JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000 Co1ÀxCrx ÕPt multilayers as perpendicular recording media Kentaro Takano,a) G. Zeltzer, D. K. Weller, and Eric E. Fullerton IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 We have investigated Co1 xCrx /Pt multilayers as potential perpendicular media capable of high-density magnetic recording. The films exhibited strong perpendicular anisotropy, coercivity values ranging from 0.3 to 6.9 kOe, and a range of coercive squareness (S*) from 0.21 to 0.96. Magnetic and atomic force microscopy were used to correlate the grain and magnetic domain structures. The degree of intergranular exchange is clearly dependent on the deposition temperature and, to a lesser extent, Cr content. Co1 xCrx /Pt multilayers are ferromagnetic at room temperature for x 30%, which is not observed in thick Co1 xCrx films. Initial recording results determined a SONR ratio of 23 dB for a Co 4 Å /Pt 5 Å 10 multilayer recorded at 10 kiloflux changes per millimeter using a standard longitudinal ring head. © 2000 American Institute of Physics. S0021-8979 00 46108-6 I. INTRODUCTION intergranular coupling, which is a primary source of transi- As magnetic recording pushes to higher areal densities, tion media noise. Each film had a 200­300 Å Pt underlayer perpendicular media1 may have advantages in thermal stabil- to serve as a template to attain the required microstructure ity over longitudinal media, and thus help to delay the arrival for perpendicular magnetization and high coercivity. Films of the superparamagnetic limit.2 Co/Pt multilayer films dem- with a range of Cr compositions 0 x 60 vol. % , sput- onstrate strong perpendicular anisotropy3 and high rema- tering pressures 3 and 10 mTorr , and deposition tempera- nence squareness, and have been used as perpendicular me- tures 30­330 °C were explored. The multilayer films with dia for magneto-optic applications.4 Recent experiments5 of the structure Co(a Å /Pt(b Å N were deposited where 4 domain wall motion suggest that Co/Pt multilayers could po- a 6, 2 b 17, and N 10 or 12. The Cr addition was tentially sustain 400 Gbits/in2. However, long-range ex- performed by substituting Cr for Co such that the Co1 xCrx change coupling often results in higher thermal stability but film thickness was held constant. As an oxidation barrier, the lower signal-to-noise recording. In Co films were capped with a 30 Å Pt layer. Film thickness val- 1 xCrx films, weakly or nonmagnetic Cr-rich regions form at the grain boundaries ues were calibrated using x-ray diffraction, and controlled that reduce the intergranular exchange coupling and result in during deposition by a computerized shutter system. The Cr lower medium noise. Therefore, it may be expected that dop- content of several films was verified using Rutherford back ing Cr into the Co layers may also act as a segregant to scattering RBS measurements. reduce media noise. Following the work by Lairson et al.,6 we investigated the viability of Co III. RESULTS AND DISCUSSION 1 xCrx /Pt multilayers as stable perpendicular media capable of high recording densi- Low-angle x-ray reflectivity curves shown in Fig. 1 con- ties. We present the structure, magnetic, and recording prop- firm the multilayer structure and bilayer periodicity of films erties of Co1 xCrx /Pt multilayers as a function of growth grown at several deposition temperatures. Due to the short parameters and chemical composition. In particular, we are bilayer periodicity ( 11 Å and enhanced interfacial rough- interested in understanding the segregation process in these ness of the films, only the first superlattice peak is observed. structures. In Co1 xCrx /Pt multilayer films with perpendicu- High-angle diffraction measurements indicate a weak 111 lar magnetization, the individual Co1 xCrx layers are less texture with a single superlattice reflection. We find no sig- than 10 Å thick. We found that the segregation dynamics nificant differences in the structure with Cr content. The in- differ significantly from that in thicker Co1 xCrx films. tensity of the superlattice peak, however, decreased with in- creasing deposition temperature, as seen in Fig. 1. This II. EXPERIMENT indicates that the reduction of the layered structure is due to greater interlayer diffusion or alloying. The present multilayer films were deposited by magne- Room-temperature perpendicular magnetization mea- tron sputtering from elemental sources onto glass substrates. surements were performed using the polar magneto-optical The substrates were mounted onto a holder, backed by a Kerr effect MOKE and a superconducting quantum inter- quartz lamp heater, that can achieve stable elevated tempera- ference device SQUID magnetometer. Films deposited at 3 tures and can rotate ( 1 Hz to improve film uniformity. mTorr Ar exhibited perpendicular magnetization but low co- The addition of chromium to the cobalt layers was achieved ercivity (H by cosputtering from elemental Co and Cr sources. The Cr C 1 kOe even at elevated deposition tempera- tures. Films deposited at 10 mTorr Ar exhibited full rema- addition is thought to serve as a segregant to reduce the nence (Mr MS) and large coercivity values up to 6.9 kOe particularly at elevated deposition temperatures. Enhanced a Electronic mail: takano@almaden.ibm.com coercivity due to elevated sputtering pressures has been ob- 0021-8979/2000/87(9)/6364/3/$17.00 6364 © 2000 American Institute of Physics Downloaded 24 Apr 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Takano et al. 6365 FIG. 3. Kerr rotation amplitude dependence on the Cr concentration of Co1 xCrx (4 Å)/Pt(7 Å)]12 films deposited at 30, 150, and 250 °C. concentration up to 20% and then decreased more rapidly FIG. 1. Low angle x-ray reflection measurements of Co 4 Å /Pt 7 Å 12 above 30% Cr. films deposited at 30, 150, and 250 °C. The arrow indicates the position of Figure 3 shows the polar Kerr rotation amplitude depen- the first superlattice peak. The short wavelength oscillations are due to the dence on the Cr concentration. The polar Kerr rotation, film thickness. which is linearly dependent on the magnetization to first order8, decreases linearly with increasing Cr concentration. The Kerr rotation is smaller for films deposited at 30 °C than served in the Co/Pd system.7 Shown in Fig. 2 are the MOKE at elevated deposition temperatures. Extending the Kerr ro- loops for Co 4 Å /Pt 7 Å 12 multilayers deposited at 30, tation curves in Fig. 3, the thin CoCr layers deposited at 150, and 250 °C. The coercivity HC and squareness (S*) of 30 °C become nonmagnetic at 40% Cr. For the elevated the magnetization loops are a nonmonotonic function of the deposition temperatures, however, a ferromagnetic signal is substrate temperature. Co/Pt films grown at 150 °C consis- observed for the Co tently exhibited very square loops (S* 0.95) and slightly 60Cr40 /Pt film and may persist for Cr concentrations greater than 50%. These concentrations are reduced HC . With increasing deposition temperature above considerably higher than previously observed in either bulk 150 °C, an increase in HC and a decrease in S* was ob- Co served. 1 xCrx alloys or Co1 xCrx films which are paramagnetic for Cr compositions greater than 24% and 34%, A series of Co1 xCrx(4 Å /Pt(7 Å) 12 films with dif- respectively.9 This suggests that the very thin Co ferent Cr concentrations were deposited at various deposition 1 xCrx layer thicknesses in these structures ( 4 Å or the proximity to temperatures: 30, 150, and 250 °C. The addition of Cr, in the Pt layers enhance the chemical segregation of Co and Cr. general, increased S* of the magnetization loops. The coer- In the CoCr phase diagram, there is a magnetically driven civity of Cr doped samples deposited at 30 and 150 °C were segregation of ferromagnetic and paramagnetic hexagonal- comparable. The coercivity, however, was considerably en- close-packed hcp -CoCr phases. However, this is limited to hanced when the deposition temperature was increased to the hcp region of the phase diagram (x 37%). The struc- 250 °C. HC values decreased slightly with increasing Cr tural transition to a sigma phase for x 37% is thought to limit the segregation process.9 However, for the thin layers in the CoCr/Pt multilayers, the CoCr structure is determined by the Pt layers, so the magnetic segregation process may ex- tend to much higher Cr concentrations. This result is consis- tent with recent observations of chemical segregation in ul- trathin Co­Cr films on W substrates.10 Thermal stability of the Co1 xCrx /Pt multilayer films was also measured using a SQUID magnetometer. The samples exhibited excellent thermal stability with typical logarithmic decay rates at 30 °C of only 0.05% ­ 0.10% of the remanent moment per decade of time. The effect of Cr doping on thermal stability was not discernible at such small decay rates. Magnetic and atomic force microscopy images of un- magnetized samples were used to correlate the grain and FIG. 2. Polar MOKE loops of Co 4 Å /Pt 7 Å 12 films deposited at 30, 150, and 250 °C. The coercivity and squareness of the loops is clearly magnetic domain structures. The degree of intergranular seg- dependent upon the deposition temperature. regation was clearly dependent on the deposition tempera- Downloaded 24 Apr 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp 6366 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Takano et al. 10 kfc/mm systematically decreased with decreasing deposi- tion temperature, which is associated with higher S* and lower HC . For Co(a Å /Pt 5 Å 10 films where 4 a 6, increasing the Co layer thickness resulted in higher remanent moments and increased HC , but decreased the SONR. In the films tested, the integrated media noise increased with in- creasing recording densities resulting from additional contri- butions of transition noise. The addition of Cr into Co1 xCrx( 5 Å /Pt 5 Å 10 films, where 13% x 31%, increased the squareness of the loops. SONR, how- ever, was not very sensitive to Cr addition in this composi- tional range. IV. CONCLUSION We have demonstrated the thermal stability and perpen- dicular magnetic recording capability of Co1 xCrx /Pt multi- layers. The multilayers films exhibited strong perpendicular anisotropy, coercivities as high as 6.9 kOe, and high thermal stability. The degree of intergranular exchange, which is re- flected in the coercivity and squareness (S*), is clearly de- pendent on the sputtering pressure, deposition temperature FIG. 4. AFM left side and MFM right side images of Co80Cr20 and, to a lesser extent, Cr composition of the multilayers. (4 Å)/Pt(7 Å)]10 films deposited at a 30 and b 250 °C. The images represent sample areas of 1 m by 1 m. Differences in intergranular exchange were also reflected in the magnetic domain structure of the films and observed di- rectly by MFM and AFM measurements. The Co1 xCrx /Pt ture. Co1 xCrx /Pt films deposited at elevated temperatures multilayers are ferromagnetic at room temperature for x exhibited smaller or finer magnetic domain structures pre- 30% which is not observed in thick Co1 xCrx films and sumably due to weakened intergranular exchange coupling. indicates differences of the segregation process or Pt polar- Figure 4 shows the atomic force microscopy AFM and ization effect in the ultrathin film limit. magnetic force microscopy MFM images of Co 1 80Cr20(4 Å /Pt 7 Å 12 films deposited at 30 and S. Iwasaki and Y. Nakamura, IEEE Trans. Magn. MAG-13, 1272 1977 . 250 °C. In the film deposited at 30 °C, the magnetic do- 2 N. Bertram and M. Williams, ``SNR and Density limit estimates: A com- mains are comprised of cluster sizes of 20 grains, whereas parison of longitudinal and perpendicular recording,'' Digests of The Magnetic Recording Conference, 9­11 Aug., 1999, San Diego, CA. un- the film deposited at 250 °C has clusters comprised of only published . 4­12 grains. The transition between the magnetic domains 3 For a recent review see M. T. Johnson, P. J. H. Bloemen, F. J. A. den occurred at the grain boundaries. Broeder, and J. J. de Vries, Rep. Prog. Phys. 59, 1409 1996 . 4 Initial spin-stand measurements were performed using a J. E. Hurst Jr. and W. J. Kozlovsky, Jpn. J. Appl. Phys., Part 1 32, 5301 1993 . Guzik test stand with a ring head for recording and a giant 5 X. Chen and M. H. Kryder, J. Appl. Phys. 85, 5006 1999 . magnetoresistance GMR spin valve head for readback. 6 K. Ho, B.M. Lairson, Y. K. Kim, G. I. Noyes, and S. Sun, IEEE Trans. However, the writing mechanism was not optimized for the Magn. 34, 1854 1998 . 7 perpendicular recording geometry. For each sample, S L. Wu, S. Yanase, N. Honda, and K. Ouchi, J. Magn. Soc. Jpn. 21, 301 ONR 1997 . isolated transition versus media noise at recording density 8 S. D. Bader and J. L. Erksine, in Ultrathin Magnetic Structures II, edited was measured for written transitions at a spatial frequency of by B. Heinrich and J. A. C. Bland Springer, Berlin, 1994 , Chap. 4. 10 kiloflux changes per millimeter kfc/mm as a measure of 9 F. T. Parker, H. Oesterreicher, and E. E. Fullerton, J. Appl. Phys. 66, 5988 comparison. Best results are 23 dB for a 1989 . 10 S.-J. Kahng, Y. J. Choi, J.-Y. Park, and Y. Kuk, Appl. Phys. Lett. 74, Co 4 Å /Pt 5 Å 10 film deposited at 330 °C. The SONR at 1087 1999 . Downloaded 24 Apr 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp