Magnetization Fluctuations and Characteristic Lengths for Sputtered CoPICr Thin Film Media

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1 HEWLETT PACKARD Magnetizatin Fluctuatins and Characteristic Lengths fr Sputtered CPCr Thin Film Media Gira J. Tarnplsky; Lung T. Tran Cmputer Peripherals Labratry H. Neal Bertram, UC San Dieg* HPL August, 1990 intergranular exchange; crrelatin length; media nise Magnetizatin fluctuatins f unifrmly magnetized media have been measured in sputtered CP thin films as a functin f Cr underlayer thickness. ncreasing underlayer thickness frm 20 m t 400 m yields decreasing magnetizatin fluctuatin nise frm 210 m t abut 25 m, similar t the knwn decrease f transitin nise. Fr every disk but ne, the nise pwer dependance n magnetizatin is well fit by the square f thewagnetizatin vs. current lp derivative, (dm/dl,y'. Thus, the nise mechanism appears t fllw a mdulatin prcess independent f nise level. Analysis f measured magnetizatin nise spectra yields apprximate crrelatin lengths which decrease with increasing Cr underlayer thickness. "Center nternal fr Accessin Magnetic Date Recrding Only Research T be published in the Jurnal fapplied Physics, MMM Supplement, January, (c) Cpyright Hewlett-Packard Cmpany 1990

3 1. ntrductin Metallic magnetic thin films prvide superir recrding perfrmance fr high density magnetic recrding. Medium nise can dminate system nise and the substantial research int identifying surces has been reviewed by Ygijl]. t has been argued that the predminant mechanism yielding thin film transitin nise is the existence f intergranular exchange cupling in these plycrystalline films[2]. Films are cmmnly prduced by first sputtering a Cr underlayer n which is sputtered a C ally. Transitin nise is knwn t decrease substantially with increasing Cr underlayer[3]. t is presumed that intergranular magnetic cupling is reduced as the underlayer thickness increases. t has been shwn [4] that transitin nise is directly related t the fluctuatins f unifrmly (de) magnetized media s that a study f unifrmly magnetized media can prvide insights int physical nise mechanisms. n [5] nise pwer was measured versus magnetizatin state fr a magnetic medium thickness series with fixed Cr underlayer thickness. The nise pwer varied as the square f the derivative f the magnetizatin curve indicating a medium inhmgeneity driven prcess[6]. Here the dependence f unifrmly magnetized medium nise n lp shape is investigated as a functin f medium micrstructure [1] by varying the Cr underlayer thickness. 2. Experimental Prcedure The media [3] used cnsists f sputtered disks f 3Q-nm-thick C 8at%P magnetic layer each, depsited ver Cr underlayers in the thickness range f 20 t 400 nm. The measurement cnditins were head-media separatin d=0.242 #Lm, media speed v= 15 mis, and a single ferrite head with turns, track width W=55 #Lm, gap length g= 1.03 #Lm, and efficiency e =90%. As in [5] a sequence f de currents were applied t the head in rder t take the medium alng 2

4 the majr lp frm ne state f saturatin magnetizatin remanence t that f the ppsite plarity. At each state f unifrm magnetizatin, nise pwer spectra were measured. The spectrawere acquired in the frequency interval 0.4 < f < 10 MHz, crrespnding t a wave number k=2tf/v interval 0.16<k<4.2p.m-1. These spectra were crrected fr gap and spacing factrs, ne pwer f track width, as well as abslute calibratin factrs such as M,lJ, channel gain and head efficiency t give a spectrum f the magnetizatin fluctuatins. The magnetizatin level which resulted frm each applicatin f (nn-saturating) de current was determined fr each disk frm a recrding measurement as described in [5]. The media magnetic prperties are listed in Table Results n Fig.1 magnetizatin fluctuatin spectra Mn(k) (eq. 6 f Ref. [5]) are pltted fr three underlayer thicknesses. The media in Fig. 1 have been reversed de demagnetized t the remanent cercive state which yields the maximum nise level. Nise spectra measured ver the entire series, and exemplified in Fig. 1, decrease in level with increasing Cr underlayer thickness. The spectral shapes d nt exhibit a simple frm with a clear lng wavelength asymptte. Over the wavelength range measured these spectra cntinuusly decrease perhaps reflecting a brad distributin f nise dmain sizes. These systems culd exhibit self-rganized behavir which cntains n scale length and pssess spectra which vary as a fixed pwer f the frequency ver a wide range[7]. Crrelatin lengths (C.L.) can nly be defined apprximately fr systems exhibiting cmplex spectral shapes. The prcedure adpted here affrds an unambiguus relative statement abut the C.L. f varius media r f varius magnetizatin states f the same medium. A cnventinally defined C.L. can be btained by Furier transfrming measured nise spectra t btain the autcrrelatin functin[8]. We define the Cil., t be the lie pint f the autcrrelatin functin. t is difficult t determine the lw frequency 3

asymptte f nise pwer spectra t be used fr the Furier transfrm. The differentiating actin f the playback prcess reduces medium nise measurements t system nise levels at lng wavelengths.

5 asymptte f nise pwer spectra t be used fr the Furier transfrm. The differentiating actin f the playback prcess reduces medium nise measurements t system nise levels at lng wavelengths. Thus the crrelatin lengths determined nly give a relative trend f nise dmain lengths in these media. The autcrrelatin functins fund are f an apprximate functinal frm Pn ap(-xl/a), were Pn is the ttal nise defined belw and A is a C.L. defined by this cnventin. Over the entire series A varies frm 3.6 #Lm t 1.6 #LD, decreasing with increasing Cr underlayer thickness. Even if the physical cncept f ur C.L. is smewhat ambiguus due t the limited knwledge f the magnetizatin fluctuatin spectra, it appears that reducing the intergranular exchange cupling reduces the size f the interactin dmains. The spectra in Fig. 1 can be integrated t yield ttal nise pwer Pn at the remanent cercive state. Pn is pltted vs. Cr underlayer thickness in Fig. 2. The units are nm crrespnding t a track-width directin effective crrelatin length times an intrinsic prcess variance[5]. The nise decreases frm abut 200 nm by an rder f magnitude in the range f Cr thicknesses frm nm crrespnding t the reprted decrease in high density transitin nise[3]. The medium magnetizatin M() btained frm lng-wavelength, nn-saturating, square-wave recrding [5] is pltted in Fig. 3 vs. reverse de current. These curves apprximately reflect the majr M-H lp. The current where the magnetizatin vanishes crrespnds t a medium field apprximately equal t the remanent cercivity He,,., which increases with increasing underlayer thickness. The curve shape becmes less steep with increasing underlayer thickness reflecting a decrease in intergranular exchange. Fr each disk, thin film medium nise varies with the level f magnetizatin attaining a sharp maximum as the magnitude f magnetizatin decreases. The ttal (integrated ver frequency) nise WWirr) m ja" W!ynt in fi 4fr the disk with 4

6 a 20 DD Cr underlayer. The nise pwer dependence n magnetizatin is cmpared in Fig. 4 with functinal frms expected fr specific nise mechanisms. n Fig. 4 dm/d and (dm/d)2 are pltted versus current. n additin, the 1-[M()/MrJ2 variatin expected f ttally independent grains is shwn. All curves are vertically scaled t yield the same maximum value. t is clear that the nise pwer varies clsely with the square f the lp derivative as fund in [5]. All media were similarly evaluated and it was fund that fr all underlayer thicknesses except 150 DD the nise pwer varied as (dm/d)2. This is seen clearly in Fig. 5 where, fr Cr underlayer thicknesses f 20, 80, and 300 nm, the nise pwer is pltted with the squared derivative f the in-situ M- lp. 4. Discussin t is remarkable that the nise pwer varies as (dm/d)2 virtually ver the whle underlayer series. A squared variatin is indicative f a mdulatin prcess [6]. Fr a Cr underlayer thickness f 150 DD the variatin appears t fall between linear and quadratic. n this instance perhaps intrinsic prcesses as discussed in [7] are relatively strng. Mdulatin prcesses can arise frm variatins in underlayer smthness r inhmgeneities in the film micrstructure leading t variatins in intergranular exchange cupling between large regins f grains. n a mdulatin prcess the strength f the mdulatin surce can vary with film grwth cnditins and therefre, as in this case, Cr underlayer thickness. The scale factr relating ttal nise t (dm/d)2 is listed in Table 1. There is nt ne invariant cnstant relating the lp derivative t the nise pwer. One shuld ntice that the factr varies within a 7% band in the nm Cr range, suggesting perhaps a cmmn nise mechanism fr these films. The data shws that in nne f the films des the nise fllw a 1 [M()/MrJ2statistical variatin due t independentgrains. C.L. are difficult t determine unambiguusly due t uncertainties at extremely lng wavelengths. We used a well-defined numerical 5

7 prcedure applied t the fully decnvlved magnetizatin fluctuatin spectra in rder t derive a C.L. withut need fr assumptins abut the spectral shape Mn(k). Fr any given disk, the C.L. thus btained shwed a maximum at the remanent cercive state, in strict crrelatin t the unifrm magnetizatin nise pwer Pn() variatin. 5. Cnclusin Nise arising frm unifrmly magnetized thin film media has been studied fr a series f media with varying Cr underlayer thickness. Magnetizatin fluctuatins are shwn t vary quadratically with magnetizatin lp derivative ver the entire series indicating the predminance f a mdulatin prcess. The driving mechanism f this prcess cannt be identified frm these measurements, but it is shwn that the mdulatin surce strength varies ver the series. Fluctuatin C.L. are estimated which decrease with increasing Cr underlayer thickness. t is shwn that the nise pwer decreases with increasing underlayer thickness in gd crrespndence with measurements f high density transitin nise. The authrs gratefully acknwledge the cperatin f Drs. E. S. Murdck and B. Natarajan wh kindly prvided the media and VSM results used in this study. They als thank Jbst Brandt fr help in data handling.

8 References 1. T. Ygi, presented at the 1990 ntermag Cnference, EEE Trans. Mag. 1990(t be published). 2. J.-G. Zhu and H. N. Bertram, EEE Trans. Mag. 24, 2706 (1988). 3. B. R. Natarajan and E. S. Murdck, EEE Trans. Mag. 24, 2724(1988). 4. H. Ai, M. Saith, N. Nishiyama, R. Tsuchiya, and T. Tamura, EEE Trans. Mag. 22, 895(1986). 5. T. Silva and H. N. Bertram, accepted fr publicatin in EEE Trans. Mag. 26 (Nv. 1990). 6. H. N. Bertram, K.. Hallamasek, and M. Madrid, EEE Trans. Mag. 22, 247(1986). 7. J.-G. Zhu and H. N. Bertram, t be presented at the 35th MMM Cnference, San Dieg, CA, Octber W. B. D. Davenprt and W. L. Rt, An ntrductin t the Thery frandm Signals and Nise,pp. 91 et seq., New Yrk, McGraw-Hill C(1958). 7

9 Table 1 Medium Magnetic Prperties and Results Cr thickness Mp He S P n A Scale Factr nm emu/cm 31 m Oe nm p.m nm/ma

10 Figure Captins Fig. 1. Unifrm magnetizatin fluctuatin spectra fr underlayer thicknesses. varius Cr Fig. 2. Ttal nise pwer at underlayer thickness. the remanent cercive state vs. Cr Fig. 3. Medium magnetizatin vs, current fr varius Cr underlayer thicknesses. Fig. 4. Ttal nise pwer and nrmalized dm/d, (dm/d)2 and 1 (M()/Mr)2 vs. current fr the 2-nm-thick Cr underlayerfilm. Fig. 5. Ttal nise pwer and (dm/d)2 fr varius Cr underlayer thicknesses. Fr each curve, the nly fit parameter is a scale factr. 9

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