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Effect of film texture on magnetization reversal and switching field in continuous and patterned (Co/Pd) multilayers

Effect of film texture on magnetization reversal and switching field in continuous and patterned (Co/Pd) multilayers
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  See discussions, stats, and author profiles for this publication at: Effect of film texture on magnetization reversaland switching field in continuous and patterned(Co/Pd) multilayers  Article   in  Journal of Applied Physics · August 2009 DOI: 10.1063/1.3173546 · Source: IEEE Xplore CITATIONS 23 READS 40 6 authors , including: Some of the authors of this publication are also working on these related projects: Magnetic Recording Media   View projectDomain Wall Memory   View projectRachid SbiaaSultan Qaboos University 158   PUBLICATIONS   906   CITATIONS   SEE PROFILE S.N. PiramanayagamNanyang Technological University 209   PUBLICATIONS   1,773   CITATIONS   SEE PROFILE All content following this page was uploaded by S.N. Piramanayagam on 15 January 2017. The user has requested enhancement of the downloaded file.  Effect of film texture on magnetization reversal and switching fieldin continuous and patterned  „ Co/Pd …  multilayers Rachid Sbiaa, 1,a  Cho Zhong Hua, 1 S. N. Piramanayagam, 1 Randall Law, 1 Kyaw Oo Aung, 1 and Naganivetha Thiyagarajah 2 1  Data Storage Institute, A * STAR (Agency for Science, Technology and Research), DSI Building,5 Engineering Drive 1, Singapore 117608, Singapore 2  Department of Electrical and Computer Engineering, National University of Singapore,Singapore 117576, Singapore  Received 31 March 2009; accepted 11 June 2009; published online 17 July 2009  We studied the reversal properties of perpendicular anisotropy   Co/Pd   multilayers with differentcrystallographic textures. In case of continuous films, an increase in the coercivity and reduction inthe switching field distribution   SFD   were observed as the growth is improved. From magneticforce microscopy, a stripe-type domain configuration was observed in films deposited at low gaspressure while a bubble-type domain was observed in high pressure deposited films. In patternedfilms, the SFD did not vary significantly for samples with different textures although a 2 kOeincrease in the switching field was measured. In patterned structures, the controllability of SFD maynot be related to the improvement of film crystallographic growth as was observed for unpatternedfilms. The results from this study indicate that local variation in the intrinsic film properties playsa major role in the SFD. ©  2009 American Institute of Physics .   DOI: 10.1063/1.3173546  I. INTRODUCTION With the current granular perpendicular magnetic record-ing   PMR   media technology, the areal recording density inhard disk drives could be increased by 30%–40% annuallyup to 700 Gbits / in. 2 or closer. The success of the PMR tech-nology is due to the existence of a soft magnetic underlayer,which images the write head. Thus improvement in writefield efficiency can allow the use of small grains that arethermally stable. This success is also due to the use of CoCrPt-oxide based media, which have a combination of good properties such as small grain size, square hysteresisloop, low noise, high anisotropy, and magnetically exchangedecoupled grains. 1–9 The limitation in the areal density for PMR is mainlydue to the conflict between thermal stability and writability.The reduction in media grain size should be accompanied byan increase in magnetic anisotropy energy to keep the ther-mal stability factor  K  u V  / k   B T   larger than 60. In this ratio be-tween anisotropy and thermal energies,  K  u  is the effectiveuniaxial anisotropy,  V   is the volume of the magnetic grain,  k   B is Boltzmann’s constant, and  T   is the operating temperature,which could reach 80 °C when the head flies on the record-ing media. On the other hand, the writing field is limited bya write pole material with a saturation magnetization nothigher than 2.4 T.As future technologies, bit-patterned media  BPM  10–14 or energy assisted magnetic recording   EAMR  have the potential to increase the areal density to 10 Tbits / in. 2 or beyond. 15–17 It appears likely that BPMcould be the first technology to be adopted after granularPMR and later EAMR may be introduced as an additionaltechnology when thermally stable magnetic islands of about4 nm in lateral size are required. In BPM, the effective vol-ume is made larger by using highly exchange coupled grainswhile in EAMR the anisotropy energy of the bit   grains   isreduced during the writing process only.The fabrication of nanosized features in a full disk withhigh throughput and low cost is one of the biggest challengesfacing BPM technology. It is most likely that the combina-tion of high resolution lithography process such electronbeam and nanoimprint lithography could help to solve theabove issues. 13,18 However, besides the nanofabrication of patterned media, the controllability of magnetic propertiesfrom one bit to the other, writing synchronization, planariza-tion, and other issues should be solved before seeing the firsthard disk drive with BPM in the market.The switching field distribution   SFD   is one key param-eter, which needs to be narrowed as much as possible toavoid the writing in error and relax the writing synchroniza-tion window. Optimizing patterned nanostructure propertiesfor better performance can be faster if one could find a cor-relation between the behavior of continuous and patternedfilms. Understanding the srcin of SFD was studied recently by several groups. 19–22 Thomson  et al. 19 were able to predictthe SFD by considering only the distribution of the mediaintrinsic  K  u . They concluded that the broadening of SFD isnot the result of patterning and there is no major contributionfrom the distribution of easy axis angle. This does not meanthat the irregularities in size and shape will not affect theSFD, but it will add an additional contribution to intrinsic  K  u effect in broadening SFD. Another attempt to explain thesrcin of SFD was reported by Lau  et al. 21 They observed theexistence of strong   200   spot by dark field electron micros-copy in all dots with low switching fields only. They con-cluded that in order to reduce the SFD, it is important tocontrol the microstructure by eliminating the trigger grainswith in-plane   100   vectors.In this paper, we focused on the effect of film growth a  Electronic mail: JOURNAL OF APPLIED PHYSICS  106 , 023906   2009  0021-8979/2009/106  2   /023906/5/$25.00 © 2009 American Institute of Physics 106 , 023906-1 Downloaded 16 Aug 2009 to Redistribution subject to AIP license or copyright; see  conditions on the SFD in thin film and in patterned struc-tures. The aim was to improve the film crystallographic tex-ture by changing the deposition condition   Ar-gas pressure  and see whether there is a change in the mean value of theswitching field  H  sw  and its distribution. II. EXPERIMENTS A series of    Co/Pd   multilayers was deposited by dcsputtering at different argon gas pressures ranging from 1.5to 5 mTorr. A laminated seedlayer of Ta  5 nm   /Pd  3 nm   / Cu  5 nm   was used to promote   111   texture of the magneticmultilayer and Pd  3 nm   /Ta  5 nm   was used as a cappinglayer for protection against oxidation. The magnetic andstructural properties of the deposited samples were investi-gated mainly by a polar magneto-optical Kerr effect  MOKE   magnetometer, an alternating gradient magnetome-ter, atomic and magnetic force microscopes   AFM/MFM  , ascanning electron microscope, and an x-ray diffractometer  XRD  . The patterned samples were made using Elionixelectron beam lithography and ion milling. Details of thenanofabrication process are reported in our previous work. 23 III. RESULTS AND DISCUSSIONSA. Continuous  „ Co/Pd …  multilayers The   Co/Pd   multilayers have the advantage to easilytailor their magnetic anisotropy by just changing the thick-ness of the Co and Pd layers. They are also widely studied asfuture magnetoresistive devices with perpendicular aniso-tropy, which can be used for magnetic random accessmemory. 24–26 Based on our previous study, we fixed thethickness of the Co and Pd layers to 3 and 8 Å, respectively,and the number of bilayers to 15. 26 Figure 1 shows the hysteresis loops obtained by theMOKE magnetometer for multilayers deposited at three dif-ferent Ar-gas pressures of 1.5, 3, and 5 mTorr. There is anoticeable increase in the coercivity  H  C   from 0.9 to 2.7 kOeas argon pressure was increased from 1.5 to 5 mTorr. Simul-taneously there was an improvement of the crystallographictexture, which is indicated by the full width half maximumof the rocking curve    50  of the   111   peak of Co as shownin Fig. 2. All the samples investigated in this paper show aclear fcc   111   peak at around 41° determined from    -2   XRD scan. By increasing the gas pressure during film depo-sition we were able to improve the growth and magneticgrain texture with    50  changing from 5.1° down to 3.8°.This improvement in crystallographic growth was ac-companied by an increase in the coercivity by threefold andalso a reduction in SFD. Figure 3 shows the normalized mag-netization to the saturation magnetization as a function of thenormalized applied field by the coercivities of each of thesemultilayers. It can clearly be seen that the sample with    50 of 3.8°   5 mTorr   shows sharper magnetization switchingcompared to the one with    50  of 4.7°   3 mTorr  . In contrast,the sample with a low crystallographic texture      50  of 5.1°  has a longer tail in the hysteresis loop.Prior to patterning these samples, we conducted a MFMscan of the continuous films at remanence states in order tounderstand their switching behavior. For this purpose, thefilms were saturated with a field of   20 kOe at first and thena positive field   closer to the  H  C    was applied for 5 s. Thisprocedure was repeated for different magnetic fields untilobtaining a reversed saturation state of the magnetic film soas to follow the change in magnetic configuration. It is im-portant to note that for each one of the three samples dis-cussed earlier, the major hysteresis loop and the dc demag-netization curve are almost identical.Series of MFM images for sample deposited at low gaspressure of 1.5 mTorr   top images   and the sample depositedat high pressure of 5 mTorr   bottom images   deposited at lowpressure are shown in Fig. 4. A clear difference in magnetic FIG. 1.   Color online   Hysteresis loops measured from polar MOKE forthree samples deposited at different Ar pressures. FIG. 2.   Color online   The coercivity and the full width half maximum,    50 , of    Co/Pd   multilayers as a function of the deposition argon pressure.FIG. 3.   Color online   Normalized magnetization vs normalized appliedfield to the coercivity for nonpatterned samples deposited at different argonpressures. The larger the deposition pressure the narrow the switching fieldis. 023906-2 Sbiaa  et al.  J. Appl. Phys.  106 , 023906   2009  Downloaded 16 Aug 2009 to Redistribution subject to AIP license or copyright; see  domain configuration in the two samples can be seen. Stripe-type domains were observed in the sample deposited at lowgas pressure. At remanence state   shown in Fig. 4  , afterapplying a 900 Oe perpendicular magnetic field, the magne-tization reversal starts by a large domain, indicated by thered semicircle. AFM image   not shown here   indicated thatthere was no defect at that position. Further increase in themagnetic field causes this domain to expand with appearanceof more stripes and almost all are connected to each other.Asthe magnetic field increases toward saturation, these stripesare swept away and their length and width are reduced untilthey disappear at around 1400 Oe. In contrast, bubblelikedomains were observed for the multilayer deposited at highargon pressure. For an applied field higher than the coerciv-ity, the size and the number of the magnetic domains are bothreduced as can be seen in MFM images taken at 3 and 3.2kOe cases. Similar reports of a change from stripe domain tobubble type of domain formation have been reported in sev-eral other systems, such as Co/Au multilayers CoPt andFePt. 27,28 B. Patterned  „ Co/Pd …  multilayers From the SEM image   not shown here  , the dots madeby electron beam lithography and ion milling were measuredto be an array of 60 nm diameter magnetic dots with 50 nmspacing. To successfully remove the 100 nm thick polym-ethyl methacrylate resist used as a mask, a multistep etchingwas conducted in order to avoid hardening the resist. Anoptimal etching time of about 3 min was required for achiev-ing a good patterned sample. For longer etching time, weobserved a damage of the patterned sample with a weakMFM signal, which could be due to the etching of the cap-ping layer and partially the magnetic layer.To evaluate  H  sw  for the patterned sample deposited atthree different gas pressures, we followed the same methodused for continuous films. We first saturated each sample athigh negative magnetic field and then applied a reversed fieldin the opposite direction in 500 Oe increments. After remov-ing the field, the MFM image was taken over a 10  10    m 2 patterned area. The patterned   Co/Pd   multilayersshow a noticeable increase in  H  sw  compared to continuousfilms. This is mainly due to the difference in switchingmechanism. In continuous film the magnetization reversaloccurs by domain nucleation followed by domain wallpropagation. On the other hand, for the 60 nm size dots inthe single domain structure, the reversal is coherent rotation  Stoner–Wohlfarth reversal  .The switching probabilities for the three samples weredetermined by counting the number of switched dots normal-ized by the total number of dots as shown in Fig. 5. The dotslocated at the edges   two or three rows   were excluded. Thisis because these dots are closer to the unpatterned area of thefilm and their magnetization reversal will be affected by thelarge magnetostatic field coming from the thin film ratherfrom the neighboring patterned dots.From Fig. 5, it can be seen that the sample deposited at 1.5 mTorr   low    50   has the smaller  H  sw  of about 13.5 kOe.This value represents an increase of 15 times as compared tothe case of continuous film. A tenfold increase has been re-ported by Smith  et al. 29 in Co/Pd multilayers. However, the FIG. 4.   Color online   MFM images of 10  10    m 2 taken at remanence states after removing the applied field indicated on each figure. The top series of images are for the sample deposited at 1.5 mTorr with    50  of 5.1°. The bottom series of images are for the sample deposited at 5 mTorr deposition pressureand having    50  of 3.8°.FIG. 5.   Color online   Switching probability as a function of applied fieldfor samples deposited at 1.5, 3, and 5 mTorr. No major difference in switch-ing probability for samples deposited at 3 and 5 mTorr but 50% of thenumber of dots switched at larger field compared to the sample deposited atlow pressure of 1.5 mTorr. 023906-3 Sbiaa  et al.  J. Appl. Phys.  106 , 023906   2009  Downloaded 16 Aug 2009 to Redistribution subject to AIP license or copyright; see  difference between the previous studies and our study is thatwe have employed a Ta/Pd buffer layer, which is known toprovide a better crystallographic texture. 30 It is well knownthat the buffer layers play a crucial role in the microstructureand magnetic properties. It has also been reported that thetexture plays a key role in the magnetic anisotropy. 31 It isquite likely that the larger increase is due to increased  K  u .We also looked at the SFD in these patterned samples inorder to check whether improving the growth of the film mayhelp in narrowing the SFD. From the derivative of theswitching probability we plotted the SFD versus the normal-ized magnetic field as can be seen in Fig. 6. Surprisingly, the three samples with different deposition gas pressures andcrystallographic textures show almost identical SFD. Refer-ence 22 indicates that the SFD is linked to the ratio betweenmagnetic anisotropy energy   K  u  and  K  u  in the assumptionthat magnetization reversal occurs by coherent rotation.From the results in Fig. 2, where an increase in pressure wasfound to result in a reduced    50  a reduced  K  u  is expected.It can also be observed from Fig. 5 that the increased pres-sure leads to an increase in switching field, and an increased K  u  is expected. Therefore, it was expected that the samplewith the smallest    50  of 3.8°   deposited at a higher Ar pres-sure   will have the narrower SFD. However, our results in-dicate no significant dependence of SFD on    50 . This resultcan be understood by having a closer look at Fig. 5, where the reversal of the dots was found to take place over a widerange of reversal field, indicating that the    H  K  , in fact, in-creased even though a reduced    50  was observed. The dis-tribution in the anisotropy constant,   K  u , may come fromtwo possibilities:   1   from a reduced    50  or   2   from a dis-tribution in the arrangement or composition of atoms. It isvery likely that the reduced    50  arising from high pressuresputtering was compensated by increased disorder in the ar-rangement of composition, leading to an increased    H  K  .These results indicate that a decrease in SFD of the dots isnot just governed by the     50 , but by other factors such aselemental composition in the dots as well.Referring to the work of Shaw  et al. , 20  Co/Pd   multilay-ers grown epitaxially by electron beam evaporation showednarrower SFD in continuous film case when compared tofilms deposited by sputtering on a Ta seedlayer. After thepatterning, a nanostructure with a Ta seedlayer had a smallerSFD compared to the epitaxial one. In our study on continu-ous films, we observed an increase in  H  sw  and reduction inSFD with improvement of film growth; i.e., for lower valuesof      50 . After patterning these films, the  H  sw  increases withthe reduction in     50  but no much difference in SFD wasmeasured. The switching mechanisms are different in con-tinuous films and patterned structures. In the first case, arather square hysteresis loop can be obtained due to magne-tization reversal by nucleation and domain wall propagationover a large area   Fig. 1  . On the other hand, magnetizationreversal in patterned dots occurs by coherent rotation.In continuous films, we observed a reduction in    50  andan increase in  H  C     H  sw   with an increase in the pressure   Fig.3  . The two samples deposited at 3 and 5 mTorr show anincrease in  H  sw  of about 2 kOe compared to the sample de-posited at 1.5 mTorr. In the case of patterned dots, the samplewith a     50  of about 5° showed a coercivity of about 13.5kOe and those with lower    50  showed a coercivity close to15 kOe. A similar trend has also been reported by Smith  et al. 29 While the switching field in the continuous films   espe-cially those deposited at low pressures   is determined by thenucleation and domain wall motion, the switching field of the patterned dots is mostly related to the  H  K    or  K  u   of thedots. Therefore, a clear trend between the reduction in     50 associated with an increase in  H  C     H  sw   for samples depos-ited at higher pressures   indicate that the texture may play animportant role in determining  K  u . IV. SUMMARY The effect of crystallographic texture in   Co/Pd   multi-layers on the switching field and SFD was investigated. Forthe continuous film case, as the film texture is improved,indicated by small     50 , there was an increase in  H  C   andreduction in SFD. From MFM images, there was also a cleardifference in magnetic configuration and domain patterns infilms deposited with low and high argon pressures. Afterpatterning the film in 60 nm size arrays, the switching fieldwas increased as the crystallographic texture is improved.However, no difference in SFD was observed for sampleswith different    50  ranging from 3.8° to 5.1°. Improving filmgrowth can improve thermal stability by increasing theswitching field, without a significant effect on the SFD. Ourstudy points out that crystallographic texture may not affectthe SFD significantly as compared to other intrinsic proper-ties. 1 G. A. Bertero, D. Wachenschwanz, S. Malhotra, S. Velu, B. Bian, D.Stafford, W. Yan, T. Yamashita, and S. X. Wang, IEEE Trans. Magn.  38 ,1627   2002  . 2 T. Oikawa, M. Nakamura, H. Uwazumi, T. Shimatsu, H. Muraoka, and Y.Nakamura, IEEE Trans. Magn.  38 , 1976   2002  . 3 H. Uwazumi, K. Enomoto, Y. Sakai, S. Takenoiri, T. Oikawa, and S.Watanabe, IEEE Trans. Magn.  39 , 1914   2003  . 4 B. R. Acharya, J. N. Zhou, M. Zheng, G. Choe, E. N. Abarra, and K. E.Johnson, IEEE Trans. Magn.  40 , 2383   2004  . 5 R. Mukai, T. Usumaki, and A. Tanaka, J. Appl. Phys.  97 , 10N119   2005  . 6 K. Unoh, R. Sinclair, E. M. T. Velu, S. Malhotra, and G. Bertero, IEEETrans. Magn.  41 , 3193   2005  . 7 J. Z. Shi, S. N. Piramanayagam, C. S. Mah, H. B. Zhao, J. M. Zhao, Y. S.Kay, and C. K. Pock, Appl. Phys. Lett.  87 , 222503   2005  . 8 S. N. Piramanayagam, J. Appl. Phys.  102 , 011301   2007  . 9 K. Srinivasan, S. N. Piramanayagam, and R. Sbiaa, Appl. Phys. Lett.  93 ,072503   2008  .FIG. 6.   Color online   SFD for the three samples shown in Fig. 6. Theapplied field is normalized to their respective coercivities. 023906-4 Sbiaa  et al.  J. Appl. Phys.  106 , 023906   2009  Downloaded 16 Aug 2009 to Redistribution subject to AIP license or copyright; see
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