JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999 Longitudinal spin-density-wave ordering in thin epitaxial chromium layers M. Rots,a) S. Demuynck, S. Cottenier, J. Dekoster, and J. Meersschaut Instituut voor Kern- en Stralingsfysica, K.U.L.-University. Leuven, Celestijnenlaan 200D, B-3000 Leuven, Belgium Thin chromium films molecular beam epitaxy grown on partly relaxed MgO/Fe substrates held below T 150 °C are found either nonmagnetic below a chromium thickness around 5.0 nm or in a single-Q magnetic domain with the longitudinal spins-out-of-plane spin-density-wave SDW ordering stable up to 500 K. The longitudinal SDW-to-paramagnetic transition, the absence of the spin-flip transition and the actual character of the SDW depends on specific growth conditions. The present results reveal the spin-flip transition in chromium as an anisotropy effect rather than an intrinsic feature of the SDW ordering. © 1999 American Institute of Physics. S0021-8979 99 58808-7 I. INTRODUCTION In order to avoid any influence on the chromium spin structure as due to a magnetic spacer layer the PAC tech- The typical itinerant magnetism of chromium remains a nique was applied on Ag/Cr multilayers.4 One expects to topic of broad interest, especially since molecular beam ep- observe in this system a magnetic state representative for itaxy MBE techniques can produce high quality epitaxial chromium in reduced dimension. These multilayers were layers and superlattices. Most novel properties of the inter- grown by molecular beam epitaxy on preannealed MgO sub- layer coupled systems were first discovered in the Fe/Cr mul- strates. The lattice mismatch for Cr and Ag on MgO is re- tilayers and therefore activated the research of chromium spectively 4.1% and 2.9%. Therefore the roughness of magnetism in reduced dimension. Buried spacer layers how- the multilayer was reduced by using a buffer layer of 10 nm ever are difficult to access, especially when antiferromagneti- Fe, deposited at 150 °C, shown by channeling experiments to cally ordered. In the nanometer thickness range even neutron be completely relaxed the lattice mismatch between Ag and scattering techniques are not very suitable. Perturbed angular Fe is only 0.8% . The face-centered-cubic fcc Ag grows on correlation PAC spectroscopy was proposed1 recently as a body-centered-cubic bcc Cr under the epitaxial relation valuable alternative for the study of the peculiar magnetism 110 Ag// 100 Cr. All samples consist of ten bilayers with in chromium, with some appealing results. It is well known varying Cr thickness. The deposition rates were 0.5 Cr and that bulk chromium orders, at the NeŽel temperature TN 0.9 Ć/s Ag and the growth temperatures 25 °C. Reflection 311 K, as an incommensurate spin density wave SDW high-energy electron diffraction RHEED patterns were re- antiferromagnet with wave vector QSDW 0.958(2 /a) corded to monitor the growth and the quality of the samples along the 100 direction. Its polarization changes from S Q was checked by x-ray diffraction XRD . Rutherford back- in the transverse SDW phase (AF1) above the spin-flip tem- scattering and channeling experiments, used4 to measure perature (TSF 123 K) to S Q in the longitudinal SDW phase strain in the epitaxial layers, prove that in the Ag/Cr multi- (AF2) below. layers the out-of-plane lattice parameter is reduced as com- The PAC experiments1 on Fe/Cr multilayers grown on pared to the in-plane lattice parameter. For the Cr layers the MgO at 150 °C found the Cr spin orientation perpendicular compression equals 4 10 3 while in the Ag layers a much to the layers and thus perpendicular to the in-plane Fe mag- larger reduction of the out-of-plane lattice parameter of 2.4% netization, independent of the Cr-layer thickness at least up is observed. The detailed structural characterization analysis to 400 nm. Furthermore we found1 the collapse of the SDW implies that the body-centered-tetragonal bct structure of for Cr thickness below about 5.0 nm. This critical thickness the chromium layer should be responsible for the actual mag- almost matches with the period of the SDW, in those mea- netic order in the Cr layers. surements observed in single Q domain. Subsequent experiments have indicated that the specific growth conditions are important for the actual type of mag- II. THE Cr MAGNETISM IN Ag/Cr 001... MULTILAYERS netic order. Indeed, in contrast to the original PAC study, for In order to characterize the magnetism in the samples, Fe/Cr multilayers grown on Nb at 300 °C with Cr thickness we use a nuclear technique based on the hyperfine interac- between 5.1 and 19 nm, both neutron diffraction data2 and tion: perturbed angular correlation spectroscopy PACs .1,3 A PAC data3 indicate a transverse SDW with an out-of-plane Q trace amount of typically 1011 radioactive 111In probes is vector and spins in-plane, but turning out-of-plane for thicker introduced into the sample by ion implantation at an energy Cr layers. In addition, below the critical thickness for the of 80 keV, thereby distributing the probes throughout the stability of the SDW, neutron diffraction experiments2 on multilayer. The experimental result of a PAC measurement is Fe/Cr superlattices find Cr partially in the in-plane AF0 state. the so-called R(t) spectrum, a more or less periodic pattern representing the nuclear Larmor precession. The method is a Electronic mail: michel.rots@fys.kuleuven.ac.be not only sensitive to the magnitude of the extranuclear elec- 0021-8979/99/85(8)/4836/3/$15.00 4836 © 1999 American Institute of Physics Downloaded 27 Mar 2001 to 148.6.178.13. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcr.jsp J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Rots et al. 4837 FIG. 3. PAC time spectra for (2.8 nm Ag/5.9 nm Cr)10 upper and (3.2 nm Ag/5.1 nm Cr)10 lower measured at 293 K. FIG. 1. PAC time spectra for (2.8 nm Ag/7.9 nm Cr)10 measured at 77 K. The upper spectrum was recorded with the sample normal perpendicular to the detector plane, the lower spectrum with the sample normal in the detec- which indeed appears as soon as magnetic order establishes. tor plane. The corresponding hyperfine field however reflects a distri- bution typical for an incommensurate SDW. The value of the hyperfine field indicates3 longitudinal polarization thus the tromagnetic fields through the frequency, but important also LSDW phase PAC is only indirectly sensitive to the Q vec- to the orientation of these hyperfine fields relative to the tor . The orientation of the magnetic field is derived from the detectors. Recent PAC experiments3 on thin films and mul- relative amplitude of the first and second harmonic in the tilayers have proven that the diamagnetic 111Cd nuclear R(t) spectrum see caption to Fig. 1 . The hyperfine field in probe indeed senses the intrinsic magnetic behavior of the Cr our samples and thus the magnetic moments in the Cr layers layer giving results compatible with neutron diffraction ex- are oriented out-of-plane. periments when measured on the same samples. The 111CdCr hyperfine fields, magnitude, and orienta- We have studied samples with varying Cr thickness at tion, measured at room temperature in the Ag/Cr system are temperatures from 4.2 up to 550 K and in different detector nicely consistent with those determined1 in Fe/Cr multilayers geometries. A typical R(t) spectrum, measured simulta- and Cr single 400-nm-thick layers grown also on MgO. As a neously in two detector geometries in order to derive the function of temperature and for all samples measured the hyperfine field orientation in a straightforward way, is shown magnitude of the hyperfine field decreases almost linearly in Fig. 1. In the analysis5 we accounted for three In probe with an average slope of 7.0(2) 10 4 K. Interestingly, locations: apart from probes at locations near to the interface, this linear behavior Fig. 2 which extends from 4.2 up to we expect probes at substitutional positions in the Cr as well 500 K, is a perfect extrapolation of the temperature depen- as the Ag layer. The probes ending up within the Cr layer dence of the integrated intensity at the 0,1, satellite mea- contribute to the oscillation seen in the spectra Fig. 1 , sured by neutron scattering6 in bulk Cr in the LSDW low temperature phase. This observation clearly proves that in the present PAC experiments the longitudinal SDW state remains stable up to 500 K, far above the spin-flip tempera- ture at 123 K in bulk Cr. At 500 K Cr is still magnetic, but this magnetic phase has completely disappeared at 550 K. Finally, while in the sample with tCr 5.9 nm the precession pattern due to magnetic ordering is still clearly present, this contribution disappears completely for the thinner layers Fig. 3 . Moreover the absence of any precession signal fur- ther proves that the collapsed SDW is not replaced by a commensurate antiferromagnetic ordering. III. CONCLUSIONS The observation of the SDW collapse at a chromium thickness comparable with the SDW period, in Fe/Cr but also in Ag/Cr suggests that this effect is an intrinsic size FIG. 2. Hyperfine field (Bhf), or equivalently the chromium magnetic mo- ment amplitude, as a function of temperature. The data points represent effect of Cr in reduced dimension. The elastic information averages over the different layer thicknesses. derived from our channeling experiments reveals that the Downloaded 27 Mar 2001 to 148.6.178.13. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcr.jsp 4838 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Rots et al. perpendicular spin orientation observed in chromium layers the spin-flip transition. Based on the present results, this can be attributed to residual tetragonal lattice distortion fa- phase transition appears as an anisotropy effect rather than voring a longitudinal SDW in a single-Q magnetic domain. an intrinsic feature of the SDW ordering. Forthcoming re- This magnetic order persists up to 500 K and has been ob- fined electronic structure calculations may give further sup- served in Cr layers with thickness at least up to 400 nm. The port on the conclusion that indeed a tetragonal distortion of ``enhanced'' NeŽel temperature in those samples is under- chromium results in a lowering of the energy for the longi- stood as the ``true'' LSDW-to-paramagnetic transition tem- tudinal relative to the transversal SDW polarization. perature, seen here without the intervening TSDW as usual in bulk Cr. The transition to the paramagnetic state should be ACKNOWLEDGMENTS rather sharp since we observe at 550 K no magnetic order. Therefore the message of this article is: very thin Cr is para- We thank Dr. K. Freitag Institut fušr Strahlen- und Kern- magnetic because the SDW collapses due to finite size effect physik, Bonn, Germany for his kind cooperation in the ion and the magnetic ordering for thicker layers is LSDW without implantations. This work was financially supported by the a spin-flip transition, due to residual tetragonal lattice distor- FWO/G.0137.95 and GOA/94.2 projects. tion. The latter bct structure is realized by MBE growth of chromium layers on a Fe/MgO buffer/substrate combination 1 J. Meersschaut, J. Dekoster, R. Schad, P. Beliešn, and M. Rots, Phys. Rev. held below 150 °C. Furthermore, the linear temperature de- Lett. 75, 1638 1995 . pendence of the chromium magnetic moment the amplitude 2 A. Schreyer, C. F. Majkrzak, Th. Zeidler, T. Schmitte, P. Bošdeker, K. of the SDW as reflected by the magnetic hyperfine field is Theis-Brošhl, A. Abromeit, J. A. Dura, and T. Watanabe, Phys. Rev. Lett. 79, 4914 1997 . an unusual behavior for a three-dimensional 3D system and 3 J. Meersschaut, J. Dekoster, S. Demuynck, S. Cottenier, B. Swinnen, and a challenge to theory. M. Rots, Phys. Rev. B 57, 5575 1998 . In conclusion, those appealing observations may possi- 4 S. Demuynck, J. Meersschaut, J. Dekoster, B. Swinnen, R. Moons, A. bly initiate further studies on the itinerant magnetism in re- Vantomme, S. Cottenier, and M. Rots, Phys. Rev. Lett. in press . 5 N. P. Barradas and J. C. Soares, Hyperfine Interact C...1, 310 1996 . duced dimension and contribute to understand the nature of 6 S. A. Werner, A. Arott, and H. Kendrick, Phys. Rev. 155, 528 1967 . Downloaded 27 Mar 2001 to 148.6.178.13. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcr.jsp