JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999 Mo¨ssbauer study of the influence of thermal treatment on giant magnetoresistance and interface structure in Fe/Cr multilayers M. Kopcewicza) Institute of Electronic Materials Technology, Wo´lczyn´ska 133, 01-919 Warszawa, Poland T. Lucin´ski and F. Stobiecki Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan´, Poland G. Reiss University of Bielefeld, Faculty of Physics, Universita¨tstrasse 25, D-33501 Bielefeld, Germany The dependence of the giant magnetoresistance effect GMR on the interface structure in Fe/Cr multilayers was studied by magnetoresistivity and Mo¨ssbauer spectroscopy. The Fe/Cr multilayers consisting of Fe 6 nm Cr 1.1 nm /Fe 3 nm 60 Cr 1.1 nm were deposited by dc magnetron sputtering. Samples were annealed for 1 h at temperatures ranging from 200 to 500 °C. The interface structure was characterized by conversion electron Mo¨ssbauer spectroscopy CEMS . Various different Fe sites: the bulk, step and perfect interface positions were identified. The evolution of the fraction of Fe atoms in different environments vs annealing temperature revealed that annealing at 300 °C induces bulk and in-plane diffusion of atoms. Higher annealing temperature causes substantial roughening of the interface related to a strong bulk diffusion of atoms. Microstructural changes observed in the CEMS spectra correspond well to the GMR measurements which reveal an enhancement of magnetoresistivity in Fe/Cr multilayers annealed at about 300 °C and its decrease due to deterioration of the interface at higher annealing temperatures. © 1999 American Institute of Physics. S0021-8979 99 19608-7 Since the discovery of the giant magnetoresistance In this article, we present a systematic study of the cor- GMR effect in Fe/Cr multilayers Mls ,1 many experimen- relation between the GMR, resistivity and interface structure, tal and theoretical studies have been performed to elucidate characterized by CEMS, for annealed Fe/Cr Mls obtained by the origin of spin-dependent transport properties.1­6 In par- the sputtering technique. ticular, the influence of the interface roughness on the GMR The Fe/Cr multilayers were deposited on oxidized Si amplitude was recognized. However, the correlation between substrates, attached to a computer controlled rotating plat- GMR, magnetization and interface structure in Fe/Cr Mls4­8 form, using dc magnetron sputtering.9 The sputtering process is still not well understood. Petroff et al.4 have found that the raises a deposition temperature by about 50 °C from ambient. Fe/Cr superlattices with low and weakly temperature- The deposition rate was monitored by quartz-crystal oscilla- dependent resistivity have very small GMR that was related tors. A 6 nm thick iron layer was deposited first as a buffer to sharp interfaces. They observed that annealing increased layer, and a final 1.1 nm thick chromium layer was used as a the residual resistivity as well as interface roughness and the capping layer. The structure of our samples is: SiOx scattering, i.e., roughening of the interfaces enhanced the Fe 6 nm Cr 1.1 nm /Fe 3 nm 60 Cr 1.1 nm . The mul- GMR effect. The results of Fullerton et al.7 have also shown tilayers were patterned into a conventional geometry for re- that the GMR is enhanced by interface roughness. In contrast sistivity measurements (1 mm 7 mm) using photolitho- to these results the largest GMR effect 220% at 1.5 K has graphic and lift-off techniques. The total film thickness was been found in Fe/Cr multilayers with sharp interfaces and measured with a profilometer. low residual resistivity.6 The interface structure was usually The as-deposited Fe/Cr multilayers were annealed for 1 investigated by x-ray diffraction see, e.g., Refs. 5 and 7 h at temperatures ranging from 200 to 500 °C in an argon which is suitable for analyzing long-range atomic order but atmosphere under normal pressure. The structure of the as- which gives ambiguous information for the lateral scale of deposited and annealed films was studied by CEMS which interfacial structure making the determination of the lateral probes the surface region about 120 nm deep. The CEMS length of roughness difficult. Rensing et al.8 have performed measurements were performed at room temperature by using a He-6%CH a conversion electron Mo¨ssbauer spectroscopy CEMS 4 gas flow electron counter. The Mo¨ssbauer spectral parameters hyperfine fields, linewidths, isomer study which demonstrated that the structural effect of anneal- shifts, quadrupole splittings were calculated by fitting ing the Fe/Cr Mls is the promotion of Cr interdiffusion into Lorentzian line shapes to the experimental data by using the the bulk of the Fe layers rather than short scale intermixing least-squares method. The isomer shifts are related to the at the interfaces. -Fe standard. The transport properties have been studied using a stan- a Corresponding author; electronic mail: kopcew dard dc four terminal method. The current (i 5 mA) was ­m@sp.itme.edu.pl 0021-8979/99/85(8)/5039/3/$15.00 5039 © 1999 American Institute of Physics Downloaded 03 Oct 2001 to 148.6.178.100. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp 5040 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Kopcewicz et al. FIG. 1. a GMR and b residual resistivity s measured at saturation field measured at 4.2 K vs annealing temperature, collected for four sample sets FIG. 2. Comparison of the magnetization loops measured at 4.2 K for Fe/Cr with the same nominal thicknesses of the Fe and Cr layers. sample at different stages of annealing. always in the plane of the sample and the magnetic field up explained in terms of roughening of the interfaces by com- to 5 T was parallel to the current. The magnetoresistivity positional mixing as a result of interdiffusion. If interdiffu- and the resistivity were measured at 4.2 K before and after sion occurs in the multilayer structures, the residual resistiv- sequential annealing. ity of the films should markedly increase due to enhanced A standard vibrating sample magnetometer VSM was electron-impurity scattering. Therefore the reduction of the used to measure magnetisation loops at 4.2 K and the exis- residual resistivity in the annealed samples can be attributed tence of an antiferromagnetic alignments of the adjacent Fe to the annealing effect in which some of the structural de- layers. fects, such as vacancies, dislocations and grain boundaries, In Fig. 1 the GMR amplitude for four sample sets with have been annihilated. nominally the same sublayer thicknesses , defined as Figure 2 shows the examples of the evolution of the H 0 M(H)/Ms curves upon annealing for as-deposited and an- GMR s H Hs , nealed at 400 and 500 °C samples. The remanent magnetiza- s H Hs tion MR for TA 400 °C was negligible. At TA 400 °C a and resistivity s , measured in the plane of the samples at small MR appeared; however, the shape of the M(H) loop saturation field Hs at 4.2 K, are shown vs annealing tempera- remained typical for antiferromagnetically coupled Mls. An- tures TA . As can be seen, the magnetoresistivity, after the nealing at 500 °C induced a significant increase of the coer- initial increase observed for the samples annealed at 270 °C, civity and remanence. Since for TA 300 °C MR 0, local is gradually reduced by the thermal treatment, although for ferromagnetic and/or biquadratic coupling due to presence samples annealed at 400 °C a considerable magnetoresistiv- of pinholes leading to a canted arrangement of the magne- ity is still observed. Annealing at 500 °C destroyed the GMR tization vectors in the adjacent Fe layers can be excluded. effect; one sample showed only an anisotropic magnetoresis- Therefore the increase of GMR amplitude observed for TA tance effect of 1.7% Fig. 1 a . The residual resistivity s , 300 °C can be due to the annihilation of structural defects determined by crystal imperfections, decreases for TA and/or reduction of the interface roughness leading to the 300 °C Fig. 1 b . Therefore, if the interface scattering increase of the mean free path of the conduction electrons. rather than the bulk one dominates in our multilayers, the Such a conclusion is confirmed by the reduction of the re- minimum s(TA) should be observed at TA for which the sidual resistivity, as noted previously for TA 300 °C Fig. interface roughness is smallest. Such a minimum in s(TA) 1 b . has been observed for TA 300 °C. For TA 400 °C, inter- The interface structure of the Fe/Cr Mls was studied by diffusion at the interfaces started to occur, resulting in a the Mo¨ssbauer spectroscopy. The CEMS spectra for the as- gradual increase of s . Atomic force microscopy AFM deposited and annealed samples are shown in Fig. 3. The measurements of the films have shown a very smooth surface spectra, interpreted in terms of the Landes et al. model,10 with a root mean square roughness hrms 0.7 nm and the were fitted with four individual magnetic components, H1, grain size D 50 nm. These values remained constant up to H2, H3, and H4, associated with different iron positions. The TA 400 °C. For the Mls annealed at 500 °C, a considerable parameters of the Mo¨ssbauer spectral components H1­H4, grain growth (D 100 nm) accompanied by the increase of such as the hyperfine fields, Hhf , isomer shifts, IS, quadru- roughness to about 3 nm was observed. pole splittings, QS, linewidths, , and the line intensity ra- These results led us to the conclusion that the changes of tios, D23, were similar in the CEMS spectra of all our the magnetoresistivity due to annealing can not be simply samples and were the following: component H1:Hhf 32.9 T, Downloaded 03 Oct 2001 to 148.6.178.100. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Kopcewicz et al. 5041 FIG. 4. Atomic fractions of Fe in a given site: bulk H1 , step H2 and H3 , and perfect interface H4 , vs annealing temperature, calculated from CEMS spectra. ing (TA 200 °C) does not affect markedly the interface structure. Annealing at 300 °C causes a significant increase of the Fe step sites and a small, but clear increase of the perfect interface sites, as shown by the increase of the H2, H3, and H4 components at the expense of the H1 one Fig. 3 c and Fig. 4 . High temperature annealing (TA 450 °C) induces strong roughening of the interface. The fraction of Fe atoms at various step sites increases substantially as evi- denced by a strong increase of H2 and H3 components Fig. FIG. 3. a CEMS spectra recorded for the Fe/Cr Mls in the as-deposited 3 d , Fig. 4 . Simultaneously, the H4 component decreases state and after annealing at b TA 200 °C, c 300 °C, and d 450 °C, respectively. which shows that the perfectly flat interface deteriorates. Such changes in the microstructure of the Fe/Cr interface suggest that two diffusion processes occur during annealing: IS 0.00 mm/s, 0.3 mm/s, corresponds to the iron atoms one related to the diffusion of atoms in the plane of the in bulk sites in Fe layer; component H2:Hhf 30 T, interface leading to its smoothing, and the other related to the IS 0.00 mm/s, 0.5 mm/s, and component H3:Hhf 24 T, volume atomic diffusion which causes roughening of the in- IS 0.05 mm/s, 0.5 mm/s, corresponds to Fe atoms in terface. At annealing temperatures of about 300 °C both pro- various step sites at the interface; component H4:Hhf 19 T, cesses occur. At higher annealing temperatures (TA IS 0.05 mm/s, 0.4 mm/s, corresponds to the Fe atoms 450 °C), a bulk diffusion strongly dominates and leads to in perfect interface positions. For all spectral components, a significant roughening of the interface. the quadrupole parameter is negligible. 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Mantl, and Zs. Kajcsos, J. interface H4 is only about 5.5%. Low temperature anneal- Magn. Magn. Mater. 86, 71 1990 . Downloaded 03 Oct 2001 to 148.6.178.100. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp