Structure and magnetic anisotropy of epitaxial fcc-Co(110) and hcp- Co(11¯00) films C. K. Lo,a) Y. Liou, C. P. Chang, I. Klik, and Y. D. Yao Institute of Physics, Academia Sinica, Taipei, Taiwan, Republic of China J. C. A. Huang Department of Physics, National Cheng-Kung University, Tainan, Taiwan, Republic of China Received 8 November 1995; accepted for publication 2 February 1996 Fcc-Co 110 and hcp-Co 11¯00 films of 200 Å thickness were grown on MgO 110 and bcc-Cr 211 /MgO 110 , respectively, by the molecular beam epitaxy method. Reflection high energy diffraction was used to in situ characterize the crystal structure. Co films grown directly on MgO 110 were pseudomorphic fcc structure. Twofold symmetrical hcp-Co 11¯00 films on top of the bcc-Cr 211 /MgO 110 were grown and confirmed. The magneto-optical Kerr effect MOKE was used to investigate the magnetic anisotropy of these films. The magnetization of these samples was found to be in-plane. The magnetization of fcc-Co films has cubic symmetry with texture induced uniaxial anisotropy. For hcp-Co 11¯00 /bcc-Cr 211 bilayer films, the magnetization was strongly anisotropic, but independent of the thickness of the Cr layer. © 1996 American Institute of Physics. S0003-6951 96 01415-1 Surface magnetism is recently a much studied topic both high energy electron diffraction RHEED with energy of 15 because of fundamental physics and the demands of high- keV was used to in situ examine the crystal structure of the density magnetostorage technology. The orientation of the film surface throughout all growth. The magnetic anisotropy magnetization for a ferromagnetic thin film is determined by of the Co films was ex situ characterized at room temperature the competition between shape anisotropy, magnetocrystal- in a magnetic field up to 2 kOe by MOKE. According to the line anisotropy, magnetic surface anisotropy, etc.1 Experi- incident plane and the orientation of the magnetization M, mentally, the magneto-optical Kerr effect MOKE has be- MOKE is classified into three configurations: polar MOKE come a powerful tool and has been widely used to search PMOKE , where M is perpendicular to the sample surface magnetic anisotropies as a function of thickness, layer modu- and lies on the incident plane; longitudinal MOKE lation, surface Curie temperature, etc.2 It is known that mag- LMOKE , where M lies on both the sample surface and the netism is sensitive to the crystalline structure.3 Due to the incident plane; and transverse MOKE TMOKE , where M surface reconstruction or epitaxial strain, the structure of an lies on the sample surface but is perpendicular to the incident artificial film may energetically favor a pseudomorphic struc- plane. MOKE depends on the magnetization of the sample ture. Co possessing metastable bcc,4 fcc,5 and stable hcp and not on the external applied field, which is just a driving phases at different preparation conditions provides an excel- lent model system for studies of structure-related magnetism. force to modulate the magnetization of the sample. In this In a previous work,6 the authors reported the properties study, only PMOKE and LMOKE were used to examine the of Co/Cr superlattices; however, their properties are far too magnetic anisotropies. The magnetic hysteresis loop was complicated, and in this letter, therefore, are presented a sys- picked up by monitoring the change of the MOKE signal on tematic analysis of the effect of crystal structure on magnetic the reflected beam as a function of the applied field. For the anisotropy of epitaxial Co films grown on MgO 110 with or LMOKE measurements, the sample can be rotated about its without a Cr buffer layer. surface normal to determine the in-plane easy and hard axes. The growths of Co and Cr films on MgO 110 were The unit cell of fcc-Co 110 , 3.44 Å 2.44 Å, has a carried out in a molecular beam epitaxy MBE system Eiko 7%­8% mismatch on both axes with MgO 110 , 4.21 EL-10A that is equipped with three independent electron Å 2.98 Å. At a rough estimate from the RHEED patterns of beam evaporators and a high energy electron gum. Prior to Figs. 1 a and 1 d , the 3D geometry of fcc-Co 110 has a tilt deposition, epitaxial grade substrates were outgassed at of 19° off axis on MgO 110 as sketched in Fig. 2 and hence 900 °C for at least 30 min under ultrahigh vacuum in the the possibility of textured fcc-Co films growth. The unit cell MBE chamber to clean the substrate surface. The tempera- of hcp-Co 11¯00 , 4.07 Å 2.51 Å, matches perfectly with tures of the substrates were kept between 300 and 350 °C that of Cr 211 , 4.07 Å 2.50 Å, as shown in Figs. 1 c and during the deposition. The base pressure of the system is 1 d , but there exists a 1.7% mismatch of Cr 211 with the lower than 1 10 10 Torr. During the growth of the super- unit cell of MgO 110 as shown in Figs. 1 e and 1 f . The lattices the pressure of the system was kept below 5 10 9 schematic diagram of the hcp-Co 11¯00 /bcc-Cr 211 on Torr, and the growth rates were kept at about 0.1 Å/s. The MgO 110 is shown in Fig. 3 Ref. 6 . The shown structural growth rate and film thickness were monitored by a quartz quality of hcp-Co is reached only after the Co thickness ex- crystal thickness monitor Leybold Inficon XTC . Reflection ceeds 15­20 Å. No PMOKE was observed for all samples, indicating a Electronic mail: cklo@phys.sinica.edu.tw strong in-plane anisotropic magnetization. Figure 4 shows Appl. Phys. Lett. 68 (15), 8 April 1996 0003-6951/96/68(15)/2155/3/$10.00 © 1996 American Institute of Physics 2155 Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/aplo/aplcr.jsp FIG. 3. Schematic diagram of hcp-Co/bcc-Cr 211 /Mg 110 . ropy of the hcp-Co 11¯00 films on the top bcc-20 Å Cr 211 / MgO 110 , where is the angle between the in-plane crystal FIG. 1. The RHEED patterns of a fcc-Co 110 /MgO 110 ; b bcc- axis MgO 001 Co 0001 and the applied field. A series of Cr 211 /MgO 110 ; c hcp-Co 11¯00 /bcc-Cr 211 /MgO 110 ; d fcc- hcp-Co 11¯00 films with a thicker Cr buffer layer were mea- Co 110 /MgO 110 ; e bcc-Cr 211 /MgO 110 ; and f hcp-Co 11¯00 /bcc- sured and showed the same behavior as in Fig. 4 c . Clearly, Cr 211 /MgO 110 . For a ­ c , the incident electron beam of 15 keV was this magnetic property originates from the crystalline anisot- along MgO 11¯0 ; for d ­ e , the e-beam was along MgO 001 . The thick- nesses of the Co and Cr films were 200 and 20Å, respectively. ropy of hcp-Co. The anisotropic behavior of the hcp- Co 11¯00 /bcc-Cr 211 superlattices on MgO 110 has also the LMOKE of Co films as a function of the azimuthal angle been confirmed by the measurement of anisotropic magne- . In Fig. 4 a , the magnetic anisotropy of the Co 110 film toresistance and reported elsewhere.8 on MgO 110 has cubic symmetry; however, there also ex- The magnetization of 200 Å Co films on MgO 110 with ists a uniaxial magnetic anisotropy due to surface roughness, or without Cr buffer layer has been shown to be in-plane. Co etc.;7 the authors estimate its magnitude at about 5% or less films grown directly on MgO 110 were pseudomorphic fcc of the cubic anisotropy energy, although in other samples not shown it may be somewhat larger up to 15% . The hcp-Co 11¯00 films on bcc-Cr 211 /MgO 110 were aniso- tropic, as seen in Figs. 4 b and 4 c . The thickness of the 6 Å Cr 211 film may not cover the substrate completely, and therefore, the specimen consists of cubic Co 100 and uniaxial Co 11¯00 clusters. This is revealed by the shape of the magnetic hysteresis loops and the nonzero loop area at any angle, as seen in Fig. 4 b , which is a superposition of Figs. 4 a and 4 c . The latter plot shows the uniaxial anisot- FIG. 4. LMOKE hysteresis loops of a 200 Å fcc-Co 110 on MgO 110 , b 200 Å hcp-Co 11¯00 on 6 Å bcc-Cr 211 /MgO 110 , and c hcp- Co 11¯00 on 20 Å bcc-Cr 211 /MgO 110 . In c , the azimuthal angle runs from 0° easy axis MgO 001 Co 0001 to 90° hard axis MgO 11¯0 112¯0 . The width of the loop at zero field is proportional to within less 5% to cos as expected for a uniaxial system. The Kerr inten- FIG. 2. Schematic diagram of fcc-Co 110 /MgO 110 . sity in the three sets of plots is not to scale. 2156 Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Lo et al. Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/aplo/aplcr.jsp structures with isotropic magnetization. A strong anisotropic P. Schuster, and J. Kirschner, J. Magn. Magn. Mater. 93, 1 1991 . magnetization of uniaxial hcp-Co 11¯00 /bcc-Cr 211 films 3 S. Chikazumi and S. H. Charap, Physics of Magnetism Wiley, New York, grown on MgO 110 was demonstrated by the angular de- 1972 . 4 pendence of the coercivity in LMOKE hysteresis loops. Ultrathin Magnetic Structure II, edited by B. Heinrich and J. A. C. Bland The authors are grateful for the financial support of the Springer, Berlin, 1994 , Chap. 1. 5 J. J. de Miguel, A. Cebollada, J. M. Gallego, S. Ferrer, R. Miranda, C. M. National Science Council of the ROC under Grant Nos. 85- Schneider, P. Bressler, J. Garbe, K. Bethke, and J. Kirschner, Surf. Sci. 2112-M-001-019 Y.L. , 85-2112-M-006-018 J.C.A.H. , 211/212, 732 1989 ; C. M. Schneider, P. Bressler, P. Schuster, J. 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Miranda, C. M. Schneider, Chang, and C. K. Lo unpublished . Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Lo et al. 2157 Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/aplo/aplcr.jsp