reached. In Fig. la & (Kerr) - UT Research Information

150

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 1, JANUARY 1987 SLTRFACE AMI VOLUME HYSERESIS OF SPiJTEWD Cccr K. H~rmes,J.C. m e r , Th.J.A. Pcfim m a t e University ofTechnology, P . 0 . b ~ 217,7500 AE Enschede, ?he N e t h e r l d .

Abstract. Surface nngnetic hysteresis, measured by P o l a r M.O. & r r effect on RF and Magnetron sputtered Cccr (81/19 a t % ) films i n the range 20 m is carrpared w i t h volume hysteresis -urd by VSM, bth w i t h external field p?rperaiicular to the surface. The surface coercivity Hcs vas found to decrease belm the volume coercivity a t a critical tkickness of 125 nm for RF films 111 and a t a k u t 1100 and @+J mn centre for and o f f a t r e MarpleLon samples respectively. The slope of the Kerr loop a t 6, = 0 increased slightly again deviating f r m the V9.I slope a t a film thickness of atout 100, 800 and 500 nm f o r RF, centre and offOentre Magnetron Cc€r samples respectively. These results are consistent with the formation of mll spikes fran the surface for f i l m thicker than a c r i t i c a l value. Order of mgnitude calculations do not o p e the fonration of such -11 reversed dcmains and the higher critical thickness f o r Magnetron samples is also in qualitative agreermkwith these calculations, due to a higher perpendicular anisotrom by a f a d o r 2 forthese f i l m . I f spike damins do exist i n Cocr the reversal mechanism is mst likely one i n which the reversed dcmains grow a t the expense of the main damins.

INECDXTPICN m e reversal mhanisn and associated w i t h it the darain f o r perpendicular m q e t i c structure of CcCr thin films recording is not clear a t present. There are tw min d e l s , -1y the particulate and the m t i n u c u s d e l . In the f i r s t me the CcCr columns that are forms3 during film d q s i t i o n a r e telieved to interact only through magnetostatic interaction. N o exchange forceacts over the m l m boundaries due t o e.g. a higher concentration of C r there. In the mntinuous d e l the reversal mechanism i s thought to take place by B l c c h walls a s in stripe damins, hindered by the m l m bundaries [2] Cn the base of Neutron Deplarisation and MagnetO4ptic Kerr m?asur-ts a third inte?mdiate d e l has been p m p e d , in vhich themgnetizationreversal takes place by spike daMin wdl mtion within the oolums [1,41. Usthe Magneto-optic K e n Effect (MDKE) the hysteresis loop of the surface of Cccr can be measured, since the penetration &pth of l i g h t = 632 nn) into CcCr i s a b u t 15 m. T i s o n of surface and volume hysteresis could reveal a p s i b l e inhmqeneity i n the magnetization d i s t r i h t i o n t h r q h c x t the thickness of the film and thus m e insight can k e gained i n the damin structure and reversal mhmisn of the Cocr f i l m . QS and the Earlier we r e p r t e d on the surfacecoercivity cpercivity of the total film H, measured w i t h a VSM as a function of filmthickness [l]. A rotatinganalyser apparatus was used to masure the K e r r rotation as a function of the applied field. The experimental set up is described in c11. using a m u l t i p l ~ r e f l e c t i c msample holder the K e r r rotation d d be measured with an accuracy of 0.002deg A mximrm & was found a t t = 80 m for RF-filmsand a t atout 10430 nm for Magnetron films (bth81/19 a t e ) . The coercivity r a t i o CR is defined by Hcs/G. For RF-films a critical film thickness tC= 125 nm was found, so that i f t < t+ then CR>landiftzt,thenCRrphology of Bing Pri-tskii's theory the c r i t i c a l film thickness for the k m t i o n of spikes is calculated beween 25 and 40 run, but strictly speaking t h i s cannot ke correct since than the length of thespikes wauld beaxw m l l e r than the wall thickness. probably the weak anisotropy assuption and theapproxination of r n interaction between top and botsurface mde by Frivomtskii does not hold for CrCr i n this case. In the particulate d e l the critical radius rc for a spherical single d a m i n particle i s also very small. According to Chicazunni [7] rc = 4.5hm. The alum diameter d and film thickness t for (RF) C d r filmsare roughly related by d = t2/3 (both i n nm) [6,91. Here we measured by (S)TEM that a b u t the same relation holds for Magnetron films. So by sinply put ' = the d c r i t i c & film thickness would be = ( 9 A s 2 180 nm for RF 300 and m for &gnetron films. Hence order of mgnitudecalculations do not w e the formationofsuch small spike dcmains i n Cocr t h i n Also the largercritical filmtkicknesses fourad for kgnetron f i h are qualitatively predicted. h e to the di-tinums character of thereported results, especially the surface ooercivity ofRF-films, it i s not likely that thep*-effect w i l l be able to explain a l l theresults. Rwever, possible a ccmtribution of this effect cannot be

x,

cccr.

'2

films.

disregarded.

Ccncluding, it can be stated that present results of c a p d . n g surface and bulk magnetic tehaviour are consistent with a spike theory i n Cocr thin films. If spike daMins do exist in Cb3, thenthereversal mechanism i s nnst likely one in which thereverseddaMins g r w a t the expenseof the main

&in

dumin.

AcaKmbzThe authors m l d like to thank J.D. their contribution to this work. ' ole conclusion is otrvious, namely that for f i l m with different surface and bulk hysteresis, the magnetization cannot be lrncgenecus throughout the film thickness during a l l stages of the hysteresis curve. me cause wuld be asuMivision of the &mim by d l reversed daMins a t thesurface(spikes),as pqxised by P r i w m t s k i i [81, or a deviation of the mgnetizationfran the perpendicular orientationas described i n the p*-effect. The theory mst explain the different hhaviour of f i h r q i the critical value. For RF-film t h i s hhaviour is a l m s t discontinucusly. Spike-chmin theory is m s t pranising, since it predicts a critical thickness a t which these damins can be created to lwer the total energy. kcording to Priwrotskii these spikespenetrate to atcut 10% of the film thickness or not a t a l l . The remment mgnetization is lcwered a t the surface only by the formation of qikes. Together with Mr coercivity the bS i s decreased. The w t o arerelated by the slqe T according to: +-i = T.%. Hence a strong decrease in H, acanpnied by a nwre gradual decrease of €& can be explained by the formaticslof spikes. Also an increase of the Icerr slope for films thicker than is consistent with I t i s w e l l knmn that basically the slop spike d d n is equal to one, due to the sheet demagnetization: % = M. The formationof spikes 1-s thedmgnetizing field, hence the slope of the hysteresis curve kam?s steeper. F4xently H u b e r t C31 renewed theinterest in branched chain structures i n thin films w i t h uniaxial anisotropy, and also a c u l a t e d the (Vi%) slope of the perpendcular hysteresis curve for such a branched chain structure. Based M t h i s

theory.

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z,