Voltage-Induced Switching Dynamics of a Coupled Spin Pair in a Molecular Junction

Abstract

Molecular spintronics is made possible by the coupling between electronic configuration and magnetic polarization of the molecules. For control and application of the individual molecular states it is necessary to both read and write their spin states. Conventionally, this is achieved by means of external magnetic fields or ferromagnetic contacts, which may change the intentional spin state and may present additional challenges when downsizing devices. Here, we predict that coupling magnetic molecules together opens up possibilities for all electrical control of both the molecular spin states as well as the current flow through the system. Tuning between the regimes of ferromagnetic and anti-ferromagnetic exchange interaction, the current can be, at least, an order of magnitude enhanced or reduced. The effect is susceptible to the tunnel coupling and molecular level alignment which can be used to achieve current rectification. Molecular spintronics is a field which aims to merge the flexibility of synthetic design of molecular compounds with novel functionalities offered by magnetic properties in conjunction with electronics circuits [1]. Magnetically active molecules have been used to demonstrate spin valve effect using external magnetic fields [2], stochastic switching between high and low conductive states by transitions between spin singlet and triplet ground states [3][4][5][6], controlled transport properties via paramagnetic atoms [7], as well as their potential for quantum based computation [8][9][10][11][12][13]. Arrays of magnetic molecules inserted between conducting leads, moreover, provide an important forum to investigate fundamental magnetic properties of finite one-dimensional Ising or Heisenberg chains [14][15][16] as well as potential for electrical and thermal control of the magnetic state. Certain classes of molecules, e.g., metal-phthalocyanines (MPc) and metal-porhyrins (MP) present chemical stability with specific optical and electrical properties make them highly appreciated for technological applications including organic field effect transistors [17,18], light emitting devices [19,20] and photovoltaic cells [21], and for fundamental studies [7,[22][23][24][25][26][27].While incorporation of magnetic elements in molecular compounds can have a significant effect on the overall molecular transport properties [7], the main established route to spintronics manipulations entails external magnetic fields [2] or ferromagnetic electrodes [28,29], often exploiting spin transfer torques from spin-polarized scattering [30] or Coulomb interaction [31]. Here, we propose a different route to molecular spintronics based on voltage induced control of magnetic interactions that allows for all electrical control of the transport properties. Deriving from local exchange interactions between the localized spin moments and the electrons in paramagnetic molecules, an indirect effective spinspin interaction is generated between the molecular spin moments through the electron tunneling between the molecules [32...

Charge Transport and Entropy Production Rate in Magnetically Active Molecular Dimer

“…This prediction is based on the (electronically mediated) indirect exchange interaction between the localized spins which controls the charge transport properties [51,52]. Similar effects were reported in Ref.…”

“…In such set-ups, the effective magnetic interaction parameter J between the two spins can be calculated using the expression, see Refs. [51,53],…”

“…For the details about the interdependence between the electronic structure in the molecular orbitals and the spin-spin exchange we refer to Refs. [51,53]. The temperatures of the leads are included in the respective Fermi function on which the exchange interaction within the spin dimer depends, see Eq.…”

“…Theoretical predictions suggest all electrical control for both reading and writing spin states in molecular dimers * Electronic address: Jonas.Fransson@physics.uu.se [51,52]. This prediction is based on the (electronically mediated) indirect exchange interaction between the localized spins which controls the charge transport properties [51,52].…”

“…[51,53] for dimers of magnetic molecules in which the effective spin-spin interactions are mediated by the properties of the delocalized electrons and extend to thermally induced magnetic and transport properties. In particular we study thermal transport and its response to changes of the magnetic configurations.…”