Molecular quantum spintronics: supramolecular spin valves based on single-molecule magnets and carbon nanotubes

Abstract

We built new hybrid devices consisting of chemical vapor deposition (CVD) grown carbon nanotube (CNT) transistors, decorated with TbPc(2) (Pc = phthalocyanine) rare-earth based single-molecule magnets (SMMs). The drafting was achieved by tailoring supramolecular π-π interactions between CNTs and SMMs. The magnetoresistance hysteresis loop measurements revealed steep steps, which we can relate to the magnetization reversal of individual SMMs. Indeed, we established that the electronic transport properties of these devices depend strongly on the relative magnetization orientations of the grafted SMMs. The SMMs are playing the role of localized spin polarizer and analyzer on the CNT electronic conducting channel. As a result, we measured magneto-resistance ratios up to several hundred percent. We used this spin valve effect to confirm the strong uniaxial anisotropy and the superparamagnetic blocking temperature (T(B) ~ 1 K) of isolated TbPc(2) SMMs. For the first time, the strength of exchange interaction between the different SMMs of the molecular spin valve geometry could be determined. Our results introduce a new design for operable molecular spintronic devices using the quantum effects of individual SMMs.

Keywords: molecular magnets; molecular quantum spintronics; nanoelectronics devices.

Figure 1

Magnetization reversal mechanisms in the TbPc₂ SMM (alkyl and pyrene substituents are omitted for reasons of clarity). (a) Scheme of the TbPc₂-SMM. The terbium ion has a J = 6 magnetic moment and an unpaired electron is delocalized over the organic part; (b) Zeeman diagram calculated for the TbPc₂ SMM ground state (J = 6, |J_z| = 6). The interaction with the Tb nucleus spin I = 3/2 splits each electronic substate through the hyperfine coupling, providing a path for Quantum Tunneling of Magnetization (QTM) at the anti-crossing of two levels; (c) Hysteresis loops of the crystallized TbPc₂-SMM (2% in the YPc₂ matrix) measured at 40 mK for different sweeping rates ranging from 1 to 280 mT.s⁻¹. QTM reflects in staircase-like steps of the hysteresis loops at low magnetic fields, each step corresponding to a level anti-crossing. Molecules, which did not undergo QTM can relax their magnetization to a lower energy state by the direct transition (DT) occurring at larger magnetic fields.

Figure 2

Electronic transport in carbon nanotube quantum dots with grafted TbPc₂ molecules. (a) Artist view of the device scheme, consisting of an electrically connected carbon nanotube junction, laterally coupled to isolated TbPc₂-SMMs; (b) Color-scale plots of the differential conductance dI/dV at temperature T = 40 mK, as a function of source-drain voltage V_sd and back-gate voltage V_g, displaying the charge stability diagram in the Coulomb blockade regime. The typical charging energy is about 20 meV; (c–d) Enlarged views of (b), showing the charge degeneracy point around V_g = 1.79 V at constant static magnetic fields (c) μ₀H = 0 T and (d) μ₀H = 1 T.

Figure 3

Conductance hysteresis loops of the supramolecular spin valve. (a–b) Differential conductance dI/dV measured at T = 40 mK as a function of in-plane magnetic field μ₀H applied respectively along (a) the easy axis direction (0°); and (b) the hard direction (90°) of magnetization. The red and blue arrows indicate the magnetic field sweep directions; (c) Color-scale plot of the dI/dV hysteresis (obtained from the difference between both field sweep directions) as a function of the applied magnetic field angle. The white color code is associated to zero hysteresis (reproducible dI/dV curves); (d) Relative disposition of the molecule with respect to the nanotube. The magnetic hard axis is 30° tilted from the nanotube axis.

Figure 4

Bias and temperature dependences of the conductance hysteresis loops: (a) Color scale map of the dI/dV hysteresis as a function of in-plane magnetic field and source drain voltage V_ds. The magnetic hysteresis are suppressed above V_ds = ± 1 mV; (b) 15 records of conductance hysteresis loops for several temperatures ranging from 0.04 to 1 K at a constant sweep rate of 50 mT/s. The curves for T > 40 mK are offset by a multiple of 200 nS for clarity.

Figure 4

Bias and temperature dependences of the conductance hysteresis loops: (a) Color scale map of the dI/dV hysteresis as a function of in-plane magnetic field and source drain voltage V_ds. The magnetic hysteresis are suppressed above V_ds = ± 1 mV; (b) 15 records of conductance hysteresis loops for several temperatures ranging from 0.04 to 1 K at a constant sweep rate of 50 mT/s. The curves for T > 40 mK are offset by a multiple of 200 nS for clarity.

Figure 5

(a) Scheme of the localized dots induced by hybridization between the molecules and the nanotube; (b) Both molecules are polarized in the same manner. It induces a Zeeman splitting identical for both sets of localized states; (c) In the antiparallel configuration the Zeeman splitting is inhomogeneous, preventing spin transport through the device, unless a bias higher than the exchange interaction is applied.