3B2v7:51c ED: Chanakshi;Brr GML4:3:1 MAGMA : 8453 Prod:Type: com pp:123ðcol:fig::NILÞ PAGN: ananth SCAN: Mangala ARTICLE IN PRESS 1 3 Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 5 7 Effect of interface structure on magnetic and magnetoresistive 9 properties of Fe/Cr multilayers 11 V.V. Ustinovn,L.N. Romashev,T.P. Krinitsina,E.A. Kravtsov,M.A. Milyaev, 13 A.V. Semerikov,V.A. Tsurin,N.V. Kourtina 15 Ural Division of the Russian Academy of Sciences, Institute of Metal Physics, 18 Sofia Kovalevskaya St., 620219 Ekaterinburg, Russia 17 19 Abstract 21 The influence of growth temperature on atomic and magnetic interface structure was traced in Fe/Cr multilayers grown with MBE at different substrate temperatures in the range 20­4801C. Proper growth was found to be possible 23 only in a narrow temperature range about 140­1801C,deviations from optimal temperature causing drastic increase in interface roughness. Giant magnetoresistive effect was shown to be in close correlation with the interface structure. 25 r 2001 Published by Elsevier Science B.V. 27 Keywords: Multilayers,metallic; Interface structure; Magnetoresistance,giant; X-ray reflectivity 29 57 31 Interface structure is believed to play an important of fine crystallites with close orientation. The crystallites 59 33 role in forming physical properties of magnetic multi- are equiaxial in the layer plane,their (0 0 1) plane being layers. By tuning interface properties of a multilayer,it parallel to the substrate plane. The interface structure 61 35 is possible to influence its macroscopic behaviour. One was investigated by combining X-ray reflectometry (XR) of the ways to make an impact on interface is to vary and M.ossbauer spectroscopy (MS). X-ray measure- 63 37 growth conditions. Growth temperature is known to be ments were made with Co Ka radiation. M.ossbauer one of the governing factors in multilayer growth [1]. In spectroscopy investigations were made in transmission 65 39 the present work,we report on the study of the geometry for absorption set-up using a 57Co source in a dependence of magnetic and transport properties of Cr matrix. Magnetic properties were studied by SQUID 67 41 Fe/Cr multilayers on their interface structure. In order and VSM magnetometry; magnetoresistance measure- to reveal the role of the interface structure,we have ments were made in standard DC four contact scheme. 69 43 studied a series of multilayers grown at different All the investigations were carried out at room substrate temperatures and therefore having different temperature. 71 45 atomic and magnetic interface structure. The layer structure and vertical rms interface rough- The investigations were carried out on a series of ness were determined by fitting X-ray reflectivity curves. 73 47 [Cr(9 (A)/57Fe(20 (A)]8 multilayers MBE grown on The X-ray reflectivity profiles were treated within a ð1 0 1 2ÞAl2O3 substrates covered by 70 (A-Cr buffer layer dynamical approach based on recurrent scheme [1],the 75 49 at different 77 51 79 53 n Corr UNCORRECTED PROOF substrate temperatures in the range 20­ rms interfacial roughness being included through 4801C. The crystalline structure was studied with Debye­Waller factors [2,3]. As an example of fitting transmission electron microscopy. The investigated results in Fig. 1 experimental and fitted reflectivity multilayers were found to consist of a large arrangement curves obtained for the multilayer grown at 2401C are shown. The structural information obtained from the X-ray esponding author. Tel.: +7-3432-444471; Fax: +7-3432- 81 55 745244 data is summarised in Fig. 2. In this figure,the rms E-mail address: ustinov@imp.uran.ru (V.V. Ustinov). roughness at Fe-on-Cr and Cr-on-Fe interfaces reduced 0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 9 2 - 7 MAGMA : 8453 ARTICLE IN PRESS 2 V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 1 (b) 57 3 59 5 61 (a) 7 63 9 65 11 67 13 Fig. 3. Growth temperature dependence of relative area Sa Fe 69 of a-Fe subspectrum in the resulting M.ossbauer specrum. 15 Insets: (a) typical multilayer M.ossbauer spectrum; (b) reduced 71 hyperfine-field Bhf distributions for multilayers grown at 20 and 17 Fig. 1. Experimental (points) and fitted (line) X-ray reflectivity 1801C. 73 spectra measured at Co Ka radiation from the Fe/Cr multilayer grown at substrate temperature T¼ 2401C: 19 75 21 77 23 79 25 81 27 83 29 85 31 87 33 89 35 91 Fig. 4. Magnetisation curves measured with a SQUID magnet- ometer for multilayers grown at 20,180 and 2801C. 37 Fig. 2. Substrate temperature T dependence of vertical rms 93 roughness at Fe-on-Cr (squares) and Cr-on-Fe (circles) inter- 39 faces in Fe/Cr multilayers deduced from XR data. spectrum with that corresponding to ``bulk'' a-Fe. The 95 MS results are depicted in Fig. 3. In the main part of the 41 figure,the growth temperature dependence of relative 97 from XR data is depicted. As follows from the X-ray area of ``bulk'' a-Fe subspectrum in resulting multilayer 43 results,the substrate temperature has a profound effect spectrum is shown. In the Insets of Fig. 3,a typical 99 on the rms interface roughness. The best interface multilayer M.ossbauer spectrum composed of ``bulk'' 45 structure was observed at growth within a narrow and interface subspectra and the corresponding hyper- 101 temperature range of about 140­1801C. The lowest rms fine-field distribution are shown. An important point is 47 interface roughness is observed in the multilayer grown that while the XR is sensitive to interface imperfections 103 at 1401C. A specific feature of the best multilayers is a originating from both interface roughness and diffuse 49 strong 105 51 107 53 UNCORRECTED PROOF difference between Fe-on-Cr and Cr-on-Fe inter- intermixing,the MS shows the last contribution only. By face structures. Whereas the Fe-on-Cr interface is sharp, comparing the XR and MS data we can conclude that the Cr-on-Fe one is diffusively intermixed at 2­3 atomic the loss of interface quality is connected with interface monolayers in thickness. Deviation from the optimal roughening at lower growth temperatures and with Fe­ growth temperature region causes interface degradation, Cr interdiffusion at higher growth temperatures. 109 preferably at the Fe-on-Cr boundary. The macroscopic magnetic and transport properties 55 Interface structure and hyperfine-field distribution were revealed to be in close correlation with the interface 111 were estimated by comparing multilayer M.ossbauer structure. In Fig. 4 magnetisation curves measured for MAGMA : 8453 ARTICLE IN PRESS V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 3 1 substrate temperature,with the maximum magnetore- 43 sistance effect being observed in multilayers with close- 3 to perfect interface structure. The multilayer grown at 45 the optimal substrate temperature of 1801C was shown 5 to display the maximum magnetoresistive effect. The 47 reduction of giant magnetoresistance with increasing 7 interface roughness is also observed in work of Schad 49 et al. [6] who observed the maximum giant magnetore- 9 sistance value at 2401C. 51 In conclusion,we have systematically studied the 11 influence of growth temperature on the interface 53 structure in Fe/Cr multilayers and traced the correlation 13 between interface properties and macroscopic behaviour 55 of multilayers. We established that there is a clear 15 correlation between magnetic interface structure and the 57 Fig. 5. The value of the giant magnetoresistance effect as a resulting magnetic and magnetoresistive properties. The function of growth temperature. The insets show magnetic field 17 maximum magnetoresistive effect is observed in multi- 59 dependences of magnetoresistance in some multilayers grown at different temperatures. The sample grown at 4801C displays layers with the best interface structure,both interface 19 insignificant magnetoresistance effect due to complete degrada- roughening and interface intermixing reducing the value 61 tion of the layer structure. of giant magnetoresistance effect. 21 63 multilayers grown at different temperatures are shown. The research was partly supported by RFBR (Grants 23 The Cr layer thickness (9 (A) in the series was chosen to No. 00-15-96745 and 01-02-17202) and MIST of RF. 65 be corresponding to the first antiferromagnetic max- 25 imum. As a general tendency we note clear evolution of 67 magnetic ordering from an antiferromagnetic structure 1. Uncited Reference 27 to a ferromagnetic one as the growth temperature 69 increases. It is interesting that in multilayers with rough [4] 29 interfaces grown at lower temperatures we found a near- 71 antiferromagnetic ordering in contrast to more perfect 31 multilayers grown at higher temperatures. The tendency 73 of slow variation of antiferromagnetic exchange cou- References 33 pling with increasing growth temperature is in line with 75 results reported earlier for similar systems (see review [5] [1] D.T. Pierce,J. Unguris,R.J. Celotta,M.D. Stiles,J. Magn. 35 and references therein). Most likely the reason for it is Magn. Mater. 2000 (1999) 290. 77 the formation of FeCr interlayer causing additional [2] V.G. Kohn,Phys. Stat. Sol. B 187 (1995) 61. 37 ferromagnetic contribution to magnetisation. [3] L. Nevot,P. Croce,Rev. Phys. Appl. 15 (1980) 761. 79 [4] R.A. Cowley,T.W. Royan,J. Phys. D 20 (1987) 61. Magnetoresistance rðHÞ ¼ ½RðHÞ Rð0Þ =Rð0Þ as a [5] B. Heinrich,J.F. Cochran,Adv. Phys. 42 (1993) 523. 39 function of growth temperature is shown in Fig. 5. We [6] R. Schad,P. Belien,G. Verbanck,C.D. Potter,H. Fischer, 81 note that the saturation magnetoresistance value rs S. Lefebvre,M. Bessiere,V.V. Moshchalkov,Y. Bruynser- 41 throughout the series depends non-monotonically on the aede,Phys. Rev. B 57 (1998) 13692. 83 UNCORRECTED PROOF