Author name used in this publication: C. S. Wei2009-2010 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Akaike, Y

Herng, TS

Lau, SP

Tanemura, M

Wang, L

Wei, C

Zhao, BC

Applied Physics Letters

Author name used in this publication: C. S. Wei2009-2010 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Stable ferromagnetism in p-type carbon-doped ZnO nanoneedlesT. S. Herng,1 S. P. Lau,2,a C. S. Wei,2 L. Wang,3 B. C. Zhao,3 M. Tanemura,4 andY. Akaike41School of Electrical and Electronic Engineering, Nanyang Technological University,Singapore 639798, Singapore2Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong3Division of Physics and Applied Physics, School of Physical and Mathematical Science,Nanyang Technological University, Singapore 637371, Singapore4Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku,Nagoya 466-8555, JapanReceived 20 August 2009; accepted 7 September 2009; published online 28 September 2009The authors report the synthesis and magnetic properties of carbon-doped ZnO ZnO:Cnanoneedles. A saturated magnetic moment of 2.16 emu /cm3 was found in the ZnO:C nanoneedles.The samples showed anomalous Hall effect and p-type conduction with a hole concentration of1.81018 cm−3. The ferromagnetism in the ZnO:C nanoneedles could be attributed to Csubstitution on the O site which introduces hole, so the p-p interaction leads to the strong spininteraction between the C atoms and carriers. It was found that the ferromagnetism and p-typeconduction in the ZnO:C nanoneedles were stable in ambient air over a period of 1 year andannealing temperature of up to 100 °C. © 2009 American Institute of Physics.doi:10.1063/1.3238289Room-temperature RT ferromagnetism has been ob-served in a wide range of oxide materials, which do notcontain ions with partially filled d or f bands. The term “d0ferromagnetism” is given to this type of materials.1 The ex-amples of d0 ferromagnets include HfO2,1,2 In2O3,2,3 TiO2,2ZnO Ref. 4 and carbon-doped ZnO ZnO:C.5,6 Amongthese d0 oxides, ZnO:C has received significant attention be-cause of its potential application in spintronics. Peng et al.7showed that direct hole doping at the anion site, such as Csubstitution on the O site, was more effective for localizingthe hole and sustaining the magnetic moment. Indeed, p-typeconduction and anomalous Hall effect have been observedexperimentally in ZnO:C thin films.6 Peng et al.7 also pre-dicted that quantum confinement effect could reduce thecritical hole concentration to induce ferromagnetism in ZnOnanostructures. However, it has been proved challenging toincorporate C into ZnO nanostructures effectively by con-ventional methods. Although graphite powder is often mixedwith ZnO powder as source materials to produce ZnO nano-structures in a vapor phase process, graphite is only acted asa reducing agent for ZnO.8 The ZnO nanostructures are al-ways free of C incorporation. Here we report the ferromag-netic p-type ZnO:C nanoneedles prepared by a low-energy500 eV ion beam technique. The magnetic and transportproperties of the ZnO:C nanoneedles are stable with agingtime which is different from most of the transition-metal-doped ZnO ZnO:TM.9,10 Furthermore, by means of anoma-lous Hall effect, the intrinsic ferromagnetism in ZnO:Cnanoneedles is probed.A highly texture 002 oriented starting material of ZnOfilms thickness 600 nm was prepared on Si substrates at300 °C by the filtered cathodic vacuum arc technique, asdescribed elsewhere.11 Prior to the fabrication of nano-needles, a thin layer of C was deposited onto the sample inorder to enhance the formation of nanostructures.12 A C plateserving as C source was placed perpendicular to the ZnOsample. An ion beam source with 500 eV Ar+ ions was irra-diated simultaneously at an incidence angle of 45° to the Cplate and the ZnO surface. Carbon was doped into the hostmaterial during nanoneedles formation. The undoped ZnOnanoneedles was prepared under the identical condition butwithout the presence of the C plate. After the fabrication ofnanoneedles, the residual C layer was removed by ethanolprior to characterization. The C content of the ZnO:C nano-needles was estimated to be 2 at. % by the energy disper-sive x-ray spectroscopy. The magnetotransport measure-ments were performed on a physical properties measurementsystem PPMS Quantum Design. The standard four probemethod was employed for Hall measurement. The magneticmeasurements were carried out with a vibrating sample mag-netometer attached to the PPMS system.Figure 1 shows the x-ray diffraction XRD patterns ofthe ZnO and ZnO:C nanoneedles. All the peaks are welldefined corresponding to a ZnO wurtzite structure. No traceof secondary phases, impurities, or graphite-related peakscan be detected within sensitivity of the XRD. The 002aElectronic mail: apsplau@polyu.edu.hk.FIG. 1. Typical XRD patterns of the ZnO and ZnO:C nanoneedles. Inset isthe SEM image of the ZnO:C nanoneedles array, showing the cone sharpwith an average length of 250–600 nm.APPLIED PHYSICS LETTERS 95, 133103 20090003-6951/2009/9513/133103/3/$25.00 © 2009 American Institute of Physics95, 133103-1Downloaded 01 Dec 2010 to 158.132.161.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionspeak shifts to a higher angle by 0.12° when comparing withthe undoped ZnO nanoneedles. This indicates a reduction oflattice constant c with carbon doping, which is expected assmaller carbon ions is incorporated into the O sites of ZnOmatrices.13 The surface morphology of the nanoneedles isshown in the inset of Fig. 1, possessing a needlelike structurewith the length ranging from 250 to 600 nm. The diameter ofthe nanoneedles in the stem part is around 100 nm.Figure 2a depicts the magnetic hysteresis loops of theZnO:C nanoneedles at 5 and 300 K. The samples showedclear hysteresis loop at RT with its saturated magnetizationMs of 2.16 emu /cm3 0.7 B /C as illustrated in theinset of Fig. 2a. At 5 K, a mild increment of Ms by 17%was observed. Nevertheless, a large enhancement in coerciv-ity was detected at low temperature. The coercivity wasfound in the order of 30 Oe at 300 K; then it surged to490 Oe at 5 K. This broadening effect is a classical behav-ior of magnetic materials at low temperature. The zero-field-cooled ZFC and field-cooled FC magnetization curves inthe temperature ranging from 5 to 350 K at dc field of 1 kOeare plotted in Fig. 2b. The curves manifest ZnO:C exhibitsa Curie temperature above 350 K. The ZFC-FC curves areconversed into one flat curve at temperature above 80 K,indicating the absence of superparamagnetism. This rule outthe possibility of C clusters being the origin of the observedferromagnetism in our ZnO:C nanoneedles. The cusp in theZFC and FC magnetization at 35 K could be attributed tospin-glass behavior.14 The spin glasslike behavior is relatedto the frustration originating from the coexistence of ferro-magnetic and antiferromagnetic interaction. However, theferromagnetic phase is still strongly dominant over the wholerange of temperatures. It is worthwhile to highlight that noferromagnetism can be detected in the undoped ZnO nano-needles, implying the observed ferromagnetism is associatedwith the presence of C.Anomalous Hall effect AHE is an important tool foridentifying intrinsic ferromagnetic semiconductor. The Hallresistivity xy is given by sum of the ordinary Hall effectand AHE term, xy =RoB+Rs0M, where B, 0, M, Ro, andRs is magnetic induction, magnetic permeability, magnetiza-tion, ordinary Hall coefficient and anomalous Hall coeffi-cient, respectively. Figure 3 shows the magnetic-field Hdependence Hall resistivity xy of the ZnO:C nanoneedles.The field was applied perpendicular to the sample’s surfaceat RT. At low magnetic fields 1 kOe, xy has largelylinear field dependence and the anomalous contributiondominates. Above 1 kOe, xy increases gradually and be-comes much less field dependent, which is dominated byordinary Hall resistance. The positive slope at high field re-gime implies the charge carriers in the ZnO:C nanoneedles isp-type with its hole concentration of 1.81018 cm−3. Itshould be noted that n-type conduction was observed in theundoped ZnO films. Thus, the observed p-type conduction inthe ZnO:C nanoneedles must be associated with the C dop-ing. The experimental result is in good agreement with the-oretical studies. If C substitutes O in the ZnO, the C 2porbital is localized and each site creates two holes.5,7 The p-pinteraction can lead to the strong spin interaction between theC atoms and carriers which can sustain the magnetic mo-ment. In addition, by subtracting the linear normal Hall ef-fect term yields a clear signature that corresponding to theAHE as shown in the inset of Fig. 3. Notably, the shape ofthe xy curve is well coincidence with the RT M-H curve asshown in Fig. 2a, confirming intrinsic ferromagnetism inthe ZnO:C nanoneedles.It is known that ferromagnetism in ZnO:TM films ornanoparticles are not stable and will degrade with time. Thedegradation could take place within hour to several monthsafter the samples were synthesized.2,9,10,15 Thus, it would beof interest to study the stability of the ZnO:C nanoneedles.The ZnO:C samples were kept in a transparent sample box inambient air for a long period of time. The time dependenceMs was measured in an interval of 3 months as plotted inFig. 4. The Ms of the ZnO:C remains constant at2.16 emu /cm3 over a period of 1 year. Our observation isstark contrast to many ZnO:TM materials where the ferro-magnetism will degrade with time.9,10,15 It should also benoted that the hole concentration of the ZnO:C nanoneedlesFIG. 2. Color online a Magnetic properties of the ZnO:C nanoneedlesmeasured at 5 and 300 K. The inset shows enlarged view of hysteresis loops.b ZFC-FC curves of the ZnO:C nanoneedles.FIG. 3. The magnetic field dependence of anomalous Hall resistivity xy ofthe ZnO:C nanoneedles measured at 300 K. The applied magnetic field isperpendicular to the sample’s plane with constant current of 0.01 mA. Thexy data after background subtraction is displayed in the inset.133103-2 Herng et al. Appl. Phys. Lett. 95, 133103 2009Downloaded 01 Dec 2010 to 158.132.161.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsremains almost constant for the entire storage period asshown in Fig. 4, suggesting the stable p-type behavior in theZnO:C system. The transient p-type behavior observed inmost of the p-type ZnO can be attributed to either the addi-tion of free electrons due to hydrogen or the creation ofcompensating defects.9 The conduction in ZnO:C nano-needles does not seem to be sensitive to ambient air or oxi-dation process. To have better understanding of its ferromag-netic stability, a series of ZnO:C nanoneedles were annealedin air for 1 h at various temperatures ranging from RT to200 °C. The Ms of the ZnO:C nanoneedles remains constantfor annealing temperature up to 100 °C as shown in theinset of Fig. 4. Nevertheless, the Ms reduced by 40% to1.3 emu /cm3 after the sample was annealed at 200 °C. Thesample annealed at 200 °C showed high resistivity and noreliable Hall measurement could be obtained. The loss ofelectronically active acceptor states could be explained bythe generation of compensating defects such as oxygenvacancies.9 The ferromagnetism in the ZnO:C nanoneedles isassociated with the hole and the hole could enhance the fer-romagnetism, which is in good agreement with the theoreti-cal studies.5,7 Further experiments are needed to clarify therelationship between p-type conduction and ferromagnetismin ZnO:C nanostructures.In summary, the origin of ferromagnetism in the ZnO:Cnanoneedles can be attributed to C doping. The ZnO:Cnanoneedles exhibited AHE with hole concentration of 1.81018 cm−3. The ferromagnetism and p-type conduction inthe ZnO:C nanoneedles were found to be stable in ambientair for more than 1 year.This work was partly supported by The Hong KongPolytechnic University Project No. 1-ZV95 and the Re-search Grants Council of Hong Kong Project No. PolyU5013/09P.1M. Venkatesan, C. B. Fitzgerald, and J. M. D. Coey, Nature London430, 630 2004.2N. H. Hong, J. Sakai, N. Poirot, and V. Brize, Phys. Rev. B 73, 1324042006.3R. P. Panguluri, P. Kharel, C. Sudakar, R. Naik, R. Suryanarayanan, V. M.Naik, A. G. Petukhov, B. Nadgorny, and G. Lawes, Phys. Rev. B 79,165208 2009.4S. Banerjee, M. Mandal, N. Gayathri, and M. Sardar, Appl. Phys. Lett. 91,182501 2007.5H. Pan, J. B. Yi, L. Shen, R. Q. Wu, J. H. Yang, J. Y. Lin, Y. P. Feng, J.Ding, L. H. Van, and J. H. Yin, Phys. Rev. Lett. 99, 127201 2007.6T. S. Herng, S. P. Lau, L. Wang, B. C. Zhao, S. F. Yu, M. Tanemura, A.Akaike, and K. S. Teng, Appl. Phys. Lett. 95, 012505 2009.7H. Peng, H. J. Xiang, S.-H. Wei, S.-S. Li, J.-B. Xia, and J. Li, Phys. Rev.Lett. 102, 017201 2009.8B. D. Yao, Y. F. Chan, and N. Wang, Appl. Phys. Lett. 81, 757 2002.9Q. Wan, Appl. Phys. Lett. 89, 082515 2006.10T. S. Herng, S. P. Lau, S. F. Yu, H. Y. Yang, K. S. Teng, and J. S. Chen, J.Phys. Condens. Matter 19, 356214 2007.11Y. G. Wang, S. P. Lau, H. W. Lee, S. F. Yu, B. K. Tay, X. H. Zhang, K. Y.Tse, and H. H. Hng, J. Appl. Phys. 94, 1597 2003.12M. Tanemura, J. Tanaka, K. Itoh, Y. Fujimoto, Y. Agawa, L. Miao, and S.Tanemura, Appl. Phys. Lett. 86, 113107 2005.13R. D. Shannon, Acta Crystallogr., Sect. A: Found. Crystallogr. 32, 7511976.14C. Liu, F. Yun, and H. Morkoc, J. Mater. Sci.: Mater. Electron. 16, 5552005.15D. Pan, J. Wan, G. Xu, L. Lv, Y. Wu, H. Min, J. Liu, and G. Wang, J. Am.Chem. Soc. 128, 12608 2006.FIG. 4. The saturated magnetic moment Ms and hole concentration of theZnO:C nanoneedles as a function of storage time. The inset shows the Msdependence of the ZnO:C nanoneedles as a function of annealingtemperature.133103-3 Herng et al. Appl. Phys. Lett. 95, 133103 2009Downloaded 01 Dec 2010 to 158.132.161.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Stable ferromagnetism in p-type carbon-doped ZnO nanoneedles

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