Lederman, D.

Shi, H.

English

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Faculty Scholarship2000Annealed Co thin films: Pit formation andmagnetic anisotropyH. ShiD. LedermanFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationShi, H. and Lederman, D., "Annealed Co thin films: Pit formation and magnetic anisotropy" (2000). Faculty Scholarship. 57.https://researchrepository.wvu.edu/faculty_publications/57Annealed Co thin films: Pit formation and magnetic anisotropyH. Shi and D. LedermanDepartment of Physics, West Virginia University, Morgantown, West Virginia 26506-6315We present a study of the morphological and magnetic transformation of hcp~0001! Co thin films,grown via sputtering on Al2O3(112¯0), which occurs when they are annealed in vacuum in thetemperature range of 500 °C,TA,600 °C. In this temperature range, the films undergo a reentrantsmoothening transition. For very thin films ~thickness t,20 nm!, this coincides with a fcc~111! tofcc~001! recrystallization, and with the appearance of rectangular pits. For thicker films, therecrystallization does not occur, even though the surface smoothens out leaving hexagonal pitsbehind. The number of pits per unit area decreases exponentially as the thickness of the film isincreased, demonstrating that interactions at the film surface and film–substrate interface cause thesmoothening and pit formation during the annealing process. Whereas the hcp~0001! and fcc~111!phases are magnetically isotropic in the film’s plane, the fcc~001! films exhibit a biaxial in-planeanisotropy. © 2000 American Institute of Physics. @S0021-8979~00!50808-1#I. INTRODUCTIONCo thin films have been intensively studied during thepast few years because of the ease of epitaxial growth ondifferent substrates, its applications in giant magnetoresistive~GMR! devices, and in the formation of magnetic nanopar-ticles. Bulk Co is unusual among the 3d transition metalelements because it undergoes a hcp to fcc martensitic tran-sition at a temperature ’415 °C.1Much of the research on pure Co films has focused onthe crystallinity and film quality of as-grown samples. It hasbeen previously reported that Co films deposited onAl2O3(112¯0) at approximately 300 °C are smoother thanfilms grown at higher temperatures,2 and that the fcc phase isstable in fine particles only at high temperatures.3 It is alsoknown that the fcc Co phase can be stabilized at low tem-peratures in Co/Ni superlattices,4 and that bcc Co can begrown on GaAs as single films5 or in Co/Fe superlattices.6We recently discovered a new reentrant smootheningtransition in pure Co films grown on Al2O3(112¯0).7 In thiscase, as-grown epitaxial films annealed at higher tempera-tures initially become rough as the annealing temperature TAis increased. However, in the temperature range 500 °C,TA,600 °C, the surface smoothens out, leavingsubmicron-sized rectangular pits behind if the film thicknesst,20 nm. This coincides with a fcc~111! to fcc~001! recrys-tallization of the lattice. A similar, thus far unexplainedsmoothening transition, which leaves nanometer-scale rect-angular pits behind, occurs in Ag fcc~111! films grown at130 K on GaAs when they are annealed at roomtemperature.8By understanding the basic mechanism responsible forthe pit formation in these materials, new lithographic tech-niques, and novel magnetic structures could be developed. Inthis paper, we study further the morphology and magneticproperties of annealed Co films to obtain further clues aboutthe pit formation mechanism. We find that the concentrationof rectangular pits in fcc~001! films decreases exponentiallywith the film thickness, indicating that the pits form becauseof a surface or surface/substrate interaction. We also find thatthicker films do not undergo the fcc~111! to fcc~001! reori-entation, although the smoothening transition does occur andhexagonal pits are formed. The fcc~001! recrystallized filmsare magnetically anisotropic in the plane, in contrast with theisotropic fcc~111! and hcp~0001! films.II. EXPERIMENTAL DETAILSThe samples were grown via dc magnetron sputtering ina high vacuum chamber with base pressure of 331027 Torr using a 3.0 mTorr argon pressure. The growthtemperature for all samples was 315 °C, and the substratewas Al2O3(112¯0). Immediately after growth, the sampleswere annealed in vacuum at different temperatures for 15min, and subsequently quenched to room temperature to pre-serve the high temperature morphology. Quenching wasachieved by turning off the heater and then making thesample holder come in contact with a small liquid nitrogenvessel. For samples grown under nominally identical condi-tions, differences in the surface morphology were due to thedifferent annealing conditions. The surface morphology wasthen studied ex situ using atomic force microscopy ~AFM!.Magnetic hysteresis loops were acquired, also ex situ, usingthe magnetooptic Kerr effect ~MOKE! at room temperature.III. RESULTS AND DISCUSSIONAn AFM image of a 6.3 nm thick sample annealed at535 °C is shown in Fig. 1. In this case, the film is orientedalong the fcc@001# direction. The pits shown in the image areas deep as the film thickness to within the AFM resolution.7We therefore believe that the pits are devoid of Co, althoughwe cannot exclude the possibility that a monolayer or tworemain. An important observation is that the sides of thedifferent rectangular pits on the sample are aligned along thesame direction. This is reasonable given that the films areepitaxial when the pits are formed. X-ray phi scans of thesample and substrate show that the sides of the pits corre-spond to the Co fcc@100# and @010# directions.Figure 2 shows the thickness dependence of the surfacemorphology. All of the samples shown were annealed atJOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 200060950021-8979/2000/87(9)/6095/3/$17.00 © 2000 American Institute of Physics550 °C. Rectangular pits form on samples with t,20 nm.For the thicker sample shown, the pits are hexagonal. X-raydiffraction revealed that thicker samples remain fcc~111! ori-ented when they undergo their smoothening transition. Sincethe film is epitaxial, the pits reflect the in-plane crystal sym-metry, which in this case is hexagonal.The number of pits per unit area, r, calculated fromAFM images, is shown in Fig. 3~a!. For the thinner samples,r decreases exponentially with increasing sample thickness.By fitting r to r5r0e2t/l, where t is the film thickness, oneobtains a characteristic length scale l51.2 nm. The physicalmeaning of l may correspond to the minimum film thicknessrequired for the pits to form, but more experiments are re-quired to verify this. Figure 3~b! shows the porosity of thefilm as a function of film thickness, calculated as the percent-age of the area that is missing from the film. For small thick-ness, the porosity seems to level off, indicating that the sizeof the pits decreases slightly for very thin films. The expo-nential dependence is an indication that the pit-formingmechanism is strongly influenced by the surface and/or thesubstrate/film interface.The magnetic properties of the thin fcc~001! sampleswhich have undergone the smoothening transition are verydifferent from those of the as-grown samples. The as-grownsamples do not display an in-plane magnetic anisotropy. Onthe other hand, the magnetic hysteresis loops of the fcc~001!samples are strongly dependent on the direction of the ap-plied field, as shown in Fig. 4. The presence of the doubleloops for angles at approximately 90° from the easy axis isan indication of biaxial anisotropy, with the magnitude of themagnetic anisotropy being greater for one of the easy axes(Q50°) than the other (Q590°).9 From in-plane x-ray phiscans,7 we find that the axis with the larger anisotropy pointsalong the fcc@110# direction, which is parallel to the in-planec axis of the Al2O3 substrate. A similar phenomenon hasbeen previously observed in Fe bcc~001! films grown onMgO, resulting from the angle of the Fe flux during deposi-tion which presumably creates a unidirectional strain.9 In ourcase the in-plane strain, if present, is not created duringgrowth, but during the post-growth annealing process.Regarding the possible mechanism for the reentrantsmoothening transition and the crystallographic changes as-sociated with it, we note that the pits are formed only onsamples grown in our high vacuum sputtering system. Wehave performed preliminary experiments on samples grownvia molecular beam epitaxy ~base pressure 7310210 Torr!,and discovered that these samples do not form rectangularpits, although hexagonal pits are sometimes observed—thatis, there is no fcc~111! to fcc~001! recrystallization. Thispoints to a surface contaminant being responsible for therecrystallization, present either during growth or immedi-ately after growth. When the contaminant desorbs at highertemperatures ~approximately 550 °C!, the surface relaxes, re-FIG. 1. Rectangular pits formed for a 6.3 nm thick Co film annealed at535 °C. The film orientation is fcc@001#. The pits are aligned along thein-plane fcc@100# direction, as determined from x-ray diffraction measure-ments.FIG. 2. AFM images of films annealed at 550 °C with different thickness t.Thinner films form rectangular pits, while thicker films form hexagonal pits.The x2y scale is 2000 nm32000 nm. The depth of the pits corresponds tothe film thickness.6096 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 H. Shi and D. Ledermancrystallizing the film if it is thin enough. The surface con-taminant therefore acts as a surfactant during the smoothen-ing process. This smoothening mechanism has been in Ptsurfaces contaminated with oxygen, which acts as a surfac-tant and thus helps create very smooth homoepitaxial Pt thinfilms.10 This possibility is currently being studied further.IV. CONCLUSIONSWe have studied the morphological and magnetic prop-erties of Co thin films grown on Al2O3 annealed at highertemperatures. The films display a remarkable surfacesmoothening transition between 500 and 600 °C. For filmsthinner than 20 nm, the smoothening transition coincideswith a recrystallization of the entire film from fcc~111! tofcc~001! and the formation of submicron rectangular pits.For thicker films, the surface can also smoothen out, and theshape of the pits that do appear is hexagonal because theirstructure remains fcc~111!. The pit concentration decreasesexponentially with the film thickness, indicating that this is asurface effect. The magnetic properties of the fcc~001!samples are highly anisotropic, consistent with an in-planebiaxial anisotropy with two unequal easy axes. A possiblemechanism for the formation of the rectangular pits is theadsorption of contaminants during or immediately aftergrowth, followed by desorption at higher temperatures. If amethod is developed for controlling the size and position ofthe pits, the pits could be used in liftoff nanolithography forquantum electronics and in developing new submicron mag-netic structures.ACKNOWLEDGMENTSThis work was supported by the National Science Foun-dation ~CAREER Award No. DMR-9734051! and theAmerican Chemical Society’s Petroleum Research Fund~Starter Grant No. 32814-G5!.1 Constitution of Binary Alloys, edited by M. Hansen ~McGraw-Hill, NewYork, 1958!, p. 471ff.2 Ch. Morawe, A. Stierle, N. Metoki, K. Brohl and H. Zabel, J. Magn.Magn. Mater. 102, 223 ~1991!.3 Y. Fukano, J. Appl. Phys. 13, 1001 ~1974!; C. G. Granqvist and R. A.Buhrman, ibid. 47, 2200 ~1976! J. P. Chen, C. M. Sorensen, K. J. Kla-bunde, and G. C. Hadjipanayis, ibid. 76, 6316 ~1994!; J. Jiao, S. Seraphin,X. Wang, and J. C. Withers, ibid. 80, 103 ~1996!.4 J. M. Gallego, S. Kim, T. J. Moran, D. Lederman, and I. K. Schuller,Phys. Rev. B 51, 2550 ~1995!.5 G. A. Prinz, Phys. Rev. Lett. 54, 1051 ~1985!.6 N. Sato, J. Appl. Phys. 67, 4462 ~1990! R. Krishnan, H. O. Gupta, H.Lassri, C. Sella, and M. Kaabouchi, ibid. 70, 6471 ~1991!.7 H. Shi and D. Lederman, Phys. Rev. B 58, R1778 ~1998!.8 A. R. Smith, K.-J. Chao, Q. Niu, and C.-K. Shih, Science 273, 226 ~1996!.9 O. Durand, J. R. Childress, P. Galtier, R. Bisaro, and A. Schuhl, J. Magn.Magn. Mater. 145, 111 ~1995!.10 S. Esch, M. Hohage, T. Michely, and G. Comsa, Phys. Rev. Lett. 72, 518~1994!.FIG. 3. Pit density ~a! and porosity ~b!, or the percentage of the AFM imageleft uncovered by the film, as functions of film thickness. The solid line is afit to r5r0e2t/l for t,14 nm, with r052.33103 mm2 and l51.2 mm. FIG. 4. Magneto-optic Kerr effect ~MOKE! at room temperature in air froma fcc~001!-oriented sample which was annealed at 550 °C which containsrectangular pits. Q is the angle between the applied magnetic field and theeasy axis in the sample plane, fcc@110#.6097J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 H. Shi and D. Lederman

Annealed Co thin films: Pit formation and magnetic anisotropy

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Annealed Co thin films: Pit formation and magnetic anisotropy

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