Building the Molecular Multiferroic

By Guanhua Hao

Background.

I want to prove that a molecular system can be multiferroic and I want to show I can make a nonvolatile, low power molecular device that has the potential to be scalable to industry. To succeed, I need the design parameters for making the “better” heteromolecular multiferroic molecular device. My approach to prove that a molecular thin film can be an easily manipulated multiferroic system is to characterize spin crossover (SCO) molecular films under various field and temperature conditions. Such molecules exhibit a transition between a low spin (LS) diamagnetic state of the metal ion to a high-spin (HS) paramagnetic state, and can, under some circumstances, exhibit ultiferroic character, as I have proven (1). By combining novel molecular spin crossover systems with molecular ferroelectrics (1), I have fabricated molecular voltage-controlled multiferroic devices. Yet the chemistry and intermolecular cooperative effects, that will determine spin state switching energetics (ideally, I want the devices to be very low power), is far from understood. Our prior work has validated that the spin state of at least one molecular spin crossover complex ([Fe(H2B(pz)2)2(bipy)]) can be voltage controlled, essentially though interactions at a heteromolecular interface (1), and we are now poised to develop a better understanding of interfacial chemistry on the molecular spin crossover transition. My research will focus on some of the ultimate “nano-challenges”: develop the understanding of the fundamental applicable physics and chemistry, so that better nonvolatile molecular multiferroic devices, with giant magneto-electric effects, can be built and fabricated “by design”.

Improve our understanding of the energy barriers to spin state switching.

For widespread applications, one key aspect of a molecular switches, with wider relevence, is non-volatile control as well as a big response to small voltages. This requires bistability of spin states in spin crossover molecular complexes. The challenge is to engineer a proper enthalpy difference between two spin states. The goal is to have the spin crossover transitions around room temperature and generate sizable hysteresis between the spin states, so as to have systems more suitable for molecular device operations at room temperature. Using interfaces, it is possible to modify the energy barrier between spin states and to tune the width of the hysteresis loop (2).

But thermal energy scale activation energies enter the spin crossover transition in other ways. Although we have conclusively demonstrated that heteromolecular spin crossover systems, as well as molecular spin crossover thin films on dielectric substrates, can be switched by X-rays (3), there are also thermal energy barriers in these systems. We need to better understand the origins of the various energy barriers and the temperature dependence to the voltage control of molecular spin state.

Insight into the intermolecular interactions.

The key to engineering molecular spin crossover devices is to understand the precise nature of the intermolecular interaction that produces such a wide range of interactions. With changing temperature, we have seen new phenomena like re-entrant spin crossover transitions (spin state changes from low spin to high spin then back to low spin with increasing temperature) from combining [Fe(H2B(pz)2)2(bipy)] with benzimidazole, “locking” in one spin state and no effect whatsoever from the various dipolar molecular additions. To truly understand and manipulate these important effects, we need insight into the local interactions.

 

  1. G. Hao et al., Appl. Phys. Lett. 114 (2019) 032901.
  2. X. Jiang et al., J. Phys.: Condens. Matter 31 (2019) 315401.
  3. X. Zhang et al., Adv. Mater. 29 (2017) 1702257.
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