10 ABSTRACT

inch'. We report on a systematic study of activation volumes and their correlation with physical grain sizes in CoCrPt media fabricated by mag- netron sputtering. Different underlayers (Cr. CrV, and NiAI) and CoCrPt layer thicknesses were used to provide a range of lateral grain sizes (about 14 nm to 21 nm), M,t values (about 0.3 to 0.7 memu/cm2), and rema- nence coercivities (about 1.5 kOe to 3.1 kOe). Two different methods, namely the field-sweep-rate dependence of coercivity and measurements of magnetic viscosity and irreversible susceptibility, were used to deter- mine the activation volumes and the results obtained from the two meth- ods are in reasonable agreement. For CoCrPt layer thickness from 10 nm to 27 nm, the activation volumes of these films range from about 3 X 10- ' * cm3 to 5 X lo- '* cm', which indicate that these films are thermally stable. Furthermore, for most samples the activation volumes are close to the volumes of the physical CoCrPt grains whose sizes were determined by TEM and X-ray measurements. This suggests that the magnetic grains in these films switch almost independently. Films with physical grain sizes small enough to approach thermal instability will also be discussed. *Research supported by NSIC, NSF, and CMRA.

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AB-04. MAGNETIC SWITCHING VOLUME AND GRAIN SIZE IN RECORDING MEDIA. L. Morellon,* Y. Kubota,** and E. E. Mariner0 (IBM Almaden Res. Ctr. K63/E3, 650 Harry Rd., San Jose, CA 95120). T. Reith and B. York (Storage Systems Div., 5600 Cottle Rd., San Jose, CA 95120)

The activation volume of magnetization reversal or magnetic switching volume ( V * ) is of great importance for the characterization of high den- sity magnetic recording media. This parameter is directly related to the thermal stability and media noise, which limit the performance of such materials.'.2 A series of quatemary alloys grown on Cr underlayers have been prepared by dc magnetron sputtering with nominal values of the remanence magnetization-thickness product. Mrt, ranging from 0.5 to 0.1 memu/cm* and with coercivities Hc -2.9-0.2 kOe, respectively. The switching volume in these films has been estimated by means of both magnetic viscosity, i.e., time dependence of the magnetization, and sweep- rate dependence of the coercivity, yielding values of the order of V* -2.5X lo-'' cm3. Transmission electron microscope ( E M ) experi- ments had Grazing Incidence X-ray Scattering (GIXS) have been utilized to investigate the microstructural characteristics of these films and to de- termine the physical grain size volume in order to correlate this physical grain size to the switching volume inferred from magnetic studies. A detailed correlation between the measured switching volumes, magnetic properties, and microstructure will be presented and implications regard- ing the thermal stability of these alloys will be discussed.

*Departamento de Fisica de la Materia Condensada-ICMA, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain. **Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA 94720. 'M. P. Sharrock, IEEE Trans. Magn. 26, 193 (1990). *P. Lu and S. H. Charap, J. Appl. Phys. 75. 5768 (1994).

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AB-OS. EFFECT OF STACKING FAULTS ON MAGNETIC VISCOSITY AND THERMAL LOSS IN THIN FILM MEDIA. P. Dova', K. O'Grady', H. Laidler', M. F. Doeme?, and M. F. Toney' ('SEECS, Univ. of Wales Bangor, LL57 IUT, UK, 'IBM Storage Systems Div., San Jos6, CA 95193, 3t13M Almaden Res. Ctr., San Josk, CA 95120)

There is currently much interest in the effects of the presence of fcc grains and other crystallographic defects on the magnetization reversal processes and recording performance of thin film media. This interest arises because of the fact that such grains have low magnetic anisotropy compared with the normal hcp phase.' This can lead to a weak-link effect that may give nse to the phenomena such as percolation and may also lead to thermal

loss effects. These effects may be enhanced by the exchange coupling between such grains and the normal grains. In this paper we have exam- ined time dependence of the bulk magnetization in an attempt to charac- terize the effects of the low anisotropy grains. We have examined two samples, one having approximately 8% fcc and the other having 17%. Both samples exhibit uniform magnetization reversal as determined from hysteresis curves and remanence curves. Their respective coercivities are 1860 and 3410 Oe. The switching field distributions are symmetrical and approximately Gaussian in form. Magnetic viscosity experiments were performed where the change in magnetization was measured over 800 seconds at field values between zero and - 2 H , following initial satura- tion in the positive direction. In each case the variation of M with f was found to be quasi-linear with In t allowing us to determine S ( H ) (= dM/dlnr). The variation of S with H is expected to follow the form of the differential of the demagnetizing remanence curve.2 However the mag- netic viscosity curve shows a significant skew towards low field values which increases with the fcc content as shown in the figure. The area under the curve corresponding to the low anisotropy grains is marked showing the origin of these effects. In the full paper the origins and con- sequences, particularly in terms of thermal loss will, be discussed.

'Gao er al., IntermaghlMM'98. 2Uren er al., IEEE Trans. Mag. 24(2), (1988), p. 1808.

0 I 2 3 4

1024

AB-06. EFFECTS OF THIN Cr INTERLAYER ON TIME DECAY OF MAGNETIZATION AND MAGNETIZATION REVERSAL FOR CoCrTaR THIN FILM MEDIA. Jian Ping Wang, Lea Peng Tan, and Thomas Y. F. Liew (Data Storage Inst., 10 Kent Ridge Crescent, Singapore 11 9260)

The effect of flash Cr interlayer on time decay of magnetization and mag- netization reversal for CoCrTaPt /Cr/ CoCrTaPt media is presented. The samples with each CO alloy layer of 12 nm and various flash Cr interlayer thickness up to 3 nm were deposited on NiP/AI substrates with 100 nm Cr underlayer. Crystal structure analysis using grazing incidence X-ray dif- fraction showed that the intensity of Co(002) reduces with increasing in- terlayer thickness which implies the c-axis tilting down into the film plane as Cr interlayer thickness increases. The time decay of magnetization was measured at various reverse magnetic field using VSM. The maximum magnetic viscosity constant occurred at around H,JHc,=0.97 increases with increasing interlayer thickness as listed in Table implying the degra- dation of thermal stability. The activation volumes calculated from the magnetic viscosity and remanence measurement decreases with increasing Cr interlayer thickness. We have used the law of approach to saturation to estimate the magnetic anisotropy which shows a decrease with increasing interlayer thickness which is in agreement with torque measurement. The distribution of rotational hysteresis losses (W,) as shown in Fig. 1 were measured using the torque magnetometry. The rotational hysteresis inte- grals (Rh) was found to decrease with increasing Cr interlayer thickness which agrees with the theoretical simulation.' The micromagnetic struc- ture investigated by using the MFM for both ac erased and remanent state and delta M measurement were correlated with the study of time decay of

ABSTRACT 11

magnetization and magnetization reversal. Table I Magnetic properties and time decay of magnetization for CoCrTaWWCoCrTaPt .

'Wei Yang and D.N. Lambeth, Digests of Intermag 97, AB-IO, New Or- leans, Louisiana, USA. *We acknowledge Dr. E. Tang, Dr. P. W. Wang, Dr. Li Tang, Dr. D. N. Lambeth, and Dr. D. H. Han for their collaboration.

FIG. 1. Rotational hysteresis loss (Wr) vs. I/H for CoCrTaPu/Cr/ CoCrTaPt thin film media.

TABLE I. Magnetic properties and time decay of magnetization for CoCrTaWCrlCoCrTaPt.

interlayer M,t (X106) S v D % Cr

Dep. H, memu/ erg/ ( x I o - ~ ) ( x I o - ' ~ ) (nm) Power (Oe) cc cm3 R, H, emu/sec cnC3 Calc.*

0.0 kW 2737 1.05 1.3 3.96 1.86 13.8 0.1 kW 2766 1.02 1.2 1.3 13 4.81 1.46 12.3 0.2 kW 2520 1.01 1.1 5.14 1.31 11.6 0.3kW 2448 0.99 1.1 1.1 11 5.19 0.38 6.35

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AB-07. DYNAMIC COERCIVITY EFFECTS IN THIN FILM MEDIA. H. J. Richter, Stella Wu. and Roger Malmhall (Seagate Technol. Recording Media Group, 47010 Kat0 Rd., Fremont, CA 94538)

Recently, thermally activated magnetization processes have been dis- cussed intensively. It is generally believed that these processes might limit the maximum achievable areal density in magnetic recording. Thermally activated magnetization processes become evident in a time dependence of the coercivity of the media. We subjected various bard disk media to field pulses of equivalent lengths between Ions and l00ps and determined their remanent coercivities. The determination of the remanent coercivity is done on a spinstand utilizing the fact that the maximum medium noise occurs when a medium is in the remanent coercive state. The duration of the shortest pulse is limited by the maximum linear velocity which can be obtained on a spinstand (U -45ds) . Using conventional VSM/AGM tech- niques, this particular time range is not accessible. Moreover, conventional techniques do not cover such a wide range of pulse lengths. It is empha- sized that the results are not influenced by any frequency effects since only DC currents are employed. The result is shown in the figure below. The calibration of the measured results is obtained by extrapolation to a con- ventional measurement with known field sweep rate. The results are com- patible with the Nee1 theory of thermally activated magnetization pro- cesses. In the full paper, we will report details of the measurement technique and additional results.

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AB-08. NUMERICAL SIMULATIONS OF THE EFFECT OF RECORD FIELD PULSE TIME ON MEDIUM COERCIVITY AT FINITE TEMPERATURES. H. Neal Bertram (Ctr. for Magnetic Recording Res., Mail Code 0401, Univ. of California-San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0401) and Oinezhi Pene (IBM COIU., _ - I

C7R/43A, 5600 Cottle Rd., San Jose, CA 95193)

Thin film media under consideration for high density recording in the range of 5 Gbitshn' and beyond will utilize extremely small grains in order to achieve acceptable signal-to-noise ratio limits. Recent measure- ments of the coercivity versus field pulse time indicate a significant effect due to thermal fluctuations even in the nanosec range.' In this talk we will present the results of numerical simulations of this phenomena by solving the Landau-Lifshitz dynamic equations for a medium comprised of a hex- agonal array of approximately 4000 grains with an in-plane uniaxial, iso- tropic crystalline anisotropy. Thermal fluctuations are added by including an effective thermal field2 at each time interval during the integration of the dynamic equations. This field has zero mean, is applied randomly in direction, and has no correlation between time steps nor between grains in the array. The field is assumed to be Gaussian distributed and have a variance given by the fluctuation dissipation theorem. Typical results for a grain size and film thickness of 10 nm. grain magnetization of 360 emukc, anisotropy field of 66000e, and a dynamic damping constant of rr=0.05, yields a 2 0 6 decrease in coercivity between pulse times of I and 100 nanoseconds. At I O nanoseconds, typical of channel bit cell times, the coercivity is 24000e compared to 29000e obtained when thermal fluctua- tions are neglected. The effect of the damping constant is complicated because the effect of pure dynamics is to yield a minimum switching time for a = O . I , whereas the thermal field variance is directly proportional to the damping constant. The maintenance of stability against thermal fluc- tuations of in-plane grain reduction by increasing the film thickness will be specifically addressed.

'F. I. Li, R. M. Metzger, and W. D. Doyle. To be published in IEEE Trans. Magn., Sept. 1997. *E. D. Boemer and H. N. Bertram. To be published in IEEE Trans. Magn., Sept. 1997.

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AB-09. COERCIVITY AND FREQUENCY DEPENDENCE OF TRACK WIDTHS AND ERASE BANDS IN THIN FILM MEDIA. L. Mei, M. E. Schabes, and N. H. Yeh (Komag, Inc., 1704 Automation Pky., San Jose, CA 95131)

To understand track width, erase bands and total write width (track width + erase bands) as a function of coercivity and frequency in thin film media, a systematic study was conducted based on triple track profile (TPF) techniques' using pseudo random sequences (PRS-TF'F). The thin film disks used for the present study have coercivities ranging from 1427 to 3180 Oe. Frequency harmonics of PRS up to 200 kfci were analyzed to reveal the frequency dependence of the track edge effects. For disks with the same coercivity, the total write width is insensitive to the change of the frequency. The track width is reduced with widened erase bands as the frequency increases. When the media coercivity changes, track width and the erase band behave differently in different frequency regions. It is no- ticed that at fixed frequency the total write width is decreased approxi- mately linearly at a rate of 0.08 p m per 100 Oe increment in disk coer- civity for the headmedia combination of these measurements. Investigations by single tone frequency recording (SF-TPF) and MIW (magnetic force microscopy) were also carried out to compare with the PRS TPF results. Regardless of the nonlinear track edge effects, all three methods show good correlation over the coercivity and frequency ranges. We therefore propose that PRS-TPF is an effective way of characterizing track edge effects in the design of future high density recording systems.

IT. Lin er al., IEEE Trans. Magn., vol. 25, pp. 710-715, 1989.