JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999 Oscillation period of the interlayer coupling for epitaxial Fe/Cr1 xVx 100... and 211... superlattices Chun-Yeol You,a) C. H. Sowers, A. Inomata, J. S. Jiang, S. D. Bader, and D. D. Koelling Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 The microscopic origin of the 18 Å period in the oscillatory interlayer exchange coupling of Fe/Cr 100 and 211 superlattices is investigated by alloying the Cr spacer with V to alter its Fermi surface. The addition of V increases or decreases in size the various Fermi surface calipers that are candidates for governing the oscillation period, such as those that span the ellipse, lens, octahedron, and nested sheets. Epitaxial sputtered superlattices of 100 and 211 orientation were grown and characterized via magnetoresistance measurements as a function of spacer layer thickness for different V-doping levels. A small decrease of the oscillation period is found experimentally which strongly implicates the N-centered ellipse as the origin of the 18 Å period in Fe/Cr superlattices. © 1999 American Institute of Physics. S0021-8979 99 67908-7 I. INTRODUCTION pling. However, Stiles9 subsequently reinterpreted the photo- emission results as being due to quantum wells that are not The oscillatory interlayer exchange coupling between spin polarized and therefore not associated with the coupling. two ferromagnetic layers through spacer layers has recently The motivation of the present work is to address this been extensively studied due to both fundamental interest in issue via alloying the Cr spacer in order to modify its Fermi the physics of giant magnetoresistance GMR materials and surface topology. It is known that adding elements having a applied interest associated with the development of new smaller V or larger Mn number of valence electrons than magnetic sensors and nonvolatile memory arrays.1 It is now Cr can be interpreted within a rigid-band approximation.11 broadly accepted that the period of the interlayer exchange Alloying effectively increases Mn or decreases V the coupling is determined by particular Fermi surface calipers Fermi energy of Cr. Since alloys of Cr­Mn exhibit antifer- of the spacer materials.2 Chromium, being a transition metal, romagnetism, we limited our experiments to alloys of V has a complicated Fermi surface with a diversity of such only. spanning vectors; however, none exhibit precisely the 18 Å Similar studies12,13 have been performed for Co/Cu su- experimentally measured value for what is commonly re- perlattices, where Cu, of course, has a much simpler Fermi ferred to as the ``long'' period. In contrast to this situation, surface. Parkin et al.12 and Bobo et al.13 modified the Cu the origin of the 2 monolayer ``short'' period of Cr is Fermi surface via alloying with Au, Fe, or Ni to investigate widely recognized as being due to the nesting vector that the variation of the oscillation period. In this system the re- also gives rise to the antiferromagnetism of Cr.3 lationship between the modification of the period and the The origin of the 18-Å-long period of the interlayer ex- Fermi-surface caliper with alloying can be explained quite change coupling exhibited by Fe/Cr superlattices of both convincingly using a simple band model. 100 and 211 epitaxial orientation4 remains controversial In the present work, we grow epitaxial superlattices of despite intensive experimental5 and theoretical6­10 investiga- Fe/Cr1 xVx(100) and 211 , with x 0.085 and 0.20, on tions. Van Schilfgaarde6 argued that aliasing of the second MgO 100 and 110 substrates, respectively, by direct- harmonics of the short-period oscillation gives rise to the current dc magnetron sputtering.4 The GMR was measured long period, and predicted a rapid decrease of the period with to monitor the periodicity as a function of spacer thickness. 10% of V for the 001 orientation. Koelling7 suggested that We find that the period of oscillation decreases only mod- the relatively isotropic d-derived lens can provide the k-space estly with increasing V doping, which strongly suggests that caliper to explain the crystallographic orientational the ellipse centered at the N point of the Fermi surface is the insensitive4 of the long period. Mirbt8 identified an likely origin of the long-period oscillation in the Fe/Cr GMR octahedron-coupling caliper based on a CsCl structure that is system. the same as the Cr body-centered-cubic bcc structure but with two atoms per unit cell to account for the antiferromag- II. EXPERIMENTAL PROCEDURE netism of Cr. Stiles9 and Tsetseris10 identified the sp-derived Fe/Cr ellipse centered at the N point of the Fermi surface as pro- 1 xVx (x 0.085,0.20) superlattices were grown by dc magnetron sputtering onto epitaxially polished single viding the underlying caliper for the long period. Li et al. crystal MgO 100 and 110 substrates. The sputtering cham- experimentally implicated the lens calipers, based on photo- ber has a base pressure of 1 10 7 Torr, and the 2 in. emission experiments that monitored the emergence of planar magnetron sputtering guns were operated in an Ar gas quantum-well features that are believed to underlie the cou- pressure of 4 mTorr and a target-substrate distance of 3.5 in. A 100 Å Cr buffer layer was initially deposited at a substrate a Electronic mail: cyyou@anl.gov temperature of 400 °C to establish the epitaxial orientation 0021-8979/99/85(8)/5889/3/$15.00 5889 © 1999 American Institute of Physics 5890 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 You et al. indicate the presence of additional phases or orientations. Rocking curves about the Fe/Cr1 xVx(100) and 211 have full-width at half-maximum FWHM of 2° for various Cr1 xVx thicknesses. The results indicate a high degree of crystallographic orientation of the samples. The magnetic properties of the alloy superlattices are also very similar to those of the Fe/Cr superlattices. The 211 -oriented alloy superlattices have both the expected bi- axial magnetocrystalline anisotropy and uniaxial anisotropy originating from the surface anisotropy.14,15 The magne- totransport was measured at room temperature. The depen- dence of GMR on the thickness of Cr1 xVx layers is shown in Figs. 1 a ­1 d for each crystalline orientation and V con- centration. As shown in Fig. 1, periods of 16(x 0.085) and 15 Å(x 0.20) for Fe/Cr1 xVx(100), and 17(x 0.085) and 16 Å(x 0.20) for Fe/Cr1 xVx(211) were found. Thus, a slight decrease in the value of the period with V doping was observed for each crystallographic orientation. IV. DISCUSSION AND RESULTS The band structure along the high symmetry and directions for Cr group transition metals is depicted in Fig. 2. The horizontal lines denote the Fermi energies of the Cr and labeled alloys. According to the rigid band approximation the addition of 10 and 20 at. % V to Cr causes the Fermi energy to decrease the changes the Fermi surface topology. The concentration dependences of the alloy topologies are shown in Fig. 3. As shown in this figure, when lowering the Fermi energy, the lens shrinks rapidly, while the ellipse ex- pands modestly and the octahedron and nesting contract modestly. Hence, the period would be expected to grow dra- matically if it is controlled by the lens, grow only modestly if FIG. 1. The spacer-layer thickness dependence of the magnetoresistance for it is due to octahedral spanning vectors, or rapidly decrease if 100 - and 211 -oriented Fe(14 Å)/Cr1 xVx (tspacer) superlattices measured at room temperature. a x 0.085 and b x 0.2 for the 100 orientation, and c x 0.085 and d x 0.2 for the 211 . with the substrate. The substrate was cooled to 100 °C and the superlattice was grown. To make the alloy layer, we fab- ricated mosaic targets. The number of layers was adjusted so that the total superlattice thickness was constant at about 1000 Å. The structures were characterized by x-ray diffrac- tion using Cu K radiation, and the magnetic properties were measured by means of the longitudinal Kerr rotation. The magnetotransport properties were studied by a standard, four-probe technique. The concentrations of V in the Cr ma- trix were determined by the induction coupled plasma ICP analytic technique. More details on the experimental proce- dure are published elsewhere.4 III. EXPERIMENTAL RESULTS Since Cr and V are completely soluble in one another over the whole concentration range, the structural properties of Fe/Cr1 xVx can be predicted to be very similar with those of Fe/Cr. The x-ray diffraction results of the Fe/Cr1 xVx superlattices are similar to those of Fe/Cr superlattice that were studied previously.4 In the high-angle x-ray diffraction FIG. 2. The band structure of Cr group transition metals based on the tight binding method. The Fermi energy for each composition is indicated as is spectra, there are no unexpected peaks present that would the lens and ellipse. J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 You et al. 5891 layer thickness was measured at room temperature. The pe- riods are found to slightly decrease with V addition, such that it becomes 16(x 0.085) and 15 Å(x 0.20) for Fe/Cr1 xVx(211), and 17(x 0.085) and 16 Å(x 0.20) for Fe/Cr1 xVx(211). With these experimental re- sults in hand and giving consideration to the theoretically anticipated evolution of the Fermi surface topology with V alloying within the rigid band picture, we can conclude that the long period is due to the caliper vector of the ellipse centered at the N point of the Fermi surface. ACKNOWLEDGMENTS Work supported by the U.S. Department of Energy, BES-Material Science, under Contract No. W-31-109-ENG- 38. And one author C.Y.Y. wishes to acknowledge the fi- nancial support of the Korea Research Foundation made in FIG. 3. Fermi surfaces of bulk Cr thick curves , Cr1 xVx , x 0.1, 0.2 thin thinner , showing cuts across the ellipse, lens and nesting region, from top to the program Year 1997. bottom, respectively. The arrows indicate the direction from a Cr rich to a more V-rich alloy. The Fermi surfaces were generated by the linearized 1 G. A. Prinz, Phys. Today , 58 1995 . augmented-plane-wave method within the virtual crystal approximation with 2 For a review of exchange coupling, see Ultrathin Magnetic Structures II, bulk parameters. edited by B. Heinrich and J. A. C. Bland Springer, Berlin, 1994 , Chap. 2 and references therein. 3 it is originated from aliasing of the nesting vector, but only E. Fawcett, Rev. Mod. Phys. 60, 209 1988 . 4 Eric E. Fullerton, M. J. Conover, J. E. Mattson, C. H. Sowers, and S. D. to shrink incrementally if controlled by the N-centered el- Bader, Phys. Rev. B 48, 15755 1993 . lipse. From the experimental result of observing a slight de- 5 Dongqi Li, J. Peason, S. D. Bader, E. Vescovo, D.-J. Huang, P. D. crease of the period for each crystalline orientation with in- Johnson, and B. Heinrich, Phys. Rev. Lett. 78, 1154 1997 . 6 creasing V addition, we conclude that the long-period M. van Schilfgaarde, F. Herman, S. S. P. Parkin, and J. Kudronvsky, Phys. Rev. Lett. 74, 4063 1995 . oscillation of Cr originates from the ellipse caliper, as argued 7 D. D. Koelling, Phys. Rev. B 50, 273 1994 . by Stiles9 and Tsetseris et al.10 8 S. Mirbt, A. M. N. Niklasson, B. Johansson, and H. L. Skriver, Phys. Rev. B 54, 6382 1996 . 9 V. CONCLUSIONS M. D. Stiles, Phys. Rev. B 54, 14679 1996 . 10 L. Tsetseris, B. Lee, and Y.-C. Chang, Phys. Rev. B 55, 11586 1997 ; 56, The period of the oscillatory exchange coupling of 11392 1997 . 11 Fe/Cr S. Komura, Y. Hamaguchi, and N. Kunitomi, J. Phys. Soc. 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