IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 3569Stacking Faults in Smaller Grain Size Perpendicular MediaHua Yuan1;2and David E. Laughlin1;2Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213 USAData Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213 USAThe effect of stacking faults (SFs) on the uniaxial magnetocrystalline anisotropy( )of perpendicular magnetic recording media isan important factor when the media grain size becomes smaller. In this study, the relationship between grain size, SFs, and the magneticproperties of the media has been evaluated. It was found that when the SiO2content in the Co-alloy granular thin film increases, thegrain size anddecrease, and the amount of SFs increases. Keeping the same oxide volume fraction and decreasing the magnetic grainsize does not further increase the amount of SFs much; however, thevalue further decreases. This is likely due to the larger volumecontent of the possible presence of a deadlayer in the outermost shell of the smaller magnetic grains. The measured thermal energybarrier()and intrinsic switching field(0)decrease for smaller grain size media, determined by fitting the DC demagnetization(DCD) measurements with Sharrock’s formula. Improving the structure and microstructure characteristics of the smaller grain sizemedia will be important to obtain good magnetic and thermal properties.Index Terms—Grain size, magnetocrystalline anisotropy, perpendicular magnetic recording media, stacking faults.I. INTRODUCTIONTHE thermal stability factor is determined by the uniaxialmagnetocrystalline anisotropyand grain volumecharacteristics of the granular magnetic thin film media.In order to obtain high areal density and high signal-to-noiseratio, good properties such as large, small grain size, andhigh thermal stability of media are required [1].of granularCo-alloy perpendicular magnetic recording media stronglydepends on its crystallographic texture and microstructure.An ideal hexagonal-close-packed (hcp) structure is shown inFig. 1. When stacking faults (SFs) are present in a thin filmwith hcp structure, they result in a lowermedia, due tothe lower anisotropy nature of the present face-centered-cubic(fcc) phase. This will lead to the nonthermally stable grains.An earlier study [2] pointed out the importance of the roleof SiOin the fcc phase increases at 30 at.% Pt addition andthe degradation ofof CoPtCr–SiO media compared withthat of CoPtCr media. On the other hand, when the media hassmaller grain size, the thermal fluctuation increases which aidsthe irreversible switching of magnetization overcoming thethermal energy barrier. Thus, magnetic viscosity phenomenacan be often observed. This becomes worse when the grain sizeanddecrease at the same time. How the amount of SFsvaries with grain size is important to understand the cause ofdegradation. In this study, the relationship between SFs,grain size,properties and magnetic after effect [3], [4] ofthe media will be discussed in detail.II. EXPERIMENTAL METHOD ANDDISCUSSIONSSamples were sputter deposited by using a four-targetLeybold–Heraeus Z-400 system with RF diode mode. Thebase pressure for deposition was5 10 torr or better.A controlled sample: a continuous CoPt alloy thin film, aManuscript received March 06, 2009. Current version published September18, 2009. Corresponding author: H. Yuan (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2009.2024954Fig. 1. High-resolution TEM and schematic illustration of the hcp structure.granular CoPt oxide media of different grain sizes weresubsequently prepared without a soft magnetic underlayer. Thedetails of the sample structure are described in Table I. Thecomposition for the magnetic alloy thin film is CoPt .DCdemagnetization (DCD) remanent curves were measured by aLakeShore vibrating sample magnetometer (VSM) to analyzethe thermal energy barrier and intrinsic switching field in themagnetic media samples. [5] The maximum field was 18 kOe.The time duration at different applied field is 25, 50, 100, 200,400, and 800 s.was measured by the hard axis methodusing VSM as well and then corrected by the demagnetizationfactor. X-ray in-planescan was used to quantify theamount of SFs by means of detecting the integrated peakintensity ratio ofusingan X’Pert diffractometer with Cu Kradiation. JOEL 2000transmission electron microscopy (TEM) was utilized to studythe microstructures of the thin films.Table I shows the sample structures, grain sizes (defined asthe mean grain-to-grain distance),intensity ratios, SFs contents, and values forall the samples in this study. In order to quantify the amountof SFs in the CoPt grains, Lorentz-polarization factor [shownin (1)] has been utilized for geometrical correction. The SFscontent can be calculated by using (1)–(4) and the measuredintensity ratio by theX-ray in-plane scans. After the calculation, sample 1: a contin-uous CoPt alloy thin film is found to have almost perfect hcp0018-9464/$26.00 © 2009 IEEEAuthorized licensed use limited to: Carnegie Mellon Libraries. Downloaded on December 1, 2009 at 16:36 from IEEE Xplore. Restrictions apply.3570 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009TABLE IMAGNETICGRAIN SIZE(DEFINED AS THEAVERAGE GRAIN-TO-GRAINDISTANCE),I(10:0)=[I(11:0) +I(220) ]INTENSITYRATIO,SFSCONTENT,KANDMVALUESOFFOURSAMPLES,RESPECTIVELYstacking and zero amount of fcc content. As 11 vol% oxide iscosputtered in the CoPt film, SFs content increases up to only2.6%. However, as 17 vol% oxide is included in the thin film,the SFs content increases dramatically up to 22.0% and abovedepending on the grain size. These calculated SFs volumefractions are quite consistent with the observed SFs quantitydirectly determined from the high-resolution TEM imagesLorentz-polarization factor(1)measured-ratio(2)(3)(4)Data in Table I generally suggests the fact that the presence oflarge amount of SFs is harmful to obtain highgranular oxideCoPt thin films. In the continuous CoPt thin film sputter de-posited at room temperature, the grain size is as large as around22 nm determined by plan-view TEM image. However, it has avery low SFs content, which results in a very highpropertyof 9.010 erg/cm . When 11 vol.% of an amorphous oxidephase such as SiOis added into the CoPt magnetic alloy
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