Thin Solid Films 375 Z . 2000 55 58 Giant magnetoresistance and structural properties in Co Cu Co sandwiches with Si and Cr buffer layers Hong-Lie Shen , Guan-Xiong Li, Qin-Wo Shen, Tie Li, Shi-Chang Zou State Key Laboratory of Functional Materials for Informatics and State Key Laboratories of Transducer Technology, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, PR China Received 16 June 1999; received in revised form 3 December 1999; accepted 3 December 1999 Abstract Cobalt 5.5 nm Cu 3 nm Co 5.5 nm sandwiches with Si and Cr buffer layers were prepared by ultra-high vacuum electron beam evaporation. A large in-plane anisotropy of the giant magnetoresistance Z . GMR effect was found in Si buffered sandwiches when the buffer layer thickness was equal to or larger than 0.9 nm. In the easy axis, the GMR effect reached a value of 5.5% with a high field sensitivity of approximately 0.7% Oe, while in Cr-buffered Co Cu Co sandwiches, the GMR effect showed only in-plane isotropic properties with a maximum GMR value of 6%. The XRD spectrum and HRTEM image revealed that there exists a Co2Si compound between the Si buffer and the lower Co magnetic layer. All these results indicate that the anisotropic GMR effect in Si-buffered sandwiches results from the cobalt silicide between the Si buffer and the lower Co magnetic layer. 2000 Elsevier Science S.A. All rights reserved. Keywords: Giant magnetoresistance; Co Cu Co sandwich; Buffer layer; Anisotropy 1. Introduction 6 . Commonly, it was considered that the buffer layer could improve the flatness of the multilayers, so that Since the discovery of the giant magnetoresistance the MR ratio was enhanced. Confusingly, there was Z . GMR effect in Fe Cr multilayers 1 , considerable also a report of large MR ratio in multilayers without attention has been paid in the literature to this effect any buffer layer 7 . Considering that a sandwich con- observed later in many other multilayers such as Co Cu sisting of two magnetic and one non-magnetic layer is 2 , Co Ru 3 , and NiFe Cu 4 . At present, the Co Cu the basic unit of multilayers, the effect and the impor- multilayers seem to be of the greatest interest because tance of a buffer layer on its GMR effect should be of the large GMR effect and of its weak temperature easily found. In this work, semiconductor material Si dependence 2,5 . Because of these properties, this and transition metal Cr were used as buffer layers for system could be a candidate for applications. Co Cu Co sandwiches. It was found that a strong It was reported that the buffer layer played an im- in-plane anisotropic GMR effect related to cobalt sili- portant role in the enhancement of the MR ratio in cide appeared in Si-buffered sandwiches, while only an magnetic multilayers. Large MR ratios were obtained isotropic GMR effect with larger saturation field in in Co Cu multilayers with a Fe buffer layer 2 and in Cr-buffered ones was found. NiFe Cu Co Cu multilayers with a Cr buffer layer 2. Experiment Corresponding author. Tel.: 86-21-6251-1070; fax: 86-21- 6251-3510. The sandwiches were prepared using a Balzers E-mail address: hlshen@itsvr.sim.ac.cn ZH. Shen.. ultra-high vacuum Z . UHV electron beam evaporation 0040-6090 00 $ - see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Z 0 0 . 0 1 1 8 0 - 9 56 H. Shen et al. Thin Solid Films 375 ( ) 2000 55 58 system. The base pressure was approximately 5 10 7 Pa and the working pressure was approximately 5 10 6 Pa. The deposition rates were 0.05 nm s. Con- ventionally cleaned Z Si . 100 wafers with a size of 5 15 mm2 were used as substrates. Cobalt 5.5 nm Cu 3 nm Co 5.5 nm sandwiches were deposited after the deposition of a Si or Cr buffer layer. The thickness of each buffer layer, ranging from 1 nm to 14 nm, was controlled by a quartz oscillating thickness monitor and was calibrated by high-resolution transmission electron microscopy Z . HRTEM . The hysteresis loop of the sam- ples was measured by a vibrating sample magnetometer Z . VSM . The film texture was investigated using an Fig. 2. Normalized hysteresis loops for Z Si . 100 Si tSi Co 5.5 nm Cu X'pert Philips X-ray diffractometer Z . XRD with a Cu 3 nm Co 5.5 nm with tSi 1.5 nm Za. and 0.6 nm Zb.. is the angle K between field H and easy axis. target and the microstructure of the sandwiches was observed by HRTEM. All the characterizations were performed at room temperature. The MR is defined as anisotropic in-plane magnetization for thick Si buffer MR Z R R . S RS where RS is the resistance at satu- layers. Shown in Fig. 2a are hysteresis loops measured ration field. The applied field H along the sample in the same sample in Fig. 1. One notes that in the easy plane was parallel to the measurement current. axis direction Z 0 . , the curve has a clear rectangu- lar shape with a coercivity of 28 Oe, meaning that the 3. Results and discussion magnetic moments could turn over quickly as the ap- plied field changes, so that we get larger sensitivity in For sandwiches with a Si buffer layer, the MR value this direction. In the hard axis direction Z 90 . , increases quickly from 0.3% to 4.8% when the Si buffer however, the magnetic moments move slowly with the layer thickness increases from zero to 0.6 nm. A strong applied field, resulting in a smaller sensitivity. The anisotropic GMR effect appeared when the Si thick- same process takes place in those sandwiches with ness was equal to or larger than 0.9 nm. Fig. 1 shows thinner Si buffer layers since they have similar magne- the typical results obtained in a Si 1.5 nm Co 5.5 tization curves ZFig. 2b. to that in Fig. 2a measured at nm Cu 3 nm Co 5.5 nm sandwich. is the angle 90 . between the applied field H and the length side. When In Cr-buffered sandwiches, the GMR effect was the field is applied along the length side Z 0 . , the found to be isotropic for all Cr buffer layer thicknesses. maximum MR value is 5.5% with a sensitivity of With increasing Cr thickness to 8 nm, the GMR value 0.7% Oe. The MR value decreases to 4.2% when the increased gradually to its maximum value of 6%. After field becomes perpendicular to the length side Z 90 . . that, the MR value decreased for much thicker Cr At the same time, the saturation field also increases buffer layers due to current shunting. Fig. 3 shows the from approximately 90 Oe to approximately 150 Oe, typical field dependence of MR and magnetization for resulting in a very small sensitivity of 0.06% Oe. For a a Cr 8 nm Co 5.5 nm Cu 3 nm Co 5.5 nm sandwich. Si thickness less than 0.9 nm, the MR value is the same It can be noticed that the saturation field is much in any direction, also with low field sensitivity. Corre- increased to approximately 200 Oe. From the hysteresis spondingly, the sandwiches presented similar loop in Fig. 3b, a coercivity of 86 Oe was obtained. In order to understand the origin of the large difference in coercivity of a Co Cu Co sandwich using either Si or Cr buffer layers, we prepared two Si 3 nm Co 5.5 nm and Cr 8 nm Co 5.5 nm bilayer samples. It was found that the coercivities of the Si Co bilayer were only approximately 30 Oe with in-plane magnetic anisotropy, while for the Cr Co bilayer, its coercivity increased to a value of approximately 200 Oe. The same coercivity enhancement was observed when Co- based alloy was grown on Cr buffer layer 8 and was attributed to the bcc Cr structure promoting the forma- tion of a hcp Co-based thin film, resulting in the hcp c-axes being distributed in the plane of the film. It is Fig. 1. The magnetoresistance curves for Z Si . 100 Si 1.5 nm Co 5.5 clear now that the magnetic anisotropy in Si-buffered nm Cu 3 nm Co 5.5 nm with Za. 0 and Zb. 90 . sandwiches comes from the lower Co magnetic layer H. Shen et al. Thin Solid Films 375 ( ) 2000 55 58 57 Fig. 4. The high-angle XRD patterns for sandwiches Z Si . 100 Si Fig. 3. The magnetoresistance curve and normalized hysteresis loop tSi Co 5.5 nm Cu 3 nm Co 5.5 nm with tSi 0, 0.6 and 1.5 nm and for Z Si . 100 Cr 8 nm Co 5.5 nm Cu 3 nm Co 5.5 nm. sandwich Z Si . 100 Cr 8 nm Co 5.5 nm Cu 3 nm Co 5.5 nm. consumption of Si to form Co2Si. This observation is in and that the Cr buffer layer enlarged the coercivity of agreement with the above XRD result and supports its neighboring Co layer, resulting in the larger coerciv- our hypothesis. ity of the sandwich. Since we did not apply any magnetic field to the 4. Conclusions sample during the deposition process and used the same procedure for the fabrication of all sandwiches, A large anisotropy of GMR effect was found in the mechanism of the in-plane magnetic anisotropy Si-buffered Co Cu Co sandwiches when the thickness appearing only in the lower Co layer of Si buffered of Si buffer layer was equal to or larger than 0.9 nm. sandwiches remains to be studied further. It may result With H parallel to the easy axis, the GMR effect from its special structure. To confirm this hypothesis, reached a value of 5.5% with a high field sensitivity of an XRD measurement was performed on Si- and Cr- approximately 0.7% Oe for a Si buffer thickness of 1.5 buffered sandwiches. As shown in Fig. 4, there is a nm, which differs from the GMR value of 4.2% and the strong peak at approximately 2 44 in all Si-buffered sensitivity of 0.06% Oe with H perpendicular to the samples, which was assigned to Z fcc-Co . 111 together easy axis. While in Cr-buffered Co Cu Co sand- with some contribution from fcc-CuZ . 111 . Moreover, wiches, the GMR effect showed only in-plane isotropic there is also a shoulder peak at approximately 2 45.3 for a Si buffer layer equal to or thicker than 0.9 nm. This peak has been attributed to Co Z 2 Si . 301 . Ac- cording to the theoretical analysis, the average atom distance in the Co Z 2 Si . 301 plane is 0.258 nm, which is close to that in Z fcc-Co . 111 , 0.251 nm. Also the atom arrangement of these two kinds of planes are all regu- lar hexagon. It is easy for Z fcc-Co . 111 to grow on Co Z 2 Si . 301 . On the other hand, there is only a good repeatability of atom arrangement in the 010 direc- tion in the Co Z 2 Si . 301 plane. The repeatability of atom arrangement is dependent on the preferred-direction, which will influence the atom arrangement in the fcc- Z Co . 111 grown on Co Z 2 Si . 301 plane, leading to the observed in-plane magnetic anisotropy. The structural properties of the sandwiches were further investigated by cross-sectional HRTEM. There were only randomly oriented polycrystalline grains in Cr-buffered sand- wiches. The Cr buffer seemed to grow in pillar-shaped grains on a Si substrate. For an evaporated Si buffer layer, it is an amorphous state as shown in Fig. 5. From the HRTEM image, it is evident there is a Co2Si layer, approximately 2 nm thick, between the Si buffer layer and the lower Co magnetic layer. The thickness of the Fig. 5. Cross-sectional HRTEM image of sandwich Z Si . 100 Si 5 -Si buffer layer is less than the set value due to the nm Co 5.5 nm Cu 3 nm Co 5.5 nm. 58 H. Shen et al. Thin Solid Films 375 ( ) 2000 55 58 properties. Its MR ratio increased to a maximum of 6% References gradually with Cr thickness up to 8 nm, and then decreased due to the shunting effect in Cr buffer layer, which is in contrast to that quick increase of MR ratio 1 M.N. Baibich, J.M. Broto, A. Fert et al., Phys. Rev. Lett. 61 with Si thickness and then a stable MR ratio in Si- Z . 1988 2472. buffered Co Cu Co sandwiches. XRD results and 2 S.S.P. Parkin, Z.G. Li, D.J. Smith, Appl. Phys. Lett. 58 Z . 1991 HRTEM observation revealed that there exists a Co 2710. 2 Si compound between the Si buffer and the lower Co 3 S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 Z . 1990 magnetic layer. All the results above demonstrate that 2304. 4 S.S.P. Parkin, Appl. Phys. Lett. 61 Z . 1992 1358. the observed Co2Si compound is responsible for the 5 D.H. Mosca, F. Petroff, A. Fert, P.A. Schroeder, W.P. Pratt Jr., anisotropic GMR effect appearing in Si-buffered R. Laloee, J. Magn. Magn. Mater. 94 Z . 1991 L1. Co Cu Co sandwiches. 6 H. 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