Resistivity dependence of magnetoresistance in Co/ZnO films
© Quan et al.; licensee Springer. 2014
Received: 11 October 2013
Accepted: 5 December 2013
Published: 6 January 2014
We report the dependence of magnetoresistance effect on resistivity (ρ) in Co/ZnO films deposited by magnetron sputtering at different sputtering pressures with different ZnO contents. The magnitude of the resistivity reflects different carrier transport regimes ranging from metallic to hopping behaviors. Large room-temperature magnetoresistance greater than 8% is obtained in the resistivity range from 0.08 to 0.5 Ω · cm. The magnetoresistance value decreases markedly when the resistivity of the films is less than 0.08 Ω · cm or greater than 0.5 Ω · cm. When 0.08 Ω · cm < ρ < 0.5 Ω · cm, the conduction contains two channels: the spin-dependent tunneling channel and the spin-independent second-order hopping (N = 2). The former gives rise to a high room-temperature magnetoresistance effect. When ρ > 0.5 Ω · cm, the spin-independent higher-order hopping (N > 2) comes into play and decreases the tunneling magnetoresistance value. For the samples with ρ < 0.08 Ω · cm, reduced magnetoresistance is mainly ascribed to the formation of percolation paths through interconnected elongated metallic Co particles. This observation is significant for the improvement of room-temperature magnetoresistance value for future spintronic devices.
The investigation of electron spin transport from metallic ferromagnets to semiconductors has been an active research field in spintronics in the past two decades [1–3]. The manipulation of carrier spins between magnetic metals and semiconductors provides improved functionality of spintronic devices such as magnetic sensors, spin transistors, and magnetic memory cells [4, 5]. Spin injection into a semiconductor reveals low efficiency in ferromagnetic metal/semiconductor films at room temperature (RT) because of a significant mismatch in conductivities [6–8].
Recently, magnetic metal/semiconductor films have been considered for their large magnetoresistance (MR) at RT, which is responsible for effective spin injection into semiconductors [9–14]. However, the origin of MR and the different influential factors for the MR effect are controversial. Yan et al. reported a large negative MR of 11% at RT in Co/ZnO films, which was ascribed to spin-dependent variable range hopping . Hsu et al. observed transverse magnetotransport transition from a negative MR of 4.6% to the anomalous Hall effect at RT and found a variation with different annealing temperatures in a Co/ZnO film . In our previous publications, we obtained a larger RT MR ratio of approximately 12.3% in a Co/ZnAlO granular film that resulted from spin-dependent tunneling through semiconductor barriers and observed that the values of MR changed with the film thickness in Co/ZnO granular films [12, 13]. By contrast, Varalda et al. investigated Fe/ZnSe films consisting of Fe-clustered particles embedded in a ZnSe matrix and observed significant negative MR only at low temperature . These inconsistent results may likely be attributed to the fact that the MR effect of magnetic metal/semiconductor films is extremely sensitive to fabrication conditions resulting in varied microstructures and defects in semiconductors. However, up to now, few experiments have been performed for the systematic study to correlate these structural properties with magnetotransport. Besides, the mechanism of MR remains unclear. Thus, investigating this issue may help us better understand the physics involved and achieve a higher MR ratio at higher temperature for practical applications.
In this work, we studied a large number of Co/ZnO films deposited at different sputtering pressures with different ZnO thicknesses and found that the MR effect is strongly dependent on the resistivity of films. We further investigated the charge transport in these films and found that conduction can be separated into three regimes, namely metallic, tunneling, and hopping regimes, with different temperature dependence. We found that among the three regimes, only the tunneling part is strongly spin dependent. This leads to a broad maximum of MR in the tunneling regime. This finding is useful in the tuning of MR values and in understanding its mechanism.
Co/ZnO films were deposited by sequentially sputtering ultrathin Co layers and ZnO layers on glass substrates at RT. Direct-current and radio-frequency powers were applied to Co and ZnO targets, respectively. The sputtering chamber pressure was reduced to 8 × 10−5 Pa before deposition. The sputtering gas was an Ar atmosphere with a range of 0.4 to 0.8 Pa. The film nominal structure is [Co (0.6)/ZnO (x)]60 (denoted as Co/ZnO; thicknesses in nanometers), where x = 0.3 to 2.5 nm is the thickness of the ZnO layer. The details of the growth have been described in a previous publication .
The thickness of the films was measured by a surface profiler. The structures of the films were analyzed using X-ray diffraction (XRD). The magnetic properties of the films were measured using a superconducting quantum interference device magnetometer with a magnetic field applied parallel to the film plane. The magnetic field dependence of MR was measured using a conventional four-probe method in the maximum applied magnetic field of 20 kOe with current in the plane at RT. The temperature dependence of resistance was measured by four-point geometry from 5 to 300 K.
Results and discussion
From the above discussions, it can be concluded that the films of samples B and C contain Co nanoparticles with different particle sizes dispersed in the ZnO matrix, and some interconnected Co particles may exist in sample A. The plane-view schematic illustrations of the three samples are shown in Figure 3. The structural, magnetic, and transport measurements strongly suggest that the MR effect in these granular films should be related to the size and spatial distribution of Co particles. In the metallic regime, the value of MR decreases with decreasing resistivity probably because of the increase in the number of interconnected Co particles. When the resistivity is less than 0.004 Ω · cm, the value of MR is almost zero. Most Co particles connect with one another and provide few opportunities for spin-polarized electron tunneling. The MR ratio is also reduced as the resistivity in the hopping regime increases, but it still remains greater than 3.7% even when resistivity reaches 3.8 Ω · cm and the volume fraction of Co calculated according to the nominal structure of Co (0.6)/ZnO (2.0) is less than 24%. This observation can be ascribed to the relatively long spin-coherence length in our material [21, 22].
We turn to the spin polarization of electron in the films, which can be estimated roughly from the Inoue-Maekawa model as follows: MR = P2m2/(1 + P2m2) , where P is the spin polarization of the tunneling electrons, m is the relative magnetization of the film, and m2 = 〈 cos θ〉. m = 1 in the saturated state, and the above equation becomes MR = P2/(1 + P2). The RT spin polarization in the tunneling regime calculated from the MR value of 8.1% is approximately 30%, which is very close to the 35% of the bulk Co metal determined by tunneling . This large RT spin polarization indicates that the transport of polarized carriers in the semiconductor ZnO is very efficient in our films.
Fitting results and mainly transport mechanism of three samples
G0 (S · cm−1)
C1 (S · cm−1 · K−1.33)
3.1 × 10−2
8.2 × 10−3
C2 (S · cm−1 · K−2.5)
4.0 × 10−4
C3 (S · cm−1 · K−3.6)
6.1 × 10−8
Straight slope (μΩ · cm/log(K))
For sample A, the resistivity as a function of temperature is shown in Figure 5e. Although the temperature coefficient of resistivity is negative below RT, the temperature dependence of resistivity between sample A and the others exhibits evident differences. The resistivity increases gradually with decreasing temperature and varies slightly from 0.0093 Ω · cm (T = 300 K) to 0.011 Ω · cm (T = 5 K). Combined with the structure of sample A, the transport process is probably dominated by metallic paths because of the large number of interconnected elongated Co particles (see Figure 3a), which decreases when the resistivity increases, accompanying an increased MR effect. The approximate linear relationship between ρ and ln T for sample A is shown in Figure 5f. The fitting value of straight slope is shown in Table 1. The same phenomenon was reported in a CoO-coated monodispersive Co cluster system corresponding to a small negative MR value in a metal/semiconductor transition regime  and in the CoFeB/MgO films, in which the sample with high magnetic metal concentration is not in the strongly localized regime of conduction and the resistivity is plotted as a linear function of log(T) . Further detailed studies are necessary and in progress to elucidate the mechanism behind this result.
In summary, the structure, magnetic properties, and MR effect were investigated in Co/ZnO films deposited by sputtering at different pressures with different ZnO contents. We observed that the MR effect is strongly related to the resistivity of the films. Based on conduction, the MR effect can be classified into three regimes: the metallic, tunneling, and hopping regimes. Large RT MR values greater than 8.1% were obtained in the tunneling regime with a range of resistivity from 0.08 to 0.5 Ω · cm. By contrast, the MR value decreases distinctly when the resistivity of the films is less than 0.08 Ω · cm (metallic regime) or greater than 0.5 Ω · cm (hopping regime). In the tunneling regime, the conduction of the films mainly has two channels: the spin-dependent tunneling channel, which gives rise to high RT MR effect, and the spin-independent second-order hopping (N = 2). In the hopping regime, the increased spin-independent higher-order hopping (N > 2) through the localized states in thicker ZnO matrix served an important function and is the main reason for the rapid decrease in tunneling MR. In the metallic regime, metallic paths between interconnected elongated Co particles impede the MR effect. These results facilitate a deeper understanding of the spin transport mechanism in metal/semiconductor granular films and are significant for the improvement of the RT MR effect in spintronic applications.
The work is financially supported by NSFC (nos. 51025101 and 11274214), the Special Funds of Shanxi Scholars Program, the Ministry of Education of China (nos. IRT 1156 and 20121404130001), and the Youth Science Foundation of Shanxi Province (2012021020–2).
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