CoPtCr-SiO2 perpendicular recording media with varying levels of SiO2 were examined by two different methods to determine the activation volume. The first is based on the sweep-rate dependence of the remanence coercivity using Sharrock's equation. The second is based on the measurement of the fluctuation field from time-dependence data, determined using a magneto-optical Kerr effect (MOKE) magnetometer. The values of V-act measured at the coercivity for both methods are almost the same, with the fluctuation field and activation volumes increasing with the SiO2 content. The difference between V-act and the grain volume measured directly from bright-field TEM images decreases as the SiO2 content increases due to the reduction of intergranular exchange coupling. The experimental results indicate that values of V-act obtained from single- and double-layered media are consistent. It was also found that the coercivity and normalized hysteresis loop slope at coercivity varied with SiO2 content, with the coercivity peaking at 8 at % SiO2 (nearly 26 vol% SiO2)

Dutson, J D

Inaba, Y

Muraoka, H

O'Grady, K

Shimatsu, T

English

Y. Inaba

T. Shimatsu

H. Muraoka

J.D. Dutson

K. O'Grady

Crossref

Activation volumes in CoPtCr-SiO/sub 2/ perpendicular recording media

White Rose Research Online

This is a repository copy of Activation volumes in CoPtCr-SiO2 perpendicular recording media.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/1824/Article:Inaba, Y, Shimatsu, T, Muraoka, H et al. (2 more authors) (2005) Activation volumes in CoPtCr-SiO2 perpendicular recording media. IEEE Transactions on Magnetics. pp. 3130-3132. ISSN 1941-0069 https://doi.org/10.1109/TMAG.2005.854850eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 3130 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005Activation Volumes in CoPtCr-SiO2Perpendicular Recording MediaY. Inaba1, T. Shimatsu1, H. Muraoka1, Member, IEEE, J. D. Dutson2, Member, IEEE, andK. O’Grady2, Senior Member, IEEEResearch Institute of Electrical Communication, Tohoku University, Sendai 980-8577, JapanPhysics Department, University of York, Heslington, York YO10 5DD, U.K.CoPtCr-SiO2 perpendicular recording media with varying levels of SiO2 were examined by two different methods to determine theactivation volume. The first is based on the sweep-rate dependence of the remanence coercivity using Sharrock’s equation. The second isbased on the measurement of the fluctuation field from time-dependence data, determined using a magneto-optical Kerr effect (MOKE)magnetometer. The values of act measured at the coercivity for both methods are almost the same, with the fluctuation field and acti-vation volumes increasing with the SiO2 content. The difference between act and the grain volume measured directly from bright-fieldTEM images decreases as the SiO2 content increases due to the reduction of intergranular exchange coupling. The experimental resultsindicate that values of act obtained from single- and double-layered media are consistent. It was also found that the coercivity andnormalized hysteresis loop slope at coercivity varied with SiO2 content, with the coercivity peaking at 8 at%SiO2 (nearly 26 vol%SiO2).Index Terms—Activation volume, CoPtCr-SiO2, fluctuation field, magneto-optic Kerr effect (MOKE), perpendicular recording media,thermal activation.I. INTRODUCTIONTHE ACTIVATION volume is the smallest unit ofmagnetization reversal, and the associated diameteris of importance as it indicates the potential recording densityof a medium. In this paper, was measured by two methodsfor CoPtCr-SiO media with varying levels of SiO . The firstmethod is based on the sweep-rate dependence of the remanentcoercivity using Sharrock’s equation [1], [2], and the second isfrom the measurement of the fluctuation field [3] determinedfrom time-dependence data measured using a highly stablemagneto-optic Kerr effect (MOKE) magnetometer [4]. A com-parison is made of the values of obtained from the twotechniques. The effect of soft magnetic under layers (SUL) onwas also studied.II. SAMPLE PREPARATIONCoPtCr-SiO media were deposited on 2.5 inch glass disksby cosputtering with Co Cr , Pt, and SiO targets using aUHV-magnetron sputtering system [5]. The composition ofCoPtCr was fixed at (Co Cr ) Pt , and SiO was addedto this CoPtCr composition in varying amounts by controllingthe deposition rate. The film thickness was fixed at 8 nm. Ru(20 nm) and Pt (10 nm) seed and preseed layers were used. Nosubstrate heating was carried out during the deposition process.CoZrNb amorphous films (200 nm) were used for the softunderlayer.III. RESULTS AND DISCUSSIONA. Magnetic PropertiesHysteresis loops were measured for all samples using aMOKE magnetometer and loops for single-layered mediaDigital Object Identifier 10.1109/TMAG.2005.854850Fig. 1. Hysteresis loops of CoPtCr-SiO single-layered media with variousSiO contents.containing 0, 5.5, 11.2, and 14.4 at.% SiO are shown in Fig. 1.To enable a comparison of the films to be made, values of thecoercivity and the normalized slope of the magnetizationcurve , as a function of the SiOcontent for both single- and double-layered media, are shownin Figs. 2 and 3.The coercivity of the films increased significantly as theSiO content increased and was coincident with the reduction of, reaching a maximum at around at%SiO (vol%SiO ). The initial increase in is due to an enhance-ment of grain isolation on the addition of SiO [5]. However,increasing the SiO content above 8 at% reduced signifi-cantly, although remained almost constant. There wereno significant differences in the magnetic properties of single-and double-layered media.0018-9464/$20.00 © 2005 IEEEINABA et al.: ACTIVATION VOLUMES IN CoPtCr-SiO PERPENDICULAR RECORDING MEDIA 3131Fig. 2. Values of H as a function of SiO content for CoPtCr-SiO single-and double-layered media.Fig. 3. Normalized slope of the hysteresis loop as a function of SiO contentfor CoPtCr-SiO single- and double-layered media.Fig. 4 shows remanence curves for a CoPtCr-SiO mediumwith 8.4at% SiO measured at sweep rates of 10 Oe/sec usinga VSM, and Oe/sec using a pulse field magnetometer. Themagnetization curve measured at 10 Oe/sec is also shown in thisfigure. The remanent coercivity at Oe/s, , is 5.95 kOe,which is larger than that at Oe/s, by about 1.8 kOe.The values of were obtained by fitting the valuesof and to Sharrock’s equation [1], [2] ( is the mag-netic anisotropy, the activation volume, the Boltzmannconstant and the absolute temperature). Fig. 5 shows thevalues of of single-layered media as a function ofthe SiO content. decreased monotonically as theSiO content increased, mainly due to a reduction of the graindiameter [5]. This result indicates that the reduction inon the addition of SiO in excess of at% is mainly dueto thermal agitation of magnetization.B. Estimation of the Activation VolumeFig. 6 shows the time dependence of magnetization in dif-ferent applied fields for the 8.4 at% SiO sample measured bythe highly stable MOKE magnetometer [4]. The fluctuationFig. 4. Remanence curves at two different sweep rates for 8.4at% SiOsingle-layered media.Fig. 5. Values ofK V =kT as a function of SiO content for single-layeredmedia.field can be calculated from these results using the waiting timemethod [3] and (1)(1)where is the difference in the applied field between twotime-dependence measurements, and and are the times atwhich the data reaches a constant value of magnetization . Toreduce experimental error, a number of points are measured inpreference to the two points described by (1). The fluctuationfield is then determined from the gradient of the applied fieldversus the time to fall to the constant level of magnetization.The values of fluctuation field , determined for each levelof SiO content for both double- and single-layered media, areshown in Fig. 7. increased as the SiO content increased,but there were no significant differences between single-layeredmedia and media with an SUL.From the fluctuation field, it is possible to determine the ac-tivation volume , if the bulk saturation magnetization isknown from (2) [6](2)3132 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005Fig. 6. Time-dependence data for single-layered media with 8.4 at% SiOmeasured by MOKE.Fig. 7. Values of fluctuation field H as a function of SiO content forCoPtCr-SiO double- and single-layered media.Values of have been determined from both the fluctua-tion field and sweep-rate methods and are shown in Fig. 8. Thevalues for single-layered media were calculated from the valuesof in Fig. 5 and , measured by torque magne-tometry. The values of grain volume , including the grainboundary thickness, have been obtained from TEM bright fieldimages, and they are also plotted in the figure.The values of in both series of media decreased as theSiO content increased. The reduction in was in qualita-tive agreement with the reduction of on SiO addition[5]. The values of were larger than due to limitedcooperative reversal. However, the difference between anddecreased as the SiO content increased due to the reduc-tion of intergranular exchange coupling [5].Fig. 8. Activation volume, for double- and single-layered media, as a functionof SiO content. The physical grain volume V is also shown.IV. CONCLUSIONThere were no significant differences in the magnetic prop-erties and between single- and double-layered media. Thevalue of decreased as the SiO content increased, whichis consistent with a reduction in the average grain size. Fur-thermore, the difference between and decreased asthe SiO content increased due to the reduction of intergran-ular coupling. The results for single- and double-layered mediawere almost the same, and we can use the values obtainedfrom single-layered media to predict the magnetic properties ofdouble-layered media.ACKNOWLEDGMENTThis work was supported in part by the IT program (RR2002)of MEXT and in part by SRC.REFERENCES[1] P. J. Flanders and M. P. Sharrock, “An analysis of time-dependent mag-netization and coercivity and of their relationship to print-through inrecording tapes,” J. Appl. Phys., vol. 62, pp. 2918–2928, 1987.[2] M. P. Sharrock, “Time dependence of switching fields in magneticrecording media (invited),” J. Appl. Phys., vol. 76, pp. 6413–6418,1994.[3] M. El-Hilo, K. O’Grady, and R. W. Chantrell, “Fluctuation fields andreversal mechanisms in granular magnetic systems,” J. Magn. Magn.Mater., vol. 248, pp. 360–373, 2002.[4] J. D. Dutson, J. Wu, K. O’Grady, N. C. Woolsey, Y. Kubota, and C. L.Platt, “Time dependence in perpendicular media with a soft underlayer,”IEEE Trans. Magn., pt. 2, vol. 40, no. 4, pp. 2504–2506, Jul. 2004.[5] Y. Inaba, T. Shimatsu, T. Oikawa, H. Sato, H. Aoi, H. Muraoka, and Y.Nakamura, “Optimization of the SiO content in CoPtCr-SiO perpen-dicular recording media for high density recording,” IEEE Trans. Magn.,pt. 2, vol. 40, no. 4, pp. 2486–2488, Jul. 2004.[6] E. P. Wohlfarth, “The coefficient of magnetic viscosity,” J. Phys. F: Met.Phys., vol. 14, pp. L155–159, 1984.Manuscript received February 7, 2005.