В работе представлены результаты исследования структурных и магнитооптических свойств
наногранулированных пленок Co-Ti-O, полученных в условиях твердотельной реакции с обменом
кислородом в слоистых структурах CoO/Ti. Показано образование гранулированных пленок со-
ставов в области и ниже порога перколяции. Обнаружены особенности магнитооптических
спектров пленок с коэффициентом заполнения магнитной фазы от 0.3 до 0.51 в сравнении со
спектрами сплошных пленок металлов.In this work the results of investigations of the magneto-optical properties of Co-Ti-O nanogranular films
obtained under conditions of the solid-state reaction with the oxygen exchange reaction in the CoO/Ti
layered structures are presented. The formation of the granular films with composition in the area of and
below a percolation threshold is shown. The magnetooptical spectra features of the obtained films with the
filling factor of magnetic phase from 0.3 to 0.52 in comparison with spectra of uniform metallic films are
found

Bondarenko, Gennadiy V.

Polyakov, Viktor V.

Polyakova, Klavdiya P.

Seredkin, Vitaly A.

Zharkov, Sergey M.

Бондаренко, Геннадий В.

Жарков, Сергей М.

Поляков, Виктор В.

Полякова, Клавдия П.

Середкин, Виталий А.

Siberian Federal University Digital Repository

Journal of Siberian Federal University. Mathematics & Physics 2010, 3(1), 111-117УДК 517.9Magnetooptical Properties of Nanogranular Co-Ti-O FilmsViktor V.PolyakovKlavdiya P. Polyakova∗Vitaly A. SeredkinGennadiy V.BondarenkoKirensky Institute of Physics, SB RAS,Akademgorodok 50, Krasnoyarsk, 660036,RussiaSergey M. ZharkovSiberian Federal University,Svobodny 79, Krasnoyarsk, 660041,RussiaReceived 10.12.2009, received in revised form 15.01.2010, accepted 11.02.2010In this work the results of investigations of the magneto-optical properties of Co-Ti-O nanogranular ﬁlmsobtained under conditions of the solid-state reaction with the oxygen exchange reaction in the CoO/Tilayered structures are presented. The formation of the granular ﬁlms with composition in the area of andbelow a percolation threshold is shown. The magnetooptical spectra features of the obtained ﬁlms with theﬁlling factor of magnetic phase from 0.3 to 0.52 in comparison with spectra of uniform metallic ﬁlms arefound.Keywords: nanogranular ﬁlms, solid-state reaction, synthesis, magneto-optical properties, Kerr eﬀect.Magnetic granular ﬁlms consisting of ferromagnetic nanoclusters embedded in an insulatingmatrix were extensively investigated in recent years because of their giant magnetoresistance [1],giant extraordinary Hall eﬀect [2], high magnetorefractive eﬀect [3], magneto-optical features[3–7] and potential magnetic recording applications [8]. It is known that the magneto-opticalproperties of composite systems depend on the ﬁlling factor (relative volume) of magnetic fractionand properties of a dielectric matrix [9]. Most of the performed research deals with metalparticles in SiO2 and Al2O3 dielectric matrixes [3, 5–7]. From this point of view the magneto-optical properties of the granular ﬁlms in TiO2 matrix with the dielectric constant exceeding thedielectric constants of SiO2 and Al2O3 are of interest [10].There exist several methods of the nanogranular structures preparation. Solid state reactionsin layered systems metal-dielectric or metal-semiconductor is one of the perspective methodsfrom the viewpoint of creating nanoparticles with narrow size distribution. Earlier it has beenshown that the products of solid-state reactions which are passing in layered Fe2O3/Al andFe2O3/Ti structures with the oxygen exchange reaction in layers are Fe nanoparticles imbeddedin dielectric matrixes Al2O3 and TiO2, respectively [11]. In this case, the oxide matrix had ananocrystalline or amorphous structure.In this work the results of investigations of magneto-optical properties of Co-Ti-O nanogran-ular ﬁlms with the ﬁlling factor X = 0.2 – 0.52 obtained under conditions of solid-state reactionwith oxygen exchange in the CoO/Ti layered structures are presented.∗pkp@iph.krasn.ruc© Siberian Federal University. All rights reserved– 111 –Viktor V.Polyakov, Klavdiya P. Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O FilmsFig. 1. X-ray diffraction pattern of the layer CoO/Ti structure (a) before and (b) after annealingTaking into account high chemical activity of the titanium in relation to oxygen, for prepara-tion of granular Co ﬁlms in a dielectric matrix of titanium oxide we used the following reaction:2CoO+ Ti = 2Co+ TiO2. (1)The CoO and Ti layers were reagents of reaction (1). It follows from the reaction equation(1), that the relative volume Х=V (Сo)/V (Co+Ti-O) of a magnetic phase (Co) in the reactionproducts is 0.4. To change the ratio of volumes of the magnetic and dielectric phases in productsof reaction it is necessary to change not only the ratio of the reagent layer thicknesses but alsothe pressure of residual gases. The ﬁlm preparation with X values near 0.4 and lower seemsto be easier comparing to X exceeding 0.4. In the ﬁrst case cobalt reduced completely and thesurplus titanium with respect to the equilibrium condition in equation (1) can be oxidized duringannealing.The CoO ﬁlms were obtained by oxidation of the cobalt ﬁlms in air at temperature 670K.The cobalt ﬁlms were prepared by thermal evaporation of Co metal on cover-glass plates invacuum 10−6 Torr at temperature 470–520K. The titanium layers were deposited on the CoOﬁlms by ion-plasma sputtering of a bulk titanium target in an argon atmosphere at a pressure(4-5)×10−5 Torr and the substrate temperature 320K. Thus, the samples for solid-state reactionconsisted of two layers. The solid-state reaction was carried out in a isothermal annealing modeat temperature 620–670K under a pressure 10−6−10−5 Torr. The layered structures with ﬁllingfactor of magnetic phase X = 0.2. . .0.52 have been obtained.The chemical composition and thickness of the obtained samples were deﬁned by X-ray ﬂuo-rescence analysis. The X-ray diﬀraction patterns were obtained with the DRON-3 diﬀractometer(CuKαradiation). The ﬁlm structure and the phase composition were studied with transmissionand diﬀraction electron microscopy using a PREM-200 transmission electron microscope. Themeasurements of saturation magnetization were carried out with a torsion magnetometer. Themagnitudes of the polar eﬀect Керра (θk) and the Faraday (θF ) eﬀects as well as their spectraldependencies were measured in magnetic ﬁelds up to 14 kOe with the technique of zero-analyzer– 112 –Viktor V.Polyakov, Klavdiya P.Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O FilmsFig. 2. Spectral dependences of the Kerr rotation angle of the nanogranular Co-Ti-O films with thefiling factor of the magnetic phase X = (1) 0.21, (2) 0.28, (3) 0.38, (4) 0.48, and (5) 0.52with double modulation of the polarization plane of the incident light. Measurements were madeat room temperature in the wavelength range of 350-1000 nm. The accuracy of measurementwas equal to ± 0.2min. The coercive force (Hc) was deﬁned from the magneto-optical hysteresisloops.For the phase identiﬁcation of products of the solid-state reactions in layered CoO/Ti struc-ture, we performed X-ray diﬀraction studies of the structure before and after reaction performedunder aforementioned conditionsIn Fig. 1а X-ray diﬀraction pattern of the layered CoO/Ti structure with the relative volumeof the magnetic phase Х = 0.4 is presented. This picture contains the reﬂection from the CoOphase. There are no titanium reﬂection in the X-ray diﬀraction pattern, which is likely indicativeof the formation of amorphous titanium. Note that research of the structure of the titanium ﬁlmsof 300 nm thickness, obtained at he same conditions, also shows the formation of an amorphousstructure.Fig.1b shows the X-ray diﬀraction pattern of the layered structure described above aftercarrying out the solid-state reaction at a temperature 670 K and a pressure 1×10−6 Torr inthe vacuum chamber. The diﬀraction picture contains a peak which can be attributed to bothcubic and hexagonal Co. Apparently, diﬀraction peaks of the titanium compounds are absentin the diﬀraction pattern, which is indicative of amorphous structure of titanium oxides. Thelatter does not contradict the data in [12] specifying the formation of the amorphous titaniumoxide ﬁlms with a thickness of up to 35 nanometers at a substrate temperature no higher than620K. In the work [13] the authors observe a sedimentation of amorphous oxide of titanium attemperature of the substrate not more than 720K.The electron microscopy investigations of the ﬁlm with X = 0.56 after carrying out the solid-state reaction, obtained on a single-crystal NaCl substrate, have shown, that the ﬁlm consists ofﬁne crystallites with the size of 10-20 nm and bigger crystallites with the size of 50-100 nm.As a result of the interpretation of the electron diﬀraction pattern, we established that theﬁlm after carrying out the reaction contains the following phases: Co (hcp), CoO, Co2TiO4.In the electron diﬀraction pattern, the reﬂections corresponding to the СoO and Co2TiO4phases have the shape of rings characteristic of polycrystals, and point reﬂections correspondingto the hcp Co phase. On the basis of this fact, we can conclude that the СoO and Co2TiO4– 113 –Viktor V.Polyakov, Klavdiya P. Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O FilmsFig. 3. Dependence of the Kerr rotation angle on the filing factor of the magnetic phase (1) in the caseof the maximum angle, (2) at the wavelength 700 nm, and (3) at the wavelength 630 nmphases are in ﬁne-grained state (the crystallite size is 10-20 nm), and form an original matrix inwhich bigger Co crystallites (the size is 50–100 nm) are located.Apparently, in this case the dielectric matrix has complex phase composition. The presenceof the CoO phase can be explained, to some extent, by the fact that, on the one hand, thereaction with oxygen exchange in the layers does not occur through all depth, and on the otherhand, there is an excess CoO at X = 0.56 with respect to the equilibrium content (X = 0.4).The formation of the Co2TiO4 compound in the dielectric matrix is unexpected likely due tothe variety of phases in Co-Ti-O system. Likely, the conditions of solid-state reaction should beoptimized.Thus, the X-ray diﬀraction and electron microscopy investigations of the ﬁlms synthesizedunder conditions of the isothermal reaction in the layered CoO/Ti structures unambiguouslyshow that metallic Co is formed among the products of the reaction (1). The presence of thedielectric matrix is conﬁrmed by the high values of the electrical resistivity of the sample in theinvestigated magnetic-phase concentration range (10−3 − 10−1Ωm).The research of magnetic properties of the obtained nanogranular Co-Ti-O ﬁlms shows thefollowing.The magnetization reversal curves of the Co-Ti-O ﬁlms measured by the magneto-opticalmethod (polar Kerr eﬀect) in a magnetic ﬁelds up to 14 kOe, for the composition with thevolume concentration of the magnetic phase X = (0.3 - 0.52) exhibit a ferromagnetic character,and curves for concentration of 0.21 does not have any hysteresis, what is indicative of thesuperparamagnetic state [11,14].The magneto-optical spectra of the obtained ﬁlms were investigated in the visible opticalrange in a magnetic ﬁeld up to 14 kOe. Fig. 2 shows the spectral dependences of Kerr rotationfor the nanogranular ﬁlms with various ﬁlling factor (volume concentration) of the magneticphase (Co). Apparently the spectral curves are nonmonotonic. For the structure with X =0.52 the curve shows resonance character with substantial increase in Kerr rotations angle in thewavelength range of 500–700 nm.Fig. 3 shows the dependence of the absolute value of maximum Kerr rotation angle and alsothe values of the angle at wavelength of 630 and 700 nm on the ﬁlling factor of the magneticphase X. It is necessary to note, that the observed features in the Kerr eﬀect spectrum of theobtained nanogranular ﬁlms are characteristic of ingomogeneous metal-isolator media [3–7].The dependencies (Fig. 3) have two regions of an increase in the rotation angle, one ofwhich is in the subpercolation region and the other region is near the percolation threshold. The– 114 –Viktor V.Polyakov, Klavdiya P.Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O FilmsFig. 4. Dependence of the specific Faraday rotation on the filing factor of the magnetic phase X at thewave-length of 630 nmﬁrst maximum was observed in work [5] and was connected, in opinion of the authors, with theinterference eﬀects. The second region is likely explained by the percolation [5, 6].The study of the Faraday rotation spectra of the obtained ﬁlms has shown the following. Thestarting layered CoO/Ti structure, which was not subjected to annealing, is characterized by thepresence of the Faraday rotation. This testiﬁes the fact that the solid-state reaction occurs evenduring deposition of the titanium layer on the cobalt oxide layer in spite of the arbitrarily coldsubstrate (320K). So, before annealing, the speciﬁc Faraday rotation of the ﬁlms with the ﬁllingfactror of the magnetic phase Х = 0.46 at a wavelength of 630 nm is 6.5 degree/µm. Annealingleads to the increase of the Faraday rotation to 12 degree/µm. The Faraday rotation spectra, aswell as the Kerr rotation spectra, are nonmonotonic. Fig. 4 shows the dependence of the speciﬁcFaraday rotation angle on the X at wavelength of 630 nm. It should be noted that there is acorrelation between the ﬁlling factor dependences of the Kerr and Faraday rotations (comparewith curve 3 in Fig. 3).Let us note the basic results of the investigations of physical properties of the nanogranularCo-Ti-O ﬁlms obtained under conditions of the solid-state reactions in layered CoO/Ti structures.The products of the solid-state reaction in layered structure CoO/Ti in the isothermal anneal-ing mode are metallic Co and amorphous Ti-O compounds. The nanogranular ﬁlms are metalliccobalt particles embedded in a dielectric matrix. The shape of the magnetization reversal curvesfor the samples with X = (0.3−0.52) testiﬁes to the ferromagnetic interaction between magneticregions.The magneto-optical spectra of the Kerr eﬀect of the ﬁlms with ﬁlling factor of magneticphase from 0.3 to 0.52 are nonmonotonic, unlike the spectrum of the uniform Co ﬁlm. In thewavelength range of 550–700 nm, the Kerr rotation angle of the ﬁlms with Х = 0.52 increases bya factor of 2–2.5 compared to conventional Co ﬁlms. The correlation between the ﬁlling factordependences of the Kerr and Faraday rotations is established.The work was supported by the Russian Foundation for Basic Research (RFBR projects no.08-02-00397-а, no. 07-03-00190-а).References[1] L.V. Lutsev, Yu.E.Kalinin, A.V. Sitnikov et. al., Electron transport in magnetic ﬁeld ofgranular amorphous silica dioxide ﬁlms with ferromagnetic nanoparticles, Fiz. Tverd. Tela,– 115 –Viktor V.Polyakov, Klavdiya P. Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O Films44(2002), no. 10, 1802 (in Russian).[2] J.C.Denardin, M.Knobel et. al., Giant Hall eﬀect in superparamagnetic granular ﬁlms, J.Magn. Magn. Mater., 262(2003), no. 1, 12.[3] V.G.Kravets, L.V. Poperenko, I.V.Yurgelevych et. al., Optical and magneto-optical prop-erties and magnetorefractive eﬀect in metal-isolator CoFe-Al2O3 granular ﬁlms, J. Appl.Phys., 98(2005), no. 4, 043705.[4] V.S. Zhigalov, R.D. Ivantsov, I.S. Edelman et. al., Faraday eﬀect in Co-Sm-O nanogranularﬁlms, Fiz. Tverd. Tela, 47 (2005), no. 6, 91 (in Russian).[5] A.M.Kalashnikov, V.V.Paul, R.V.Pisarev et. al., Optical and magneto-optical propertiesof CoFeB/SiO2 and CoFeZr/Al2O3 nanogranular structures, Fiz. Tverd. Tela, 46(2004), no.11, 2092 (in Russian).[6] Z.S. Jiang, G.J. Jin, J.T. Ji et. al., Magneto-optical Kerr eﬀect in Fe-SiO2 ﬁlms, J. Appl.Phys., 78(1995), no. 1 , 439.[7] A.V.Kimel, R.V.Pisarev, A.A.Rzhevsky, Magneto-optical investigation of granular ﬁlms ofoxide silica with CoNbTa particles, Fiz. Tverd. Tela, 45 (2003), no. 2, 269 (in Russian).[8] G.I. Frolov, Film medium for memory devices with superdense magnetic record, JTP,71(2001), no. 8, 50.[9] M.Abe, M.Gomi, Magneto-optical eﬀect and eﬀective dielectric tensor in composite materialcontaining magnetic ﬁne particles or thin layers, Jpn. J. Appl. Phys., 23(1984), no. 12, 1580.[10] Yu.A.Dynnik, I.S. Edelman, T.P.Mopozova et. al., Faraday eﬀect in Co/TiO2 ﬁlms, JETPLett., 65(1997), no. 7-8, 531.[11] V.G.Miagkov, K.P. Polyakova, G.N.Bondarenko, V.V.Polyakov, Granular Fe-Al2O3 ﬁlmsprepared by self-propagating high temperature synthesis, J. Magn. Magn. Mater., 258-259(2003), no. 2, 358.[12] L.Holland, Vacuum Deposition of Thin Films (New Work, 1956; Gosenergoizdat, Moscow,1962).[13] M. Lottiaux, C.Boulesteix et. al., Morphology and structure of TiO2 thin layers vs. thicknessand substrate temperature, Thin Solid Films, 170(1989), no. 1, 107.[14] D.Kumar, J.Narayan, A.V.Rvit et. al., High coercivity and superparamagnetic behavior ofnanocrystalline iron particles in alumina matrix, J. Magn. Magn. Mater., 232(2001), no. 1,161.– 116 –Viktor V.Polyakov, Klavdiya P.Polyakova..... Magnetooptical Properties of Nanogranular Co-Ti-O FilmsМагнитооптические свойства наногранулированныхпленок Co-Ti-OВиктор В.ПоляковКлавдия П.ПоляковаВиталий А.СередкинГеннадий В.БондаренкоСергей М.ЖарковВ работе представлены результаты исследования структурных и магнитооптических свойствнаногранулированных пленок Co-Ti-O, полученных в условиях твердотельной реакции с обменомкислородом в слоистых структурах CoO/Ti. Показано образование гранулированных пленок со-ставов в области и ниже порога перколяции. Обнаружены особенности магнитооптическихспектров пленок с коэффициентом заполнения магнитной фазы от 0.3 до 0.51 в сравнении соспектрами сплошных пленок металлов.Ключевые слова: наногранулированные пленки, твердотельные реакции, синтез, магнитооптиче-ские свойства, эффект Керра.– 117 –

Magnetooptical Properties of Nanogranular Co-Ti-O Films

Magnetooptical Properties of Nanogranular Co-Ti-O Films

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