Single-electron transport and magnetic properties of $\mathrm{Fe}\text{−}\mathrm{Si}{\mathrm{O}}_2$ nanocomposites prepared by ion implantation

The electric transport, magnetic, and magnetotransport properties of $\mathrm{Fe}\text{−}\mathrm{Si}{\mathrm{O}}_2$ nanocomposites prepared by Fe-ion implantation into silica were investigated. The structural studies revealed bcc Fe nanoparticles of an average size of $3\mathrm{nm}$ dispersed in a $100\text{−}\mathrm{nm}$-thick nanocomposite layer formed within the silica substrate. Using special thin-film electrodes that were only $100\mathrm{nm}$ apart, in-plane electrical measurements were performed in a temperature range of $4–300\mathrm{K}$. Though no external gate electrode was used, single-electron transport phenomena (Coulomb blockade and Coulomb staircase) were observed at $4\mathrm{K}$. The presence of Coulomb steps in $I\text{−}V$ curves implies that the electric transport was realized by the tunneling of electrons via a random quasi-one-dimensional chain of a few isolated iron nanoparticles. The magnetic properties of the nanoparticles were determined by surface effects and by the superparamagnetic behavior. The nanoparticles exhibited enhanced anisotropy and were dipolarly interacting. However, the tunneling current was found to be independent of external magnetic field; i.e., no tunneling magnetoresistivity (TMR) was measured at $77\mathrm{K}$. At this temperature the nanoparticles were superparamagnetic. Presumably, a low volumetric concentration of Fe nanoparticles $(<14\%)$ and a spin-flip process due to residual single Fe atoms present in the silica barriers were responsible for the absence of the TMR effect.

Figure 1

(a) Cross-sectional bright-field TEM micrograph of the $\mathrm{Fe}\text{−}\mathrm{Si}{\mathrm{O}}_2$ composite, (b) corresponding particle-size distribution, (c) SAED pattern with indices corresponding to the bcc structure of Fe, and (d) high-resolution TEM micrograph of the $\mathrm{Fe}\text{−}\mathrm{Si}{\mathrm{O}}_2$ sample.

Figure 2

(Color online) (a) SEM view of the four-probe pattern, (b) sketch of the single pattern showing the mesa structure, before opening the top gold layer by electromigration, and (c) the gold strip open circuited by electromigration and the corresponding temperature profile along the stripe.

Figure 3

Time dependence of the composite resistivity determined from $I\text{−}V$ curves as $dI∕dV$ against bias voltage plots at $V=0$. In some cases a parallel magnetic field of $1.2\mathrm{T}$ was applied.

Figure 4

(a) A typical nonlinear $I\text{−}V$ curve measured at room temperature and (b) some samples exhibiting a rectifying behavior (solid circles) that disappeared after several voltage sweeps (open triangles).

Figure 5

(a) Steps in the $I\text{−}V$ curve measured at $4.2\mathrm{K}$ and (b) detail of the same curve at negative bias.

Figure 6

Schematic illustration of a double junction governing the transport in a random quasi-1D chain of several Fe nanograins.

Figure 7

Differential conductivity plotted against bias voltage depicts more clearly the steps in the $I\text{−}V$ curve shown in Fig. 5a.

Figure 8

(Color online) Hysteresis loops measured at $4\mathrm{K}$ (solid square symbols) and $77\mathrm{K}$ (circle symbols). Inset: detail of the loops in the low-magnetic-field region.

Figure 9

Temperature dependence of the ZFC and FC magnetization measurements.