Charge-switchable molecular magnet and spin blockade of tunneling C. Romeike, 1 M. R. Wegewijs, 1 M. Ruben, 2 W. Wenzel, 2 and H. Schoeller 1 1 Institut für Theoretische Physik A, RWTH Aachen, 52056 Aachen, Germany 2 Institut für Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany Received 10 December 2004; revised manuscript received 5 July 2006; published 5 February 2007 We analyze a model for a metal-organic complex with redox orbitals centered at both the constituent metal ions and ligands. We focus on the case where electrons added to the molecule go onto the ligands and the charge fluctuations on the metal ions remain small due to the relatively strong Coulomb repulsion. Importantly, if a nonzero spin is present on each metal ion it couples to the intramolecular transfer of the excess electrons between ligand orbitals. We find that around special electron fillings, addition of a single electron switches the total spin S tot = 0 to the maximal value supported by electrons added to the ligands, S tot =3/2 or even S tot =7/2 for metal ions with spin 1/2. This charge sensitivity of the molecular spin is a strong correlation effect due to the Nagaoka mechanism. Fingerprints of the maximal spin states, as either ground states or low-lying excitations, can be experimentally observed in transport spectroscopy as spin blockade at low bias voltage and negative differential conductance and complete current suppression at finite bias, respectively. DOI: 10.1103/PhysRevB.75.064404 PACS numbers: 75.50.Xx, 73.23.Hk, 31.25.Qm, 71.10.Fd I. INTRODUCTION Recent experiments on metal-organic grid complexes, consisting of rationally designed ligands and metal ions as building units, have exhibited interesting electrochemical, 1–3 magnetic, 4–7 and transport properties. 8 By self-assembly the metal ions and ligands arrange in a rigid, highly symmetric grid see Fig. 1. Due to their different nature, electron orbit- als can often be roughly attributed to either the metal ions or the ligands. Such a separation has been used successfully to describe the low-temperature intramolecular spin coupling of Co-2  2 grids 9,10 and Mn-3  3 grids 5–7 for a fixed charge state as well as the electrochemical properties of Mn,Fe,Co,Zn-2  2Refs. 11–13 and Mn-3  3 grids 2 and scanning tunneling microscopy STM measurements on a Co-2  2 grid. 8 For such complexes it is well known 14 that both the pyridine ligands as well as the metal ions can be reduced. Which type of redox site is preferred depends on chemical details which can be controlled, mainly by substi- tution of metal ions and modification of the ligand. 11 This raises the interesting question of whether magnetic states can be associated with extra electrons added to the ligands. An- other question concerns the effect of the motion of the excess electrons 15 on different equivalent redox sites. Finally, the observation of high-spin states in three-terminal transport ex- periments on single molecules 16–25 is of interest. In such a setup, the number of electrons on the molecule can be con- trolled electrostatically with a gate voltage, which opens up the possibility of single-molecule spin switching. In addition, the bias voltage induces a current which is sensitive to the spin. Here we analyze a phenomenological low-temperature model for a 2  2 grid molecule consisting of four ligands “holding” either four metal zero-spin ions Fe grid or four spin-1 / 2 ions Co grid. This is sketched in Fig. 2. We show that i the molecular spin can be highly sensitive to the charge added to the ligands and can therefore be switched electrically, ii if open-shell metal ions are present between the ligands their spin degrees of freedom may also be switched, and iii the spin splitting gives rise to clear fin- gerprints in tunneling spectroscopy due to spin blockade physics. For the particular geometry and connectivity of the redox orbitals in a 2  2 grid, the Nagaoka mechanism 26 becomes effective, but only for special numbers of added electrons. Due to the strong electrostatic interaction the de- localization of an extra hole or electron relative to half fill- ing favors a fully polarized background of all other elec- trons. This may dominate over the antiferromagnetic superexchange processes. In the context of band magnetism the relevance of the Nagaoka mechanism is limited due to its lattice-type dependence 27 and its strong charge sensitivity. Only for a single additional electron or hole relative to a half-filled band can the spin-polarization effect be guaran- teed. In single-molecule devices, however, the strong charge sensitivity is of great interest, since the issues of the control of the charge and the geometry can be overcome. First, the advanced rational design of supramolecular structures allows complex “lattice” types to be realized. 1,28 Second, due to the energy and charge quantization one can modulate the total charge of a molecule by a single electron. 16–25 FIG. 1. Color online Structure of the 2  2-grid-type complex. PHYSICAL REVIEW B 75, 064404 2007 1098-0121/2007/756/0644048 ©2007 The American Physical Society 064404-1