Thin Solid Films 518 (2010) 3945–3948

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Thin Solid Films

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Morphological and structural investigations of Co–MgF2 granular thin films grown bythermionic vacuum arc

V. Ionescu a, M. Osiac b,⁎, C.P. Lungu c, O.G. Pompilian c, I. Jepu c, I. Mustata c, G.E. Iacobescu b

a Ovidius University, 900527, Constanta, Romaniab Faculty of Physics, University of Craiova, 200585, Craiova, Romaniac National Institute for Laser, Plasma and Radiation Physics, Bucharest-Magurele, 077125 Romania

⁎ Corresponding author.E-mail address: m_osiac@yahoo.com (M. Osiac).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.02.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 June 2009Received in revised form 1 February 2010Accepted 12 February 2010Available online 19 February 2010

Keywords:Thermionic vacuum arcThin filmsCo–MgF2 plasma

Co–MgF2 granular films were deposited in thermionic vacuum arc plasma with simultaneous ignition ofplasma in Co and MgF2 vapours. The samples were investigated by transmission electron microscopy whichrevealed Co grains of a few nm in diameter embedded in the MgF2 matrix. The crystalline phase for Co andMgF2 was studied by electron diffraction after selecting a typical area of the sample. Low-angle x-raydiffraction method was used to verify the bulk crystalline structure of the samples. The surface morphologiesof the films were investigated by atomic force microscopy. The magneto-optical longitudinal Kerr rotationspectra of the films were also measured and compared.

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1. Introduction

Combinations of metal–nonmetal granular soft magnetic filmssuch as Co–Al2O3, Co–MgO, Fe–SiO2, and Fe–SrF2 have attracted muchinterest in the last years, due to their electrical and magneticproperties, such as high electrical resistivity, low coercivity, loweffective magnetic anisotropy, and very weak inter-particle magneticinteraction.

Good magnetic properties including susceptibilities of 10−3 up to10−1, electrical resistivity over 500 µΩ cm, saturation flux densities ofabout 1 T, and coercivities as low as 10 A/m have been obtained [1–4].For example, Co–MgF2 granular system is used as thin film inductor inpower converters for microprocessors, with fast response, highefficiency and small size. Magnesium fluoride (MgF2) was chosen asan insulating material because of its low evaporation point. Thismaterial has a high resistivity due to the wider gap and smallerdielectric constant than e.g. MgO and Al2O3, and it has a tendency toget separated from the co-evaporated metal [5,6].

Networks of magnetic tunnel junctions are formed in ferromag-netic metal (FM)-insulator granular films composed of metallicnanodots embedded in the insulating matrix. The tunneling magne-toresistance effect due to spin-dependent tunneling between neigh-bouring FM dots has been intensively studied [7–9].

In the present study we propose the use of a thermionic vacuumarc (TVA) deposition technique for coating smooth, nanostructuredCo–MgF2 granular thin films onto glass substrates. We investigated

two samples placed at two different positions relative to the anode ofthe discharge that produces the Co plasma.

2. Experimental details

The method used here based on TVA for deposition of granularfilms was described in details elsewhere [10,11]. It uses two identicalset-ups which produce two separate electron beams emitted byexternally heated cathodes. Each cathode is made from a tungstenwire with a diameter of 1 mm. The electron beams are accelerated byhigh anodic voltages and bombard simultaneously the cruciblesplaced at the anodes, as shown in Fig. 1. The crucibles contain each afew grams of Co and MgF2, respectively. By applying a high voltagebetween 1 and 6 kV on each of the anode–cathode sets, two differentplasmas are formed in pure Co and MgF2 vapours. The chamber ispumped down by mechanical and diffusion pumps until a pressure of2.7*10−3 Pa is obtained.

Both plasmas expand in the high vacuum chamber and reach thesurface of the substrates of the samples, which are grounded.

The currents used to heat the cathode filaments are between 40and 55 A. The current of the discharge ignited in Co vapours isIdisch=0.7 to 0.8 A at a voltage Udisch=400 V. In the case of thedischarge ignited in MgF2 vapours, the current and the voltage wereIdisch=0.4 to 0.5 A and Udisch=300–400 V, respectively. The deposi-tion rate rd and film thickness d were measured and controlled in situusing a FTM7 quartz microbalance. The thickness of the Co–MgF2coatings was 200 nm.

The film substrates were rectangular pieces of industrial glass withan area of 15×15 mm2. The studied thin film samples were located atP1 above and just in front of the Co crucible and P2 at mid-distance

Fig. 1. Experimental set-up for Co–MgF2 thin film deposition.

Fig. 2. Cross-sectional TEM images for: a) sample P1; b) sample P2; and c) SAED patternfor sample P1.

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between the anodes as illustrated in Fig. 1. The distance between theCo crucible and P1 sample was 250 mm, while the distance betweenthe Co crucible and P2 sample was 275 mm. The substrates weremaintained at T=300 K during deposition.

The transmission electron microscopy (TEM) analyses werecarried out using a Philips CM120ST microscope operating at 100 kVwith Cs=1.2 mm and ≈2 Å resolution. The x-ray diffraction (XRD)patterns were obtained using a Shimadzu model 6000 diffractometeroperating with Cu Kα radiation (40 kV, 30 mA). The atomic forcemicroscopy (AFM) data were recorded in non-contact mode using aPark XE-100 equipment (silicon tip with conical shape). The AFMscans were taken over the area 5×5 μm2. We used the horizontal lineby line flattening as planarization method.

Finally, the magnetic properties of the films were investigated vialongitudinal Magneto-Optic Kerr Effect (MOKE), using p-polarized,633 nm He–Ne laser light (10 mW power) as an incident light. Theincident beam was modulated with an electro-optic-modulator.The magnetic field was applied parallel to the films and lays in theincidence plane of He–Ne laser.

3. Results and discussion

The TEM image of P1 sample, presented in Fig. 2(a) shows anetwork-like structure of nanoscale grains of Co (dark areas) andthin intergrain boundaries of MgF2 (light areas). The grain size isabout 3–4 nm in diameter with intergrain regions of 3–6 nm width.The TEM image of P2 sample presented in Fig. 2(b) shows the presenceof some very fine dark spots (Co) fixed in a lighter area surface (MgF2

Fig. 3. XRD patterns of: a) sample P1; and b) sample P2.

Fig. 5. MOKE measurements on samples P1 and P2 at room temperature.

Fig. 4. AFM top view for a scan area of 5×5 µm2 in a) sample surface P1; and b) samplesurface P2.

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structure). We could not notice the existence of Co nanograinsembedded in MgF2 matrix in this case. Coonley et al. [6] reported asimilar structure for a granular soft magnetic Co–MgF2 film preparedby evaporative co-deposition onto glass substrate, with 41% volume ofCo. In their study, the Co grain size was between 2.7 nm and 3.7 nm indiameter, and the intergrain region widths in the range of 1 to 2 nm.

For sample P1 the well defined diffraction rings shown in SAEDpattern (Fig. 2(c)) correspond to the interplanar distances of0.212 nm and 0.194 nm. These values fit well with the distances ofthe (100) and (101) diffraction planes for the hexagonal-close-packed(hcp) phase of Co. This measurement proves the presence ofcrystalline structure of Co grains in the MgF2 matrix of sample P1.

Low-angle XRD analyses were performed to establish the presenceof crystalline phases in the coatings. In the XRD patterns of bothsamples P1 and P2 the 2θ diffraction peaks can be assigned to (110),(111), (210), (211) and (301) crystalline planes of MgF2 tetragonalphase (Fig. 3). In Fig. 3(a), we can see for sample P1 the smooth slopeof the XRD curve in the 44°–47° range. The Co lines inserted from thedatabase JCPDS—International Centre for Diffraction Data, whichoverlap this angle range, allowed us to assign this slope to thepresence of Co in the MgF2 matrix. Fig. 3(b) shows that by XRDmeasurements it is not possible to detect the presence of Co in sampleP2.

The topographical AFM images and subsequent statistical dataanalysis, including the calculation of the root mean square roughness(Rq), gave detailed information about the surface morphology. Themorphological AFM images of samples P1 and P2 are presented inFig. 4(a) and (b), respectively. The Rq values are 3.8 nm for P1 and4.7 nm for P2. The in-plane size of the surface asperities was found tobe almost uniform for both samples, only a small number of theseasperities being higher than 10 nm for P1 and higher than 20 nmfor P2. Similar results were reported by Hosoya et al. [1] in the case ofFe–SrF2 co-evaporated granular films deposited onto MgO substrates.

In Fig. 5 we present the MOKE hysteresis loops (longitudinalmode) at room temperature for samples P1 and P2. It can be observedthat the saturation magnetization for sample P1 is about two timeshigher than that of the P2 sample; the coercivities resulting fromMOKE measurements are: 0.038 T for P1 and 0.054 T for P2.

4. Conclusions

In order to obtain Co–MgF2 composite thin films in plasmasgenerated in the vapours of Co andMgF2materials employing the TVAtechnique the substrates are usually placed at 250–275 mm above theevaporating anodes. From the SAED pattern obtained by TEM analysiswe could reveal the presence of hcp crystalline phase of Co and ofnanometrical Co grains embedded in the MgF2 insulating matrix, onlyin the film placed just above the anodewhich produces the Co plasma.The XRD patterns in the bulk of the same film revealed a randomorientation of Co in the MgF2 crystalline matrix. This sample has a lowsurface roughness with crack-free and densely packedmicrostructure.The best magneto-optical properties are obtained for the filmwith the

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Co nanograins fixed in the MgF2 structure. The Co–MgF2 granular thinfilm with the thickness of about 200 nm was deposited onto glasssubstrates placed within the Co plasma.

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