Tuning magnetoresistance and exchange coupling in ZnO by doping transition metals Yu-feng Tian, Yong-feng Li, and Tom Wu Citation: Appl. Phys. Lett. 99, 222503 (2011); doi: 10.1063/1.3664116 View online: http://dx.doi.org/10.1063/1.3664116 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i22 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 99, 222503 (2011)
Tuning magnetoresistance and exchange coupling in ZnO by doping transition metals Yu-feng Tian, Yong-feng Li, and Tom Wua) Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
(Received 23 August 2011; accepted 6 November 2011; published online 28 November 2011) A systematic study focused on the magneto-transport properties of transition metal (Cu or Co) doped ZnO thin films is performed to elucidate the role of doping on tuning the band structure and exchange coupling in wide band gap oxides. Detailed theoretical fittings suggest that the negative magnetoresistance (MR) originates from the spin-dependent scattering due to the high-order sp-d exchange interaction, while the positive MR can be well described by a model invoking two spin split subbands. Our results suggest that with different dopants both the electronic band structure C 2011 American Institute of Physics. and the exchange coupling in ZnO can be rationally tailored. V [doi:10.1063/1.3664116]
Wide band gap oxides are technologically important materials and characterized by a large band gap often in the UV regime, e.g., 3.6 eV for ZnO.1 By creating new levels within the band gap, doping is not only the most common tool to tailor the transport characteristics of semiconductors,2 but also opens venues towards modified magnetic properties.3–7 ZnO has been extensively studied because it has long spin coherence time and length.8–10 Comparing with the conventional magnetic measurements on transition metal (TM) doped ZnO which often suffer from artifacts,11 magneto-transport, in particular magnetoresistance (MR), measurements are effective to reveal the doping-modified magnetic interactions.12–14 But although there have been various models proposed to explain the observed magneto-transport behaviors,15–21 a thorough understanding is rather challenging. In this work, by quantitatively analyzing the magnetotransport properties of undoped, Cu-doped, and Co-doped ZnO, the origin of both negative and positive MR is elucidated. Furthermore, it is clearly revealed that both the band structure and the sp-d exchange coupling in ZnO can be tailored by TM doping, which underscores the importance of rational materials design in achieving the desired functionalities in doped wide band gap oxides. Since the magnetic interactions in ZnO depend sensitively on the carrier localization and the defect characteristics, we systematically adjusted our preparation conditions to tailor the spin-related transport properties. A commercial pure ZnO target was used for the preparation of reference undoped samples. For TM-doped ZnO targets, high purity powders were ball milled and sintered as described in previous reports.22 Samples were prepared on a-cut (110) sapphire substrates using pulsed laser deposition under high vacuum (104 Pa) at 400 C. The thickness of all deposited films was determined to be about 90 nm by using x-ray diffraction (XRD) and reflectivity (Smartlab, Rigaku Japan). The TM doping levels were characterized by using energy dispersive x-ray spectroscopy (EDS).
a)
Electronic mail:
[email protected].
0003-6951/2011/99(22)/222503/3/$30.00
The XRD pattern of Zn0.95Cu0.05O is shown in Fig. 1(a) as an example of the ZnO based films, and all the diffraction peaks can be assigned to the ZnO phase with a wurtzite structure (space group P63/mmc). To further confirm the epitaxy relationship and to check the film quality, we performed reciprocal space mapping (RSM), and the data shown in Fig. 1(b) revealed the high expitaxial quality of the ZnO films. The epitaxial relationship was determined to be (001) ZnO// (110) Al2O3 and (110) ZnO//(001) Al2O3. As shown in Fig. 2(a), in the high temperature regime, the exponential decrease in resistivity with increasing temperature is a clear signature that all the studied films maintain the semiconducting behavior. At low temperatures, variable range hopping (VRH) transport was observed as shown in Fig. 2(b).23 The Hall effect measurements were carried out to determine the carrier concentration and mobility, and the higher resistivity in the doped samples can be mainly attributed to the reduced carrier concentration (Table I). It is well known that electronic properties of semiconductors sensitively depend on the ratio of the mean distance between donors r ¼ ð1=4pnÞ1=3 to the effective Bohr radius aB , which is 1.7 nm for ZnO.12 In the diluted limit as in Co and Cu doped samples (Table I), i.e., r aB , electrons are bound to individual donors, and low temperature transport is proceeded through phonon-assisted VRH between occupied and empty states, which is consistent with the observed Mott VRH behavior (Fig. 2(b)).
FIG. 1. (Color online) Typical XRD h2h (a) and the corresponding RSM (b) data of the Zn0.95Cu0.05O film. “S” and “F” stand for substrates and film, respectively. 99, 222503-1
C 2011 American Institute of Physics V
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Appl. Phys. Lett. 99, 222503 (2011)
FIG. 2. (Color online) Resistivity (q) versus temperature T (a) and T1/4 (b) curves measured on the studied samples. Solid straight lines in (b) are theoretical fittings according to the Mott VRH model.
Figure 3 shows the normalized MR of the studied samples with the magnetic field applied perpendicular to the film plane. Both undoped and Cu doped samples are dominated by negative MR (Figs. 3(a) and 3(b)), but in Zn0.95Co0.05O a positive MR component takes over with decreasing temperatures (Fig. 3(c)). Negative MR is often observed in TM-doped ZnO in the weak localization regime, i.e., kF l ¼ hð3p2 Þ2=3 =ðe2 qn1=3 Þ > 1, where kF is the Fermi wave vector, l is the mean free path, h is the Planck constant, e is the electron charge, q is the resistivity, and n is the electron concentration.15 It has previously been quantitatively explained by the destructive effect of magnetic field on the interference between scattered electrons.15,16 However, in our case, the calculated values of kFl at 5 K for all samples are much smaller than 1 (Table I). As a result, the studied films are not in the weak localization regime. On the other hand, the observed negative MR in Fig. 3 can be explained by the model proposed by Khosla and Fischer, where the third-order expansion of the s-d exchange Hamiltonian was considered.24 Correspondingly, a semiempirical expression of the negative MR is given as14,18,24,25 qH ¼ 1 a2 lnð1 þ b2 H 2 Þ; q0
(1)
a2 ¼ A1 JDðeF Þ½SðS þ 1Þ þ < M2 >; " 4 # glB 2 2 2 2 2JDðeF Þ b ¼ 1 þ 4S p : akB T g
(2) (3)
Here, A1 represents the contribution of spin scattering, J is the exchange interaction integral, DðeF Þ is the density of states at the Fermi level, S is the spin of the localized magnetic moment, < M > is the average magnetization, g is the effective Lande factor of the localized magnetic moment, lB is the Bohr magneton, a is a numerical constant on the order of unity, and kB is the Boltzmann constant. TABLE I. The resistivity ðqÞ, electron concentration ðnÞ, mobility ðlÞ, mean distance between donor ðrÞ, and calculated kFl of the studied samples. All are 5 K data. Material ZnO Zn0.95Cu0.05O Zn0.95Co0.05O
P (Xm)
n (m3)
l (m2/vs)
r (nm)
kFl
4.09 104 8.37 104 3.97 103
4.1 1025 6.5 1024 4.5 1024
3.72 104 1.15 103 3.49 104
1.798 3.324 3.757
0.28 0.25 0.06
FIG. 3. (Color online) MR data measured on pure ZnO (a), Zn0.95Cu0.05O (b), and Zn0.95Co0.05O (c) samples with the applied field perpendicular to the film plane. Solid lines are the theoretical fitting results as discussed in the main text. (d) Fitting parameter b in Eq. (3) versus the inverse temperature. The linear lines are a guide to the eyes.
According to Eq. (3), the fitting parameter b should be inversely proportional to the temperature, which has been regarded as a evidence for this MR mechanism.14,25 Indeed, as revealed in Fig. 3(d), the linear relationship between b and T 1 was clearly observed for all samples under investigation. Previous first-principles calculations indicate that the 3d states of the TM are located at different locations in the ZnO band structures, hybridizing with the Zn and O orbitals.26 A larger magnetic moment is obtained in Co-doped ZnO compared to the Cu case, reflecting a stronger exchange interaction. To explain the positive MR, we used a two-band model with reference to the multiple subbands in doped ZnO. In this model, due to the sp-d exchange interactions the conduction band in ZnO splits into two subbands for spin-up and spin-down carriers with different mobilities.18 The distribution of carriers in the two subbands is dictated by the temperature, the magnetic field dependent spin-splitting effect, and the thermal activation energy. Such a semiempirical two band model predicts a positive MR,18 qH c2 H 2 ¼1þ ; q0 1 þ d2 H2 c2 ¼
d2 ¼
r1 r2 ðl1 þ l2 Þ2 ðr1 þ r2 Þ2 ðr1 l2 r2 l1 Þ2 ðr1 þ r2 Þ2
(4)
;
(5)
;
(6)
where r1 ðr2 Þ and l1 ðl2 Þ are the conductivity and mobility of carriers in the majority (minority) band. Least-squares fit to the MR data of Co-doped sample were performed by combining Eqs. (1) and (4) into qH =q0 a2 lnð1 þ b2 H 2 Þ þ c2 H 2 =ð1 þ d2 H 2 Þ. As can be seen in Fig. 3(c), the agreement between the theoretical
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fitting and the experimental is excellent in the whole temperature range. The obtained fitting parameters are c ¼ 0:41457 and d ¼ 0:77199 for Zn0.95Co0.05O at 5 K. Considering Eqs. (5) and (6), also that the total conductivity r ¼ r1 þ r2 and the total carrier concentration n ¼ n1 þ n2 , we obtained that r1 ¼195:538(Xm)1, l1 ¼2:712104 m2/Vs, n1 ¼ 4:499 1024 m3, r2 ¼56:350 (Xm)1, l2 ¼0:994 m2/Vs, n2 ¼3:536 1020 m3 for the two subbands. From the theoretical fitting, we can conclude that the conduction is dominated by the majority subband (n1 ), whose carrier concentration is about four orders larger than that of the minority band (n2 ), though the mobility of the majority band is much lower. Furthermore, there is a clear temperature dependent crossover from the positive MR to the negative MR in Zn0.95Co0.05O at about 50 K. It can be well explained as the result of decreasing splitting between the spin polarized subbands with increasing temperature, which makes the negative MR become more pronounced at high temperatures. In summary, we conducted a comparative study on the magneto-transport properties of undoped and TM-doped ZnO samples. Good agreement between experimental and theoretical results not only explains the origin of negative and positive MR, but also indicates that both the exchange coupling and the band structure of wide band gap oxides can be rationally tailored by doping different TM elements. We acknowledge the financial supports from the Singapore National Research Foundation. 1
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