Effect of cotunneling and spin polarization on the large tunneling magnetoresistance effect in granular ${\mathrm{C}}_{60}$-Co films

The tunneling magnetoresistance (TMR) effect of granular ${\mathrm{C}}_{60}$-Co films at low temperatures and also at small applied voltages is studied in the current-perpendicular-to-plane (CPP) geometry to elucidate the spin-dependent tunneling process, bringing about a remarkably high magnetoresistance (MR) as compared to the expectations from the conventional theory on sequential tunneling. The current-voltage characteristics showed ohmic and power-law dependences in the Coulomb blockade regime, which are interpreted as the occurrence of cooperative tunneling (so-called cotunneling) through a few to several Co nanoparticles. The zero-bias MR ratios are in the range of 50%–90% at a few degrees Kelvin and show strong and unconventional temperature dependence depending on the temperature range. Furthermore, the spin polarization of tunneling electrons evaluated based on the cotunneling model is in the range of 50%–80%, suggesting that the enhanced spin polarization of tunneling electrons at the interface between Co nanoparticles and a ${\mathrm{C}}_{60}$-based matrix (C${}_{60}$-Co compound) is crucial for large TMR effects.

Figure 1

(a) Schematic drawing of the device structure of the CPP sample and (b) optical microscope image of the cross area of the top and bottom Ag electrodes with perpendicular orientation.

Figure 2

MR-$H$ curves of the ${\mathrm{C}}_{60}$Co${}_6$ sample, measured at 4.2 K. The data were obtained under the applied voltages of $V$ = 15, 50, and 100 mV, respectively. The inset shows the enlargement of the low-field region for the curve at $V$ = 50 mV.

Figure 3

(a) I-V and (b) MR-$V$ characteristics of the ${\mathrm{C}}_{60}$Co${}_6$ sample, measured at temperatures of 3.5, 6, 10, and 15 K. The solid lines in (a) are obtained by fitting with the linear ($I\propto V$) and power-law ($I\propto V^{\mathrm{\alpha }}$) relationships at lower and higher voltages, respectively. Numbers in italics indicate the exponents $a$ obtained from the power-law fits. The solid lines in (b) are fitted using Eq. (2).

Figure 4

Electrical resistivity ρ as a function of $T^{-1/2}$. The data are shown for the ${\mathrm{C}}_{60}$Co${}_6$, ${\mathrm{C}}_{60}$Co${}_7$, and ${\mathrm{C}}_{60}$Co${}_8$ samples (circles, triangles, and squares). The solid lines show the fits with the ($\rho \propto T_0$/$T$)${}^{-1/2}$ relationship for the respective samples.

Figure 5

Logarithmic plot of MR${}_0$ as a function of temperature. The data show the values obtained at $H$ = 50 kOe (C${}_{60}$Co${}_7$) and 70 kOe (C${}_{60}$Co${}_6$ and ${\mathrm{C}}_{60}$Co${}_8$). The symbols represent the same as in Fig. 4.

Figure 6

Cotunneling order $j$ as a function of $T$. The symbols represent the same as in Fig. 4. The solid lines show fits with Eq. (3) for the respective samples.

Figure 7

Temperature dependence of the spin polarization $P$ multiplied by $m$. The symbols represent the same as in Fig. 4. The fitted results with Eq. (5) are represented by the solid lines for the respective samples.