APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 8 19 AUGUST 2002 Crystallographic orientation of Cr in longitudinal recording media and its relation to magnetic anisotropy Antony Ajana) and Iwao Okamoto Advanced Magnetic Recording Laboratory, Fujitsu Laboratories Ltd. 10-1 Morinosato-Wakamiya, Atsugi 243 0197, Japan Received 9 April 2002; accepted for publication 18 June 2002 A specific growth of Cr layer grains is found to exist when grown on the mechanically textured NiP­Al substrates used for longitudinal recording. High resolution transmission electron microscopy analysis of a large number of individual Cr grains indicate a Cr 110 preferential growth along the textured direction groove or circumferential direction . This particular orientation of the Cr underlayer is found to be the cause of an in-plane magnetic anisotropy of the Co based magnetic layer. The temperature dependence of this in-plane magnetic anisotropy study indicated the importance of the specific crystallographic orientations of both the underlayer and the magnetic layer. © 2002 American Institute of Physics. DOI: 10.1063/1.1500433 Longitudinal magnetic recording media made on circum- graphic symmetry. Also, mechanical scratches create surface ferential mechanical textured scratch NiP/substrate, are topography with two-fold in-plane degeneracy, making it dif- known to show superior high density recording performance. ficult to get the lattice orientation effects from grazing inci- When deposited at higher substrate temperatures dence x-ray diffraction or selected area diffraction. To avoid ( 200 °C), an epitaxial orientation relationship of the above-mentioned modulation effects, instead of looking Co 1120 0002 Cr 002 110 /NiP/Al is easily obtained. at a collective ensemble of Cr grains, we looked at individual Samples made on a mechanically textured NiP layer, in ad- grains and examined the 110 direction in relation to the dition, show an in-plane magnetic anisotropy of the groove direction using high resolution transmission electron Co 1120 layer. This anisotropy is quantified as the orienta- microscopy HRTEM . The method of observing a Cr grain's tion ratio OR , a dimensionless quantity, calculated by the crystallographic orientation is schematically represented in ratio of the coercivities or remanent magnetization when Fig. 2. For this study, a layer of 5-nm-thick Cr was deposited the applied magnetic field is along circumferential easy on the mechanically textured NiP layer. The TEM sample axis and radial directions, respectively. Although the origin was mounted on the platform and the electron beam was of OR is so far not clearly understood, interesting studies moved across the groove directions at an angle of 30° to have been reported which are beneficial for the future devel- opment of magnetic recording.1­4 The two possible main scan six to eight grains, taking diffraction patterns of each of mechanisms, suggested for this in-plane magnetic anisotropy, them. In order to observe the single grain diffraction images, are stress on the magnetic layer,1 and the c-axis Co 0002 an e-beam of diameter 3 ­ 4 nm was used, which is less direction preference of orientation along the groove than the average grain diameter of Cr grains (5 6 nm). The direction.2,3 However, there are no explanations for this par- photograph is taken in such a way that the circumferential ticular predominant c-axis orientation of the Co grains in the direction is parallel to one side of the photograph. This helps magnetic layer along the mechanical groove direction. It was us to obtain the crystallographic orientation of Cr with re- pointed out that this predominant orientation of the c-axis spect to the circumferential direction. The diffraction pattern may be due to a particular nucleation of Cr.3 In this letter, we thus obtained is shown in Fig. 2 b . It is easy to identify the explain in detail how this particular growth of Cr was ob- Cr 110 zone axis direction as shown in Fig. 2 b . served, which in turn helps the orientation of Co 0002 along Diffraction patterns of more than 100 grains were taken the circumferential direction and gives rise to an OR. We and the 110 orientation with respect to circumferential di- have also studied the OR with respect to temperature to un- rection was estimated. The statistics thus obtained is illus- derstand the mechanism of in-plane magnetic anisotropy. trated in Fig. 3. Figure 3 clearly shows that number of Cr The above-mentioned epitaxial relationship of the Cr un- derlayer and Co magnetic layer is shown in Fig. 1, which explains the good lattice matching for Co c-axis Co 0002 along the Cr 110 direction. For the observed c-axis growth along the circumferential direction to occur, Cr lattice orien- tation has to be anisotropic in the plane; more precisely, Cr 110 has to be along the scratch direction circumferen- tial . This is because of the epitaxial growth of Cr and Co layers as mentioned above. The main difficulty in establish- ing the in-plane anisotropy of Cr is due to its high crystallo- FIG. 1. Epitaxial and interatomic spacing relationship for Cr 002 and Co 1120 . Note that lattice mismatch between Cr and Co lattice along the a Electronic mail: antony@flab.fujitsu.co.jp c-axis Co 0002 direction is 0.5% and that perpendicular to it is 5.4%. 0003-6951/2002/81(8)/1465/3/$19.00 1465 © 2002 American Institute of Physics Downloaded 21 Jan 2003 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp 1466 Appl. Phys. Lett., Vol. 81, No. 8, 19 August 2002 A. Ajan and I. Okamoto FIG. 4. Expected 4 fold and experimentally observed two fold Co c-axis in plane orientation on the anisotropic Cr 110 . Inset shows the experimen- tal c-axis population of Co 1120 texture, highest along the circumferential AC and lowest along the radial directions BD . ever, experimentally we only see a preference of c-axis ori- entation along circumferential direction and not along the radial direction two-fold solid lines in Fig. 4 .2,3 If there FIG. 2. a Schematic representation of TEM observation of Cr grains on were any other angle of preference in the plane, then the mechanically textured NiP­Al substrates and b diffraction pattern from a distributions shown in Figs. 3 and 4 of Cr 110 and Co 0002 single grain with arrows indicating Cr 110 direction. would show a maximum at that corresponding angle. Figure 5 shows the typical in-plane bright field TEM grains with the 110 direction along 0° circumferential di- image of magnetic grains of Co alloy CoCrPtB layer whose rection and 90° radial direction are prominent over any c-axis distribution is shown in Fig. 4. From the large number other direction, within the plane of the film. Since Cr 002 of grains we have studied, it is clear that the c-axis of the has a square lattice structure as shown in Fig. 1 , circumfer- grains shown as white arrows near the mechanical texture ential and radial directions are equivalent and hence indistin- line shown as black arrows is mostly in the same direction. guishable. This preference of the Cr lattice is an important Also along the scratch lines grain boundaries are formed. It result, believed to be random within the plane. Although the is evident from different c-axis orientation of neighboring reason for the Cr 110 towards the texture lines is not clear, grains on either side of texture line, that they are not origi- we believe it may be either to reduce the surface energy or to nating from a single grain with the texture line acting as a grow less stressed lattice structure. It is interesting to note stacking fault direction. that specific crystal orientation induced by scratches are also It is a known fact that magnetostriction parallel to the Co found for the thin film transistor systems.5 Recent observa- c-axis direction is zero and is relatively higher in the perpen- tion of OR on samples made using the skewed angle depo- dicular direction and highest at 60° to c-axis.7,8 However, sition of Cr layer6 with no scratches also shows the impor- CoPt systems show an increase in magnetostriction,9 al- tance of specific orientation requirements of Cr grains. though for the c-axis direction it is still the lowest. Ross The Co grains formed on this particular Cr 110 orien- et al.10 have reported that along the groove direction, mag- tation are expected to have the four-fold periodicity equal netic grains undergo higher compressive stress than that of preference along circumferential and radial directions of its the radial direction for a thick layer of CoCrTa alloy system, c-axis circles in Fig. 4 due to the epitaxial growth. Figure 4 although the stress induced magnetic anisotropy is much shows the schematic representation of the expected and the lower than the crystalline magnetic anisotropy. The above experimentally observed Co c-axis orientation on top of the factors indicate that Co nucleation on 110 oriented Cr above-mentioned Cr 110 002 anisotropic orientation. How- grains will happen when the c-axis is along the groove di- rection. Thus the grains may form by minimizing the lattice distortion by aligning the c-axis along the texture direction. If it were the other way, with the c-axis along the radial FIG. 3. Cr 110 orientation on the textured substrate. Note that 110 di- rection is likely to lie along circumferential (0°) or radial direction (90°) FIG. 5. HRTEM image of Co grains of CoCrPtB 20 nm /Cr 5 nm /NiP­Al. than any other angle. Inset shows the schematic of the same population of White arrows are indications of each grain's c-axis orientation, near to the 110 of Cr 002 . AC indicate the circumferential and BD indicate the radial mechanical texture lines shown as black arrow . Note the Cr rich grain directions. boundary formation along the texture lines. Downloaded 21 Jan 2003 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp Appl. Phys. Lett., Vol. 81, No. 8, 19 August 2002 A. Ajan and I. Okamoto 1467 400 K value ( 1.31). In Figs. 6 b and 6 c , M ­ H loops measured at 5 and 400 K, respectively, are shown. It is in- teresting to note that the loop shapes are significantly differ- ent, and the radial loop closes predominantly outside the cir- cumferential loop more so at 400 K than at 5 K. Although only 5 and 400 K data are shown, the changes in loop shape are gradual going from 5 K to 400 K. Khanna et al.4 have pointed out that the radial loop closes outside the circumfer- ential loop due to the incomplete magnetization along the texture lines. Since the grain boundaries are more magnetic at lower temperatures larger exchange and less magnetic at higher temperatures weaker exchange , the radial loop would predominantly close outside the circumferential loop at higher temperatures, in agreement with Ref. 4. Similarly, the stress state at lower temperatures is higher, hence OR is expected to be larger. Even though the exchange and stress values are higher we do not find any increase in the OR at 5 K; instead a nearly constant OR value was observed. This near constant value indicates the c-axis distribution as the cause of OR, as mentioned previously. This result is thus consistent with the TEM observation2­4 and also the specific FIG. 6. a Temperature dependence of orientation ratio for 20 nm CoCrPtB Cr orientation mentioned previously. Since the OR is deter- on 5 nm layer, b and c show the M ­ H loop shapes at 5 and 400 K, mined by measuring the coercivity, thermal effects sets in respectively y-axis is M in 10 4 emu . Note radial loop closes outside circumferential loop shown as arrows at 400 K and inside at 5 K. when the thickness or grain sizes are smaller. Hence we be- lieve a slight increase in the OR value at high temperature is due to the thermal instability detailed results will be dis- direction, lattice distortion along texture lines would have cussed elsewhere . been increased due to the high stress along the groove direc- In conclusion, we report evidence for strong lattice ori- tion. The above mentioned c-axis preference would happen entation anisotropy in the plane of the film for both under- mostly for grains near the texture lines. This tendency of Co layers and magnetic layers grown on the mechanically tex- grains to avoid lattice distortion along the circumferential tured NiP­Al substrates. Complementary effects from direction is also evident from the Cr rich grain boundary specific Cr 110 orientation and stress anisotropy within the formation along the groove direction Fig. 5 . It is important magnetic layer, induced by mechanical texturing, are be- to point out that such grain boundary formations are not seen lieved to be helping the Co grains c-axis nucleation along the for the Cr underlayer, but are unique only to the magnetic texturing direction. Results from temperature dependence of layer. We have verified this c-axis orientation along the tex- the OR are consistent with the observation of c-axis distri- turing direction for a large number of films studied, and the bution as the cause of OR. results were similar to those shown in Figs. 4 and 5. Away from the texture lines, c-axis orientation of Co grains is com- Authors thank T. Yoshioka of JEOL, Japan Ltd. for TEM paratively more random. This would also explain the in- observation, Dr. E.N. Abarra and Dr. B.R Acharya for helpful crease in the OR values with an increase in density of me- discussions. chanical texture lines.1,4 For different Pt containing systems the stress or magnetostriction values are different, hence with 1 different alloy compositions, the OR value could be slightly K. E. Johnson, M. Mirzamaani, and M. F. Doerner, IEEE Trans. Magn. 31, 2721 1995 ; T. P. Nolan, R. Sinclair, R. Ranjan, and T. Yamashita, J. different. Appl. Phys. 73, 5117 1993 . Apart from the above-mentioned crystalline in-plane an- 2 H. Kataoka, J. A. Bain, S. Brennan, and B. M. Clemens, J. Appl. Phys. 73, isotropy, we have studied the temperature dependence of the 7591 1993 ; S. Guruswamy, J. E. Shield, and M. Taracki, Scr. Metall. 39, OR. This was carried out to estimate the changes in OR 647 1998 . 3 A. Ajan, B. R. Acharya, E. N. Abarra, and I. Okamoto, IEEE Trans. Magn. possibly due to intergranular exchange predicted in Ref. 4 , 37, 1508 2001 . stress or strain, and c-axis distribution. Since the intergranu- 4 G. Khanna, B. M. Clemens, H. Zhou, and H. N. Bertram, IEEE Trans. lar exchange and magnetoelastic values are significantly Magn. 37, 1468 2001 . 5 higher for Co8 and CoPt systems at lower temperatures, we M. L. Swiggers, G. Xia, J. D. Slinker, A. A. Gorodetsky, G. G. Malliaras, R. L. Headrick, B. T. Weslowski, R. N. Shashidhar, and C. S. Dulcey, expect large increase in OR at lower temperatures. On the Appl. Phys. Lett. 79, 1300 2001 . other hand, if c-axis distribution is causing the anisotropy, 6 S. Furukawa, M. Shibamoto, M. Sakai, T. Endou, and N. Watanabe, OR values are expected to remain constant since c-axis dis- MMM Conference, Seattle, Washington, November 2001, BC-05. 7 tribution of grains, once formed, is independent of tempera- B. D. Cullity, Introduction to Magnetic Materials Addison-Wesley, Mas- sachusetts, 1972 , p. 260. ture. Figure 6 a illustrates the circumferential and radial co- 8 Landolt-Bornstein series, edited by H. P. J. Wijn Springer, Berlin, 1986 , ercivity values and OR for 20 nm CoCrPtB layer on the 5 nm Vol. III/19, p. 50. Cr layer, measured using a superconducting quantum inter- 9 R. Nishikawa, T. Hirosaka, K. Igashi, and M. Kanamaru, IEEE Trans. ference device, from 5 to 400 K. From Fig. 6 a , it is clear Magn. 25, 3890 1989 . 10 C. A. Ross, M. E. Schabes, R. Ranjan, G. Bertero, and T. Chen, J. Appl. that the OR is nearly constant from the 5 K value 1.27 to Phys. 79, 5342 1996 . Downloaded 21 Jan 2003 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp