PHYSICAL REVIEW B VOLUME 62, NUMBER 14 1 OCTOBER 2000-II Structural characterization of the CoÕCr multilayers by x-ray-absorption spectroscopy Y. H. Liou,1 W. F. Pong,1,* M.-H. Tsai,2 K. H. Chang,1 H. H. Hseih,1 Y. K. Chang,1 F. Z. Chien,1 P. K. Tseng,1 J. F. Lee,3 Y. Liou,4 and J. C. A. Huang5 1Department of Physics, Tamkang University, Tamsui 251, Taiwan 2Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 3Synchrotron Radiation Research Center, Hsinchu Science-based Industrial Park, Hsinchu 300, Taiwan 4Institute of Physics, Academic Sinica, Taipei 115, Taiwan 5Department of Physics, National Cheng Kung University, Tainan 701, Taiwan Received 11 November 1999; revised manuscript received 4 May 2000 We have performed Cr and Co K-edge x-ray-absorption measurements to investigate the dependence of local electronic and atomic structures on the Cr-layer thickness in epitaxial Co(11¯00) 40 Å /Cr 211 (tCr) (tCr 2, 3, 5, 7, and 9 Å multilayers. The Cr K x-ray-absorption near-edge fine structure XANES spectra of the Co/Cr multilayers indicate an abrupt transition of the Cr layer from hcp to bcc structure when the thickness of the Cr layer is increased to exceed 5 Å or three atomic layers. Our results offer an upper limit for the ability of the Co/Cr interface to stabilize the hcp structure in the thin Cr layer. The numbers of nearest-neighbor and next-nearest-neighbor atoms in the Cr and Co layers determined by extended x-ray-absorption fine-structure measurements performed at the Cr and Co K edge, respectively, are consistent with the XANES results. Magnetic multilayers have attracted a great deal of atten- existence of fcc Co/Cr-alloy structure at the interface may tion over the last decade because of their peculiar magnetic not be likely because the fcc structure is incompatible with properties and their technological and fundamental either hcp(11¯00) or bcc 211 atomic arrangement. The importance.1 The oscillatory variation of the interlayer ex- x-ray-absorption spectrum is very sensitive to the local envi- change coupling with respect to the separation between two ronment around the absorbing atom, which can be used as a ferromagnetic layers in the multilayer systems is particularly fingerprint of the crystallographic structure and allows us to of interest.2 For epitaxial Co/Cr multilayers, previous works study the local structures in multilayer systems.11,12 In this showed that the magnetic properties have the characteristics work we measure the local electronic and atomic structures of giant and anisotropic magnetoresistance.3­5 It was also of a series of Co/Cr multilayers and characterize the varia- found that the magnetic and magnetotransport properties of tion of local electronic and atomic structures with respect to the magnetic multilayers are strongly affected by their elec- the Cr-layer thickness. This study may help us understand the dependence of the local electronic and atomic structures tronic and atomic structures.6 The reflection high-energy in the epitaxial Co/Cr multilayers on the Cr-layer thickness. electron diffraction RHEED analyses by Vavra et al.7 and X-ray absorption spectra of the Co/Cr multilayers were Henry et al.8 showed that the Cr layer in epitaxial Co/Cr measured using a double-crystal Si 111 monochromator at multilayers exhibited an abrupt transition from bcc structure the wiggler beamline, with an electron-beam energy of 1.5 to close-packed structure fcc or hcp at a Cr-layer thickness GeV and a maximum stored current of 200 mA at the Syn- of 5 Å. This property was attributed to the interfacial chrotron Radiation Research Center SRRC in Hsinchu, energy that stabilizes the thermodynamically less favored Taiwan. The absorption spectra of the Co/Cr multilayers and densest atomic arrangement. However, they obtained differ- thin-film CoCr alloy at the Cr and Co K edges were mea- ent local atomic structures around Cr when the Cr layer is sured using the fluorescence mode with the Lytle detector at very thin. Vavra et al. found that pseudomorphically grown room temperature. The spectra of the reference Cr and Co Cr layers are constrained coherently to the hcp structure of foils were obtained in transmission mode. All the spectra the underlying Co, while Henry et al. reported that no were collected with step energy of 0.5 eV in the x-ray- pseudomorphism occurs at the Cr/Co interface, but it is more absorption near-edge structure XANES region and of 2 eV likely that interdiffusion resulted in forming a close-packed in the extended x-ray-absorption fine-structure EXAFS re- CoCr alloy. Another investigation also found the existence of gion. Samples of 24x Co 40 Å /Cr 2 Å and interdiffusion at the Cr/Co interface of the sputtering grown 20x Co (40 Å)/Cr (tCr) multilayers with tCr 3, 5, 7, and 9 Co/Cr multilayers.9,10 Different orientations and preparation Å and a 50 Å Mo buffer were deposited on the MgO 110 conditions of the thin-film samples used in these studies substrates. X-ray-diffraction results indicate that the Co and might be the cause of the different interfacial structures. The Cr layers deposited in the alternating Co/Cr multilayers mismatch of the atomic arrangement at the interface depends mainly have hcp/bcc structure in the Co(11¯00)/Cr 211 ori- on the orientation of the multilayers. Recently, based on entation. The details of the preparation of the Co/Cr multi- structural characterization Huang et al.3 found that though layers and x-ray-diffraction determination of the orientation bcc and hcp structures have different atomic arrangements, of these multilayers have been described elsewhere.3­5 Co(11¯00) and Cr 211 planes match extremely well in sym- Figures 1 and 2 show, respectively, the Cr and Co K-edge metry and lattice parameters. In this crystal orientation, the XANES spectra obtained for the Co/Cr multilayers, refer- 0163-1829/2000/62 14 /9616 5 /$15.00 PRB 62 9616 ©2000 The American Physical Society PRB 62 STRUCTURAL CHARACTERIZATION OF THE . . . 9617 FIG. 1. Normalized Cr K near-edge absorption spectra for epi- taxial Co 40 Å /Cr (t FIG. 2. Normalized Co K near-edge absorption spectra for epi- Cr) (tCr 2, 3, 5, 7, and 9 Å multilayers. a Co foil offset Co K edge , b CoCr alloy, c t taxial Co 40 Å /Cr (tCr) (tCr 2, 3, 5, 7, and 9 Å multilayers. a Cr 2 Å, d tCr 3 Å, e t Co foil, b CoCr alloy, c tCr 2 Å, d tCr 3 Å, e tCr 5 Å, f Cr 5 Å, f tCr 7 Å, g tCr 9 Å, and h Cr foil. tCr 7 Å, g tCr 9 Å, and h Cr foil offset Cr K edge . ence CoCr alloy, and Cr and Co foils. For all the spectra, rehybridization.15 The two-peak features b1 and b2 labeled zero energy was selected at the inflection point of the thresh- by vertical arrows in the Cr K-edge XANES of the Co/Cr old in the spectra. The zero energies correspond to absolute multilayers with a Cr-layer thickness tCr less than 5 Å energies of 5989.0 and 7709.1 eV, respectively, for the Cr closely resemble those of the Co foil with a hcp structure. In and Co K edges. Due to the bulk sensitivity of fluorescence contrast, the single-peak feature in the Cr K-edge spectra of measurements, the spectra in Figs. 1 and 2 predominantly the Co/Cr multilayers with tCr 5 Å resembles the single reflect the bulk absorption of the Co/Cr multilayers. The nor- sharp feature b* located in the region between peaks b1 and malized EXAFS oscillations (k) are weighted by k3 for b2 in the spectrum of the Cr foil with a bcc structure. The both Cr and Co K edges, and the corresponding Fourier features in the Cr K-edge XANES of the CoCr alloy are transforms FTs of the k3 data for the Co/Cr multilayers much broader and appear to be least resolved in comparison and reference samples are shown in Figs. 3 and 4, respec- with those of the Co/Cr multilayers and Cr and Co foils. The tively. Further analysis involved the use of a combination of first derivatives of the XANES spectra of the Co/Cr multi- the multiple-scattering EXAFS computer program FEFF6 layers, CoCr alloy, and Cr and Co foils are shown in the Ref. 13 and the nonlinear least-squares-fitting computer inset of Fig. 1. A general trend of the change from the single program FEFFIT.14 As also shown in Figs. 3 and 4, the quality peak b* to the double peaks b1 and b2 can be easily seen of the fit for the nearest-neighbor NN and next-nearest- when the Cr-layer thickness decreases in the Co/Cr multilay- neighbor NNN bond lengths is quite good. ers. This trend clearly indicates a local structural transition at The part of the Cr K-edge XANES spectra of the Co/Cr tCr 5 Å in the Co/Cr multilayers. It may suggest that multilayers, reference CoCr alloy, and Cr foil and of the pseudomorphic Cr films can be grown on Co and can be offset Co K-edge XANES spectra of the Co foil between constrained coherently into the hcp structure in thin layers labels b1 and b2 as shown in Fig. 1 can be attributed to the (tCr 5 Å). In contrast, the Cr layer prefers to be bulk like dipole 1s-to-4p transitions above the Fermi level. The two when its thickness is greater than 5 Å. The intensity of small bumps in the region from about 0 to 10 eV above the bumps a1 and a2 in the Cr K-edge XANES spectra remains edge labeled as a1 and a2) are primarily due to the Cr and nearly constant. The Co K-edge XANES spectra of the Co 1s-to-3d transition through the Cr and Co p-d Co/Cr multilayers shown in Fig. 2 contain relatively well- 9618 Y. H. LIOU et al. PRB 62 FIG. 4. Fourier transform amplitudes of the EXAFS k3 data at FIG. 3. Fourier transform amplitudes of the EXAFS k3 data at the Co K edge for Co/Cr multilayers. a Co foil, b CoCr alloy, c the Cr K edge for Co/Cr multilayers. a Co foil, b CoCr alloy, c tCr 2 Å, d tCr 3 Å, e tCr 5 Å, f tCr 7 Å, g tCr 9 Å, tCr 2 Å, d tCr 3 Å, e tCr 5 Å, f tCr 7 Å, g tCr 9 Å, and h Cr foil. Final fit of theory to the NN bond lengths open and h Cr foil. Final fit of theory to the NN and NNN bond lengths circles . The inset represents the Co K-edge EXAFS oscillation k3 open circles . The inset represents the Cr K-edge EXAFS oscilla- data. The coordination number of Co was fixed at 12 in fitting a tion k3 data. In fitting a model compound to the experimental model compound to the experimental EXAFS because of a very EXAFS, the coordination number of NN Co was fixed at 12 for small contribution from Cr atoms. tCr 5 Å with a hcp structure, while the coordination numbers of NN Cr and NNN Cr were fixed at 8 and 6, respectively, for t spectra have a common feature marked by shaded peaks. Cr 5 Å with a bcc structure. Other spectra do not have this feature. Since the spectra of Figs. 3 a and 3 c ­3 e belong to the Co foil and Co/Cr resolved two-peak features b1 and b2 , which resemble those multilayers with thin Cr layers (tCr 5 Å), respectively, our of the Co foil. The first derivatives of the Co K-edge XANES FT spectra show that the thin Cr layers (tCr 5 Å) are more spectra of the Co/Cr multilayers, CoCr alloy, and Co and Cr likely to have the same hcp structure of the Co foil. This foils are also shown in the inset of Fig. 2. agrees with XANES results and further confirms that the Figures 3 and 4 show the Cr and Co K-edge FTs of the epitaxially grown tCr 5 Å Cr layers in the Co/Cr multilay- k3 data for the Co/Cr multilayers, CoCr alloy, and Cr and ers are constrained to be Co metal like as reported by earlier Co foils. The first peaks in the FT spectra shown in Fig. 3 RHEED studies.7,8 The similarity between the XANES spec- appear to have roughly the same location, though they have tra of the reference CoCr alloy and Co metal suggests that different heights and full widths at the half maximum. How- the CoCr alloy has a hcp structure, in agreement with our ever, the peaks at a distance larger than 3 Å appear to x-ray-diffraction measurement. The FT spectra shown in Fig. differ significantly and can be attributed to differences in the 3 have the characteristic of splitting two-neighbor shells in average environment in farther away shells between the two the region between 3.8 and 5.0 Å for tCr 5 Å. The splitting tCr 5 Å and tCr 5 Å cases. The features in the FT spectra two-neighbor shells are also found in the FT spectra of the of the Co/Cr multilayers with a thick Cr layer (tCr 5 Å) Co K edge for the Co/Cr multilayers, CoCr alloy, and Co foil have a single peak near 4.5 Å. They resemble closely that of as shown in Fig. 4. In Fig. 4 the splitting two-neighbor shells the Cr foil with a bcc structure.11 On the other hand, the local are nearly at the same position for the Co/Cr multilayers, atomic structures of thin Cr layers (tCr 5 Å) in the Co/Cr CoCr alloy, and Co foil, but the heights are obviously larger multilayers and CoCr alloy are quite similar to that of the Co for thinner Cr layers than for thicker Cr layers in the Co/Cr foil with a hcp structure. In the region between 6.8 and 7.3 Å multilayers. This property can be primarily attributed to the in the FT spectra shown in Figs. 3 a and 3 c ­3 e , the decrease of the structural order due to NNN bond-length dis- PRB 62 STRUCTURAL CHARACTERIZATION OF THE . . . 9619 tortion in the Co layer caused by the thick bcc Cr layer. Cr-Cr couplings through d orbitals render the bcc structure to A best-fit procedure is applied to the first main peaks in become more favorable. Based on the above argument, the the Cr K-edge EXAFS FT spectra to obtain NN and NNN observed ultrathin three-atomic-layer critical thickness of bond lengths using the one- and two-shell models for t the bcc Cr layer seems to suggest that the interdiffusion of Cr 5 Å and t Co and Cr atoms at the interface is less likely or at least is Cr 5 Å, respectively. The Cr atoms are found bonded with 12 NN Co atoms at 2.50 0.01 Å, which is a limited to very few layers. The physical reason is that inter- characteristic of the hcp structure, for t diffusion reduces the number of Cr atoms surrounding a Cr 5 Å. For tCr 5 Å, the Cr atoms are bonded with 8 NN Cr atoms at given Cr atom and tends to destabilize the bcc structure, so 2.49 0.01 Å and 6 NNN Cr atoms at 2.78 0.02 Å for t that a thicker Cr layer is required to be stabilized in the bcc Cr 7 Å and at 2.84 0.02 Å for t structure. Based on structural characterization, Huang et al.3 Cr 9 Å indicative of a bcc structure. The 2.84 Å NNN bond length is close to that of the found that though bcc and hcp structures have different bulk Cr metal of 2.88 Å. For t atomic arrangements, Co(11¯00) and Cr 211 planes match Cr 5 Å the Cr layer contains 3 atomic layers of Cr. The central Cr layer is sandwiched extremely well in symmetry and lattice parameters. The unit between two Cr side layers, so that the nearest neighbors of cell of Co(11¯00), 4.07 Å 2.51 Å, matches perfectly that of the atoms in this layer are all Cr atoms. On the other hand, Cr 211 , 4.07 Å 2.50 Å. The one-atom-thick hcp Cr(11¯00) the side-layer Cr atoms have some bcc-type Cr nearest layer is expected to have a similar excellent match and have neighbors and some hcp-type Co nearest neighbors. The real negligible strain energy. For the two-atom-thick hcp sample may even contain both hcp and bcc phases. Thus, in Cr(11¯00) layer, the strain energy is associated with the dis- this case, the EXAFS data were fitted with a combination of tortion of the NN bond angles from those of the bcc Cr 211 1/3 hcp and 2/3 bcc coordinations. The NN and NNN bond structure. For semiconductors the NN bonding is covalent lengths obtained are 2.50 0.01 and 2.84 0.02 Å, respec- and directional and the bond-angle distortion gives rise to tively. They are consistent with those determined for other significant strain energy.17 In contrast, for transition metals Cr-layer thicknesses. The single-shell model is fitted for the the strain energy associated with the bond-angle distortion is first main peak of the Co K-edge FT spectra for all Co/Cr given rise by directional d-orbital couplings, which is not as multilayers. The Co atoms are found bonded with 12 NN Co significant as the nondirectional metallic bonding through atoms at 2.49­2.50 Å indicative of a hcp structure. itinerant electrons. Thus the strain energy in the two-atom- Transition metals with a fcc structure have a closed or nearly closed outermost d shell, while those with a hcp struc- thick hcp Cr(11¯00) layer is of second order. During the ture have a few singly occupied d orbitals.16 Both fcc and deposition of the third Cr layer, this strain energy can be hcp structures are the densest structure, in which each atom easily overcome by the energy transferred from the adsorp- has 12 NN atoms and the coupling between atoms are domi- tion energy of the approaching gas-phase Cr atoms. Thus a nated by metallic bonding through itinerant electrons, which three-atom-thick bcc Cr 211 layer can be easily formed. are not directional. In contrast, the transition metals with a In summary, our Cr and Co K-edge XANES measure- bcc structure have from 3 to 6 d electrons in the outermost d ments for the epitaxial Co 40 Å /Cr (tCr) (tCr 2, 3, 5, 7, shell. Since the 5 d orbitals are directional, the contribution and 9 Å multilayers show an abrupt transition of the Cr of d orbitals to the coupling between two neighboring atoms layer from hcp structure to bcc structure at 5 Å. Our results is also directional, which renders the bcc structure with a offer an upper limit for the ability of the Co/Cr interface to smaller coordination number of 6 becomes more favorable stabilize the hcp structure in the thin Cr layer. The hcp-to- than the fcc/hcp structure. 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