Indium-fin oxide (ITO)-Fe-cluster composite films were fabricated by plasma-gas-condensation-type cluster beam deposition and RF helicon sputtering method, and the structural, optical, electrical and magnetic properties of the films were investigated. The optical transmittance of about 90% is obtained for the Fe cluster layer thickness of 10 nm although it is decreased with increasing the cluster layer thickness. The resistivity is further decreased by embedding metalic Fe clusters into ITO matrix and a resistivity value of about an order of 10(-4)Omega cm was obtained. Moreover, the ITO-Fe cluster composite films show ferromagnetism at room temperature even for a cluster layer thickness of 10 unit

Hihara, T.

Morikawa, H.

Nozawa, N.

Peng, D. L.

Sumiyama, K.

彭栋梁

IEEE Transactions on Magnetics

Xiamen University Institutional Repository

3406 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005Ferromagnetic, Transparent and ConductingITO-Fe-Cluster Composite FilmsD. L. Peng, K. Sumiyama, N. Nozawa, T. Hihara, and H. MorikawaDepartment of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, JapanIndium-tin oxide (ITO)-Fe-cluster composite films were fabricated by plasma-gas-condensation-type cluster beam deposition and RFhelicon sputtering method, and the structural, optical, electrical and magnetic properties of the films were investigated. The opticaltransmittance of about 90% is obtained for the Fe cluster layer thickness of 10 nm although it is decreased with increasing the clusterlayer thickness. The resistivity is further decreased by embedding metalic Fe clusters into ITO matrix and a resistivity value of about anorder of 10 4 cm was obtained. Moreover, the ITO-Fe cluster composite films show ferromagnetism at room temperature even for acluster layer thickness of 10 nm.Index Terms—High electrical conductivity, indium-tin oxide (ITO) Fe-cluster composite films, room-temperature ferromagnetism,transparency.I. INTRODUCTIOND ILUTED magnetic semiconductors have attracted consid-erable attention because of their potential for novel tech-nological applications utilizing both the semiconductor and fer-romagnetic characteristics, the so-called “spintronics” [1]–[3].Most of the Curie temperatures of these diluted magnetic con-ductors are much lower ( K) than the room tempera-ture, being not appropriate for technical application. Recently,co-doped anatase TiO thin films were reported to be ferromag-netic even above 400 K [4], [5], while they have high resistivity( cm).For developing ultimately new multifunctional devices,moreover, it is desired to utilize simultaneously optical,electrical and magnetic properties of materials and theirinter-correlation effect. In conventional materials, however,good optical transparency, high electrical conductivity andferromagnetism above room temperature are usually incom-patible. Since indium-tin oxide (ITO) thin films have lowresistivty, high transmittance, and good etching properties,which have been widely used as optoelectronic devices such astouch panels, flat panel display and thin film solar cells, we tryto prepare the ITO-Fe-cluster composite film by film sputteringdeposition combining with magnetic cluster beam deposition,and study their optical, electrical and magnetic properties.II. EXPERIMENTFe clusters were prepared by the plasma-gas-condensation(PGC)-type cluster beam deposition apparatus (Fig. 1), whichis based on plasma-glow-discharge vaporization (sputtering)and inert gas condensation techniques [6]. The apparatus iscomposed of the three main parts: a sputtering chamber, acluster growth region and a deposition chamber. The back-ground pressure of all chambers was Pa. Duringcluster deposition, a large amount of high purity Ar and HeDigital Object Identifier 10.1109/TMAG.2005.855216Fig. 1. Schematic drawing of PGC-type cluster deposition apparatus and RFhericon sputtering method. TMP, MBP, and CMP represent turbo-molecularpump, mechanical booster pump, and compound molecular pump, respectively.gases of mol/s were introduced con-tinuously into the sputtering chamber and evacuated by amechanical booster pump (MBP) through a nozzle, makingthe sputtering chamber pressure approximatelyPa. Fe atoms sputtered into the inert gas space are deceleratedby collisions with inert gas atoms and collide with each otherto form Fe clusters. We can control the mean cluster sizeby changing the flow rate of Ar and He gases from 7 to16 nm with a standard deviation less than 10% of [7]. ITOfilms were generated by a hericon sputtering system in thedeposition chamber. Fe clusters and ITO films were depositedon a substrate in a sandwich form (ITO nm/Fe-cluster-layer – nm/ITO nm) [Fig. 2(a)] or in a multilayer formnm nm nm[Fig. 2(b)], where the subscript numbers indicate the effec-tive thicknesses . The substrate temperature was kept at. The obtained ITO-Fe composite films wereannealed at and K for 2 h in a vacuum of aboutPa. The effective thickness of Fe cluster layerswas measured using a quartz crystal thickness monitorand the thickness of ITO films was obtained by the cali-brated relation between the input power, deposition time and0018-9464/$20.00 © 2005 IEEEPENG et al.: FERROMAGNETIC, TRANSPARENT AND CONDUCTING ITO-Fe-CLUSTER COMPOSITE FILMS 3407Fig. 2. (a) and (b) Schematic drawing of the ITO-Fe-cluster composite filmswith a sandwich form and a multilayer form. (c)–(f) TEM image of single Fecluster layer with different effective thicknesses.Fig. 3. Plan-view SEM image of the ITO-Fe-cluster composite filmswith (a) the sandwich form (ITO-Fe10 nm) and (b) the multilayer form(ITO  Fe2:5 nm  4).thickness. The structure and morphology were measured byX-ray diffraction, transmission electron microscope (TEM)and scanning electron microscope (SEM), optical transmit-tance by a spectrophotometer, and electrical resistivity by aconventional dc four-probe method. Magnetic measurementwas performed using a super-conducting quantum interferencedevice magnetometer.III. RESULTS AND DISCUSSIONIn this work, the size-monodispersed Fe clusters withnm were used. Fig. 2(c)–(d) show the TEM images of singleFe cluster layer with different effective thicknesses. In thoseimages, individual Fe clusters are distinguishable though theytouch and overlap each other. Fig. 3 shows the plan-view SEMFig. 4. X-ray diffraction pattern of the ITO-Fe-cluster composite films withthe sandwich form (ITO-Fe10 nm).Fig. 5. Optical transmission spectra of the ITO film and the ITO-Fe-clustercomposite films after heat treatment at (a) Ta = 623 and (b) 743 K.images of the samples with the sandwich form and the multi-layer form. The ITO-Fe-cluster composite films thus obtainedhave a clear polycrystalline structure in which the grain sizeranges from 30 to 50 nm. The X-ray diffraction measurement(Fig. 4) indicates that the ITO-Fe-cluster composite films arethe bixbyite phase.Fig. 5 shows transmission spectra of the ITO and ITO-Fe-cluster composite films after heat treatment at (a) and(b) 743 K. For both heat treatment conditions, the optical trans-mittance is decresed with increasing the Fe cluster layer thick-ness. As shown in Fig. 5(b), however, it can be seen that afterheat treatment was carried out at K, the transmittance3408 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005Fig. 6. Electrical resistivity  as a function of temperature T for (a) theITO film and (b) the ITO-Fe-cluster composite films with the sandwich form(ITO-Fe10 nm) and the multilayer form (ITO  Fe2:5 nm  4).Fig. 7. In-plane hysteresis loops of the ITO-Fe-cluster compositefilms with the sandwich form (ITO-Fe10 nm) and the multilayer form(ITO  Fe2:5 nm  4). The inset shows the data for low field regions.is not so much degraded and reaches above 90% for the samplewith a 10 nm Fe cluster layer for nm. This feature isattributable to the fact that Fe clusters are much smaller than thewavelength of visible light.Fig. 6 shows the electrical resistivity, , as a function of tem-perature for the ITO and ITO-Fe-cluster composite films. For theITO film [Fig. 6(a)], the temperature coefficient of resistivity isnegative below room temperature. For the ITO-Fe-cluster com-posite films [Fig. 6(b)], the resistivity exhibits a minimum be-tween 5 K and room temperature. Clearly, by embedding Feclusters into ITO matrix, the resistivity is decreased by an orderof magneitude in comparison with that of the ITO film and itsvalue is of an order of cm, which is three orders of mag-nitude smaller than that of the Co-doped TiO thin films [4], [5].Fig. 7 shows the magnetization curves at room temperaturefor the samples with the sandwich form (ITO-Fe10 nm) andthe multilayer form nm . The hysteresis isobserved, revealing that the present ITO-Fe-cluster compositefilms are ferromagnetic at room temperature. Moreover, both thesamples with the sandwich form and the multilayer form makeno clear difference in their magnetic properties. In the inset ofFig. 7, we plotted the low-field region of the loops showing co-ercivity values of about and 132 Oe, respectively.IV. CONCLUSIONWe have studied the optical, electrical and magnetic prop-erties of the ITO-Fe-cluster composite films prepared by com-bining the film sputtering deposition with the PGC-type clusterbeam deposition. The present experimental results show that itis possible to produce a film material with high optical transmit-tance, low resistivity and ferromagnetism at room temperatureby embedding Fe clusters into ITO matrix.ACKNOWLEDGMENTThis work was supported by Grant-in-aid for IntellectualCluster Project supported by the Ministry of Education, Sci-ence, Culture and Sports, Japan, Aichi Prefecture and AichiScience and Technology Foundation.REFERENCES[1] H. Ohno, “Making nonmagnetic semiconductors ferromagnetic,” Sci-ence, vol. 281, pp. 951–956, Aug. 1998.[2] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. waag,and L. W. Molenkamp, “Injection and detection of a spin-polarized cur-rent in a light-emitting diode,” Nature, vol. 402, pp. 787–789, Dec. 1999.[3] Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno, and D. D.Awschalom, “Electrical spin injection in a ferromagnetic semiconductorheterostructure,” Nature, vol. 402, pp. 790–792, Dec. 1999.[4] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fuku-mura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, and H.Koinuma, “Room-temperature ferromagnetism in transparent transitionmetal-doped titanium dioxide,” Science, vol. 291, pp. 854–856, Feb.2001.[5] S. R. Shinde et al., “Ferromagnetism in laser deposited anataseTi xCoO films,” Phys. Rev. B, vol. 67, pp. 115 211(1)–115 211(6), Mar.2003.[6] H. Haberland, M. Karrais, and M. Mall, “A new type of cluster andcluster ion source,” Z. Phys. D, vol. 20, pp. 413–415, Jan. 1991.[7] D. L. Peng, T. Hihara, K. Sumiyama, and H. Morikawa, “Structuraland magnetic characteristics of monodispersed Fe and oxide-coated Fecluster assemblies,” J. Appl. Phys., vol. 92, pp. 3075–3083, Sep. 2002.Manuscript received February 3, 2005.

Ferromagnetic, transparent and conducting ITO-Fe-cluster composite films

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Ferromagnetic, transparent and conducting ITO-Fe-cluster composite films

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