Nonvolatile Voltage Controlled Molecular Spin-State Switching for Memory Applications

Abstract

Nonvolatile, molecular multiferroic devices have now been demonstrated, but it is worth giving some consideration to the issue of whether such devices could be a competitive alternative for solid-state nonvolatile memory. For the Fe (II) spin crossover complex [Fe{H2B(pz)2}2(bipy)], where pz = tris(pyrazol-1-yl)-borohydride and bipy = 2,2′-bipyridine, voltage-controlled isothermal changes in the electronic structure and spin state have been demonstrated and are accompanied by changes in conductance. Higher conductance is seen with [Fe{H2B(pz)2}2(bipy)] in the high spin state, while lower conductance occurs for the low spin state. Plausibly, there is the potential here for low-cost molecular solid-state memory because the essential molecular thin films are easily fabricated. However, successful device fabrication does not mean a device that has a practical value. Here, we discuss the progress and challenges yet facing the fabrication of molecular multiferroic devices, which could be considered competitive to silicon.

Dynamics of Spin Crossover Molecular Complexes

“…The limited fraction in the HS state at short time scales and the long time scales for much higher fractions transitioning to the HS state is a concern for device integration considerations [ 8 ]. This may be addressed by size scaling as discussed next…”

“…The capability to fabricate SCO materials in a small ensemble of molecules (nanoparticles) has opened the way to use the SCO phenomena in more applications as this may mean faster “write” speeds as discussed below. This could be important to many applications [ 8 ], and we suggest that faster write speeds may be accessible. In the previous section, we summarized the current understanding of the spin state switching dynamics in solids, where the equilibrium photo-induced HS state can proceed through three processes (photo, elastic, and thermal switching) with various time scales (ps, ns, and μs, respectively).…”

“…Transition metal-based spin crossover (SCO) molecular complexes have attracted much interest due to their remarkable bistable properties and potential applications [ 1 , 2 , 3 , 4 ], in particular for molecular memory devices [ 5 , 6 , 7 , 8 ]. A better understanding of the mechanisms responsible for their change of spin state is motivated by the ultimate goal of investigating charge transfer through a single molecule junction [ 9 , 10 , 11 , 12 , 13 ].…”

“…In the context of device applications, the speed of the transition between the molecular magnetic states is a key parameter for a figure of merit [ 8 ]. In characterizing switching of the molecular spin state, the intrinsic switching speed of spin crossover complexes is of paramount importance to establishing the limits to the write delay time in molecular devices.…”

“…This spin crossover phenomenon can be observed for transition metal ions in the first transition series with d 4 to d 7 configurations [ 3 , 30 ]. The change in the electronic configuration of the molecular complex, for each electronic spin state resulting from the SCO phenomena, can drastically modify the physical properties like the molecular structure, the optical properties, the magnetic susceptibility, the volume [ 1 , 30 , 31 , 32 , 38 , 39 ] and conductivity [ 8 ]. For SCO molecular complexes with transition metal ions with d 6 electronic configuration, the spin state transition is between a low spin (LS, , 1 A 1 ) diamagnetic state (ground state) of the central metal ion to high spin (HS, , 5 T 2 ) paramagnetic state (excited state).…”