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Direct evidence for stress-induced (001) anisotropy of rapid-annealed FePt thin filmsS. N. Hsiao, S. H. Liu, S. K. Chen, T. S. Chin, and H. Y. LeeCitation: Appl. Phys. Lett. 100, 261909 (2012); doi: 10.1063/1.4730963View online: /10.1063/1.4730963View Table of Contents: /resource/1/APPLAB/v100/i26Published by the American Institute of Physics.Related ArticlesFast domain wall dynamics in MnAs/GaAs filmsAppl. Phys. Lett. 101, 072408 (2012)Sizeable magnetic circular dichroism of artificially precipitated Co clusters in amorphous carbonAIP Advances 2, 032142 (2012)Non-volatile voltage control of magnetization and magnetic domain walls in magnetostrictive epitaxial thin films Appl. Phys. Lett. 101, 072402 (2012)Ferromagnetic properties of GdN thin films studied by temperature dependent circular polarized spectroscopy Appl. Phys. Lett. 101, 072403 (2012)Influence of intergranular exchange coupling on the magnetization dynamics of CoCrPt:SiO2 granular media J. Appl. Phys. 112, 033918 (2012)Additional information on Appl. Phys. Lett.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsDownloaded 23 Aug 2012 to 140.112.24.228. Redistribution subject to AIP license or copyright; see /about/rights_and_permissionsDirect evidence for stress-induced(001)anisotropy of rapid-annealed FePt thin filmsS.N.Hsiao,1,a)S.H.Liu,2S.K.Chen,2,a)T.S.Chin,2and H.Y.Lee1,a)1Scientific Research Division,National Synchrotron Radiation Research Center,Hsinchu30077,Taiwan2Department of Materials Science and Engineering,Feng Chia University,Taichung40724,Taiwan(Received17March2012;accepted10June2012;published online28June2012;corrected5July2012)Roles of rapid thermal annealing(RTA)on the evolution of crystallographic anisotropy of single-layered FePtfilms have been characterized.We observed a huge biaxial tensile stress of2.18GPainduced with increasing heating rate from0.5to40K/s.The result is a transition of orientationfrom(111)to perfect(001)texture.The later then degrades at heating rates!80K/s due tomorphological variation.The advantages of RTA are to induce tensile stress by densificationreaction within a very short time and to simultaneously impede thickness-dependent dynamic stressrelaxation.V C2012American Institute of Physics.[/10.1063/1.4730963]In the past decades,rapid thermal annealing(RTA)tech-niques have solicited considerable attention in materials engineering,in particular thefield of low-dimensional sys-tems,e.g.,thinfilms,nanoparticles,nanowires,e of RTA has proved it as an efficient way to control the meso-scopic features including phase transition,grain growth,and crystallization of amorphous phase as well as preferred ori-entation.In recent years,RTA has also been applied to treat various magnetic materials,for instance,soft magnetic mate-rials,spintronic devices,magnetic recording materials,and high energy density permanent magnet.1For the develop-ment of ultra-high density perpendicular magnetic recording, considerable efforts have been devoted to the fundamentals and engineering of L10FePt alloys with perpendicular ani-sotropy.Attributed to its high magnetocrystalline anisotropy ($7Â107erg/cm3)L10FePt phase has a very small critical size against superparamagnetism.2Generally,as-synthesized FePt nanoparticles and thinfilms are disordered fcc structure exhibiting magnetically soft properties.A high temperature annealing is vital to induce phase transformation from fcc to ordered L10phase.RTA has been reported to improve order-ing transformation of FePt nanoparticles and thinfilms.3,4 The alignment of[001]easy axis perpendicular tofilm plane is of particular importance in perpendicular magnetic record-ing.However,the direct growth of FePtfilms on amorphous substrates will typically result in random or a(111)preferred orientation.RTA has been regarded as an effective way to induce a highly[001]orientated FePtfilms composing of a multilayered structure nearly a decade ago.5Thereafter,there have been startling discoveries in rapid-annealed FePt multi-layered systems with oxide additives.6–9In light of these dis-coveries,there has been an emerging consensus that residual stress and RTA in FePt systems may play important roles to mediate or induce(001)preferred orientation.10–12Rather surprisingly,there have been no direct quantitatively experi-mental accesses as yet to correlate between residual stress and RTA.And it remains a question how significant RTA can influence crystallographic re-orientation of FePt,or more general speaking the reorientation of preferred orientation during RTA-ordering.In this letter,our purposes were to provide direct quanti-tatively evidences of stress-induced(001)anisotropies in single-layered FePtfilms by using synchrotron radiation x-ray diffraction.We also measured the magnitude of induced residual stress during RTA at heating rates ranging from0.5 to110K/s.The change of crystallographic orientations,mag-netic properties,and stress evolution during RTA were also measured and correlated to orientation change.In addition, dependence of preferred orientation on heating rate of FePt films with various thicknesses was also explored in detail.Single-layered FePt thinfilms were prepared on Corning 1737glass substrates using rf-magnetron sputtering with a background pressure lower than2Â10À7Torr.Sputtering deposition was performed at room temperature using an Fe50Pt50alloy target.Film thickness was varied from10to 40nm.Chemical composition of thefilms was determined by inductive-coupled plasma to be Fe52Pt48(61at.%).After dep-osition,the samples were immediately subjected to an infrared lamp RTA to800 C for0s,meaning that the power is termi-nated when sample-temperature reaches800 C.The heating rates were varied from0.5to110K/s(note that the heating rate of0.5and2K/s can be achieved by regular annealing furnace, i.e.,not a RTA).A thermocouple was settled in touch with the sample surface to directly measurefilm-temperature during RTA.In-plane residual stress/strain was obtained by a modi-fied sin2w method(so-called asymmetric sin2w method or cos2a sin2w method.w is the angle between surface-normal of the sample and scattering vector,while a is defined as the Bragg angle at w¼0Àthe grazing incidence angle c).13The use of asymmetric diffraction in grazing incidence mode can effectively increase the diffraction volume,which facilitates measurements of residual stress in thinfilm with strong tex-ture.In this study,c wasfixed at3 and beam energy was 8.0keV(k¼1.5498A˚),supported by BL17B station,Taiwan Light Source.The more detailed experimental procedures about stress measurements can be found elsewhere.14Crystal-lographic structure of the samples was examined by x-raya)Authors to whom correspondence should be addressed.Electronic addresses:pmami.hsiao@,skchen@.tw,and hylee@.tw.0003-6951/2012/100(26)/261909/4/$30.00V C2012American Institute of Physics100,261909-1APPLIED PHYSICS LETTERS100,261909(2012)diffractometry (XRD)using in-house Cu K a source (18kW)with Bragg-Brentano geometry.Surface morphology of the samples was observed by field-emission scanning electron mi-croscopy (FE-SEM).Magnetic properties of the films were measured at room temperature by a vibrating sample magne-tometer with a maximum applied field of 12kOe.The dependence of lattice spacing of (111)-reflection on cos 2a sin 2w for the 10-nm-thick as-deposited film and the annealed films with different heating rates are shown in Fig.1(a).The error bar stands for peak-fitting error.Note that there are some missing points in the plots for the films annealed with heating rates 20–80K/s (cos 2a sin 2w ¼0.061and 0.227).This indicates a strong film texture with a narrow distribution of (111)reflection.Data of d-spacing were not obtainable even taken through the asymmetrical scanning mode and synchrotron radiation.Fortunately,the linearity of the plots is still convincible with high R 2factors (goodness-of-fitting)larger than 0.93,indicating uniform biaxial stresses of the samples.Residual stresses (r )of the annealed FePt films versus different heating rates are illustrated in Fig.1(b).With increasing heating rate from 0.5to 40K/s,r dramatically increases from 0.69to 2.87GPa,which then decreases to 0.81GPa at 110K/s.Note that there is a stress relaxation for samples RTA at 0.5,2,and 110K/s,respectively,where the re-sidual stresses are even lower than that of as-deposited state.Figure 2shows XRD profiles of the 10-nm-thick films RTA-annealed up to 800 C with different heating rates.Theannealed films exhibit L 10structure due to the appearances of the superlattice (001)reflections.Ordering parameter,S ,is calculated from the ratio of the integrated intensities of superlattice and fundamental reflection,using (001)/(002)and (003)/(002),proposed by Warren.15The S values are higher than 0.87for annealed films.To further investigate the degree of orientation,a semi-quantitative method,so-called Lotgering orientation factor (LOF),was adopted.16For (001)orientation of a FePt filmLOF is defined as LOF ¼(p Àp 0)/(1Àp 0),where p and p 0are P (00l )film /P (hkl )film and P (00l )powder /P (hkl )powder ,respectively.Hence,LOF represents the degree of (001)pre-ferred orientation with a value from a certain negative value to 1.The negative value indicates a preferred orientation other than (001),0represents a random distribution state,1refers to a perfect (001)orientation.Dependence of LOF on heating rate of RTA annealing is shown in Fig.3(a).The LOF value of the sample heated at 0.5K/s is À0.138,indicat-ing a (111)preferred orientation.With increasing heating rate from 0.5to 40K/s,the LOF abruptly jumps to unity,i.e.,a perfect (001)orientation.The LOF deteriorates to 0.838when the heating rate is 110K/s.In order to further investi-gate the distribution of (001)orientation,the rocking curve of the (001)reflection was determined.Fig.3(b)illustrates the full-width-at-half-maximum (FWHM)of the (001)reflection as a function of heating rate.The FWHM for sam-ples RTA-annealed at 2and 0.5K/s is not available,because the orientation distribution is too broadened to extract a peak.The FWHM curve indicates a strong consistent trend with that of LOF,i.e.,the higher the LOF the narrower distri-bution is the (001)peak.Highly [001]-oriented structure with LOF 1and FWHM 1.57is obtained from FePt films with the heating rate 40K/s.The magnitude of FWHM is comparable to that of the epitaxial-grown FePt films on MgO single crystal substrate and is much smaller than that on the textured underlayers.17–19FIG.1.(a)d -spacing of (111)reflection vs cos 2a sin 2w for the FePt thin films prepared at different heating rates.(b)Residual stress as a function of heating rate.The initial stress of room-temperature-deposited films is indicated by red dashline.FIG.2.X-ray diffraction patterns for annealed FePt films with different heating rates.From the results above,we note the following facts:(1)The development of (001)orientation strongly depends on the accumulation of tensile stress induced by rapid anneal-ing.(2)A large magnitude of r ($2.18GPa)is required for the evolution of orientation from (111)to a full (001)texture for the 10-nm-thick FePt films.(3)The (001)texture degrades with increasing r at heating rate !80K/s.In our previous works,we demonstrated that the accumulation of the tensile stress plays a crucial factor in the development of (001)orientation,12which explains the effect of r on (001)orientation.In addition,we noticed that the as-deposited film has a high stress level,0.93GPa in tension,with a preferred orientation of (111)(not shown).This strongly suggests a mechanism of grain-growth driven evolution of (001)orientation from (111).To comprehend the origin of RTA-induced (001)orientation in FePt film,we studied the dependence of the growth of crystallite size on heating rate.Since the ramp time for samples with different heating rates is not the same,it is more appropriate to depict average growth rate of crystalline size,G ¼(D ÀD o )/t ,instead of grain size itself.The D and D 0represent the crystalline size of FePt films,examined by x-ray peaks breadth using Stoke-Wilson method,20after and before annealing,respectively;t is ramp time,e.g.,heating rate of 40K/s,t ¼20s.The G as a function of heating rate is shown in Fig.3(c).An exponential increase in G from 9.4Â10À4to 0.73nm/s is manifest as heating rate is increased from 0.5to 110K/s.This result sug-gests that the importance of rapid annealing process is to induce abnormal grain growth and/or annihilate defects within a very short time.The outcome is to generate a huge residual stress in tensile direction by the densification reac-tions.21,22And this in the meanwhile significantly alleviates the influence of dynamic stress-relaxation during ramping.As a consequence,it is expected that the tensile stress gener-ated is greater in samples with higher heating rate.This prevails as yielding do not occur in grains and there is no considerable morphological variation,e.g.,from continuousfilm to island-like structure.Our experimental data fit the ex-pectation except for the samples with heating rates !80K/s where the morphology of the films dramatically change from continuous to discontinuous state,as indicated in Fig.4.The irreversible morphology change explains the recession of residual stress as well as (001)preferred orientation.12This also implies that the resultant stress exceeds the critical stress governed by particular orientation and grain size for a given thickness to sustain elastic deformation.Figures 5(a)–5(d)present in-plane and out-of-plane hys-teresis loops of the FePt films with different RTA heating rates.It should be noted that the maximum applied field of 12kOe is not sufficient to magnetically saturate the FePt films.Yet the trends of the magnetic properties are still con-vincible in complying with texture evolution.With increas-ing heating rate from 0.2to 40K/s,the magnetic anisotropy changes from an isotropic to perpendicular state as LOF of the samples increases from À0.138to unity.Enhanced per-pendicular magnetic properties with large out-of-plane coer-civity (H c )of 10.3kOe and small in-plane H c of $300Oe were obtained in the sample RTA at 40K/s.When heating rate is increased to 110K/s,the in-plane magnetization re-appears due to the degradation of LOF.To further investigate the effects of rapid-annealing on crystallographic orientation of the FePt films,we studied the thickness-dependence of LOF on heating rates of FePt films.As indicated in Fig.6,the effect of heating rate on LOF also exists in samples with different thicknesses.We identified a striking difference that needs to be addressed.The fact that LOF curves reduce to a lower level becomes more significant with increasing film thickness,except the 10-nm-thick film that has a network structure.This fact implies that a faster heating rate is likely required to mediate perpendicular ani-sotropy for thicker FePt films.In combining the effects of strain energy and interfacial energy (free surface energy and film-substrate interfacial energy),these two orientation-dependent driving forces compete with each other to domi-nate film-texture.For thicker films whose grain size is small compared to film thickness,the grains with orientation of high moduli will transfer certain load to adjacent grains with lower moduli to decrease their strain energy.23Besides,FIG.3.Dependence of (a)LOF,(b)FWHM of the (001)reflection,(c)aver-age crystalline growth rate versus heatingrate.FIG.4.SEM images of the annealed FePt films prepared at the heating rate of (a)0.5,(b)40,(c)80,and (d)110K/s.grains in thicker films are expected to have a lower yield stress.23,24These factors provide additional routes to release their residual strain for thicker FePt films.This thickness effect becomes more significant if the heating rate is not suf-ficiently high to suppress the relaxation.The inset of Fig.6shows the dependence of r on film thickness for samples RTA at a heating rate of 20K/s.It clearly shows a decrease in r from 2.87GPa to 1.01GPa as increasing film thickness from 10nm to 40nm.This is consistent with the arguments above.However,it is important to note that thickness-dependent ordering behavior and dimensionality of grain growth during RTA are quite possible to influence the evolu-tion of texture in FePt films.Further investigations are required to fully characterize other thickness-dependent phenomena of the rapid-annealed FePt films.In summary,in this letter we demonstrate for an unam-biguous and direct evidence of crystallographic origin of stress-driven reorientation in the FePt thin films induced by rapid-annealing process.By manipulating the heating rate from 0.5to 40K/s,a huge increase of biaxial tensile stress of 2.18GPa is induced in the 10-nm-thick FePt films.At this high stress state,the minimization of texture-dependent strain energy provides extra driving force for the evolution of (001)grains from the original (111)by a mechanism ofgrain-growth.The outcome is a transition of orientation from a (111)to a full (001)texture.The advantages of rapid annealing are to induce tensile stress by densification reac-tion within a very short time and,in the meanwhile,to allevi-ate dynamic stress relaxation.When the samples are RTA at !80K/s,the irreversible morphology variation leads to the relaxation of elastic strain energy and the degradation of reorientation away from full (001).The thickness-dependence of (001)re-orientation on heating rate is shown to associate with strain relaxation behavior that is proved brisk for thicker films.That is to say a higher heating rate is required to induce tensile stress and impede dynamic relaxa-tion in thicker films.This fundamental investigation also pro-vides useful knowledge for the understanding of RTA effects on magnetic materials from the viewpoint of thin film stress.The authors are happy to acknowledge Dr.Fu-Te Yuan for useful discussion.This work was supported by the National Science Council of Taiwan,under the Grant Nos.NSC99-2632-E-035-001-MY3,NSC101-3113-E-035-001,and NSC99-2221-E-213-002-MY3.We also acknowledge the technical supported from National Synchrotron Radia-tion Research Center for synchrotron x-ray source and instruments.1Z.Q.Jia and J.P.Liu,J.Phys.D:Appl.Phys.39,R227(2006).2D.Weller, A.Moser,L.Folks,M.E.Best,L.Wen,M.F.Toney,M.Schwickert,J.U.Thiele,and M.F.Doerner,IEEE Trans.Magn.36,10(2000).3H.Zeng,S.Sun,R.L.Sandstrom,and C.B.Murray,J.Magn.Magn.Mater.266,227(2003).4S.Jeong,Y.-N.Hsu,ughlin,and M.E.McHenry,IEEE Trans.Magn.37,1299(2001).5H.Zeng,M.L.Yan,N.Powers,and D.J.Sellmyer,Appl.Phys.Lett.80,2350(2002).6K.Kang,Z.G.Zhang,C.Papusoi,and T.Suzuki,Appl.Phys.Lett.84,404(2004).7Y.-C.Wu,L.-W.Wang,and i,Appl.Phys.Lett.93,242501(2008).8S.T.T.Ichitsubo,T.Uchihara,and E.Matsubara,A.Fujita,K.Takahashi,and K.Watanabe,Phys.Rev.B 77,094114(2008).9S.Ishio,T.Narisawa,S.Takahashi,Y.Kamata,S.Shibata,T.Hasagawa,Z.Yan,X.Liu,H.Yamane,Y.Kondo,and 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