Transport through anisotropic magnetic molecules with partially ferromagnetic leads: Spin-charge conversion and negative differential conductance

Florian Elste and Carsten Timm

Phys. Rev. B 73, 235305 – Published 6 June 2006

Abstract

We theoretically investigate inelastic transport through anisotropic magnetic molecules weakly coupled to one ferromagnetic and one nonmagnetic lead. We find that the current is suppressed over wide voltage ranges due to spin blockade. In this system, spin blockade is associated with successive spin flips of the molecular spin and depends on the anisotropy energy barrier. This leads to the appearance of a window of bias voltages between the Coulomb blockade and spin blockade regimes where the current is large and to negative differential conductance. Remarkably, negative differential conductance is also present close to room temperature. Spin-blockade behavior is accompanied by super-Poissonian shot noise, such as in nonmagnetic quantum dots. Finally, we show that the charge transmitted through the molecule between initial preparation in a certain spin state and infinite time strongly depends on the initial spin state in certain parameter ranges. Thus the molecule can act as a spin-charge converter, an effect potentially useful as a read-out mechanism for molecular spintronics.

Received 16 January 2006

DOI:https://doi.org/10.1103/PhysRevB.73.235305

Authors & Affiliations

Florian Elste^1,* and Carsten Timm²

¹Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
²Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA

^*Electronic address: felste@physik.fu-berlin.de

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Vol. 73, Iss. 23 — 15 June 2006

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Images

Figure 1
(Color online) Two-dimensional plots of the differential conductance $d I ∕ d V$ in the vicinity of one particular degeneracy point at low temperatures. (a) $d I ∕ d V$ for Zeeman splitting $B = 0.05 eV$ . (b) $d I ∕ d V$ for vanishing magnetic field. The model parameters are $J = K_{2} = 5 meV$ . When the magnetic field is switched on, the CB regime and the SB regime are separated by a finite window of bias voltage for which the conductance is high. Since it is low (governed by $D_{↓}^{R} ∕ D_{↑}^{R}$ ) in the SB regime, NDC must occur where the conductance drops again. (c) Scheme of the molecular energy levels with all allowed transitions involving the states with $n = 0$ and $n = 1$ electrons in an external magnetic field. In the SB regime all transitions are energetically possible if the bias voltage $V$ is large enough. However, the rates for a spin-down electrons tunneling from the molecule into the right lead (curved arrows with slash) are strongly suppressed due to the small density of states.Reuse & Permissions
Figure 2
(Color online) (a) Current-voltage characteristics in the vicinity of the CB threshold assuming $K_{2} = J = B = 0.05 eV$ . The inset shows the current for $T = 0.02 eV$ over a broader voltage range. (b) Energy level scheme for the $n = 0$ and $n = 1$ spin multiplets as a function of the magnetic quantum number $m$ . The bias voltage is just high enough to allow the transition between the many-particle states with spin $m = S$ and $m = S + 1 ∕ 2$ . The steady-state current is highly spin polarized, since the tunneling of spin-down electrons is thermally suppressed. This regime corresponds to the plateau with enhanced current in (a).Reuse & Permissions
Figure 3
(Color online) Temperature dependence of the current-voltage characteristics at high temperatures for vanishing magnetic field, exhibiting NDC. In (a) we assume $K_{2} = J = 5 meV$ , corresponding to the spin multiplets schematically shown in Figs. 1, 2 except that now $B = 0$ . In (b) we assume a large exchange interaction $J = 0.1 eV$ and vanishing magnetic anisotropy $K_{2} = 0$ . (c) Energy level scheme for the $n = 0$ and $n = 1$ multiplets as a function of the magnetic quantum number $m$ for the case in (b).Reuse & Permissions
Figure 4
(Color online) Current noise spectrum. Shown are the normalized correlation functions $S_{LL} (ω) ∕ S_{LL} (ω = \infty)$ for three different bias voltages representing the three transport regimes: $V = 1.8 V$ (CB), $V = 2.2 V$ (SB), and $V = 7.0 V$ (conducting regime). The zero-frequency limit gives the Fano factor $F = S (ω \to 0) ∕ 2 e ∣ I ∣$ which tends to 3 in the SB regime, if we assume the one lead to be half metallic, $D_{↓}^{R} ∕ D_{↑}^{R} \to 0$ .Reuse & Permissions
Figure 5
(Color online) (a) Excess transmitted charge $Δ Q^{L}$ as a function of bias voltage in the vicinity of the first CB step for different polarizations of the right lead: $D_{↓}^{R} ∕ D_{↑}^{R} = 0.5$ , 0.3, 0.1, and 0.01. We assume $J = 4 meV$ , $K_{2} = 1 meV$ , vanishing magnetic field and an initial state with $n = 0$ and $m = 2$ . The steady-state current for $D_{↓}^{R} ∕ D_{↑}^{R} = 0.1$ is also shown. The inset shows $Δ Q^{L}$ as a function of the anisotropy constant $K_{2}$ , where the bias voltage $V$ corresponds to the arithmetic mean of the first two excitation energies. (b) Energy level scheme of the $n = 0$ and $n = 1$ multiplets. The molecule is prepared in the initial state with spin $m = S$ .Reuse & Permissions

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