(a) Electrical resistivity as a function of temperature for sample B. The inset shows the dependence of ln ρ on T −1/2; the solid line represents the linear fit result. (b) Illustrations of the theoretical fits of conductivity as a function of temperature for sample B obtained from Equations 1 and 2. (c) Electrical resistivity as a function of temperature for sample C.

(d) Conductivity as a function of temperature for sample C; dotted line is the fitting curve obtained from Equation 2. (e) GDC973 Electrical resistivity as a function of temperature for sample A. (f) Electrical resistivity vs logarithmic temperature for sample A. Figure 5c shows the temperature dependence of the resistivity of sample C located in the selleckchem hopping regime. At low temperatures, an almost temperature-independent tunneling regime is observed. The direct tunneling may represent an important contribution to the total conductance at low temperature, see more which is similar to the result reported by de Moraes et al. [29]. Figure 5d shows the temperature dependence of the conductivity of sample C and the curve

fitted by Equation 2. It is obvious that not only the second-order hopping (γ = 1.33) but also the third-order hopping (γ = 2.5) and fourth-order hopping (γ = 3.6) evidently become non-negligible because a thicker ZnO barrier results in spin-independent higher-order inelastic hopping (see Figure 3c). In order to compare the fitting results of the tunneling and hopping regimes, the resulting parameters fitted by Equation 2 for samples B and C are given in Table 1. It can be seen that the number of localized states of sample C (N = 4) increases as compared to sample B (N = 2). Consequently, a much higher-order hopping gradually prevails during the transition from the tunneling regime to the hopping regime, which apparently suppresses the MR effect at RT (shown in Figure 1). Also, the tunneling activation energy PTK6 (E) estimated from Δ is 1.64 meV for sample B. With the ZnO content increasing, the value appreciably increases to 44.3 meV due to smaller Co particles and thicker ZnO barriers between Co particles, which consists with the decrease of MR effect in the hopping regime with

more defects. Table 1 Fitting results and mainly transport mechanism of three samples   Sample 1 Sample 2 Sample 3 Applied model Equation 2 Equation 2 Linear fit N 2 4 – G 0 (S · cm−1) 219.1 31.2 – C 1 (S · cm−1 · K−1.33) 3.1 × 10−2 8.2 × 10−3 – C 2 (S · cm−1 · K−2.5) – 4.0 × 10−4 – C 3 (S · cm−1 · K−3.6) – 6.1 × 10−8 – ∆ (K) 104.7 2,832.4 – E (meV) 1.64 44.35 – Straight slope (μΩ · cm/log(K)) – - −849.1 Mainly transport Tunneling Hopping Metallic paths The temperature dependence of conductivity of samples B and C are fitted by Equation 2, as shown in Figure 5b,d. The relationship between resistivity and ln T for sample A is fitted linear in Figure 5f. For sample A, the resistivity as a function of temperature is shown in Figure 5e.