Thin films of barium ferrite with perpendicular magnetic anisotropy produced by laser ablation deposition

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1 Thin films of barium ferrite with perpendicular magnetic anisotropy produced by laser ablation deposition H. J. Masterson, J. G. Lunney, and J. M. D. Coey Department of Pure and Applied Physics, Trinity College, Dublin 2, reland R. Atkinson, 1. W. Salter, and P. Papakonstantinou Department of Pure and Applied Physics, The Queens University of Belfast, Belfast, BT7 NN, Northern reland (Received 7 July 1992; accepted for publication 14 December 1992) Barium ferrite thin films with perpendicular magnetic anisotropy have been prepared on heated (001) sapphire substrates by laser ablation deposition in an oxygen atmosphere using a KrF excimer laser. X-ray diffraction patterns reveal highly oriented films of the magnetoplumbite phase with a c-axis angular spread of 1.8. Scanning electron micrographs reveal that these films are polycrystalline with a crystallite size of 0.4,um. There is no surface particulate contamination. The saturation magnetization was similar to bulk barium ferrite, and the coercivity was 145 mt. The magneto-optical properties of the films are compared with those derived from the optical and magneto-optical constants of the bulk material.. NTRODUCTON l. EXPERMENT Magnetic thin films with perpendicular anisotropy are of great interest as potential media for ultrahigh-density magnetic or magneto-optical recording.* This is possible for well-oriented materials2 due to the small internal demagnetizing fields which permit sharp transitions between regions of opposite magnetization. The magnetoplumbite structured barium ferrite (BaFe,,Ors) is a candidate for this application. t has high chemical and mechanical stability, along with substantial uniaxial magnetocrystalline anisotropy energy (typically K1 =0.34 MJm3 at room temperature). This is sufficient to produce perpendicular anisotropy in films where the c axis is oriented normal to the film plane. The condition for perpendicular anisotropy requires that the magnitude of the perpendicular anisotropy energy (equal to Kt for perfect c-axis alignment) exceeds the demagnetizing energy &,i@ of the film. Previously, thin films of barium ferrite produced by dc sputtering3 and rf sputtering2 4Y5 have been shown to form a perpendicular magnetic system under the appropriate deposition conditions. Co-Ti substituted barium ferrite thin films have also been deposited with perpendicular anisotropy by sputtering6 and thermomagnetic recording was demonstrated in them. These lilms are potentially useful for magneto-optic memory due to an enhanced magnetooptical effect at wavelengths of 700 and 1800 nm resulting from electronic transitions associated with Co + ions which occupy tetrahedral sites in the crystal. Here we describe thin films of barium ferrite, produced by laser ablation deposition (LAD). This technique has been previously successful in depositing iron oxide lilm~,*~~ and in particular high-t, superconductors. lo The preparation, structure, and magnetic properties of the films are presented. n addition, the magneto-optical performance of a film produced by this method is compared with theoretical predictions based upon the optical and magnetooptical constants of the bulk material. Laser ablation targets were formed by cold pressing a mixture of BaCOs and cr-fe,03 into disc-shaped pellets and then sintering in a furnace at 1100 C! in air for up to an hour. Films were deposited by focusing 200 mj, 23-ns pulses from a KrF excimer laser (248 nm) onto the target to give a fluence of 2 Jcm pulse at 30 Hz. The laser beam was incident on the target at angle of 30 from the normal and the substrate was set at a distance of 3 cm from the target. The substrates were (001) single-crystal sapphire (Al,O,). Typically, pulses were required to deposit a fllm 0.25 pm thick. To achieve the best film orientation, the substrate was heated to a temperature of >800 C by mounting it on a heated surface with conductive silver paste and monitoring the temperature using an optical pyrometer. Deposition was carried*out in an oxygen atmosphere of 0.1 mbar. At pressures substantially lower than this, the barium ferrite phase was not formed due to incomplete oxygenation. The film mass was estimated by measuring the film thickness by scanning electron microscopy. As the film thickness varied across the area of the specimen, it was measured at four different locations and the mass was determined to within an accuracy of 10%. The normal incidence magneto-optic Kerr rotation and Kerr ellipticity were measured on a thin film using a fully automatic Kerr polarimeter operating in a photometric mode using a rotating analyzer before a fixed analyzer technique. The precision of the method is typically f deg in the visible region of the electromagnetic spectrum RESULTS AND DSCUSSON Visibly, the films presented a smooth surface of good optical quality which were sufficiently hard to resist scratching with a steel blade. The morphology of a typical fihn is shown in the scanning electron micrographs of Fig. 1. The particulate contamination, usually associated with the LAD process, was completely absent. Figure 1 (a) shows the overall surface texture of the film, and a film 3917 J. Appl. Phys. 73 (8), 15 April American nstitute of Physics 3917

2 (a) Target W Ts = 700! C FG. 1. SEMs of a (OOl)-oriented barium ferrite thin film showing (a) the surface texture due to the polycrystalline nature of the film and a Elm thickness of approximately 0.25 pm (70 tilt), (b) the individual crystallites, seen in some cases to possess the hexagonal shape of the magnetoplumbite structure ( 15 tilt ). Ts = 840 C edge with a typical thickness of 0.25 pm. Crystallites with grain sizes of about 0.4 pm in diameter account for this morphology. n Fig. l(b) the individual crystallites are visible and are seen in some cases to possess an hexagonal shape characteristic of the magnetoplumbite phase. Figure 2 shows the x-ray diffraction patterns of an oriented film, along with the partially oriented film, and the target material for comparison. The x-ray diffraction patterns were taken in the conventional 8-20 scanning mode. For the well-oriented film the only intense peaks in the x-ray pattern are the (006)) (008), and (00,14) peaks of the magnetoplumbite phase, indicating a film which is oriented with the c axis (the magnetic easy direction) perpendicular to the film plane. This situation is realized as the hcp oxygen planes of the barium ferrite are preferentially aligned parallel to the hcp oxygen plane in the (001) plane of the sapphire. The degree of misorientation is shown by the rocking curve in Fig. 3. An angular spread of only 1X, measured by width of the curve, indicates a highly oriented f&n. Figure 4 shows the magnetization curves of the film, measured using a vibrating sample magnetometer. The inplane and perpendicular magnetization curves are shown together. The figure indicates that the easy direction of magnetization is along the normal to the film plane. Table lists the magnetic properties of the film (values for u, M, and K,, are accurate to better than 10%; where relevant, values have been corrected for shape anisotropy). The saturation magnetization is within a few percent of the value for the bulk material, suggesting stoichiometric barium ferrite. A coercivity field pdi, of 145 mt is comparable to the highest value achieved in thin films produced by conventional sputtering.5 This high degree of magnetic hardness permits a correspondingly high recording density. This can be understood by considering the effect of the demagnetizing field (which exists in films with a nonperfeet c-axis texture) between adjacent bits of opposite mag Two-Theta (degrees) FG. 2. X-ray diffraction pattern of (a) the barium ferrite laser ablation target, (b) a partially oriented thin Elm of barium ferrite, and (c) a fully oriented thin film of barium ferrite (T,=substrate temperature) Theta (degrees) FG. 3. X-ray rocking curve on the (008) peak of an oriented barium ferrite film showing an angular spread in orientation of J. Appl. Phys., Vol. 73, No. 8, 15 April 1993 Masterson et al. 3918

3 80 6 s 40-2 E 3 0.z E s? r r-r ~ -.L L-LL, -1.o o Field (T) A i : g0 % ii 4 k L -4 J -a a00 FG. 4. Magnetization loops of a (COl)-oriented barium ferrite thin film, measured perpendicular (1 ) and parallel (11 ) to the film plane. netization. The observation of some in-plane remnance a, of 12 JTkg (a,ja,=o. 18; Fig. 6) suggests the presence of a non-c-axis texture in the film. Parasitic textures are undesirable as they lead to a deterioration in the properties of the perpendicular recording medium. Depositing the films at a lower substrate temperature [while still maintaining (001) c-axis orientation] may reduce the presence of such textures, as has been shown in the case of rf-sputtered (001 )-oriented CoCr films. 2 However, a corresponding undesirable increase in the c-axis angular spread may also occur. A balance between the above effects would be required in an optimal perpendicular recording medium of barium ferrite. A. Magneto-optical properties t is interesting to examine the magneto-optical properties of the thin-film material and to compare its performance with that predicted on the basis of the dispersion of the complex refractive index N( =n+ik) and magnetooptic Voigt parameter Q of the bulk material. Recently these parameters have been determined for a series of single-crystal platelets of pure and CoD-doped barium ferrite using ellipsometry and normal incidence Kerr polarimetry.13 Such a comparison is important since it may provide information on the reliability of the optical and TABLE. Magnetic properties of a (thll)-oriented barium ferrite film. Saturation magnetization 0, 66 JTkg Saturation magnetization M, 0.35 MJTmJ Coercive field (LoHc 145 mt Perpendicular remnance o;, 29 JTkg n-pane remnance ~4, 12 JTkg Perpendicular anisotropy constant Ku 0.38 MJm3 Anisotropy field && 2.2 T Demagnetizing field pa, 0.44 T c i,o t.u Ti.- = a, -3 t d Calculated curves using bulk constants - - using reduced k FG. 5. Comparison of the measured and predicted dispersion of the normal incidence Kerr rotation (a) and Kerr ellipticity (b) for an area of a barium ferrite fihn of thickness 210 nm. magneto-optical constants N and Q and on the physical and topological structure of thematerial in its ablated thinfilm form. Although measurements of magneto-optic Kerr rotation and Kerr ellipticity are considered reliable, being made at normal incidence with a small optical beam size, ellipsometry failed to produce complete optical data because the probing optical beam sampled an area of the specimen over which the film. thickness was not uniform. While n and k agreed reasonably well with bulk measurements at short wavelengths (A < 500 nm), where the material is highly absorbing and measurements are therefore independent of film thickness, at longer wavelengths the values of the constants become unreliable as the tims become increasingly transparent. The measurements of complex Kerr rotation are shown in Figs. 5 (a) and 5(b) together with theoretical curves based on the fundamental constants of bulk barium ferrite. t has already. been established that the largest in J. Appl. Phys., Vol. 73, No. 8, 15 April 1993 Masterson et al. 3919

4 0.4, Calculated CUN~S using bulk constants - - using reduced k #* _ 0.3 -,f 8 * c -we-= a ; -.< r -. 5 r *x r; : d, i= Y, J..* L e -LA FG. 6. Calculated reflectance of an area of a barium ferrite film of thickness 210 nm. trinsic magneto-optical activity of this material occurs at short wavelengths ( ~400 nm) and that this is associated with transitions of Fe3+ ions. This fact may be observed in Fig. 5 (a) where the Kerr rotation reaches 0.1 deg at 400 nm. Additional important features seen in Fig. 5 are the large Kerr rotation and ellipticity peaks which occur at about 600 nm. t is stressed that this feature is not an intrinsic property of the material but results from a combination of low optical absorption and a film optical thickness of d2 that occurs at this wavelength. The consequence of this, as shown in Fig. 6, is that there exists a reflectance minimum at a wavelength of 625 nm that enhances the complex Kerr rotation. For the bulk material where the reflectance is high ( > 20%), the Kerr rotation would normally be very much smaller at this wavelength. Since the spectral position of the peaks in Fig. 5 are determined by satisfying the half-wave condition, they may be used to determine the mean thickness of the area of the!ilm sampled by the optical beam, providing the real part of the refractive index for the bulk is applicable. n this case we found 210 nm to be the optimum value. Two sets of theoretical curves are shown in Fig. 5, one corresponding to the predictions using the bulk constants13 and one corresponding to a fit obtained using the same constants but with a slight reduction ( -0.1) of the absorption coefficient k, uniformly applied across the spectrum. n both cases the major peaks occur at the same spectral positions. However, their magnitude is not predicted from the bulk constants and this is most likely due to errors in the measured k values reported previously. 3 These errors may have been due to poor surface conditions associated with the single-crystal facets leading to anomalous ellipsometric functions. On the other hand, it may be possible that the absorption in the ablated films is reduced compared with that of the bulk. n either case it is clear that the predicted dispersion curves for the complex Kerr FG. 7. Polar Kerr effect hysteresis loop for a (OOl)-oriented thin film of barium ferrite. rotation are good, providing the lower k values are used in the magneto-optical calculations. The enhanced magneto-optical performance of this material at long wavelengths illustrates well the usefulness of low optical loss magnetic media for recording purposes. t has been shown that where the ratio of nk is large, there exists enormous potential for improving the readout sensitivity through the use of well-designed quadrilayer structures.15 To demonstrate this point directly and to confirm the VSM measurements, Fig. 7 shows the hysteresis curve of the sample obtained through the differential detection of the Kerr effect at normal incidence using radiation of wavelength 633 nm. Again an open loop structure is observed and the noise on the magneto-optical signal is seen to be small. V. CONCLUSON Polycrystalline thin films of barium ferrite with perpendicular magnetic anisotropy have been deposited by LAD on heated (001) sapphire substrates. The films are of good optical quality possessing excellent mechanical hardness, and are free from the particulate contamination usually associated with the LAD process. The film thickness is nonuniform, a problem which could be overcome by continuously scanning the substrate through the plume of ab-. lated material. The degree of film orientation is excellent, being superior to that achieved in films of barium ferrite produced by conventional sputtering techniques. However, an in-plane remnance of 18% of the saturation magnetization indicates the presence to some degree, of a non-c-axis. texture which is undesirable for magnetic recording purposes. The problem might be remedied by depositing the films at a lower substrate temperature. Comparison of the magneto-optical properties of the films with those predicted from bulk constants indicate that the film is not too dissimilar from the bulk form. A high degree of magnetic hardness combined with the fact that the material has a low optical loss at wavelengths of current laser diode technology make these films attractive for magneto-optic recording J. Appl. Phys., Vol. 73, No. 8, 15 April 1993 Masterson et al. 3920

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