Spin Manipulation in Co-Doped ZnO

The magnetoresistance of n-type conducting, paramagnetic Co-doped ZnO films prepared by pulsed laser deposition on sapphire substrates has been studied experimentally and theoretically. Positive magnetoresistance (MR) of 124% has been observed in the film with the lowest electron concentration of 8.3·1017 cm−3, while only a negative MR of −1.9% was observed in the film with an electron concentration of 9.9·1019 cm−3 at 5 K. The positive MR is attributed to the quantum correction on the conductivity due to the s-d exchange interaction induced spin splitting of the conduction band. The negative MR is attributed to the magnetic field suppressed weak localization [1]. Voltage control of the electron concentration in Schottky diodes revealed a drastic change of the magnetoresistance and demonstrated the electrically controllable magnetotransport behavior in Co-doped ZnO [2]. The magnetically controllable spin polarization in Co-doped ZnO has been demonstrated at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom electrode and Co as a top electrode [3]. There spin-polarized electrons were injected from Co-doped ZnO to a crystallized Al2O3 layer and tunnelled through an amorphous Al2O3 barrier. Our studies demonstrate the spin polarization and manipulation in Co-doped ZnO.

strong (n < n c ) and weak (n > n c ) localization regime [1]. In this paper we will focus on the spinsplitting of the conduction band due to sd-exchange interaction between free charge carriers and localized magnetic moments, the electrically controllable magnetoresistance and magnetically controllable spin polarization in Co-doped ZnO thin films, Schottky diodes and magnetic tunnel junctions, respectively.
Experimental Results and Discussion. Magnetotransport measurements in the classical van-der-Pauw geometry represent a fast and precise method for investigating conducting, magnetic thin films. In n-or p-conducting ZnO the s-(3d,4f) or p-(3d,4f) exchange interaction, respectively, leads to spin polarization and may cause positive magnetoresistance. We modeled the corresponding spin polarization from the magnetotransport data. However, because of different mechanisms influencing the magnetotransport properties, superimposed positive and negative magnetoresistance may be observed in dependence on temperature and magnetic field strength. An advanced magnetotransport modelling approach should combine different mechanisms for example magnetic polarons and electron localization causing negative magnetoresistance and s-d and s-f induced spin splitting of the conduction band causing positive magnetoresistance in magnetic, n-type conducting ZnO films.
Spin-splitting of the Conduction Band. We modeled the spin polarization in ZnO:Co and ZnO:Mn from the decrease of positive magnetoresistance with the free electron concentration [8]. Positive magnetoresistance has been interpreted in terms of the influence of the sd-exchange interaction on the disorder-modified electron-electron interactions [9,10] and may be attributed to magnetic scattering of spin polarized free electrons. The electron concentration of spin up ↑ or spin down ↓ electrons can be calculated by: where m * is the effective mass of electrons, E f the Fermi energy, E c is the bottom of the spin up ↑ or spin down ↓ conduction band, respectively, and k B the Boltzmann constant. Due to the splitting of the conduction band, there is an energy gap δ between the spin-up and spin-down polarized conduction band (Fig. 1a). The position of the Fermi level E f was adjusted to obtain the electron concentration n, which equals n↑+n↓. The modelling has been performed under the assumption that the free electron model is valid for n ranging from 10 17 to 10 20 cm -3 [8].

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There exists no experimentally determined value for the conduction band splitting δ in magnetic ZnO and we use 1, 10, and 100 meV as initial values for modelling the spin polarization P ( Fig.  1(a)). P is defined by (n↑-n↓)/( n↑+n↓). For δ=100 meV, P is nearly independent on n. For δ=10 meV, P keeps 1 for n<10 20 cm -3 and drops with n increasing above 10 20 cm -3 . For δ=1 meV, P continuously decreases with increasing n and almost drops to zero for n>10 20 cm -3 . By comparing the dependence of the simulated spin polarization P and of the measured positive magnetoresistance on temperature and electron concentration, we determined a splitting of the conduction band E c in ZnO:Co and ZnO:Mn below 10 meV [8].
Electrically Controllable Magnetoresistance. Diluted magnetic semiconductors provide the capability of controlling the charge and spin of the charge carriers simultaneously. The clear understanding of the magnetotransport properties of DMS in an external magnetic and electric field is important for future spintronics applications. A Zn 0.96 Co 0.04 O film with low electron concentration (about 1.5×10 17 cm -3 at 21 K) on a highly conducting Zn 0.99 Al 0.01 O layer has been deposited on an a-plane sapphire substrate by PLD [2]. To study the magnetoresistance of depleted, highly insulating Co-doped ZnO an Au ohmic contact and a Pd Schottky contact were deposited on the Zn 0.99 Al 0.01 O and Zn 0.96 Co 0.04 O layer, respectively. Positive magnetoresistance of 30 % with a current of 10 -6 A was observed at 5 K (Fig. 2). The positive MR decreases drastically at 5 K and changes to negative MR at 50 K with increasing current, which is considered to be due to the bias voltage control of the electron concentration in the Zn 0.96 Co 0.04 O layer. The concentration of free electrons in the undepleted Co-doped ZnO has been measured by capacitance-voltage measurements and amounts to 1.5×10 17 cm -3 at 21 K. The concentration of remaining free electrons in the depleted Co-doped ZnO has been estimated to be about 3×10 13 cm -3 from the correspondingly measured resistivity (about 90 Ω·m measured at 5 K with 1 µA) and using the electron mobility in Co-doped ZnO [7]. This work demonstrates the electrically controllable magnetotransport behavior in Co-doped ZnO Schottky diodes due to the electrically controllable electron concentration n and the dependence of the magnetoresistance in Co-doped ZnO on n [2].
Magnetically Controllable Magnetoresistance. Future developments will also tackle the exploitation of magnetic oxides in magnetic random access memory (MRAM) devices based on

12th INTERNATIONAL CERAMICS CONGRESS PART F
tunnel magnetoresistance (TMR) structures where the magnetization can be switched by an external magnetic field. The magnetically controllable spin polarization in Co-doped ZnO has been demonstrated at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom electrode and Co as a top electrode on a nonconducting substrate [3]. Before depositing the Co-doped ZnO bottom electrode, we deposited an Al-doped ZnO layer as a buffer layer on the sapphire substrate. At low field the small negative magnetoresistance (MR) of the bottom electrode (not shown here) might originate from the Al-doped ZnO [9]. However, the deposition parameters of Co-doped ZnO were chosen to prepare the highly conductive film, which will also lead to the low field negative MR [7]. The positive MR at intermediate field and negative MR at high fields originate from the Co-doped ZnO [7]. In our junction structure, the current will then go perpendicular to the film through the Al 2 O 3 barrier layer to the Co top electrode. The mainly interesting phenomenon is the low field butterfly positive MR behaviour, as shown in Fig. 3. Contradicted to the normally observed TMR effect, double peaks have been observed in each swept curve which are located at both sides of 0 T. As one can see, with increasing applying current, the MR effect becomes weaker, which is typical TMR effect [11]. With applying 1 µA, the butterfly positive MR disappears, and the junction was broken, and no butterfly positive MR at low field can be observed as shown in Fig. 2(d). After the application of 1 µA, no butterfly positive MR at low field can be observed with smaller current. As an example the first ZnO-based TMR structure revealing magnetic field switching is shown in Fig.  3. (red) and from 6 T to -6 T (blue) [3].
The MR curves at high field (H > 1 T) are very similar to the MR curves probed on ZnCoO thin films. The low field butterfly positive MR behavior reveals double peaks located at both sides of 0 T with the lower peak at the starting field and the higher peak at the end field for each sweep direction. When applying a current of 1 µA the junction was destroyed and the butterfly positive MR disappears [3]. Note that novel MRAM devices based on TMR structures reveal currentinduced switching of magnetization.

Summary and outlook.
Insulating and n-type conducting, diluted magnetic ZnO films have been prepared by pulsed laser deposition and the electric and magnetic controllability of magnetoresistance has been demonstrated. The magnetotransport properties of magnetic ZnO depend on the species and concentration of magnetic ions, the concentration of free charge carriers and the film thickness. The large positive magnetoresistance in n-type conducting ZnCoO is attributed to the quantum correction on the conductivity due to the sd-exchange interaction induced spin-splitting of the conduction band. The sd-exchange induced splitting of the conduction band being proportional to the exchange coupling constant α is less than 10 meV. By comparing the exchange coupling constants α and β in II-VI compound semiconductors it is expected that the pdexchange interaction induced splitting of the valence band being proportional to the exchange coupling constant β is larger than 30 meV, thus producing a finite spin polarization in p-type conducting, magnetic ZnO even above room temperature. Room temperature ferromagnetism in diluted magnetic ZnO could pave the way to exploit the spin in addition to charge in future ZnObased spinelectronics devices. Because Co ions are isovalent dopants in ZnO, additional acceptor dopants have to be added, e.g. by implanation, and electrically activated by pulsed laser annealing under oxygen overpressure, otherwise oxygen tends to evaporate from the ZnO surface. Therefore, the incorporation of acceptor dopants above their solubility limit is the main challenge for the fabrication of ferromagnetic ZnO. Because Co and Mn shows a very low diffusity in ZnO over the entire temperature range, from a themodynamical point of view p-type conducting Co-and Mndoped ZnO spintronics devices will be stable enough for room temperature applications [12]. The thermodynamic stability of laser annealed p-type conducting magnetic semiconductors has already been proven by the hysteretic magnetotransport of spin-polarized holes in accordance with magnetization below the Curie temperature in Mn-doped Ge [13] and in Mn-doped GaAs [14].