Large Magnetoresistance in Diode Assisted ZnCoO Device.

Abstract

Magnetoresistance (MR) effect is at the heart of modern information storage, which represents the electrical resistance change ratio of materials or devices under the application of external magnetic field. In order to realize large MR value, lots of promising materials and structures were proposed. Extraordinary MR (EMR) [1], [2] in some non-magnetic semiconductors has gained much attention owing to the large MR magnitude and linear magnetic field dependence, which can be ascribed to inhomogeneity of carrier density or mobility. Recently, large MR value was realized by exquisitely coupling the magnetic response of materials and the nonlinear transport effect of diode [3], [4]. However, these devices could not satisfy the requirement of high sensitivity at low magnetic field and small work current simultaneously. Here, we utilized the negative MR and large resistivity of ZnCoO magnetic semiconductor films to realize high magnetic-field sensitivity with small work current at room temperature. In our experiments, ZnCoO magnetic semiconductor films of 60 nm were cut into a stripe with 7 mm in length and 2 mm in width. As shown in Fig. 1(a), linear voltage-current (V-I) curves and about -13% MR values were observed. Here, MR is always defined as MR $=( \mathrm {V}_{H}- \mathrm {V}_{0})/ \mathrm {V}_{H,}$ where $\mathrm {V}_{H}$ and V0 are the voltage detected with and without applied magnetic field respectively. Then a Zener diode was connected between electrode 1 and 4 to fabricate a diode assisted ZnCoO device (DAZD). Fig. 1(b) shows the V-I curves and MR value of the DAZD with 9.1V Zener diode under different magnetic fields. A sharp increase in voltage by several orders was observed at critical transition current $\mathrm {I}_{C}=0.375$ mA under 0T, which is directly related with the Zener Diode. When the applied current is smaller than 0.375 mA, the voltage dropped on the diode is smaller than the critical voltage $(\mathrm {U}_{C})$ of Zener diode and the diode could be considered as open circuit, which results in a small measured voltage because of the voltage attenuation with the increase of distance. When the voltage dropped on the diode is larger than $\mathrm {U}_{C}$, the diode can be considered as short circuit and electrode 4 and 1 turn into short-connected, which results in a sudden increase in the detected voltage. As shown in Fig. 1(b), the MR value of the DAZD is high up to -1539% at 0.1T, -3958% at 0.4T, -5359% at 1T and -6850% MR at 6T with small work current, which is a higher sensitivity in low magnetic field compared with the Si, Ge, GaAs nonmagnetic semiconductor. As shown in Fig. 1(c), the MR value and the shape of MR curve could be tuned by the applied current. For $\mathrm {I}=0.380$ mA and $\mathrm {I}=0.428$ mA, the MR value is almost the same $(\sim -850$%), but the former one is much higher than the later one in magnetic field sensitivity and the shape of the MR curves changes from sharp to obtuse. Generally, the MR value of DAZDs was closely related to the transport property of Zener diode and ZnCoO films. Here, we measured the MR value of DAZDs connected by Zener diodes with different threshold voltage. With the increase of Zener diodes' threshold voltage, the MR $_{max}$ value increases at first, then remains nearly unchanged as shown in Fig. 2(a). This is because that the Zener diodes' sharpness of resistance transition increases with the increase of $\mathrm {U}_{C}$ and keeps the same when $\mathrm {U}_{C}$ larger than 6.8V as shown in Fig. 2(b). This work may have potential application in the area of magnetic sensor industry with the advantage of low power consumption.