We have studied electrical conductivity, sigma, and magnetoresistance in a CoO-coated monodispersive Co cluster assembly fabricated by a plasma-gas-aggregation-type cluster beam deposition technique. The temperature dependence of sigma is described in the form of log sigma vs 1/T for 7<T <80 K. The magnetoresistance ratio (rho(0)=rho(3T))/rho(0) increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase (by a factor of 6) is much larger than those observed for conventional metal-insulator granular systems. These results are ascribed to the Coulomb blockade effect in the monodispersed cluster assemblies. (C) 1999 American Institute of Physics. [S0003-6951(99)01901-4]

Hihara, T.

Konno, T. J.

Peng, D. L.

Sumiyama, K.

Yamamuro, S.

彭栋梁

Applied Physics Letters

Xiamen University Institutional Repository

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 1 4 JANUARY 1999Characteristic tunnel-type conductivity and magnetoresistancein a CoO-coated monodispersive Co cluster assemblyD. L. PengCREST of Japan Science and Technology CorporationK. Sumiyamaa)Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, JapanS. Yamamuro and T. HiharaCREST of Japan Science and Technology CorporationT. J. KonnoInstitute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, Japan~Received 5 August 1998; accepted for publication 27 October 1998!We have studied electrical conductivity,s, and magnetoresistance in a CoO-coated monodispersiveCo cluster assembly fabricated by a plasma–gas–aggregation-type cluster beam depositiontechnique. The temperature dependence ofs is described in the form of logs vs 1/T for 7,T,80 K. The magnetoresistance ratio (r02r3T)/r0 increases sharply with decreasing temperaturebelow 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase~by a factor of 6! is muchlarger than those observed for conventional metal–insulator granular systems. These results areascribed to the Coulomb blockade effect in the monodispersed cluster assemblies. ©1999American Institute of Physics.@S0003-6951~99!01901-4#ainngteisnectv-ta, tsnucrsaoneceetecoftiv-as-c-tun-Cono-rmsticrtionThem-wththergeer,ledreandam-dino-ov-t ofngeni-Giant magnetoresistance~GMR! has been observed instructure of two ferromagnetic layers separated by a thinsulator~FM/I/FM!.1 Such GMR arises from a spin-dependetunneling effect. Electron tunneling between two ferromanetic electrodes through an insulating layer depends onrelative orientation of the magnetizations of the electrodWhen the relative orientation of the magnetizationchanged by applying a magnetic field, tunnel-type magtoresistance~TMR! is expected to occur. Although this effewas discovered by Julliere2 in 1975 and subsequently in seeral other FM/I/FM junctions,3,4 large and reproducible TMRratios have been found only very recently.5,6In FM-I granular systems, where the magnetic megranules or clusters are embedded in an insulator matrixTMR effect has also been detected7 and a marked TMR ef-fect has been reported recently in FM-I granular system8For these materials, when the metal cluster concentratiojust below the percolation threshold, the electrical condtion is dominated by tunneling between metallic clustewith a tunnel resistance enhanced by the Coulomb blockat low temperatures and also dependent on the relativeentation of the magnetization in the two clusters. The tuncurrent between randomly oriented magnetic granulessmaller than that between the magnetically aligned onSince conventional granular materials have been produby codeposition of a metal and an oxide and subsequprecipitation of magnetic granules on the substrate, thernormally wide distribution of the cluster size and interclusdistance~namely the tunnel-barrier thickness!. In such het-erogeneous granular systems, the temperature dependenthe low-field conductivity is expressed in the form3exp(2b/Ta) with a51/2 for a wide temperature range.9,10a!Electronic mail: sumiyama@snap8.imr.tohoku.ac.jp760003-6951/99/74(1)/76/3/$15.00Downloaded 12 Feb 2010 to 130.34.135.83. Redistribution subject to AIP-t-hes.-lhe.is-,deri-eliss.edntisre ofIn this study, we have measured the electrical conducity and TMR in oxide-coated monodispersive Co clustersemblies. Cobalt~II! oxide, CoO, is an antiferromagnetisemiconductor with a Ne´el temperature of 293 K. The roomt mperature resistivities of CoO single crystals are 108– 1015V cm, and the activation energies are 0.73–1.35 eV.11 Thisletter discusses the transport properties, spin-dependentneling and Coulomb blockade effect in the CoO-coatedcluster assembly. In particular, we emphasize that the modispersed Co cluster size distribution and a nearly unifothickness of the CoO shells give rise to the characterifeatures in their electrical conductivity and TMR. Ousamples are fabricated with a plasma–gas–aggrega~PGA! type cluster beam deposition apparatus.12 It is a com-bination of sputtering and gas–aggregation techniques.PGA type cluster beam deposition apparatus is mainly coposed of three parts: a sputtering chamber, a cluster groroom and a deposition chamber. The vaporized atoms insputtering chamber are decelerated by collisions with a laamount of Ar gas~the Ar gas flow rate:RAr5250– 500~sccm! injected continuously into the sputtering chamband are swept into the cluster growth room, which is cooby liquid nitrogen. The clusters formed in this room aejected from a small nozzle by differential pumping andpart of the cluster beam is intercepted by a skimmer, athen deposited onto a sample holder in the deposition chber (1025– 1024 Torr!. Using this system, we obtainemonodispersed transition metal clusters of 6–14 nmsize.12 In this study, we introduced oxygen gas into the depsition chamber during the depositing to form CoO shells cering the Co clusters, as schematically shown in the inseFig. 1. For a constantRAr , the gas pressure in the depositiochamber can be adjusted by changing the flow rate of oxygas (RO2). The initial stage of clusters deposited on the m© 1999 American Institute of Physics license or copyright; see http://apl.aip.org/apl/copyright.jspioal1pytitidmveaedather-e.R-guabu-busnse–vesre.uchemceon-fm--rome-en-K:nd-usearri-MR-d77Appl. Phys. Lett., Vol. 74, No. 1, 4 January 1999 Peng et al.crogrids were observed by a Hitachi HF-2000 transmisselectron microscope~TEM! operating at 200 kV. Figure 1shows a TEM image of the clusters produced atRAr5500sccm andRO251 sccm. As shown here, the clusters aremost monodispersed, with the mean diameter of aboutnm. The cluster size was found to be insensitive to the desition time andRO2 . The electron diffraction pattern clearlindicated the coexistence of Co and CoO phases, whilehigh resolution image displayed Co clusters covered wCoO.13 The cluster assemblies were formed on a polyimfilm at room temperature with a thickness of about 100 nas measured by a quartz thickness monitor. Using a contional four-probe method, the electrical resistivity was mesured at a constant voltage because the ohmic law ceashold at low temperatures. The MR was measured in theplied field parallel to the electrical current direction.Figure 2~a! shows the temperature dependence ofelectrical resistivityr(T) for the sample prepared atRAr5500 sccm andRO251 sccm. In the CoO-coated Co clustassembly, the temperature coefficient of resistivity~TCR! isnegative below room temperature, andr(T) decreases dramatically with decreasing temperature below 10 K. As sefrom the inset of Fig. 2~a!, the resistivity at 4.2 K is fourorders of magnitude larger than that at room temperaturethe pure Co cluster assemblies, on the other hand, TCpositive in the range of 20,T,300 K and becomes slightlynegative below 20 K. At 4.2 K, the resistivity of the CoOcoated Co cluster assemblies is about ten orders of matude larger than that of the pure Co cluster assemblies. Sa huge resistivity value and negative TCR in the clustersemblies are attributable to the tunnel-type conductiontween metallic Co clusters via CoO shell layers.The low-field tunnel-type electrical conduction of granlar materials was discussed by Neugebauer and Web14Most simply, the conductivity is expressed as follows:s}exp~22ks2Ec/2kBT!, ~1!wheres is the tunnel-barrier thickness between the two clters,k is the tunneling exponent of electron wave functioin the insulator,k5@2m* (f1EF2E)/\2#1/2; m* is the ef-FIG. 1. TEM image of the clusters~with mean cluster diameter513 nm!prepared on a carbon-coated microgrid atRAr5500 sccm andRO251 sccm.The inset shows schematic drawing of the CoO-coated monodispersecluster assembly, showing Co cores~dark shaded! and surrounding CoOshells.Downloaded 12 Feb 2010 to 130.34.135.83. Redistribution subject to AIPn-3o-hehe,n--top-enInisni-chs-e-.-fective electron mass,f is the barrier height,E is the elec-tron energy,EF is the Fermi level and\ is Planck’s constant.Ec is the electrostatic energy required to create a positivnegative charged pair in two clusters by tunneling, and girise to the Coulomb blockade effect at a low temperatuWhen the applied voltage and thermal energy are msmaller thanEc , electrons or holes cannot tunnel from onneutral cluster to another without exciting their states frothe Fermi level to the levels higher thanEc . When clustersare monodispersed with a uniform intercluster distan~namely the same barrier thickness!, Eq.~1! predicts a simplehopping type temperature dependence for the low-field cductivity: s(T)}exp(2Ec/2kBT).14 As shown in Fig. 2~b!, alinear dependence of logs vs 1/T is observed in the range o7,T,80 K for the present CoO-coated Co cluster asseblies. This temperature dependence ofs(T) is different fromthe form of logs vs 1/T1/2 commonly observed in conventional granular systems. The charging energy estimated fthe plot of logs vs 1/T is Ec56 meV, corresponding to thethermal energy ofkBT with T'70 K. This implies that whenT.70 K, normal activation type conduction is restored bcause the charging energy is overcome by the thermalergy. In addition, as seen from Fig. 2~b!, the conductivityabruptly increases with increasing temperature above 80the estimated activation energy is 0.02 eV atT.150 K. Thisbehavior is ascribable to the small and/or large polaron bahopping conduction in the CoO semiconductor layer becathere are usually a large number of defects and excess cers in the transition metal oxides.Figure 3 shows the temperature dependence of theratio @rH502r3T#/rH50 for the CoO-coated Co cluster assembly prepared atRAr5500 sccm andRO251 sccm. TheCoFIG. 2. ~a! Temperature dependence of the electrical resistivityr at zeromagnetic field for the CoO-coated Co cluster assembly prepared atRAr5500 sccm andRO251 sccm and logr vs T in the inset.~b! The logarith-mic conductivity logs as a function ofT21. license or copyright; see http://apl.aip.org/apl/copyright.jspthe-ethOstFitu2vittiorreno-dsmbette-Cinso.sinias.s int-de-lti-forisomri-ev.to,ci..F.78 Appl. Phys. Lett., Vol. 74, No. 1, 4 January 1999 Peng et al.inset of Fig. 3 shows the magnetic field dependence ofMR at 4.2 K for this sample. The MR gradually decreaswith the increase in magnetic field up toH53 T and satu-rates atH.3 T. This result is well correlated with the magnetization curve~not shown here!. Because of the exchanginteraction between the antiferromagnetic CoO shell andferromagnetic Co core, the magnetization curve of the Cocoated Co cluster assembly hardly saturates, in contrathe behavior of the pure Co cluster assembly. As seen in3, the MR ratio increases slightly with deceasing temperain the range of 25,T,300 K, while below 25 K it increasesmarkedly with decreasing temperature, i.e., from 3.5% atK to 20.5% at 4.2 K. Comparing Fig. 2~a! with Fig. 3, thereis a good correlation between the MR ratio and the resistiat low temperatures: both the resistivity and the MR raincrease drastically with decreasing temperature. Suchmarkable enhancement of the MR ratio at low temperatuwas also observed in micrometer-sized ferromagnetic tunjunctions,15 single-electron transistors consisting of ferrmagnetic metals~Ni/NiO/Co/NiO/Ni!16 and double ferro-magnetic tunnel junctions with small ferromagnetic islan(Co/Al2O 3/Co).17 Moreover, it should be noted that, ashown in Fig. 3, the MR ratio increases by a factor of 6 fro25 to 4.2 K in the present CoO-coated Co cluster assemThis increase is predicted by Takahashi and Maekawa18 andis much larger than the reported ones for the ferromagnjunction with the Coulomb blockade effect16 and the FM-Igranular systems with broad distributions of size and ingranule distance.19 Thus, the enhanced MR effect is attributed to the monodispersiveness of the cluster size in thecoated Co cluster assembly, which led to the more distCoulomb blockade effect.Figure 4 shows the bias voltage dependence of the retivity and the MR ratio at 4.2 K for the CoO-coated Ccluster assembly prepared atRAr5500 sccm andRO250.44sccm. As one can see in Fig. 4, the resistivity is hugedecreases more than 1 order of magnitude with increabias voltageVB at VB,10 V. On the other hand, the MRratio decreases by a factor of 2 with the increase in bvoltage below 40 V and is found to be insensitive toVBabove 40 V. The decrease of the MR ratio withVB is not asmarked as that observed in ferromagnetic tunnel junction5,6FIG. 3. Temperature dependence of the MR ratio atH53 T for the CoO-coated Co cluster assembly prepared atRAr5500 sccm andRO251 sccm.The inset shows the field dependence of the MR ratio at 4.2 K.Downloaded 12 Feb 2010 to 130.34.135.83. Redistribution subject to AIPese-tog.re5ye-selsly.icr-o-ctis-ItgsSince the present MR measurement is on many junctionthe series, suchVB dependence of the MR ratio is attribuable to the following aspects:~1! the fine structure in the spinpolarized density of states, which is related to the rapidcrease of the MR withVB in classical junctions, is blurred bythe Coulomb blockade effect; or/and~2! the spin polarizedelectrons are affected by spin flip scattering through mustep tunneling.This work has been supported by Core ResearchEvolutional Science and Technology~CREST! of Japan Sci-ence and Technology Corporation~JST!, and partly by aGrant-in-Aid for Scientific Research A1~Grant No.085 050 04!. The authors appreciate Dr. M. Sakurai for huseful comment. They are also indebted to the support frLaboratory for Development Research of Advanced Mateals of IMR.1J. S. Moodera and L. R. Kinder, J. Appl. Phys.79, 4724~1996!.2M. Julliere, Phys. Lett. A54A, 225 ~1975!.3S. Maekawa and Ga¨fvert, IEEE Trans. Magn.18, 707 ~1982!.4J. C. Slonczewski, Phys. Rev. B39, 6995~1989!.5J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. RLett. 74, 3273~1995!.6T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater.139, L231 ~1995!.7J. S. Helman and B. Abeles, Phys. Rev. Lett.37, 1429~1976!.8H. Fujimori, S. Mitani, and S. Ohnuma, Mater. Sci. Eng., B31, 219~1995!.9P. Sheng, B. Abeles, and Y. Arie, Phys. Rev. Lett.31, 44 ~1973!.10H. Fujimori, S. Mitani, S. Ohnuma, T. Ikeda, T. Shima, and T. MasumoMater. Sci. Eng., A181/182, 897 ~1994!.11M. Roilos and P. Nagels, Solid State Commun.2, 285 ~1964!.12S. Yamamuro, K. Sumiyama, M. Sakurai, and K. Suzuki, Supramol. S~in press!.13T. J. Konno, D. L. Peng, S. Yamamuro, and K. Sumiyama~unpublished!.14C. A. Neugebauer and M. B. Webb, J. Appl. Phys.33, 74 ~1962!.15K. Ono, H. Shimada, S. Kobayashi, and Y. Ootuka, J. Phys. Soc. Jpn65,3449 ~1996!.16K. Ono, H. Shimada, and Y. Ootuka, J. Phys. Soc. Jpn.66, 1261~1997!.17L. F. Schelp, A. Fert, F. Fettar, P. Holody, S. f. Lee, J. L. Maurice,Petroff, and A. Vaures, Phys. Rev. B56, R5747~1997!.18S. Takahashi and S. Maekawa, Phys. Rev. Lett.80, 1758~1998!.19S. Mitani, H. Fujimori, and S. Ohnuma, J. Magn. Magn. Mater.177–181,919 ~1998!.FIG. 4. Bias voltage dependence of the resistivityr and the MR ratio at 4.2K for the CoO-coated Co cluster assembly prepared atRAr5500 sccm andRO250.44 sccm. license or copyright; see http://apl.aip.org/apl/copyright.jsp

Characteristic tunnel-type conductivity and magnetoresistance in a CoO-coated monodispersive Co cluster assembly

https://dspace.xmu.edu.cn/bitstream/2288/59462/1/Characteristic%20tunnel-type%20conductivity%20and%20magnetoresistance%20in%20a%20CoO-coated%20monodispersive%20Co%20cluster%20assembly.pdf

Characteristic tunnel-type conductivity and magnetoresistance in a CoO-coated monodispersive Co cluster assembly

Abstract

Similar works

Full text

Available Versions

Xiamen University Institutional Repository