R.F. Magnetron sputtered CoCr films (79/21 at%) with various thicknesses are magnetically characterized. The domain structure is observed by digitally enhanced Kerr microscopy and depends on the Hc/Hk values of the samples. For low and high coercivity films a comparison is made between the measured VSM hysteresis, domain period and a theoretical domain model. The domain shape is a function of the magnetic history of the sample and the bending created by the deposition process

Hubert, A.

Lodder, J.C.

Popma, Th.J.A.

Wind, D.

English

Popma, T.J.A.

NARCIS 

Domain studies of CoCr with perpendicular anisotropy

University of Twente Research Information

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 5 ,  SEPTEMBER 1987 2055 DOMAIN STUDIES OF C o C r  WITH PERPENDICULAR ANISOTROPY J . C .  Lodder, D.  Wind, Th.J.A. Popma, A. Hubert* *University  of  Erlangen-Narnberg,   Martensstrasse 7 ,  D-852 Erlangen,  FRG. University  of  Twente,  P . O .  Box 2 1 7 ,  7500 AE Enschede, The Nether lands.  Cbtract. R.F. blagnetron sputtered CoCr films (79121 at%) with various thicknesses are magnetically characterized. The dorcain structure is observed by digi ta l ly  enhanced Kerr microscopy and &penas on the Hc/Hk values of the samples. For low and high axxcivity  films a comprison is made between the measured VSN hysteresis, domain period and a theoretical domain d e l .  The b i n  shape is a function of the magnetic history  of  the sample and the benaing created by the  deposition process. INI'RODUCPION The reversal mechanism of magnetization i n  sputtered CoCr layers  for  perpendicular recording has sti l l  not been mnpletely  clarified. Studying the  damin  configuration can contribute to  the solution of th i s  problem. The magnetic properties of CcCr  layers are strongly influenced @ variations  inmicrostructure, mrpholcqy and chemical karogeneity. Initial  layer  effects, C r  segregations a t  the colunni bwndaries and surface are very important for the final tzhaviour of the danains. The mrpblogy also depends on the sputtering  mthcd and parameters used (mainly substrate tanperature and Ar gas  pressure). Fran the perpendicular (e.a. hysteresis loop, measured with the VSM, we have i n  sane cases, observed a typical shoulder which indicated bubble-stripe dmains. In order to  understand mre atout the micragnetics of such layers one has to correlate the morphology, rmqnetic prameters and domains. b r e n t z  transmission  electron lrricrosc~py and digi ta l ly  enhanced Kerr microscopy have been lsed  for  damin  observations i n  CoCr layers. A 1 MeV TIP4 has t o  be used for observing Cocr layers thicker than 0.1pm, up to  a mimum of 0.3pm [1,2].  A specially designed sample stage to exclude a perpendicular  lens  field  fron  the specimen during b r a i n  observations is essential [1,31. Different d m i n  amfigurations w i t h  in-plane magnetization are  observed in   thin (< 60 nm) CoCr layers C2-41. Tkis can be explained by variations in  the  preparation  methds and consequently differences in mrphology, structure and chemical h q e n e i t y .  None of the samples used has a wel oriented hcpaxis. For a gxd textured layer (Co77Cr23), having a thickness of  50 nm, stripe  damins w i t h  a width of 0.1pm were observed [21. A maze CfiMin pattern, Iupm fran bubble-like materials, is observed i n  films of 8.1 pm thickness having a caoposition of Co82Cr18 [51. Very thin layers (30 mn) w i t h  in-plane magnetization show the  so-called  ripple  structure  in  CF9Crll C41 and Co79Cr21 C5l. Different damin forms  were observed i n  films w i t h  much batter orientation and increased thickness (>0 . lpm)  C2-51. A magnetic black-and-white dot  structure  with a distance between the dots of the same order as  the colunmar diameter has been shown by C31. In this case the film mrphology (one column mnsists of several   crystall i tes) is  totally  different fm the m a x e d  C r y s t a l  structure present in [2] which shws a s t r ipe chnain width i n  the order of the crystal size. CoCr films i n  the range between 17-30 at% C r  w i t h  a C r  segregation into the Co~uItm boundaries shOd dot-like Lorentz h g e s  which have the Same Size  as the c o l m  diameter [SI. Stripe d m i n  widths (< 0.1 pm) having the s a m  size  as  the  colm-diameter are also abserved in  [I]. I t  w a s  a lso shcwn that  frcsn relatively thick films  (0.3 prd, which are mre realistic for  remrding applications, the d m i n  width is the satre as the calm size [21 and it is suggested that the d m i n  structure consists  of a chain of crystals. Based on neutron depolarization experiments and M.O. toeasurmts  of the surface of our C& films it can ke concluded that the  damin width i s  proportional to hX, there x is atout 0.8 for RF magnetron sputtered films C6-101. This value is  closer to  the  calculated m e r  of 2/3 for a branched domain structure [ill than 112 predicted  for the Kittel configuration [l2]. I t  can also  be concluded tha t  reversed spike  damins can e x i s t  and w i l l  influence  the reversal d e  above the  critical  thickness of magnetron sputtered CoCr layers (5Ld0-808 nm) cl01. Unfortunately (spike-) dorains cannot be r e a l l y  v i s u a l i d  by neutron depolarization. EXPERIMENTAL PRCCEWRE The CoCr layers are deposited on Si-substrates by RF mgnetron sputtering, having a 3" target, back pressure  1.5~10-7 nbar and P a r  = 6x10-3 mbar. The fi lm  carpsit ion is around 21 a t% C r  and the thickness (h) is determined by XRF. Magnetic treasurements are carried out by VSM and torque magnetcmeter. -strate  material is  si (100) single  crystal 10x10 mn2. -ins are observed by a digi ta l ly  enhanced Kerr microscop. lhe  layers  are covered with an evaporated 114 x ZnS layer. FOughness and beding  of the  deposited  substrate can be nmsured w i t h  the Slcan Dektak  3030 Stylus instmnent. Column diamter and lrorpholcgy are determined by STE3 observation using a Jeol 280 cx microsmpe. The relevant sample p r c p r t i e s  are given in  Table I. Table  1: P r o p e r t i e s  of the CoCr samples. 1 Sample d m i n  Pmr Hcjm Hk K l  Hc h Ms kA/m shape nm % kA/m J/m3*l& kA/m nm 1 lo? 660 1.5 525 13.2 8 520  402 2 3 410 short 730 3.2 555 14.8 18 700 423 4 790 1.7 575 16.0 10 760  441 stripes 910 1.4 555 14.3 8 1230 _____- 6 5 413 820 6.5 615 16.4 40 1436  426 8 l ike  840 8.9 675 18.4 60 700  434 7 stripes 960 4.0 595  15.5 24 1300 415 dot- 850 7.5 6 1 F  16.0 46  1220 - M.O. KERR OBSERVATIONS The f i r s t  Kerr observations on CoCr layers  with  divergent p r q r t i e s  a r e  published i n  C131. There a specially designed digitally enhanced Kerr  microsmpe w i t h  an irrage processing system is used. The films used have a c a p s i t i o n   f r m  17 to 21 a t% C r ,  thicknesses f r m  96-622 nm and coercivities from 10- 60 kAjm with one sample of 90 kA/m. They a l l  s W  a m r e  o r  less st r ipe-dmin  s t ructure   a t  rsmnence. It was concluded from this that a l l  samples reversed by dmin-wall  m v m t   a s  a continuous medium. Using the s a m  microscope mentioned above, w caw to  the conclusion with our preliminary resul ts  C141 that we have to  divide our Wr layers  into 3 categories m e l y :  low H c  (long  stripe), medium H c  w i t h  short s t r ipes  (20<Hc<40 kA/m) and high  coercivity  films w i t h  a dot-like daMin structure. The damin behaviour is clearly connected w i t h  the shoulder in  the  perpndicular  hysteresis loop. The Kerr observation methcd has its limitations i n  the resolution (- 2016 m) and depth of information ( -15 nm) . An advantage i n  relation to transmission mrentz microscopy is that (surface) mynet ic   infomtion can be obtained  fran a l l  thicknesses witlwut any thinning  techniques w i t h  their  consequent disadvantages. A caparison between both  observation  methds is mntioned i n  [SI, i.e. the Kerr image of an 0.6pm thick sample (20 at% C r )  and a Lorentz imge of C3.1pm thick (18 a t% C r )  btl~ showing a stripe-like dcrnain pattern. The interpretation of these h g e s  is not so simple. The differences in thickness, chemical cornposition and eventually remwal f rm the or iginal  substrate w i l l  have an i q o r t a n t  influence on the d m i n  bhaviour. Besides, observation of double layered samples j.s mripletely  impssible  with  the Lorentz methcd. Another addntage of Kerr microscopy i s  the  possibility  of applying fields up to 8C30 kA/m. For most of the Wr layers the d m i n  process throuch the whole hysteresis loop can be observed [13,141 within  the  limitation  of  the  resolving pcwer. 0018-9464/87/0900-2055$01,00~1987 IEEE 2056 EXX'EKIE.1ENTAL RESULTS A series of samples was  nnde and the measured  and calculated results fran Kerr observations, VSY and torque magnetometer a re  given in table I. The @factor of our samples is derived from Q - K1/0.5 /lo E@, For Q>l the perpendicular axis will thus be the direction of preferred uwgnetization in  the case of a uniform rmgnetized layer.  Inthe demagnetized s ta te   the mgnetization is s p l i t  up into  damins. The condition for  stable  perpendicular magnetization Kl>Ed is expressed in   the Q- factor. For our samples Q varies frmn 1.30-1.50. l'he d m i n  shape is not dependent on th i s  Cbrange, but on the coercivity of the layers. Of course, the Hc is strongly dependent on the reversal mechdnisrn and the magnetic mqholoyy (Cr segregation at  the column koundaries).  In  order to explain  the observed s t r ipe  dmins  we have used the Kay and Ehz I d e l  C151. The tsm relevant equations wtiich have t o  be solved are explicitely given i n  C41. The parameters used are: film thickness (h), material parameter ( h ) , domain period (P I  and p = 1+Ms/2Kl which  was kept constant a t  1.70. Bsth the  equations  are  solved  for  several  pairs  of h / h  and H/Ms values. In Fig. 1 tlle relation M/bls VS. H / n S  and i n  Fig. 2 P/h Vs. H/Ms a re  shown. In both plots (see broken l ines)  h/), is used as  a parameter. Fran VSM measurements we know the hysteresis loop and frmn this we detennine the slope T = dM/dIl for IN43  or €T?Hc. This i n i t i a l  slope is the mst relevant parameter. An  other relevant parameter P is measured fran the &rr plwtographs as  a function of the  applied  field. 0.8 "T 0.6.' 0.4- 0,2!/ ' Calculated by Measur  VSM __ -I' -H/Ms 1501 100 'I Measured -I1 1 , 0 0 2  0 4  0.6 0.8 1.0 H/Ms---c DDC-0 S6 7- Fig. 2 P / ~ V S .  H,':G (see caption Fig. I ) .  Both P and T are detennined idependently am3 must agree simul- taneously w i t h  the theoretical calculation for a certain h/h in both nomlized figures 1 and 2. For films w i t h  a l a  H c  (81) and a high fic ( # 6 )  the M/Pk vs. H/Ms values  calculated (dotted l ine)  and measured (solid line) are given i n  Fig. 1. lhe measured slope is also i n  good ayreentent w i t h  the  calculat- ed lille for h / h =  25 i n  Fig. 2. For th i s  sample x =  20.8 run, Uw=/roMs2h = 4.2*10-3 J / n 2 ,  A = (l/Kl). (Uw/4)2 = 8.4~10-12 J / m  and 1 ='i%/Kl = 25 run. Another low Elc film ( # 2 )  has  also k e n  measured and calculated. Ekre we fomd h/h = 55 as the   best   f i t .  For sample #6 (high Hc) h~ could not f i . t  bot31 curves  (see Fi.gs. 1 I 2 ) .  We have also conpared the size and s h a p  of the domain period of #2 in the raMnent state after E a d  demaynetiziny fields (Fig. saturation w i t h  perpendicular 3) .  Fig. 3 milain  configurations of  a low mrc iv i ty   f i lm   i n  remnent ( l e f t )  and demagnetized (r ight)  state. At z-amnerit s t a t e  Pmr = 910 nm and af te r  demagnetization Po = 60fJ nm. This minirnum energy position  of  the  stripes results  in a  very good parallel  orientation. m a i n  cbservations i n  the field free state show the magnetic history of the sample. Also different  damin  configurations can be &served after  partial  saturation and applying a f ie ld  under an angle Cl31. For a high €IC sample ( 8 6 )  the dot-like domain shape a t  deim-tgnetizing field can be roughly given as Po = 665 nm. Typical shoulders i n  VSM and M.O. Kerr hysteresis loops are  not seen in the high tIc films. "he calculated d e l  by means of P and T is  not i n  gcwd agreement for  such films (see Figs. 1,2). hk have t o  reali-ze that the n t e t h c d  we used makes the following assmnptions namely: the domain-wall energy i s  independent of h, the  wall volune is neglected and, l a s t  but  not least, the coercivity is zero. Stresses have a l o t  of influence on the damin configuration. The bending of the saryles is measured; A l ine scan over the total sample leugth i n  two orthqonal directions i s  rnade.  The curvature  (e.9.  the  strain) i s  not always isotropic over a certain  area and can vaq  f r an  place t o  place  in  the same W l e .  \le found that samples have a different bending in  the  orthcqonal directions  shming  straight  stripe  damins (see Fig. 4a). More or less meandered d m i n s  a r e  seen i n  Fig. 4b hhich is rnade  from a position directions. w i t h  equal bending i n  tm a )  asymmetrical (xzy) b) symnetrical ( x y )  Fig.  4 DceMins of #2 a t  Ha = 260 kA/m a s  a fun&ion Of the bending i n  two orthogonal directions (xy). A t  the m n t  we have the impression that  stress in  O u r  mgnetrm sputtered films is higher than i n  our RF sputtered ales [4]. Consequently the s t ress  w i l l  have t o  be studied in mre detai l  in relation to the total anisotrom factor in the films. MoRElKlLcGy Cross-sections were studied w i t h  the STDl mfie of the electron microscope. ?'he fractional  surfaces were prepared by cooling the  sanples in  liquid N2 before breaking thm, t o  obtain a brittle fracture. The columnar-like mrpholgy can be clearly seen i n  a l l  our films indepndent of the Hc/Hk value. 20.57 of the coercivity. Factors l ike  stress and magnetic history play an important role. For low Hc material the stripe aOmain theory is i n  good agreement w i t h  the measured P values fran Kerr  observations and the hysteresis loop. me r-ent daMin (dot, stripe) of all our films consists of several c o l m s  . A direct  correlation between the  type of damins and sone aspects of the macroscopicalhysteresis loop (€IC, shoulder) can b= made (see also [14, 241). Stress plays an important role  in  the  damin behaviour. REFEmw [l] K. Ouchi and S .  Iwasaki, IEEE Trans. Magn.  MAG-18, 1110, [2] P.J. G d y  and M. Mi,. J. Magn.  Magn. Mat., 40, 154 C31 M. Ohkoshi and T. Kusuda, Japan. J. Appl. Phys. 22, L130, [41 T. Wielinga, Thesis,  University of Write, Enschede, The [SI H. Hoffinann, IEEE Trans. Magn.  -22, 472, (1986). [6] K. Hemnes, J.C. Lcdder, M.Th. Rekveldt and W.H. Kraan, [7] W.H. Kraan, M.Th. Rekveldt, K. H e m e s  and J.C. Lodder, C8] W.H. Kraan, M.Th. Rekveldt, K. H m s  and J.C. Lodder, C91 K. Hmes, W. Lisowski, J.C. Lcdder, L.J. Hanekanp and [l0] K. H-s, J.C. -der and Th.J.A. P o p %  IEEE Trans. [ll] A. Hubert, IEEE Trans. Magn. -21, 1604, (1985). [l2] C. Kittel, W s .  Rev. 70,  965, (1946). E131  F. Sdnnidt and A. Hubert, J. Magn.  Magn. Mat., 61, 307, (1986). c143 J.C. Lcdder, D. Wind, G.E. V. Dorssen, Th.J.A. Popra and A. Hubert, IEEE Trans. Magn. MAG23, 214, (1987). [151 C. K a ~ y  and U. E m ,  Philips Res. Repts. 15, 7, (1960). [161 I. Privorotskii, Thedynamic Theory of Camin [171 J.D. Livingston, J. Appl. Phys. 52, 2544, (1981). [181 G.A. Jones and F. Farnswrth, Phys. Stat. Sol. 79, 970, E191 J. Daval and D. W e t ,  IEEE Trans. Magn. MAG-6, 768, [201 T. T m k a ,  K. Yazawa, H. Masuya, IEEE Trans. Magn. MAG21, [ Z l ]  D.J. Craik and E.D. Isaac, P m .  Phys. Scc. 76, 160, [221 L.F. Bates, D.J. Craik and E.D. Isaac, P m .  Phys. Soc 79, [231  E.P. Wohlfarth, MRM Conference, Italy, (1986). [241 L i  Cheng-Zhang and J.C. -der, Paper BA-08, I n t e m g  '87. (1982). (1983). (1983) . Netherlands, ( 1983 ) . J. Phys. D: -1. Phys. 17, L157, (1984). Physica 1368,  440, (1986). IEEE Trans. Magn. W 2 3 ,  65, (1987). Th.J.A. P o p ,  J. phys. D: -1. Phys. 19, 1311, (1986). p.Bgn. MAG-23, 150, (1987). Structures, J. Wiley & Sons, New York, (1976). (1962). (1970). 2090, ( 1985 ) . (1960). 970, (1962). Ebr samples #3, 5 and 8 respectively an average colunm size D (measured at  the surface) of 40, 45 and 70 m is determined. Bnsequently Pnrr/D values are 20, 21 and 12. Fran another low (#2 and high  (66)  coercivity  film we have measured Po (see abvel  and using these values we  came to the conclusion that  me darnin consists of 4-8 columns, for these films. We also have to  realize that the column size is a function of the layer thickness. Studying the fractional surfaces it can be seen that the three films shaw a typical fractured plane close to the  substrate. This is interpreted  as belonging to the initial layer which is also seen i n  the in-plane W graphs of the s a w  samples. COEECIVITY 'Ihe coercivity  in  ferranagnetic  layers  with  perpendicular anisotropy is determined by two factors. 1. The type of magnetization process involved i.e. is the mg- wtization reversed by (coherent)  rotation or by damin-wall m v m n t ?  2. Morpholosy of the layer i.e. shape and dimensions of the crystallites (cohmms), nature of the boundaries, surface and initial  layer  properties. In fact these two factors are strongly interrelated and their W r t i e s  are  strongly  influenced by the  deposition parameters such as  temperature, P a r  etc.. Based on general -ledge a b u t  f e rmgne t i c   f i lms  we can conclude that  the presence of s t r ipe damins in CoCr must  be quite cumwn because the layer splits p into such a configuration by reducing its demgnetizing energy. Theoretical support for  reversed danains in  uniaxial thin  layers w i t h  perpendicular  anisotropy is provided by [11,16]. Frcm research on p e m e n t  tmgnets, Supporting argwents for the relation coercivity and reversal mechanism are given by [17]. In  polycrystalline and/or columnar-like tmterials the critical damin  size is diff icul t  to estimate because it is influenced by the presence of neighbowing px-ticles and their interactions.  If  the magnetization reversal takes  place by coherent  rotation Hc must be equal to the Hk. In UK case a l l  the layers have Hc<Hk. An explanation for  th i s  effect can be made by intrcducing  the s m a l l e d  fanning, a r l i n g  and buckling modes. These can in principle, only occur in layers in which the magnetic anisotropy is mainly prduced by shape anisotropy. In C& the main anisotrapy is  mnfigurated by magneto crystalline anisotropy. On the other h d  experimental work on high coercivity Co or Co-P layers Cl8-201  shaws danain structures independent of the source  of the magnetic anisotropy. The presence of a dcmain structure and the results of'other measurements like hysteresis loss and Hc vs. angle are not c q l e t e l y  i n  l i n e .  This could  perhaps be explained by the phenanena of interaction damins which were f i r s t  introduced by [Zl] .  H e r e  particle exchange forces  are negligible and the magnetic mcments are mutually aligned by nagnetostatic  nteractions [22]. Recently t h i s  rmlel was mntioned as a possible mechanism in Cocr layers C231. For low €k the nucleation density is much 1-r than for high Hc ones. It is clearly sham that nuclei  str ipe out at  the t ips of the -in a f te r  decreasing the applied f ie ld  [13,14J. The cbservable size of the  dots (caning fran  saturation)  in  high H c  films is determined by the  resolving pder of the Kerr microscop.  Nevertheless  these  dots do not expand very much due b the  very  close surrounding  of neigNxxlring dots. The reversal i n  one calm could be, depending on its dimensions, initiated by a reversed  damin. These small sizes cannot be resolved by Kerr microscopy and it can be accepted that  such h i n s  are  no longer  present i n  thinned TEN specimens. The prticle Nhaviour  without exchange coupling is suggested by C r  segregation at  the column boundary. A t  the m e n t  only prelimary resulsts  are  vailable  fran Scanning Auger Microspectroscopy (SAM) and wet chemical etch procedures. Fran the latter we found only qualitatively a,higher C r  content a t  the surface than in  the bulk of a l l  our samples. Fmn  a SAM msur-t of a fractional surface of a high coercivity film he came the tlie conclusion that there is a Cr segregation a t  the m l m  boundaries. Different damin configurations can be expected as  a function 

Domain studies of CoCr with perpendicular anisotropy

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