JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001 Magnetic and magneto-optical properties of PdÕCrÕCo multilayers Y. J. Tanga) and C. Y. Wong Magnetics InfoTechnology Center (MAGIC), Materials Characterization and Preparation Facility, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China X. Zhou Department of Physics, GuiZhou Normal University, GuiYang 550001, China Recent studies found that the Pt/Co/Pt trilayers can be used as a unit in combination with nonmagnetic or magnetic layer, X X Pd, Ag, Cu, and Ni , to enhance the perpendicular magnetic anisotropy of the films, reduce the Curie temperature, and alter the magneto-optical properties. The effects of intercalating Cr into Pd/Co multilayers on the magnetic and magneto-optical properties are studied in this article. The perpendicular magnetic anisotropy Ku and the coercivity Hc of the system decrease rapidly with increasing the Cr thickness (XCr) up to 0.4 nm, and change slightly when Cr thickness further increases. The dependence of the coercivity Hc on the Cr thickness, which obeys the law: H 2.66 c (XCr) XCr , indicates that the magnetization reversal is controlled by domain wall moving, mainly due to the interface roughness. Large decrease of the Kerr rotation k of the Pd/Cr/Co multilayers compared with pure Pd/Co multilayers is also found in the wavelength ranging from 200 to 800 nm. As it is well known, the large anisotropy and Kerr rotation in Pd/Co system are mainly caused by the polarization of Pd atoms due to nearby Co atoms. As the intercalating of Cr layer between Pd and Co layer, the average polarization of Pd atoms will be reduced largely. As a matter of fact, the Cr atoms can also be polarized by nearby Co atoms, which, however, seems to take a minor effect on the anisotropy and Kerr rotation of the system. © 2001 American Institute of Physics. DOI: 10.1063/1.1357149 I. INTRODUCTION II. EXPERIMENT Co/Pt and Co/Pd multilayers MLs have attracted much Glass/Pd 40 nm / Co/Cr (XCr)/(Pd) modulated multi- layers were deposited on water-cooled glass substrates by attention for their potential application as high-density re- magnetron sputtering. The background vacuum was better versible magneto-optical MO recording media due to their than 5 10 5 Pa and the Ar gas pressure was 0.5 Pa. All perpendicular magnetic anisotropy PMA , high coercivity, samples were grown on a 40 nm thick Pd buffer layer, with square hysteresis loops, and large Kerr rotations.1­8 the thickness of Co and Pt layers fixed at 0.33 and 0.8 nm, G. A. Bertero9 has suggested recently that the Pt/Co/Pt respectively. The Cr layer thickness XCr was varied from 0 to trilayers can be used as a unit in combination with nonmag- 0.66 nm. The number of the modulation period of all netic layer, X X Pd, Ag, Cu, and Ni , to enhance the per- samples is 20. The modulation character and interface struc- pendicular magnetic anisotropy of the films. Furthermore, it tures of the samples were analyzed by small- and large-angle is also found that the Curie temperature of Pt/Co/Pt Ni film x-ray diffraction XRD method. The magnetic properties was largely reduced, and the Kerr rotation was enhanced at were characterized using an alternating gradient force mag- lower wavelengths, compared with pure Pt/Co film.10 netometer with the applied magnetic field perpendicular to On the other hand, it was also found11 in Co the film plane. The perpendicular anisotropy constant Ku was xCr1 x /Pt determined from torque curves measured at an applied field films that the non- or weak-ferromagnetic Cr-rich phases of 10 kOe at room temperature. MO characterization was formed at the grain boundaries may reduce the exchange carried out using a variable wavelength Kerr rotation system coupling between the grains, which leads to the reducing of with the wavelength ranging from 200 to 800 nm. the noise when used as perpendicular magnetic recording media. In this article, we have investigated the effects of inter- III. RESULTS AND DISCUSSION calating Cr into the Co/Pd multilayers on the magnetic and Small- and large-angle XRD measurements were per- magneto-optical properties. We found that the perpendicular formed for samples with the structure Glass/Pd magnetic anisotropy Ku , the coercivity Hc , and the Kerr (40 nm)/(Co/Cr(XCr)/Pd) (XCr 0 ­ 0.66 nm), which were rotation k of the system decrease rapidly with increasing the presented in Fig. 1. From the small-angle XRD spectra, one Cr thickness (X can clearly see a strong first-order (n 1) peak for almost all Cr) up to 0.4 nm. the samples, confirming the layered structure in the film. With the increasing of the Cr layer thickness, the first-order a Author to whom all correspondence should be addressed; electronic mail: peak shifts gradually to low angle, attributed to the increas- yjtang@ust.hk ing of the modulation period length. 0021-8979/2001/89(11)/7062/3/$18.00 7062 © 2001 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. 89, No. 11, 1 June 2001 Tang, Wong, and Zhou 7063 FIG. 2. Cr layer thickness XCr dependence of the PMA constant Ku and the coercivity Hc of Glass/Pd 40 nm / Co/Cr(XCr)/Pd) multilayers. The calcu- 2.66 FIG. 1. Small- and large-angle XRD pattern of Glass/Pd 40 nm / lated results with the equation: Hc XCr is also presented in Fig. 2 as dashed line. Co/Cr(XCr)/Pd) multilayers with the Cr layer thickness XCr varying from 0 to 0.66 nm. (X Pd, Ag, Cu, and Ni layer monolayers adjacent to the All the samples exhibit a Pd 111 peak due to Pd buffer Co layers are introduced at the interfaces where the crystal layer. In previous studies, the face-centered-cubic fcc 111 symmetry is broken, resulting in extra spin­orbit coupling texture structure is generally found in CoPt alloy films and which leads to extra magnetic anisotropy. However, with Co/Pt MLs, which is necessary for PMA. The Pd­Co 111 increasing the spacer thickness, the population of the polar- peak in Pd/Co MLs could mainly come from the Pd/Co su- ized Pd atoms adjacent to Co atoms will also decrease, which perlattice character if one supposes a sharp interface between may result in the decrease of the spin­orbit coupling, leading the well-matched Pd and Co lattice. The position of the to the drop of the perpendicular magnetic anisotropy. This Pd­Co 111 peak is between fcc Pd 111 and fcc/Co 111 could be clearly seen from Fig. 2. As the Cr thickness in- reflections. One can also find two high-angle satellite peaks creases, the perpendicular magnetic anisotropy constant Ku with the increasing of Cr thickness, which is also a clear drops quickly. As a matter of fact, Cr atoms can also be evidence for a modulated periodic structure. polarized by adjacent Co atoms. However, this polarization The PMA constant K effect is much smaller than that of Pd atoms, which is evi- u can be obtained by torque mea- surement. According to Torque measurement, if the applied dent from our study. field is large enough, one obtains The coercivity Hc dependence on the Cr thickness of the samples is also presented in Fig. 2. The interface roughness T VKueff sin 2 , 1 takes an important role in determining the coercivity of mul- where T is the torque experienced by the sample under the tilayers. P. Bruno13 found that Hc X 2.5, where X is the applied field, V is the volume of the magnetic layer, and is the angle between the magnetization and the normal direc- tion of the film; Kueff is the effective PMA constant, and K 2 ueff K 2 M s , 2 where Ku is the PMA constant and Ms is the saturation mag- netization. Kueff value can be obtained through the torque curve by: Kueff Tpeak /V, where Tpeak is the peak value of T on the torque curve. In Fig. 2, the PMA constants Ku are plotted against the Cr layer thickness for all samples. For pure Pd/Co multilay- ers, Ku is about 9.25 105 J/m3. Ku drops quickly with in- creasing Cr thickness. When the Cr thickness is larger than 0.4 nm, Ku changes slightly. The origin of the perpendicular anisotropy in Co/Pt mul- tilayers has been attributed to a lack of cancellation of orbital moment due to the symmetry breaking presence of interfaces.12 Based on this idea, G. A. Bertero and co- workers suggested that additional magnetic anisotropy can be introduce in multilayers by introducing extra structure FIG. 3. Spectral dependence of the Kerr rotation k of Glass/ and/or chemical anisotropy at the interfaces.9 Thus, extra X Pd 40 nm / Co/Cr(XCr)/Pd) multilayers. Downloaded 24 Apr 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp 7064 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Tang, Wong, and Zhou spacer thickness in Pd/Co/Pd trilayer. He suggested that the layer in Co/Pd multilayers could reduce the average polar- magnetization process is mainly controlled by the domain ization of Pd atoms and spin­orbit coupling, leading to the wall movement, where the main obstacle comes from the decrease of Kerr rotation. Both Kerr rotation k and PMA interface roughness. The relationship between coercivity and constant Ku drop more quickly with increasing Cr thickness the spacer thickness was also found later by S. T. Purcell and in our samples as compared to those of Pt/Co/Pt/Ni multi- co-workers in Pd/Co/Pd trilayer.14 In our study, we found layers, which might be due to the nonmagnetic character of that H 2.66 c XCr , which is almost the same as previous re- Cr atom. sults. The simulated curve calculated with this equation is also presented in Fig. 2. As one can find through our XRD IV. CONCLUSIONS analysis, with increasing the Cr thickness, the intensity of the We found that the PMA Ku , the coercivity Hc , and the peaks at small-angle XRD patterns is getting larger and the Kerr rotation k of the system decrease rapidly with increas- satellite peaks at large-angle XRD patterns appear more ing the intercalated Cr thickness (XCr) in Co/Cr(XCr) Pd clearly, indicating an increasing quality of the layered struc- multilayers. This is ascribed to the reducing of the the aver- ture and the reducing of the interface roughness of the mul- age polarization of Pd atoms with the intercalating of Cr tilayers. Since the domain wall could move more easily between Pd and Co layer. when the interface is sharper, the coercivity will drop quickly, which could be seen in Fig. 2. 1 C. L. Canedy, X. W. Li, and G. Xiao, Phys. Rev. B 62, 508 2000 . 2 The MO properties are presented in Fig. 3. The enhance- G. A. Bertero, R. Sinclair, C.-H. Park, and Z. X. Shen, J. Appl. Phys. 77, 3953 1995 . ment of Kerr rotation k at lower wavelengths was observed 3 D. Weller, H. Bra¨ndle, and C. Chappert, J. Magn. Magn. Mater. 121, 461 and similar to that of Pd/Co multilayers, which is due to the 1993 . hybridization of strongly spin-polarized Co/d states with 4 E. R. Moog, J. Zak, and S. D. Bader, J. Appl. Phys. 69, 880 1991 . 5 spin­orbit-splitted Pd d states.15 It has also been pointed out3 W. B. Zeper, H. W. van Kesteren, B. A. J. Jacobs, and J. H. M. Spruit, J. Appl. Phys. 70, 2264 1991 . that the chemical and structural ordering, especially at the 6 C.-J. Lin, G. L. Gorman, C. H. Lee, R. F. C. Farrow, E. E. Marinero, H. interfaces, accompanied by the substantial electronic struc- V. Do, H. Notarys, and C. J. Chien, J. Magn. Magn. Mater. 93, 194 ture changes can also modify the MO Kerr spectra drasti- 1991 . 7 cally. P. F. Carcia, J. Appl. Phys. 63, 5066 1988 . 8 C. Chappert and P. Bruno, J. Appl. Phys. 64, 5763 1988 . In Pt/Co/Pt/Ni multilayers,16 an increase of the Kerr 9 G. A. Bertero and R. Sinclair, Appl. Phys. Lett. 64, 3337 1994 . rotation with increasing Ni layer thickness was generally 10 G. Srinivas and S.-C. Shin, Appl. Phys. Lett. 69, 3086 1996 . found. In our study, however, an overall decrease of the Kerr 11 K. Takano, G. Zelter, D. K. Weller, and E. E. Fullerton, J. Appl. Phys. 87, rotation 6364 2000 . k with increasing Cr thickness in all the measured 12 L. Neel, J. Phys. Radium 15, 225 1954 . wavelength range was found. The common microscopic ori- 13 P. Bruno, Phys. Rev. B 39, 865 1989 . gin of both the Kerr rotation 14 k and PMA Ku is spin­orbit S. T. Purcell, M. T. Johnson, N. W. E. McGee, W. B. Zeper, and W. coupling. A functional relationship between K Hoving, J. Magn. Magn. Mater. 113, 257 1992 . u and Kerr ro- 15 tation S. Uba, L. Uba, A. N. Yaresko, A. Y. Perlov, V. N. Antonov, and R. k arising from the exchange polarized Pt and Pd was Gontarz, Phys. Rev. B 53, 6526 1996 . also established.3 As discussed above, the intercalating of Cr 16 G. Srinivas and S.-C. Shin, Thin Solid Films 301, 211 1997 . Downloaded 24 Apr 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp