Journal of Magnetism and Magnetic Materials 198}199 (1999) 689}691 Magnetic properties of thin Cr layers in multilayer systems studied through Sn MoKssbauer probes K. Mibu *, S. Tanaka , T. Kobayashi , A. Nakanishi , T. Shinjo Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan Department of Physics, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan Abstract The magnetic properties of thin Cr layers in epitaxial [Cr(t! )/Sn(2 As)] (t! "5, 10, 20, 30, and 40 As) and [Fe(10 As)/Cr(5 As)/Sn(2 As)/Cr(5 As)] multilayers were studied through Sn conversion electron MoKssbauer spectro- scopy. It was found that the magnetic ordering temperature of Cr in both Cr/Sn and Fe/Cr/Sn/Cr systems is much higher than the NeHel temperature of bulk Cr (i.e. 311 K). It also turned out that the magnetic hyper"ne "elds at the Sn nuclear sites are quite di!erent, i.e., 11}13 T for the Cr/Sn multilayers and 2 T for the Fe/Cr/Sn/Cr multilayer at 300 K, in spite of the similarity of the local crystallographic structure of the Sn probe layers. Possible magnetic structures of the Cr layers are inferred from the size and direction of the magnetic hyper"ne "eld transferred at the Sn nuclear sites. 1999 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial multilayers; MoKssbauer spectroscopy; Magnetic hyper"ne "eld The magnetic properties of thin Cr layers are known to The epitaxial Cr/Sn and Fe/Cr/Sn/Cr multilayers were be di$cult to clarify, because Cr is basically antifer- prepared on MgO(0 0 1) substrates in an ultrahigh vac- romagnetic, where information from the magnetization uum (10\ Torr range). The substrate temperature was measurements is limited, and there is no proper experi- kept at 2003C during the deposition. A Cr(50 As) layer mental method using a Cr nuclear probe. Since Sn is was deposited as a bu!er layer prior to the depositon of a nonmagnetic element, having limited miscibility with multilayers. [Cr(t! )/Sn(2 As)] (t! "5, 10, 20, 30, and Cr in the equilibrium states, the Sn MoKssbauer nu- 40 As) and [Fe(10 As)/Cr(5 As)/Sn(2 As)/Cr(5 As)] multi- cleus can be an appropriate probe to study magnetic layers with Sn enriched Sn layers, where the Sn layer properties of Cr [1]. We have studied the magnetic prop- thickness roughly corresponds to a monatomic layer, erties of epitaxial Cr/Sn and Fe/Cr/Sn/Cr multilayers were prepared for Sn MoKssbauer measurements. through Sn MoKssbauer spectroscopy. It was found From the re#ection high energy electron di!raction pat- that the magnetic hyper"ne "elds at the Sn nuclear sites terns during the "lm growth and the X-ray di!raction are sensitively in#uenced by the magnetic properties of patterns after the preparation, it was con"rmed that the the Cr layers. The magnetic transition temperature and Cr layers and the substrate have the structural relations possible magnetic structures of the Cr layers are dis- MgO(0 0 1)//Cr(0 0 1) in the growth direction and cussed on the basis of the size and direction of the MgO[1 0 0]//Cr[1 1 0] in the "lm plane, and that the Sn hyper"ne "eld transferred at the Sn nuclear sites. probe layers grow epitaxially with the Cr layers [2]. The Sn MoKssbauer spectra were measured by means of conversion electron MoKssbauer spectroscopy, using a gas-#ow counter with He#1%(CH ) CH gas at * Corresponding author. Tel./fax: #81-774-38-3109. room temperature and a gas-"lled counter [3] with E-mail address: mibu@scl.kyoto-u.ac.jp (K. Mibu) He#2%CH gas at 100 K and H gas at 15 K [4]. 0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 2 0 1 - 3 690 K. Mibu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 689}691 A Ca SnO source was used to obtain -rays of 23.8 keV and the direction of the incident -rays was set parallel to the "lm normal. The Sn MoKssbauer spectrum for [Cr(30 As)/Sn(2 As)] measured at 300 K is shown in Fig. 1(a). The spectrum indicates a magnetically split six-line pattern, suggesting that the Sn nuclei feel very large magnetic hyper"ne "elds. The spectrum was "tted with six-line components with a distribution of magnetic hyper"ne "elds [2]. The obtained distribution of magnetic hyper"ne "elds is shown in Fig. 1(b). The isomer shift (relative to that for CaSnO ) was "tted to be 1.56 mm/s; this is a reasonable Fig. 1. (a) Sn MoKssbauer spectrum for [Cr(30 As)/Sn(2 As)] at value when the Sn atoms are sandwiched with Cr atoms. room temperature. The "tting line is also shown by the solid The hyper"ne "eld at the peak in the distribution curve line. (b) The distribution of magnetic hyper"ne "elds obtained was estimated to be 11 T. The result that large hyper"ne from the "tting of the spectrum for [Cr(30 As)/Sn(2 As)]. "elds are observed even at room temperature shows that the magnetic ordering temperature of the Cr layers is much higher than the NeHel temperature of bulk Cr (i.e., 311 K). Possible reasons of the increase of the magnetic transition temperature would be strain in the Cr layers inherent to the epitaxial Cr/Sn system and an interface e!ect or a "nite size e!ect that is intrinsic to the multi- layers. Note that a preliminary analysis of the X-ray patterns shows that the lattice spacing in the Cr layers appears to be somewhat larger than that in bulk Cr. It is not clear, at the present stage, whether the observed large hyper"ne "elds are due to a possible enhancement of Cr magnetic moments at interfaces [5] or not; the relationship between the size of the magnetic moment at the interface Cr sites and the size of the Fig. 2. Sn MoKssbauer spectra for [Cr(t transferred hyper"ne "eld at the Sn nuclear sites is not ! )/Sn(2 As)] (t! "40, 20, 10, and 5 As) at 300 K. straightforward. The Sn MoKssbauer spectra for [Cr(t! )/Sn(2 As)] (t! "40, 20, 10, and 5 As) measured at 300 K are shown in Fig. 2. All the spectra show the same extent of mag- netic splitting indicating that they have similar distribu- tion of magnetic hyper"ne "eld. The hyper"ne "elds at the peak in distribution were estimated to be about 11 T for [Cr(40 As)/Sn(2 As)], 12 T for [Cr(20 As)/Sn(2 As)], and 13 T for [Cr(10 As)/Sn(2 As)]. The thickness of 40 As cor- responds to one half of the wavelength of the spin density wave (SDW) in bulk Cr at room temperature [5]. The fact that the size of the hyper"ne "eld is not much dependent on the Cr layer thickness between 5 and 40 As Fig. 3. Sn MoKssbauer spectra for [Fe(10 As)/Cr(5 As)/Sn(2 As)/ implies that the Cr layer has a commensurate antifer- Cr(5 As)] at 300, 100, and 15 K. romagnetic structure rather than an incommensurate SDW structure in this thickness range. The intensity ratio of the six lines of the spectra for the samples with For [Fe(10 As)/Cr(5 As)/Sn(2 As)/Cr(5 As)], where the t! "20, 30, and 40 As was "tted to be 3 : 1 : 1 : 1 : 1 : 3, crystallographic circumstances of the Sn layers are which means 1cos 2"0.6, where is the direction of thought to be the same as those for [Cr(5 As)/Sn(2 As)], the hyper"ne "eld relative to the "lm normal. In an the magnetic splitting in the Sn MoKssbauer spectrum antiferromagnetic Cr "lm with (0 0 1) orientation, either at 300 K (in Fig. 3) was found to be smaller than that for perpendicular magnetic moments or in-plane magnetic [Cr(5 As)/Sn(2 As)]. The hyper"ne "eld was estimated to moments are expected to be stabilized if the magnetos- be 1.7 T at the maximum in distribution, and the distri- trictive e!ect is su$ciently large. Neither of them is, bution width was almost the same as that for however, the case in the present system. [Cr(5 As)/Sn(2 As)]. The spectra at 100 and 15 K are also K. Mibu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 689}691 691 temperature, but due to a di!erence in the magnetic properties of the Cr layers at 0 K. At the present stage, it is not clear whether this di!erence originates from an intrinsic di!erence in the electronic structures or from a di!erence in the strains of the Cr layers. The magnetic frustration e!ect accompanied by steps at the Fe/Cr interface also would reduce the size of Cr magnetic mo- Fig. 4. Temperature dependence of the hyper"ne "eld (at the ments [9] and hence the hyper"ne "elds at the Sn nuclear maximum in distribution) obtained from the Sn MoKssbauer sites. Further study is required to solve this problem. spectra for [Fe(10 As)/Cr(5 As)/Sn(2 As)/Cr(5 As)] in Fig. 3. The authors would like to thank Drs. T. Ono, and N. Hosoito for fruitful discussions during this work. This shown in Fig. 3. The temperature dependence of the work was partially supported by a Grant-in-Aid for hyper"ne "eld at the peak in the distribution estimated Creative Basic Research from Monbusho. from the spectra in Fig. 3 is shown in Fig. 4. It appears that the magnetic transition temperature of the Cr layers in the Fe/Cr/Sn/Cr multilayer is also higher than the References NeHel temperature of bulk Cr. On the magnetism of thin Cr layers in Fe/Cr multilayers, contradictory results have [1] S.M. Dubiel, J. Magn. Magn. Mater. 124 (1993) 31. been obtained from di!erent experimental techniques. [2] K. Mibu, S. Tanaka, T. Shinjo, J. Phys. Soc. Japan 67 (1998) 2633. Perturbed angular correlation experiments have in- [3] K. Fukumura, R. Katano, T. Kobayashi, A. Nakanishi, dicated that the Cr layers are nonmagnetic when the Cr Y. Isozumi, Nucl. Instr. and Meth. A 301 (1991) 482. layer thickness is smaller than 60 As [6], and neutron [4] K. Fukumura, A. Nakanishi, T. Kobayashi, Nucl. Instr. and di!raction measurements have shown that incommen- Meth. B 86 (1994) 387. surate SDW order is suppressed when the Cr layers are [5] E. Fawcett, Rev. Mod. Phys. 60 (1998) 209, and references thinner than 42 As but that commensurate antiferromag- therein. netic order exists instead [7,8]. The present result that [6] J. Meersschaut, J. Dekoster, R. Schad, P. BelieKn, M. Rots, the Sn nuclei in [Fe(10 As)/Cr(5 As)/Sn(2 As)/Cr(5 As)] feel Phys. Rev. Lett. 75 (1995) 1638. "nite hyper"ne "elds at 300 K is consistent with the [7] E.E. Fullerton, S.D. Bader, J.L. Robertson, Phys. Rev. Lett. neutron di!raction experiments, although there might be 77 (1996) 1382. [8] A. Schreyer, C.F. Majkrzak, Th. Zeidler, T. Schmitte, an e!ect originating from the inserted Sn layers in the P. BoKdeker, K. Theis-BroKhl, A. Abromeit, J.A. Dura, Fe/Cr/Sn/Cr system. The di!erence in the hyper"ne "elds T. Watanabe, Phys. Rev. Lett. 79 (1997) 4914. between the Cr/Sn and the Fe/Cr/Sn/Cr multilayer at [9] D. Stoe%er, F. Gautier, J. Magn. Magn. Mater. 147 (1995) 300 K is not due to a di!erence in the magnetic transition 260.