Particulate structure of L10 ordered ultrathin FePt films for ...

APPLIED PHYSICS LETTERS 92, 132508 2008Particulate structure of L1 0 ordered ultrathin FePt filmsfor perpendicular recordingA. Perumal, 1,a Y. K. Takahashi, 1 T. O. Seki, 2 and K. Hono 1,21 National Institute for Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan2 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, JapanReceived 27 October 2007; accepted 11 December 2007; published online 3 April 2008We report the investigation of microstructure and magnetic properties of ultrathin FePt 1–10 nmfilms grown on oxidized Si substrates with a 10 nm MgO100 intermediate layer. The isolated FePtparticles with high degree of L1 0 ordering and large magnetocrystalline anisotropy could befabricated with a thickness of less than 5 nm. Perpendicular coercivity of isolated FePt particles iscontrollable between 15 and 25 kOe. These results are promising and highly desirable for magneticrecording applications. © 2008 American Institute of Physics. DOI: 10.1063/1.2830708Equiatomic FePt thin films with a L1 0 ordered structurereceive much research interest because of its large magnetocrystallineanisotropy 710 7 ergs/cc that is required forthe development of next generation ultrahigh-densityperpendicular magnetic recording media. 1,2 Since the lastdecade, extensive studies such as the effect of alloyingcomposition, 3,4 and growing FePt film on various underlayers,MgO, 5 PtMn, 6 CoAg, 7 have been carried out to optimizetheir microstructure and magnetic properties. Many of thepreviously reported perpendicular anisotropy FePt films wereprepared directly on MgO100 substrates. 8–11 However, theuse of the MgO substrates is not suitable for practical applicationsbecause of their cost. In recent years, great effortshave been devoted to fabricate the L1 0 ordered FePt films oneconomically viable substrates such as glass or oxidizedSi 5,6,12 with various underlayers. 5–7 However, the FePt filmsoften showed large in-plane anisotropy without MgO singlecrystalline substrates. Hence, the search of an appropriateunderlayer that enhances the 001 FePt texture is stronglydesired to obtain large perpendicular anisotropy in FePtfilms. 5,13 At the same time, reduction of the L1 0 orderingtemperature of the FePt films is required to use glass or Sisubstrates. Introducing tensile stress, 14 dynamic stress 15 andan extra anisotropy through exchange coupling 6 have beenreported to be effective to reduce the ordering temperature.The aim of this work is to investigate the way to obtainstrong perpendicular anisotropy in FePt films using MgOpolycrystalline underlayers that were sputter deposited onthermally oxidized Si substrates. The choice of the MgO isbased on the fact that it is known to form a strong 001texture when sputter deposited on amorphous substrates.FePtx nm/MgO10 nm x=1,2,3,4,5,7,and10films were fabricated on oxidized Si100 substrates held atvarious substrate temperatures. The MgO layer with 10 nmthickness was deposited at 100 °C substrate temperature byrf sputtering a MgO target. Subsequently, the FePt films withvarious thicknesses were deposited by dc sputtering aFe 50 Pt 50 alloy target at 500 °C. The nominal thicknesses ofthe FePt and MgO films were adjusted based on the sputteringtime. The base pressure of the chamber was better than110 −6 Pa. All the films were deposited at 10 mTorr Ar gaspressure. The composition of the FePt grains was estimatedby energy dispersive x-ray spectroscopy. The structure of thea Electronic mail: a.perumal@nims.go.jp.films were examined using x-ray diffraction XRD patterns.Plane-view and cross sectional microstructures of the filmswere characterized by Philips CM200 and FEI Technai F30transmission electron microscopes TEM, respectively.Room temperature magnetic property of the films was examinedby a superconducting quantum interferometer deviceMPMS Quantum design magnetometer with an appliedfield up to 55 kOe.Figure 1a shows the XRD patterns of theFePtx nm/MgO10 nm films with various FePt thicknessesfabricated on oxidized Si substrates. All the peaks areindexed as an L1 0 ordered FePt and 200 textured MgOfilms except for the nonindexed peaks of the substrate. Allthe films show a highly textured MgO200 peak at 2=43°. For the 1 nm thick FePt film, we observe only onepeak at about 48.4°, which corresponds to neither 2002=47.3° nor 002 2=48.93° of L1 0 ordered FePt. Inaddition, the intensity of the 001 superlattice reflection isvery weak. This indicates that the degree of L1 0 order is veryFIG. 1. a XRD patterns of FePtx nm/MgO10 nm films. b XRD patternsof 10 nm MgO layer deposited on oxidized Si substrate i at 100 °Cand ii at 100 °C and annealed at 500 °C for 5 min.0003-6951/2008/9213/132508/3/$23.0092, 132508-1© 2008 American Institute of PhysicsDownloaded 06 Apr 2008 to 144.213.253.16. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

132508-3 Perumal et al. Appl. Phys. Lett. 92, 132508 2008FIG. 4. Out-of-plane line with closed circles and in-plane line with opencircles magnetic hysteresis loops of FePtx nm/MgO10 nm films withvarious FePt thicknesses.4 nm show a linear variation with the applied field at a highfield region, but exhibit a tiny hysteresis around zero fieldswith a coercivity of 1.4 kOe. This might arise from the presenceof a small c-axis distribution of about 5 to 10% in theperpendicular direction. By increasing the film thicknessfrom 3 to 10 nm, the in-plane M-H loops show larger hysteresisand the in-plane coercivity increases dramaticallyfrom 1.4 to 15 kOe, respectively. As described earlier, thisrapid increase in in-plane coercivity might result from theincrease in either the fct FePt200 or the FePt111 orientationswith increasing FePt thickness. However, the exactamount of contribution of the FePt orientation to thelarge in-plane coercivity is not clear at this moment. Theanisotropy field H k for each FePt film was calculated byextrapolating the in-plane hysteresis loops. Subsequently,the magnetocrystalline anisotropy was calculated usingK u =M S H k /2 and listed in Table I. The FePt 4 nmfilm has a large K u value of 3.910 7 =84092 000/2 ergs/cc. The drastic increase in the perpendicularcoercivity and magnetocrystalline anisotropy can be attributedto the high degree of L1 0 ordering in FePt by varyingits thickness. The initial magnetization curves of all thefilms were difficult to saturate except for the 10 nm thickFePt film. This is due to the change in the morphology of thefilm from a particulate to an interconnected island structure.However, the magnetic field necessary to saturate the film isstill larger than 30 kOe. The dependence of magnetic propertieson the FePt film thickness is supported well from ourmicrostructural results.We have also fabricated the FePt films with the samethicknesses directly on the single crystal MgO100 substratesunder similar experimental conditions to understandthe development of additional FePt200 and FePt111 grainorientations with increasing FePt thickness in FePtx4 nm/MgO10 nm films. The XRD patterns of the FePt1–10 nm films on the MgO100 substrate showed strong001 and 003 superlattice reflections without any additionalFePt grain orientations. Thus, the formation ofadditional fct FePt200 and FePt111 grain orientationsin the FePtx5 nm/MgO10 nm films deposited on anoxidized Si substrate can be attributed to the inadequate texturingof the 10 nm MgO underlayer, as shown in Fig. 1b.Note that such grain orientations may also result from theexistence of the small grain size in the MgO underlayer.Nevertheless, we could fabricate the particulate structuredFePt 1–5 nm films on the 10 nm MgO200 underlayerwith high degree of L1 0 ordering, large magnetocrystallineanisotropy and controllable perpendicular coercivity.In summary, this work has demonstrated the possibilityof fabricating particulate ultrathin FePt films on 10 nm thinMgO100 underlayer with controllable magnetic propertiesat a substrate temperature of 500 °C on economically viablesubstrates for the practical application of perpendicular recording.The increase in FePt thickness over 5 nm inFePt/MgO films results in the development of various FePtgrain orientations, in contrast to the FePt films prepared onMgO 100 substrates.One of the authors A.P. acknowledges financial supportfrom the Japan Society for the Promotion of Science JSPSin the form of JSPS postdoctoral fellowship. 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