscopy over different features in the bilayer showing the reduction in HOMO-LUMO gap for the HS species.

Local conductance mapping has also allowed us to visualize spatial correlations of spin-state in "like-spin domains" [8]. An example is shown in Figure 2 where a conductance map at a sample bias 1 V below the Fermi level shows meandering domains of high spin species that tend to aggregate together. These patterns are reminiscent of "like-spin" and elastic domains arising in simulations of spin transitions [8].

Figure 2. left insets: Simulated STM images using the Tersoff-Hamman approach of High-spin and low-spin isomers near the HOMO; The image on the right is a local conductance map showing the aggregation of "like-spin" molecules into meandering domains.

*This work was funded by the NSF Phase I Center for Chemical Innovation: Center for Molecular Spintronics (CHE-0943975).

References

[1] Sanvito, S. Chem. Soc. Rev. 2011, 40, 3336-3355.

[2] Gutlich, P.; Garcia, Y.; Goodwin, H. A. Chem. Soc. Rev. 2000, 29, 419-427.

[3] Baadji, N.; Sanvito, S. Phys. Rev. Lett. 2012, 108, 217201.

[4] Gopakumar et al., Angew. Chem., Int. Ed. 51, 6262 (2012).

[5] Pronschinske et al., Nano Lett. 13, 1429 (2013).

[6] Miyamachi et al., Nat. Commun. 3, 938 (2012).

[7] Calzolari et al., J. Phys. Chem. B 116, 13141 (2012).

[8] Nicolazzi et al, Phys. Rev. B 85, 094101 (2012).