Ferromagnetic electrodes for studies of spin-dependent transport in quantum dots and single molecules
We have fabricated devices consisting of magnetic permalloy (Ni81Fe19) electrodes that allow for the study of spin-dependent transport through quantum dots. Using these devices, we seek to understand how the conductance through a quantum dot is modified as we vary the source-drain and gate voltages as well as the relative magnetizations of the electrodes. We are also working towards using spin-transfer-driven ferromagnetic resonance to study the dynamical properties of spins in molecules and in metal nanoparticles.
Joshua Parks and Dan Ralph
Recent theory predicts a wide array of phenomena that arise from the interplay between spin-dependent transport and single-electron physics in nanometer-scale quantum-dot systems . However, due to the technical challenges involved in coupling a quantum dot to ferromagnetic leads, the number of experimental studies to date has been rather limited [2-4]. We are working to develop strategies for reliably contacting molecules and nanoparticles to ferromagnetic electrodes with well-defined properties, so as to study the tunneling magnetoresistance (TMR) associated with different transport mechanisms and to also measure spin dynamics. The improved understanding to be gained from this research has direct relevance for magnetic memory applications, as the small size of nanoparticles and molecules may enable spin transfer to excite switching and steady-state dynamics at much smaller currents.
Towards this end, we have extended the techniques of Ref.  and have fabricated magnetic devices on top of an oxidized aluminum gate using aligned steps of electron-beam lithography. The devices consist of 32 nm-thick permalloy electrodes bridged by a short 100 nm-wide region, and are fabricated on top of 16 nm-thick gold pads (Fig. 1). These pads in turn are connected to larger gold leads, which can be contacted to DC and high-frequency measurement sources. The shapes of the magnetic electrodes were designed based on micromagnetic modeling, which showed that ellipses promote shape anisotropy and allow the magnetic moments of the electrodes to be manipulated controllably between parallel and antiparallel alignment. In figure 2, we show measurements of the switching characteristics of electrodes in which a nanoscale gap was created by electromigration. Clear regions of high and low resistance suggest the formation of well-defined parallel and antiparallel states.
We are at the initial stages of developing devices that contain magnetic FePt nanoparticles (Angang Dong, Chris Murray) or C60 molecules bridging the electrode gap. The molecules are incorporated by allowing them to adsorb on the electrode surface prior to electromigration, and the nanoparticles are incorporated by drop-casting or by assembly using thiolated ligands. The inset of figure 3 shows an example of a single FePt nanoparticle bridging the electrode gap. In the I-V curve for the device, we observe a Coloumb blockade characteristic (Fig. 3), which is a signature of single-electron tunneling. We seek to study how the current through such a device is modified as we change the relative magnetic moments of the electrodes, and how the TMR varies as a function of bias and gate voltages in different transport regimes. In addition, we anticipate that by applying an rf current, we can excite spin precession dynamics via the spin-transfer torque mechanism . We will seek to understand and control the high-frequency dynamics by varying the externally applied magnetic field, the local exchange field, and the applied current.
Figure 1: Scanning electron micrograph of elliptical permalloy (Ni81Fe19) electrodes contacted to Au pads, fabricated on an oxidized aluminum gate.
Figure 2: Magnetic switching characteristics of permalloy electrodes separated by a nanoscale gap created by electromigration (shown in the inset). The regions of low resistance correspond to parallel magnetic alignment of the electrodes, and regions of high resistance correspond to antiparallel alignment.
Figure 3: I-V characteristics of a device containing a 7 nm FePt nanoparticle (inset), showing a region of current suppression (Coulomb blockade).
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Last updated: 19-June-2010