Abstract. The domain structure of relatively thick RF-sputtered CoCr (81 : 19 at 9%) layers with a columnar morphology in the range of thickness 1-5 pm was investigated using a polarised neutron beam. On the basis of the results a model of straight domains running through the entire film thickness has to be rejected. Arguments are given for a serpentinelike domain structure with a domain width about the same as the column diameter (0.1-0.3 pm), resulting in a 'chain of columns' model. Sputtered CoCr layers have already been applied as a perpendicular recording medium with extremely high density (Iwasaki 1980). The morphology and crystal structure and therefore the magnetic properties of the film depend strongly on the deposition and growth conditions.Our films consist of conically shaped crystals or columns with c-axis textured hcpstructure (Lodder et al 1983). The crystal or column diameter increases almost as the square root of the film thickness. In order to understand the magnetisation process, domain studies are of great importance. If the exchange force is restricted to the columns the domain size coincides with the crystal diameter. The columns will act as single domain particles and a rotational switching mechanism is to be expected for the magnetisation process. In this case the domain structure is determined by a minimum of the magnetostatic energy and will appear as a kind of checkerboard pattern formed by the individual columns. In the case of a continuous model, where the exchange forces extend over the crystal boundaries, the magnetisation process is due to domain wall motion which is more or less hindered by the column boundaries.The domain dimensions of both particle and continuous models are too small to be observed by Kerr microscopy or the Bitter technique. The most suitable method for observing small domains is Lorentz transmission electron microscopy. A 1 MeV microscope has to be used for observing CoCr layers thicker than 0.1 pm, up to a maximum of 0.3 pm Iwasaki 1982, Grundy andAli 1983). A specially designed sample stage to exclude an external perpendicular field from the specimen during domain observations is essential Iwasaki 1982, Ohkoshi andKusuda 1983).Different domain configurations with in-plane magnetisation are observed in thin (<60 nm) CoCr layers (Grundy and Ali 1983, Ohkoshi and Kusuda 1983, Wielinga 1983. This can be explained by variations in the preparation methods. None of these films have a well oriented hcp c-axis. For a good textured layer (Co77Cr23), with a

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 for 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 for Magnetron samples is also in qualitative agreermkwith these calculations, due to a higher perpendicular anisotrom by a f a d o r 2 for these f i l m . I f spike damins do exist i n Cocr the reversal mechanism is m s t likely one i n which the reversed dcmains grow a t the expense of the main damins.INECDXTPICN m e reversal m h a n i s n and associated w i t h it the darain structure of CcCr thin films for perpendicular m q e t i c recording is not clear a t present. Us-the Magneto-optic K e n Effect (MDKE) the hysteresis loop of the surface of Cccr can be measured, since the penetration &pth of light (x = 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 m h m i s n of the Cocr f i l m . Earlier we r e p r t e d on the surface coercivity QS and the cpercivity of the total film H, measured w i t h a VSM as a function of film thickness [ l ] . A rotating analyser 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. For RF-films a critical film thickness tC = 125 nm was found, so that i f t < t+ then C R > l a n d i f t z t , t h e n C R < l .?he presence of a critical thickness in Magnetron films was not d e a r fran the reported data, therefore additimal samples were hgnetron sputtered, also to cover the thicker films regions, and results are reported here. SecOnaly we will clmpare the slope of the easy-axis hysteresis l o o p measured with the Kerr effect & v9.1 a t 6, = 0 and M = 0 respectively.?he f u n c t i o n a l depedeme of the VSd slope on film thickness for RF-films was explained by Wielinga using a mtinuous &rain d e l C2l. ~ubert expanded the theoretical basis of this analysis t o bran&ed daMin structures in Cocr C3I.?he C c C r Samples were sputtered frm alloyed COcr (81/19 at%) targets of 4" and 3"inch diatreter respctively, on S i (100) substrates i n a Leybold Heraeus RF-sputter apparatus (7,400) either with or withcut Magnetron facilities. W e will distinquish here between RF-and Magnetron films. RF-films were sputtered in batches of 4 samples under the optimized sputter C5l while for Magnetron films the optimum parameters for cbtaining ya-3 perpendicular anisotropy were determined a t P k = 6 x m&r and V m = -250 V. The target substrate distance w a s 5 and 3 an respctively. As is w e l l knc...

The magnetic hysteresis curve at the surface O f RF-and magnetron-sputtered CoCr (81,'19 a t. %). in the thickness range of 20-2500 nm. was measured with a rotating-analyser apparatus using the magneto-optic Kerr effect. The Kerr rotation of CoCr films (13-19 a t. V Cr) decreases with increasing Cr content. and depends slightly on wavelength. showing a faint minimum between 550 and 600 nm. The surface hysteresis is compared with the bulk hysteresis as measured with a \'sM. For RF films the maximum surface coercivity is higher than the bulk coercivity. being 120 and 95 kA m" respectively for 80 nm thick films but an abrupt decrease in only the surface coercivity was found at I = 125 nm. The coercivity of magnetron-sputtered CoCr deviates from that of RF-sputtered films. Until a maximum coercivity is reached at-1 pm. the surface coercivity is about 20% higher. but at =2 pm both coercivities decrease strongly. The existence of reversed domains within the main domains of CoCr is proposed. and the reversal mechanism is thought to be one in which the reversed domains grow at the expense of the main domain.

No abstract

Polarised neutrons (A = 0.47 nm) are transmitted through sputtered CoCr films. From the measured diagonal elements of the depolarisation matrix it appears that the effective thickness her ~ of the films is about 50% of the actual thickness. When a soft magnetic NiFe film is deposited before or after deposition of the CoCr, he, is 67 and 60% of the actual thickness, respectively, due to the disappearance of the flux closure in the CoCr film. From the angular dependence of the diagonal elements compared with computer simlation the "chain of column" model is supported. Our results indicate that magnetic flux is closed by "branched" spikes at least on the substrate side of the CoCr film.