Giant positive magnetoresistance in Co-doped ZnO nanocluster filmsY. F. Tian, J. Antony, R. Souza, S. S. Yan, L. M. Mei, and Y. Qiang Citation: Applied Physics Letters 92, 192109 (2008); doi: 10.1063/1.2921051 View online: http://dx.doi.org/10.1063/1.2921051 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Anisotropic magnetism and spin-dependent transport in Co nanoparticle embedded ZnO thin films J. Appl. Phys. 114, 033909 (2013); 10.1063/1.4815877 The effects of group-I elements co-doping with Mn in ZnO dilute magnetic semiconductor J. Appl. Phys. 111, 123524 (2012); 10.1063/1.4729530 High temperature ferromagnetism and optical properties of Co doped ZnO nanoparticles J. Appl. Phys. 108, 084322 (2010); 10.1063/1.3500380 Magnetoresistance of Co-doped ZnO thin films J. Appl. Phys. 99, 08M124 (2006); 10.1063/1.2172194 Room temperature ferromagnetic and ultraviolet optical properties of Co-doped ZnO nanocluster films J. Appl. Phys. 97, 10D307 (2005); 10.1063/1.1846991

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Giant positive magnetoresistance in Co-doped ZnO nanocluster filmsY. F. Tian,1,2 J. Antony,1 R. Souza,1 S. S. Yan,2 L. M. Mei,2 and Y. Qiang1,a�

1Department of Physics, University of Idaho, Moscow, Idaho 83844, USA2School of Physics and Microelectronics, Shandong University, Jinan, Shandong 250100, People’s Republicof China

�Received 8 April 2008; accepted 16 April 2008; published online 15 May 2008�

We have studied nanostructures and magnetoresistance of 0.5%, 12%, and 30% Co-doped ZnOnanocluster films which were deposited by a third generation sputtering-gas-aggregation source onSi wafer. Microstructure analysis is performed by x-ray photoelectron spectrometer, transmissionelectron microscopy, and x-ray diffraction and shows a uniform mean nanocrystallite size of 20 nmwith perfect wurtzite ZnO structure. Magnetoresistance �MR� increases at 5 K with Co dopingconcentration, that is, 0.5% Co-doped ZnO with 469% MR, while the other two samples have 744%and 811%. The large positive MR is explained by the suppression of spin-dependent hopping pathswhen localized states with onsite correlation undergo a relatively large spin �Zeeman� splitting in amagnetic field due to strong s , p-d interactions in Co-doped ZnO nanocluster films. © 2008American Institute of Physics. �DOI: 10.1063/1.2921051�

Recently, widespread interest has been generated in giantmagnetoresistance �GMR� due to its applications in tech-nologies such as high-performance read heads, nonvolatilememories, and other state-of-the-art storage devices.1–3 Thedevelopment of semiconducting system exhibiting GMR of-fers many advantages in device design and fabrication. TheMR of such devices contains contribution from the magneticfield dependence of the material parameter and geometry.3–5

For most doped semiconductors, the MR is usually observedat low temperature.6–8 The carriers in ferromagnetic semi-conductor are spin polarized. Depending on the relative en-ergy scales of the Coulomb interaction, spin-spin exchangeinteraction, and Zeeman energy in the magnetic field, posi-tive and/or negative MR was observed in magnetic semicon-ductor films.9,10 Different semiconductor materials are dopedwith transition metals such as Co, Ni, and Fe to form dilutedmagnetic semiconductors �DMSs�.2–6 Among them, zincoxide is a well known n-type wide-bandgap semiconductorextensively studied for spintronic devices because of thepossibility of incorporating the magnetic degrees offreedom.11,12 There are reports confirming the ferromagneticproperties of DMSs based on ZnO.13–15 The transport andmagnetic properties of DMSs strongly depend on the filmpreparation, doping concentration, and temperature.3,4 Theexperimental and theoretical8,16 works have been reported oncobalt doped ZnO thin films showing the positive and nega-tive MRs depending on Co concentration and temperature,which reveals cobalt as one of the best doping candidates forspintronic devices. Recently, MR of Zn0.90Co0.10O filmwhich was prepared with pulsed laser deposition was foundto be less than 80% in the temperature range of 5–10 K,8

while we obtained 744% MR at 5 K for 12% Co-doped ZnOcluster films, which showed the unique characteristic ofclusters.

In this paper, we address the nanostructure, magnetiza-tion, and MR of Co-doped ZnO nanocluster films. We findthat the giant positive MR which increases with increasing

Co doping concentration can be explained well by spin effectinduced by Zeeman splitting.

The nanocluster films of zinc oxide doped with cobaltwere deposited on the Si �110� substrate by using thethird generation magnetron-sputtering-aggregation clustersource.13,17,18 The cluster source is based on a gas aggrega-tion tube having a high-pressure magnetron sputtering guninside. In order to prepare Co-doped ZnO nanocluster films,we used 99.995% pure Zn target, which is 5 mm thick andhas diameter of 75 mm. Co pellets are inlaid into the sput-tering region of the Zn target. To obtain 0.5%, 12%, and 30%Co-doped ZnO, we calculated the sputtering rate of Co andZn to find out the size and numbers of Co pallets inlaid intoZn target. The aggregation tube and sputtering gun arecooled down to 5 °C during the deposition. The sputtered Znand Co atoms travel in the presence of Ar, He, and O2 gas asto form Co-doped ZnO clusters at room temperature due tocondensation and oxidation. The prepared samples are opti-cally transparent with slightly yellow color because of clus-ter assembly. More details can be found elsewhere.13,17,18

Transmission electron microscopy �TEM� images ofsamples were taken with a JEOL JEM 2010F microscope.

a�Author to whom correspondence should be addressed. Electronic mail:youqiang@uidaho.edu. FIG. 1. TEM image of 30% Co-doped ZnO deposited on Si substrate.

APPLIED PHYSICS LETTERS 92, 192109 �2008�

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X-ray diffraction �XRD� measurements were accomplishedby using Philips X’pert MPD system equipped with Cu x-raysource. For surface and atomic compositions, x-ray photo-electron spectroscopy �XPS� measurements were performedby using a Physical Electronics Quantum 2000 ScanningESCA Microprobe based on focused monochromatic Al K�x-ray �1486.7 eV� source.

Figure 1 shows a low resolution TEM image of the 30%Co-doped ZnO nanocluster film. The average size of theseclusters is about 20 nm. The average cluster size of the othertwo samples is the same as observed in 30% Co-doped ZnOnanocluster film as listed in the Table I. Surface analysis byXPS reveals that the major elements present in the Co-dopedZnO nanocluster samples are Zn, Co, O, and C, as shown inFig. 2. Co 2p state is observed in the XPS data which meansthat the Co cation is in the +2 oxidation. No metal Co isdetected in XPS data. The carbon signal comes from thebackground. Figure 3 shows XRD pattern, which demon-strates high crystalline wurzite ZnO and is essentially iden-tical to those of bulk ZnO. By combining the XPS and XRDdata, we can say that there are no metal Co or CoO nanopar-ticles in the cluster films.

All samples show significant ferromagnetic behavior.Figure 4 shows a magnetic hysteresis loop of 30% Co-dopedZnO and demonstrates that the coercivity �Hc� of the samplesdepends on the Co doping. At 5 K, the coercivities of threesamples are also listed out in the Table I. It is obvious thatthe coercivity of Co-doped ZnO samples increases with in-creasing Co dopant concentrations. It should be pointed outthat the magnetic remanence of the 30% Co-doped ZnOsample is also higher than the other samples.

The electrical transport properties of the cluster filmswere measured by four-probe method by using supercon-ducting quantum interference device �Keithley 236 as currentsource and Keithley 2000 as voltage measurement device�

from 5 to 400 K. The sensing current was in the film plane.The applied field was also in the film plane and was parallelto the current direction. They all showed semiconductortransport behavior, which means that resistance increasedwith a decreased temperature. The room temperature resis-tance was on the order of tens kilohms. The MR was definedas MR= �R�T ,H�−R�T ,0�� /R�T ,0��100%, where R�T ,H�and R�T ,0� are the resistances with and without appliedmagnetic field at certain temperature T.

The most remarkable result exhibited in Fig. 5 is thegiant positive MR. By comparison of the results of threedifferent samples in Fig. 5, it can be seen that MR is lowerfor less Co doping. The MR measured at 5 K for 30% Co-doped ZnO sample is 811%, for 12% Co-doped ZnO it is744%, and 469% for 0.5% Co-doped ZnO. Since the appliedmagnetic field in our experiments is in the plane of film andparallel to the current flow, the orbital effects should nothave contribution to the observed large positive MR. Thepositive MR can be explained as a spin effect caused by thesuppression of spin-dependent hopping paths. In Co-dopedZnO nanocluster films, O vacancies which are known asshallow donor level can supply s , p spin carriers, and dopedCo can supply local moments �spins�. If the s , p electrons arestrongly exchange coupled to local spins via a ferromagnetic

FIG. 2. XPS spectra of 30% Co-doped ZnO.

FIG. 3. �Color online� X-ray diffraction pattern of 0.5% Co-doped ZnO.

TABLE I. Summary of experimental data for all samples.

Co concentrationin ZnO �%�

Average clustersize �nm�

Coercivityat 5 K �Oe�

Magnetoresistanceat 5 K �%�

0.5 20 83.8 46912 19.8 138.2 77430 20 183.3 811

FIG. 4. Hysteresis loop for 30% Co-doped ZnO at 5 K. Insert figure showsthat the corectivity increases with cobalt concentration.

192109-2 Tian et al. Appl. Phys. Lett. 92, 192109 �2008�

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s , p-d exchange interaction, the Zeeman splitting of the hop-ping s , p carriers in modest fields may be comparable to thethermal energy �kBT�. In this case, the relatively large Zee-man splitting is believed to be responsible for the large posi-tive MR when some fraction of localized states can be occu-pied by two electrons.9 The presence of onsite correlationenergy U �assumed to be smaller than the bandwidth of thelocalized state distribution� leads two types of site contributeto the conduction �types A and B�. The sites of type A aresingly occupied or unoccupied state and have energies �Aclose to Fermi energy EF. The sites of type B are doubleoccupied states, which accommodate two electrons with op-posite spins due to the onsite Coulomb repulsion U at allrelevant magnetic fields. Hopping between two single occu-pied A and B sites requires a pair of sites with antiparallelspins, as does the reverse process involving a doubly occu-pied B site and an unoccupied A site. The probability offinding pairs of sites with antiparallel spins decreases withincreasing magnetic field, resulting in positive MR. The re-sulting MR is predicted to obey

R � exp��T0

T�1/d+1

f�x�� ,

where f�x� is a universal function of a single scaling param-eter x=uBH / T�T0 /T�1/d+1. In the weak field limit, it can bededuced by using perturbative approach that ln�R�T ,H�� /�R�T ,0���uBH /T. As can be seen from inset Fig. 5�a� thatln�R�T ,H�� / �R�T ,0�� has a linear dependence on magneticfield in the small field and quickly turn to saturation. Also,this was consistent with the theoretical prediction that f�x�becomes a constant in the strong field regime. As a result ofincreasing s , p-d exchange interaction with increasing Codoping concentration, the MR also increases as shown inFig. 5. No room temperature MR was found because thethermal fluctuations of charge carriers may be dominant

over s , p-d interactions at high temperature. However,some inconsistencies with the theory still remain. The linearrelationship between ln�R�T ,H�� / �R�T ,0�� and T−1 was notexperimentally found. Instead, the MR shows a complextemperature dependence which quickly decreases with in-creasing temperature as shown in Fig. 5�b�, which is not wellunderstood right now.

As a summary, we have demonstrated a giant positiveMR effect in Co-doped ZnO nanocluster films on Si sub-strate. A maximal value of 811% MR is recorded for 30%Co-doped ZnO nanocluster films. The positive MR effectshows remarkable dependence on concentration of cobalt inZnO. The large positive MR is explained by the suppressionof spin-dependent hopping paths when localized states withonsite correlation �double occupancy states� undergo a rela-tively large spin �Zeeman� splitting in a magnetic field due tostrong s , p-d interaction in Co-doped ZnO nanocluster films.

This research work is supported by DOE-EPSCoR �DE-FG02-04ER46142� and DOE-BES �DE-FG02-07ER46386�.TEM, XRD, and XPS measurements were performed at theEnvironmental Molecular Sciences Laboratory �EMSL�, anational scientific user facility sponsored by the Departmentof Energy’s Office of Biological and Environmental Re-search, located at Pacific Northwest National Laboratory�PNNL�. S.S.Y. gives an acknowledgement to Chinese NSFGrant No. 50572053.

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FIG. 5. �Color online� Field dependence of magnetoresistance for allsamples measured at 5 K. Inset �a� magnetic field dependence ofln�R�T ,H�� / �R�T ,0�� and �b� temperature dependence of MR for allsamples.

192109-3 Tian et al. Appl. Phys. Lett. 92, 192109 �2008�

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