VOLUME 84, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 6 MARCH 2000 Reduction of Magnetic Moments in Very Thin Cr Layers of Fe Cr Multilayers: Evidence from 119Sn Mössbauer Spectroscopy K. Mibu,1,* M. Almokhtar,1 S. Tanaka,1 A. Nakanishi,2 T. Kobayashi,2 and T. Shinjo1 1Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan 2Department of Physics, Shiga University of Medical Science, Otsu Shiga 520-2192, Japan (Received 3 September 1999) Fe Cr multilayers with monatomic Sn layers embedded in the Cr layers were grown epitaxially on MgO(001) substrates, and the magnetic hyperfine field at the 119Sn nuclear sites was examined using Mössbauer spectroscopy. It was found that nonzero hyperfine field is induced at the Sn sites at room temperature and that the value reduces drastically from 10 to 2 T when the Cr layer thickness decreases from 80 to 10 Å. The result indicates that the Cr layers are magnetically ordered even when the thickness is very small and that the magnetic moments of Cr become smaller as the Cr layer thickness decreases. PACS numbers: 75.70.­i, 75.25.+z, 76.80.+y Since the discovery of antiferromagnetic coupling be- prepared on MgO(001) substrates using an ultrahigh- tween Fe layers through a Cr layer in Fe Cr Fe trilayers vacuum deposition system with e-gun heating. A [1] and giant magnetoresistance in Fe Cr multilayers [2], Cr(50 Å) buffer layer was deposited on the MgO(001) considerable attention has been paid to the magnetic prop- substrate prior to the deposition of a multilayer. Then, an erties of Cr in Fe Cr multilayer systems. When the Cr Fe 10 Å Cr tCr Sn t multilayer (t 1 Sn Cr tCr2 Cr layer thickness is around 10 Å, a strong antiferromagnetic tCr 1 t # 80 Å, t 1 Cr2 Sn 0.5, 1, or 2 Å) with 119Sn coupling appears between Fe layers through the interven- enriched 85% Sn layers was grown on the buffer ing Cr layer. However, it is still open to discussion whether layer. Here a thickness of about 1.7 Å for Sn corresponds the Cr layers are magnetically ordered or not in this thick- to a monatomic layer. The period of Fe Cr Sn Cr ness region. Since Cr metal is basically paramagnetic or was repeated 29 or 39 times, then the deposition was antiferromagnetic [3], there are not many experimental ended with an additional Fe(10 Å) layer. The substrate methods to get effective information about the magnetic temperature was kept at 200 ±C during the deposition. properties of Cr layers embedded between ferromagnetic The pressure during the film growth was in the 1029 Torr Fe layers. Experimental works using perturbed angular range and the deposition rate was set around 0.3 Å s, so correlation (PAC) [4], neutron diffraction [5,6], and x-ray that the contamination during the deposition is thought magnetic circular dichroism (XMCD) measurement [7] to be negligibly small. These growth conditions are have been performed to elucidate the magnetic structure of the same as those for the previously reported epitaxial the Cr layers. The results from these experimental meth- Cr Sn multilayers [9]. The 119Sn Mössbauer spec- ods were, however, somewhat contradictory. For example, tra were measured by means of conversion electron in the region where the Cr layer thickness is thinner than Mössbauer spectroscopy, using a gas-flow counter with 42 Å, the PAC experiment indicated that the Cr is non- He 1 1% CH3 3CH at room temperature and a gas-filled magnetic, whereas the neutron diffraction measurements counter with He 1 2%CH4 at 100 K and H2 at 15 K showed that the Cr has a commensurate antiferromagnetic [10]. A Ca 119mSnO3 source was used to obtain g rays of structure. The XMCD result was explained with a model 23.8 keV and the direction of the incident g rays was set where only Cr atoms close to the Fe interface acquire a parallel to the film normal. From reflection high energy significant magnetic moment. In any case, it is difficult electron diffraction and x-ray diffraction measurements, to elucidate magnetic properties of Cr layers sandwiched it was confirmed that the MgO, Cr, and Fe have the between ferromagnetic Fe layers, especially when the Cr structural relation MgO(001) Cr(001) Fe(001) in the layer thickness is very small. In the present work, 119Sn growth direction and MgO 100 Cr 110 Fe 110 in Mössbauer spectroscopy was applied to obtain comple- the film plane. The Sn atoms are thought to be located at mentary information to the PAC and the neutron diffrac- the substitutional sites of bcc Cr(001) planes, and forming tion measurements. Since Sn is a nonmagnetic element, a somewhat strained bcc lattice together with the Cr the 119Sn Mössbauer nucleus can be an appropriate probe atoms, as in the case of Cr Sn multilayers [9]. The lattice to study magnetic properties of Cr [8]. Monatomic Sn lay- spacing between the Cr(001) planes was estimated to be ers were embedded in the Cr layers of Fe Cr multilayers, 1.44 6 0.01 Å, and that between the Cr and Sn planes and the magnetic hyperfine field induced at the 119Sn nu- was 1.57 6 0.04 Å. The in-plane (100) lattice parameter clear sites was measured using Mössbauer spectroscopy. was somewhat larger than that for bulk Cr (i.e., 2.88 Å), The Fe Cr multilayers with a Sn layer embedded ranging from 2.88 to 2.91 Å. The details of the structural in each Cr layer (i.e., Fe Cr Sn Cr multilayers) were study will be published elsewhere [11]. 0031-9007 00 84(10) 2243(4)$15.00 © 2000 The American Physical Society 2243 VOLUME 84, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 6 MARCH 2000 The use of 119Sn Mössbauer spectroscopy for the a Cr matrix [8]. The value of hyperfine field is larger than study on magnetic properties of Cr layers has al- that reported for dilute Sn in Cr. The magnetic hyperfine ready been demonstrated for Cr Sn multilayers in field at the Sn site mainly reflects the spin polarization Ref. [9]. In Fig. 1(a), the 119Sn Mössbauer spectrum for of 5s conduction electrons at the nuclear site, whereas Cr 20 Å Sn 2 Å multilayer at 300 K is shown as a the magnetic moment of Cr is mainly caused by the spin typical spectrum for Cr tCr Sn 2 Å multilayers. A polarization of 3d electrons in the atomic site. In the case magnetic splitting with quite a large hyperfine field is of dilutely dissolved Sn in a Cr or Cr-rich alloy matrix, observed in the spectrum. The appearance of a large the size of the hyperfine field at the Sn site is thought hyperfine field at room temperature indicates that the mag- to be proportional to that of the magnetic moment of netic ordering temperature of the Cr layers is much higher surrounding Cr atoms [12]. For layered structures, the than the Néel temperature of bulk Cr (i.e., 311 K). The neighboring effect might be different from that for a single spectrum was fitted with six-line components with a impurity site. According to a band calculation on Cr Sn distribution of magnetic hyperfine field. The hyperfine multilayers by Oguchi and Momida [13], the magnetic field has a Gaussian-like distribution with the maximum moments of Cr adjacent to the Sn layer are somewhat around 12 T and no component at zero field [Fig. 1(b)]. enhanced in comparison with those inside the Cr layer, The isomer shift (relative to that for CaSnO3) was fitted and the hyperfine field at the Sn site is also enhanced to be 1.56 mm s, which is a reasonable value when the proportionally to the magnetic moment of the contacting Sn atoms are sandwiched with Cr atoms. The distribution Cr atoms. Thus the observed big hyperfine field at the Sn of hyperfine field in Fig. 1(b) gives information about the sites appears to be connected with the enhancement of Cr magnetic structure of Cr around the Sn layer. Suppose the magnetic moments at the interface. multilayer has an ideal crystallographic structure, i.e., a The 119Sn Mössbauer spectra for Fe 10 Å Cr tCr 1 monatomic Sn layer is embedded in a stack of bcc Cr(001) Sn 2 Å Cr tCr multilayers (t t 40, 20, 10, 2 Cr1 Cr2 planes. When the Cr layer has an antiferromagnetic struc- and 5 Å), where Sn(2 Å) is embedded at the center of ture (including the incommensurate spin-density-wave Cr tCr layer (tCr 80, 40, 20, and 10 Å), measured at (SDW) antiferromagnetic structure with the Q vector 300 K are shown in Fig. 2(a). A big change in the spectra perpendicular to the film plane), the magnetic moment is observed when tCr is varied between 80 and 10 Å. of Cr is aligned parallel in a (001) plane and antiparallel Each spectrum was fitted with six-line components with between adjacent (001) planes. If the magnetic moments a distribution of magnetic hyperfine field. The obtained of atomic Cr(001) planes in both sides of the Sn layer distribution is shown in Fig. 2(b). The hyperfine field were oriented to the opposite direction with each other, at the peak in the distribution curve was estimated to be the magnetic hyperfine field transferred at the Sn site 10 T for Fe 10 Å Cr 40 Å Sn 2 Å Cr 40 Å and would be canceled to zero. Therefore, the observation of 2 T for Fe 10 Å Cr 5 Å Sn 2 Å Cr 5 Å . Thus, a finite size hyperfine field indicates that the magnetic the hyperfine field drastically reduces as the Cr layer moments of atomic Cr planes that are sandwiching the Sn layer are oriented to the same direction and that this magnetic structure is established through the Sn layer. The Gaussian-like distribution is probably due to the deviation of the crystallographic structure from the ideal layered structure. The Gaussian-like feature also proves that all the Cr atoms that contact the Sn layer basically have the same size of magnetic moment. The existence of SDW with the Q vector in the film plane, for example, would result in an Overhauser-type distribution of hyperfine field, as observed for dilutely dissolved Sn in FIG. 2. (a) 119Sn Mössbauer spectra for Fe 10 Å Cr tCr 1 FIG. 1. (a) 119Sn Mössbauer spectrum for Cr 20 Å Sn 2 Å Sn 2 Å Cr tCr multilayers (t t 40, 20, 10, and 2 Cr1 Cr2 multilayer at 300 K. The peak positions for the magnetically 5 Å) at 300 K. The peak positions for the magnetically split split sextet are a guide to the eye. (b) The distribution of sextet are a guide to the eye. (b) The distribution of hyperfine hyperfine field obtained from the fitting of the spectrum. field obtained from the fitting of the spectra. 2244 VOLUME 84, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 6 MARCH 2000 thickness decreases. Since the Sn layer is sandwiched the hyperfine field vs temperature curve to higher tempera- between Cr layers, it appears reasonable to conclude that tures, it appears that the magnetic transition temperature (i) the decrease of the magnetic hyperfine field at the of the Cr layers in Fe 10 Å Cr 5 Å Sn 2 Å Cr 5 Å Sn sites reflects the reduction of the magnetic moments is over 400 K, which is much higher than the Néel of Cr around the Sn layer, and that (ii) this change of temperature of bulk Cr. Since the hyperfine fields at room magnetic moments occurs not only around the Sn layer temperature is even larger for other samples, the magnetic but throughout the Cr layer. In the Cr Sn multilayers, ordering temperature of Cr is probably much higher than the hyperfine field at the Sn sites increases a little as the that of bulk Cr as well. Moreover, the change in the Cr layer thickness becomes smaller, i.e., around 10 T for magnetic hyperfine field in Fig. 3 is larger than the tem- Cr 80 Å Sn 2 Å and 13 T for Cr 10 Å Sn 2 Å perature dependence of Fe layer magnetization. This fact [9]. The in-plane (100) lattice parameter is estimated to also shows that the effect due to the conduction electron be 2.90 Å for Fe 10 Å Cr 40 Å Sn 2 Å Cr 40 Å spin polarization induced by the Fe layer, as observed and 2.91 Å for Fe 10 Å Cr 5 Å Sn 2 Å Cr 5 Å , in Fe Au Sn Au multilayers [14], is not dominant in whereas that is 2.90 Å for Cr 80 Å Sn 2 Å and 2.94 Å Fe Cr Sn Cr multilayers. for Cr 10 Å Sn 2 Å . The reduction of the magnetic Figure 4 shows the change of the Mössbauer spectra as hyperfine field is observed only for the Fe Cr Sn Cr a function of the position (i.e., the depth from the Fe inter- multilayers, although the strain is prominent for the Cr Sn face) of the Sn probe layer for Fe 10 Å Cr 44 Å mul- multilayers. Therefore, the strain would not be the main tilayers with Sn(1 Å) at 300 K. The size of the hyperfine reason for the reduction of magnetic moments in the field is not much dependent on the position of the Sn layer present case. In Fe Au Sn Au multilayers, where Sn so long as the Cr layer thickness tCr is fixed. This re- layers are inserted in nonmagnetic Au layers, the hyperfine sult implies that the size of the magnetic moment of Cr field increases as the Au layer thickness decreases [14]. is almost the same throughout the layer. This is reason- The conduction electrons in the Au layer are spin polarized able because neutron diffraction measurements indicate the by the ferromagnetic Fe layer, and the spin polarization existence of a commensurate antiferromagnetic structure gets smaller as the distance from the Fe interface becomes in the present thickness region at room temperature [5,6]. larger. In the Fe Cr Sn Cr multilayers, the hyperfine Note that the insertion of Sn would also act to stabilize the field reduces as the Cr layer thickness decreases, so that commensurate antiferromagnetic phase. The fact that the such a polarization effect from the ferromagnetic layer is hyperfine field is not much dependent on the depth from not the main origin of the observed change of hyperfine the Fe interface again shows that the effect due to the con- field. duction electron spin polarization induced by the Fe layer The Mössbauer spectra for Fe 10 Å Cr 5 Å [14] is not dominant in the Fe Cr Sn Cr multilayers. The Sn 2 Å Cr 5 Å multilayer were measured also at low direction of the magnetic hyperfine field (hence that of the temperatures. The hyperfine field at the peak in the distri- magnetic moments of Cr) was also estimated from the in- bution, which was obtained from the spectra, is plotted as tensity ratio of magnetically split six-lines in the spectra. a function of temperature in Fig. 3. The hyperfine field in- It appears that the magnetic hyperfine field is almost in creases a little as the temperature decreases, but the value the film plane when the Sn layer is situated near the Fe Cr is merely around 3 T at 15 K. Therefore, the difference of the spectra at 300 K shown in Fig. 2(a) is not simply due to the change of the magnetic transition temperature of Cr, but due to the change of Cr magnetism in the temperature range from 0 K. From the extrapolation of FIG. 4. 119Sn Mössbauer spectra for Fe 10 Å Cr tCr 1 Sn 1 Å Cr tCr multilayers (t 1 t 44 Å, 2 Cr tCr1 Cr2 FIG. 3. Temperature dependence of the hyperfine field (at tCr 22, 5, and 2 Å) with the Sn layer in the different depth the maximum in distribution) for Fe 10 Å Cr 5 Å Sn 2 Å 1 from the Fe Cr interface at 300 K. The peak positions for the Cr 5 Å multilayer obtained from the 119Sn Mössbauer spectra. magnetically split sextet are a guide to the eye. 2245 VOLUME 84, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 6 MARCH 2000 The reason why the Cr magnetic moments reduce as a function of Cr layer thickness is thought to be either an intrinsic (thin film) band effect or a frustration effect. A band calculation of epitaxial Fe Cr Sn Cr multilayers with an ideal layered structure is now in progress [13]. The magnetic frustration effect accompanied by steps at the Fe Cr interface would reduce the size of Cr magnetic moments [16] and hence the hyperfine field at the Sn nu- clear sites. This frustration effect is thought to become dominant as the Cr layer thickness becomes smaller (i.e., a frustration and size effect). Further study is required to FIG. 5. 119Sn Mössbauer spectra for (a) Fe 10 Å Cr 5 Å conclude which effect is more dominant for the reduction Sn tSn Cr 5 Å multilayers (tSn 2, 1, and 0.5 Å) and of the magnetic moments. The reduction of magnetic mo- (b) Fe 10 Å Cr 22 Å Sn tSn Cr 22 Å multilayers (tSn ment would make the neutron diffraction signal weaker and 2, 1, and 0.5 Å) measured at 300 K. the PAC signal more or less nonmagnetic. Thus, the Möss- bauer results link the contradictory results from the PAC interface, and it gradually turns toward the perpendicular and the neutron diffraction measurements on Fe Cr mul- direction as the position becomes closer to the center of tilayers. Although the Mössbauer spectroscopy requires a the Cr layer. This tendency is surprisingly consistent with large amount of probe atoms, it gives effective information the result from the PAC experiment on Fe Cr multilayers about the local magnetism of Cr especially for the samples with thicker Cr layers [4] and the neutron diffraction mea- with thin Cr layer thickness, where both PAC and neu- surements on Cr films with Fe cap layers [15], although tron diffraction measurements are getting experimentally the insertion of Sn would more or less influence the mag- difficult. netism of the Cr layers. The authors thank Dr. T. Oguchi, Dr. S. M. Dubiel, and The dependence of the Mössbauer spectra on the Sn Dr. N. Hosoito for fruitful discussions during this work. layer thickness is shown for Fe 10 Å Cr 5 Å Sn tSn This work was partially supported by a Grant-in-Aid for Cr 5 Å and Fe 10 Å Cr 22 Å Sn tSn Cr 22 Å Creative Basic Research from Monbusho. multilayers in Figs. 5(a) and 5(b). The hyperfine field tends to decrease a little as tSn is reduced from 2 to 1 Å and stay constant from 1 to 0.5 Å. A big difference in the size of hyperfine field between the samples with different Cr layer thickness remains even when tSn is reduced to 0.5 Å. *Electronic address: mibu@scl.kyoto-u.ac.jp The spectra for Fe 10 Å Cr 5 Å Sn 1 Å Cr 5 Å [1] P. Grünberg et al., Phys. Rev. Lett. 57, 2442 (1986). and Fe 10 Å Cr 5 Å Sn 0.5 Å Cr 5 Å look like [2] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988). a single-line pattern, but the linewidth (2.9 mm s) is [3] For magnetic properties of Cr in general, see E. Fawcett, larger than that for a paramagnetic peak 1 mm s . Rev. Mod. Phys. 60, 209 (1998), and references therein. Therefore, the Cr layers in these samples are also re- [4] J. Meersschaut et al., Phys. Rev. Lett. 75, 1638 (1995). garded as magnetic at room temperature. For Fe 10 Å [5] E. E. Fullerton, S. D. Bader, and J. L. Robertson, Phys. Rev. Lett. 77, 1382 (1996). Cr 5 Å Sn tSn Cr 5 Å multilayers, where a strong [6] A. Schreyer et al., Phys. Rev. Lett. 79, 4914 (1997). antiferromagnetic coupling between Fe layers and a large [7] M. A. Tomaz et al., Phys. Rev. B 55, 3716 (1997). magnetoresistance effect are expected to appear when [8] S. M. Dubiel, J. Magn. Magn. Mater. 124, 31 (1993). the Sn layer thickness is zero, the magnetoresistance [9] K. Mibu, S. Tanaka, and T. Shinjo, J. Phys. Soc. Jpn. 67, ratio decreases as tSn increases from 0 to 2 Å; this is 2633 (1998). because the antiferromagnetic coupling is weakened and [10] K. Fukumura, A. Nakanishi, and T. Kobayashi, Nucl. spin-independent electron scattering is increased by the Instrum. Methods Phys. Res., Sect. B 86, 387 (1994). existence of Sn layers. The basic features of Mössbauer [11] K. Mibu et al. (unpublished). spectra, on the other hand, do not change much when [12] S. M. Dubiel, J. Cies´lak, and F. E. Wagner, Phys. Rev. B t 53, 263 (1996); (private communication). Sn decreases from 2 to 0.5 Å. (Note, in comparison, the big dependence on the Cr layer thickness.) Therefore, [13] T. Oguchi and H. Momida (unpublished). [14] T. Emoto, N. Hosoito, and T. Shinjo, J. Phys. Soc. Jpn. 66, the change in the size of the magnetic moments of Cr 803 (1997). as a function of the Cr layer thickness is thought to be a [15] P. Bödeker et al., Phys. Rev. Lett. 81, 914 (1998). common and essential feature for both Fe Cr Sn Cr and [16] D. Stoeffler and F. Gautier, J. Magn. Magn. Mater. 147, Fe Cr multilayer systems. 260 (1995). 2246