Magnetic properties of nanostructured thin films of transition metal obtained by low energy cluster beam deposition V. Dupuis, J. P. Perez, J. Tuaillon, Vincent Paillard, P. Mélinon, A. Perez, B. Barbara, L. Thomas, S. Fayeulle, J. M. Gay
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V. Dupuis, J. P. Perez, J. Tuaillon, Vincent Paillard, P. Mélinon, et al.. Magnetic properties of nanostructured thin films of transition metal obtained by low energy cluster beam deposition. Journal of Applied Physics, American Institute of Physics, 1994, 76 (10), pp.6676 - 6678. 10.1063/1.358165. hal-01660437
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Citation: J. Appl. Phys. 76, 6676 (1994); doi: 10.1063/1.358165 View online: http://dx.doi.org/10.1063/1.358165 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v76/i10 Published by the American Institute of Physics.
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Downloaded 27 Apr 2013 to 130.113.111.210. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions Magnetic properties of nanostructured thin films of transition metal obtained by low energy cluster beam deposition V. Dupuis, J. P. Perez, J. Tuaillon, V. Paillard, P. MGlinon, and A. Perez Dgpartement de Physique des Matiriaux, Universitg Lyon, 1-69622 Weurbanne C&dex, France B. Barbara and L. Thomas Laboratoire de Magndtisme Louis Nt?el, CNRS BP166X-38042 Grenoble Cgdex, France S. Fayeulle Dgpartement Mat&aux Mkanique Physique, Ecole Centrale de Lyon, BP 16369131 Ecu&, France J. M. Gay CRh4C2, Campus de Luminy, CNRS-13397, Marseille Ct?dex 13, France Clustersof iron, cobalt, andnickel areproduced in a laservaporization source. The size distributions of the incident clusters are checkedby time-of-flight massspectrometry before deposition at low energy.Studying the near threshold photoionization,Co, and Ni, clusters exhibit an icosahedral structure while for iron, no clear structure emerges.Neutral clusters were depositedon different substratesat room temperaturewith thicknessesup to 100 nm in view to determinetheir structure and magnetic properties.A limited coalescenceof the clusters is observedfrom high-resolution transmission electron microscopy. No icosahedronhas been observed but cuboctahedronand interfacetwins betweenadjacent particles have been clearly identified in Ni films. Grazingincidence x-ray diffraction experimentsreveal a classicalphase with grain size around6 and 4 mn for Fe and Ni fYms, respectivelybut an anomalousfee phasefor Co fdms and a very low grain size of 2 nm. The density of films determinedby x-ray reflectivity was estimatedto representonly 60%-65% of the bulk density.Magnetic behaviors studied by ferromagneticresonance and SQUID magnetization measurementshave been interpreted using the correlated spin glass model. Miissbauer spectra performed on Fe films at zero field revealed the presenceof 20% of iron in the form of thin nonmagneticoxide skin surroundingFe grains which allow to fmd 2.2 ,LLBper magneticiron atom in agreementwith macroscopicmagnetic measurements.Nevertheless we found an anomalous reducedatomic moment for Ni film.
INTRODUCTION Our challengein depositingtransition metal clustersis to synthesizenew phaseswhere the anomalouscrystallographic Recently,magnetic properties such as exchangecoupling structures of free clusters would be kept and to study the or giant magnetoresistancemainly observedin metallic mul- specific magnetic behavior of these weakly correlatedenti- tilayers have been detectedin other nanostructuredsystems. ties on a substrate.Once more, we show that our technique For example,Berkowitz et a1.l and Xiao et al.’ observedgi- leads to a random compact cluster stacking (RCCS).3Thus ant magnetoresistancesin ultrafine Co-rich precipitate par- magnetic results could be interpretedby random anisotropy ticles in a Cu-rich matrix. These sampleswere preparedby model with a scale law and in terms of localization of spin coevaporationtaking advantageof the low solubility of Cu in waves. Co. However, though this techniqueis limited to nonmiscible components,the adjustablecluster diameteris a new param- eter in addition to the distancebetween particles as in thin EXPERIMENT film layers. Thus, studies on clusters and cluster assembled Our cluster source is based on the technique of laser materialsare of increasinginterest. vaporization.3-5Roughly, a plasma is created in a vacuum The laser vaporization source of the laboratory3allows cavity by Nd-YAG laserlight. Synchronizedwith the laser,a the obtentionof an intensecluster beamof any size distribu- high pressure(5 bars) helium pulse, injected in the cavity by tion (from few to a thousandatoms per cluster) and the syn- a nozzle, thermalizesthe plasma and cluster growth occurs. thesis of cluster assembledmaterials, even of the most re- The nascentclusters are then rapidly quenchedduring the fractory and of the most complex ones. The cluster size following isentropic expansioninto vacuum (lo-’ Torr). distribution is checked by time-of-flight mass spectrometry Cluster size distributions are analyzedin a time-of-flight before deposition. Our source producing cold clusters with massspectrometer. Studying the nearthreshold photoioniza- low kinetic energy,incident clustersdo not fragment on the tion (performedwith a frequency-doubledtunable dye laser substrateand may conservetheir intrinsic structures.Thus pumpedby a XeCl excimer laser), mass spectraof Co, and we succeeded in the stabilization of very small size Ni, clusters exhibit oscillations and a series of magic num- fullerenes (C&L&J, never previously observedexperimen- bers in=13,55,147,309,561,...) corresponding to an tally. We clearly evidenced that deposited carbon clusters icosahedralor cuboctahedralatomic shell structurein the ob- presentedthe in flight-clusters fingerprint? tained massrange (50-800 atomsper cluster). A finer analy-
6676 J. Appt. Phys. 76 (lo), 15 November 1994 0021-8979/94/76(10)/6676/3/$6.00 0 1994 American Institute of Physics
Downloaded 27 Apr 2013 to 130.113.111.210. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions sis allows us to conclude for the icosahedralstructnre.6 For iron clusters, the results are not so simple, indicating a com- r/ petition between different regimes.6 --, I,, I...I, .I Then, free neutral clusters are deposited with a kinetic energy in the lo-20 eV range on different substratesat room temperaturewith thicknessesup to 100 nm in view to deter- mine their structure and magnetic properties.
SAMPLE CHARACTERIZATION The typical size of supported clusters obtained from high-resolution transmissionelectron microscopy (HRTEZM) was about 2-6 nm for an initial size distribution centered around 150 atoms for Fe and 300 atoms for Co and Ni clus- ters, respectively. No icosahedronwas observedbut cuboc- tahedra and interface twins between adjacent particles was clearly identified in Ni films. Quasisphericalgrain morphol- ogy existed in Fe film which could correspond to a bee rhombic dodecahedron Cl101 according to the Wulff’s theorem.7 Grazing incidence x-ray diffraction (GIXD) ex- periments exhibit a classicalbee phasefor Fe fI.lms but a fee phasefor both Co and Ni with a grain size extractedfrom the -12 -8 -4 0 4 8 12 peak width of about 6, 4, and 1.5 nm, respectively,in agree- VELOCITY mm/s ment with electronic diffractions and TEM observations.The classical structure of cobalt being hcp, the fee phase ob- FIG. 1. Mkbauer spectra obtained on a Fe,,, film at 300 K. served in Co films might be related to the icosahedralstrut- ture of the incident cluster beam. In fact, the icosahedronis expected to be the precursor of the fee crystal. The small The macroscopic magnetic behavior of our films has grain size and the reminiscence of a free cluster structure been studiedusing ferromagneticresonance (FMR) and mag- confirm the limited coalescenceprocess due to a weak diffu- netization measurements(SQUID). FMR curves roughly tra- sion of metallic clusters on the substrateeven at room tem- duced a thin film behaviorrl but revealed several resonance perature. magnetic fields due to anchorageof spin waves at the surface Rutherford backscattering spectroscopy showed that (when the applied field is perpendicularto the surface of the theseporous films are composedof 2070~30%of oxygen for film). The coercive field at 300 K is about 100 Oe for cobalt, 70%-80% of metals. The density of the films, determinedby 50 Oe for iron, and lower than 10 Oe for nickel and increases x-ray scattering at very low angle in 8/28 mode and from at 10 K up to 1000 and 500 Oe for Co and Fe films, respec- rocking curve around the critical angle of the total reflectiv- tively. The value of the saturationmagnetization was related ity, was estimated to represent only 60%-65% of the bulk to the density value and to the quantities of oxides. For iron density.’ in agreementwith Miissbauer results, the classical atomic moment of 2.2 ,uB per magnetic iron atom is retrieved. On the contrary we found a strongly reduced value in Ni film. MAGNETIC PROPERTIES The atomic moment per Ni atom has been estimated to be Mijssbauerspectra performed at room temperatureon Fe equal to l/4 of the bulk value (taking into account the fiIm films without magnetic field revealed the presence of the density). By extrapolating the magnetization curve versus sextet of the metallic iron (with hyperfine field around 332 temperature,the Curie temperatureT, has been found to be KOe and representing80% of the signal) and of two doublets around 350-400 K. Thus the magnetization reduction can correspondingto 20% of ferric oxide (Fig. 1). Whereasthe not be uniform, otherwise T, would be much more reduced. common isomer shift of both nonmagneticsignals is equal to This could be due to the presenceof dead magnetic layers 0.4 mm/s compared to the metallic iron, the quadrupolar similar to that observedin Fe films and/or to antiferromag- splittings respectively equal to 0.9 and 2.4 mm/s allow us to netic coupling betweenNi particles via Ni shell. Magnetore- differ two types of oxide. The first one is identified as a thin sistance measurementsand magnetization under high mag- layer of nonstoechiometricFeaO, or/and a mixture of sto- netic field are in progress in view to see respectively an echiometric Fe304 and y-FelOT phases’ (in agreementwith important negative magnetoresistanceand a second transi- the GIXD experiments on the‘most oxidized fllmsrO). The tion of the saturation magnetization. second one, not identified so far could be related to a free- We fitted the approachto saturationof the magnetization cluster oxidation. These results allow to describe the sup- using the Chudnovsky model.12$13The experimental law of ported clusters as pure iron core surroundedby a thin skin (2 approachto saturationin magnetic systemsis perfectly fitted or 3 monolayers7)of non-magneticoxide. On the other hand, by the formalism of the random-anisotropyamorphous mag- the intensity ratio of the sextuplet (321123) evidencesa ran- nets. In the Chudnovsky’s model,” a physical parameteris dom spatial distribution of the magnetizationat zero field. defined
J. Appl. Phys., Vol. 713, No. 10, 15 November 1994 Dupuis et a/. 6677
Downloaded 27 Apr 2013 to 130.113.111.210. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 1 1.5 , ! ! /
-2500 0 -2500 5000 0.85 El (Oe)
FIG. 3. Magnetization vs temperature obtained on a Ni,, film. 0.8 L 0 5000 10000 15000 H (04 perature(maybe correlatedto a Tr+, of Ni oxide). At low temperaturethe random anisotropy fluctuations dominate FIG. 2. Magnetization law in approaching saturation obtained on a Fers, (with a Hm2law) whereasat high temperaturethe exchanges film at 300 K. dominate(with a H ml/2law), the transition occurring around 200 K. In conclusion, these first results lead us to pursue this study to elucidatesome other characteristicmagnetic param- etersof the layers (T, , e.g., antiferromagneticcoupling in Ni films, and completemagnetic study on promising Co films). In particular,x-ray absorptionmeasurements will allow us to whereR, is the distanceover which the local anisotropyaxes Iocally describecrystallographic and magnetic atomic envi- arecorrelated and A is the exchangeconstant. The parameter ronmentin view to explain the anomalousatomic moment in A is the critical boundarybetween the correlatedspin glass Ni films. Experimentswith cooled substratesare in progress (CSG) regime (X
Published without author corrections
6678 J. Appl. Phys., Vol. 76, No. IO, 15 November 1994 Dupuis et al.
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