RAPID COMMUNICATIONS PHYSICAL REVIEW B VOLUME 57, NUMBER 10 1 MARCH 1998-II Spin-density-wave magnetism in layered chromium studied by perturbed-angular-correlation spectroscopy J. Meersschaut, J. Dekoster, S. Demuynck, S. Cottenier, B. Swinnen, and M. Rots Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Received 24 November 1997 Results obtained by perturbed-angular-correlation PAC spectroscopy on epitaxial chromium thin films are compared with data obtained from the same samples by neutron-diffraction experiments. We report on the study of a 250-nm-thick Cr single layer and an Fe/Cr multilayer with Cr thickness of 25 nm grown on Nb/Al2O3 11 02 . The chromium single layer film orders below the NeŽel temperature as a longitudinal (AF2) spin density wave antiferromagnet with the spins out of plane, while the Cr in the Fe/Cr multilayer orders as a transversal (AF1) spin density wave antiferromagnet with the spins in plane. In addition we observe a minority volume fraction in a commensurate AF0 phase, stable up to at least 400 K. The present results prove that PAC probes the intrinsic magnetic features of chromium and therefore validate this technique as a complementary approach towards the understanding of the chromium magnetism in thin films and Fe/Cr multilayers. S0163-1829 98 51210-6 In spite of the already extensive literature, the typical itin- by magnetron sputtering on MgO Ref. 6 or by molecular erant magnetism of chromium remains a topic of broad in- beam epitaxy MBE on Nb/Al2O3,7 suggest that very thin terest. It is well known1 that bulk chromium orders, at the chromium layers are not paramagnetic but rather adopt the NeŽel temperature TN 311 K, as an incommensurate spin- commensurate antiferromagnetic structure below the critical density-wave SDW antiferromagnet with wave vector chromium thickness. A problem arose also on the issue of QSDW 0.958(2 /a) along the 100 direction a lattice the SDW polarization. Neutron-diffraction experiments on constant, Q is defined as the propagation vector of the uncovered or Cu-capped chromium films approximately 300 SDW . Its polarization changes from S Q in the transverse nm thick find a longitudinal out-of-plane SDW as observed SDW phase (AF1) above the spin-flip temperature in the PAC study. When the film is covered by a 2.0 nm (TSF 123 K) to S Q in the longitudinal SDW phase (AF2) ferromagnetic i.e., Fe capping layer the Q vector reorients below TSF . The crystal structure above TN is body centered into in plane but the magnetic moments still point out of cubic, but transforms for a single Q sample from orthorhom- plane. For relatively thick chromium layers, the Fe and Cr bic to tetragonal by cooling through TSF . The commensurate moments are thus oriented perpendicular to each other as antiferromagnetic structure (AF0), in which the spins at the was also reported for Fe/Cr on MgO.4 However in contrast to corner and at the center of the bcc unit cell are of equal the PAC study, for Fe/Cr multilayers with Cr thickness be- magnitude but point in opposite directions, may be stabilized tween 5.1 nm and 19 nm neutron diffraction data7 indicate a in alloys. This phase was incorporated also in the magnetic transverse SDW with an out-of-plane Q vector and spins in phase diagram of strained chromium.2 plane. Recently it became possible to produce high quality epi- In view of the above controversy, new data to illustrate taxial layers and superlattices. Most novel properties of the the compatibility of the various methods in determining the interlayer coupled systems were first discovered in the Fe/Cr character of the Cr magnetic ordering are imperative. The multilayers. This progress activated the research of the chro- perturbed-angular-correlation spectroscopy technique on mium magnetism in reduced dimension. Perturbed-angular- bulk polycrystalline chromium was found8 to reproduce the correlation PAC spectroscopy on ion implanted Fe/Cr mul- important transitions revealed by neutron diffraction studies. tilayers gave the first evidence3,4 that the Cr layers lose the However, up to now no direct comparison between the two bulk Cr spin-density-wave magnetism for thickness below techniques has been done on single crystalline, single-Q 5.0 nm. Moreover, while the magnetization in the Fe layer is samples. Therefore the PAC method has been applied to along the in-plane 010 or 001 axes, for Cr-thickness study the same chromium films as used for the neutron dif- above 7.5 nm the observed polarization of the spin-density fraction experiments. The samples were grown9 by MBE to a wave corresponds to chromium spins along the 100 direc- thickness of 250 nm film and 25 nm multilayer: tion normal to the layers,4 thus perpendicular to the Fe spins. 2 nm Fe/25 nm Cr 10 on the 50 nm Nb/Al2O3 11 02 This type of ordering has been seen in Cr films grown on buffer/substrate system. The epitaxial relationship was MgO up to a thickness of at least 40 nm, irrespective of the checked9,10 during the growth by in-situ reflective high- presence of Fe.4 energy electron diffraction RHEED spectroscopy and con- Transport measurements on sputtered epitaxial Fe/Cr firmed by x-ray diffraction measurements. superlattices5 confirmed the suppression of magnetic order- Transport measurements are frequently used to determine ing below a Cr thickness of 4.2 nm. On the other hand, the antiferromagnetic ordering temperature in chromium and neutron diffraction experiments on Fe/Cr multilayers, grown chromium alloys.1 A clear magnetic contribution to the re- 0163-1829/98/57 10 /5575 4 /$15.00 57 R5575 © 1998 The American Physical Society RAPID COMMUNICATIONS R5576 J. MEERSSCHAUT et al. 57 The PAC technique4 identifies, through the hyperfine in- teraction parameters, the microscopic surrounding of a nuclear probe in this case 111Cd . Each probe environment is characterized by a Larmor frequency, , proportional to the local hyperfine field. Because a diamagnetic probe such as Cd senses a transferred hyperfine field only, the experi- ment is sensitive to the magnetic moments in the near envi- ronment of the probe. In the measurements, the Larmor pre- cession frequency may occur in the spectra together with its second harmonic. The relative amplitude of both frequencies is determined by the direction of the hyperfine field relative to the detector geometry. One expects the magnetic hyperfine field to be collinear with the local magnetization of the matrix.4 The polarization of the SDW then follows immedi- ately and unequivocally from a PAC experiment. The propa- gation vector Q cannot be determined directly. However, in Ref. 4 we deduced the type of antiferromagnetic ordering AF1 versus AF2 from the hyperfine field value, the latter being enhanced in the longitudinal phase.8 By the present experiments we will prove that indeed the S as well as the Q orientation can be derived from the observed hyperfine field. The experimental PAC time spectra are fitted to the ex- FIG. 1. The temperature dependence of the resistivity and the pression derivative of the resistivity for the 250 nm Cr/ 50 nm Nb/Al2O3 11 02 film. The NeŽel temperature is determined by the inflection point in the d vs dT curve at (330 10) K. R t f CSDW ane n Ct cos n Ct n 0,2 sistivity, , is observed below the paramagnetic- antiferromagnetic phase transition temperature, TN . Also in thin films this method can be applied to determine the phase f ISDW bne n ItJ0 n It 1 n 0,2 transition temperature.5 Figure 1 a shows the resistivity vs temperature for the present 250 nm Cr film, measured using to yield the magnitudes of the hyperfine fields and their rela- the standard four-contact dc method. The NeŽel temperature is tive fractions. The first term accounts for a commensurate found by the inflection point in the d /dT vs T curve at antiferromagnetic (AF0) ordering described by a frequency (330 10) K, as shown in Fig. 1 b , where the resistivity component with mean value C and Lorentzian distribution amounts to 10.08 cm. Similar results are obtained for the width C . The second term accounts for an incommensu- 2 nm Fe/25 nm Cr 10 multilayer. As observed earlier,4 the rate SDW antiferromagnetic ordering AF1 or AF2 . The resistivity anomaly in the Cr layers is considerably broad- zero-order Bessel function, J0(n It), results from the Over- ened compared with the singularity in observed at TN in a hauser distribution1,4 in the hyperfine field. chromium single crystal. Comparison with the results of For the PAC experiments the nuclear probe 111In 111Cd Geerkens11 suggests that the pronounced anomaly at 330 K is incorporated in the sample in trace quantities by ion im- indicates a bulklike antiferromagnetic-to-paramagnetic phase plantation at an energy of 80 keV. For these implantation transition, rather than a transition from the incommensurate conditions, the implantation profile of the 111In probes as to commensurate magnetic phase. estimated by the TRIM code Ref. 14 has a projected range of The present samples were characterized7 by synchrotron 18 nm and a longitudinal straggling of 7 nm. In the x-ray scattering and high-angle neutron-diffraction experi- multilayer sample, therefore, we do not expect to observe a ments. The combination of both techniques accurately significant signal due to probes in the Fe environment. The determines7 the spin orientation and the Q vector of the PAC time spectrum was measured simultaneously in two SDW, however not yet simultaneously the magnetic moment detector geometries, different by the actual orientation of the and the absolute volume fraction of any coexisting phases. In sample normal the 100 direction relative to the plane of the 250-nm-thick chromium single film, a dominant longitu- the detectors. In the perpendicular geometry, the sample nor- dinal out-of-plane spin-density wave has been observed12 at mal is perpendicular to the detector plane, whereas in the low temperatures, together with a minority out-of-plane in-plane geometry the sample normal points at 45° in be- transverse incommensurate phase. At temperatures above tween the detectors.4 330 K the incommensurate spin-density wave flips, at least The PAC time spectra measured at 77 K on the 250-nm- partially, to the commensurate AF0 phase with the spins out thick Cr layer together with their simulation according to Eq. of plane.13 For the Fe/Cr multilayer, a transverse SDW with 1 are shown in Fig. 2. The main contribution to the experi- in-plane spins is observed, associated with a minority AF0 mental spectrum explicitly shown is well reproduced by a spins in plane phase. At temperatures above 330 K the Cr distribution of the Overhauser type second term in Eq. 1 layers in the 2 nm Fe/25 nm Cr 10 superlattice are in a para- and thus corresponds to an incommensurate spin-density magnetic state coexisting with a commensurate phase with wave. In the 250-nm-thick chromium film we observe a the spins in plane. single frequency for this fraction in both geometries. In the RAPID COMMUNICATIONS 57 SPIN-DENSITY-WAVE MAGNETISM IN LAYERED . . . R5577 FIG. 2. Comparison of the PAC spectra measured in two detec- tor geometries for a and b the 250 nm Cr/ 50 nm Nb/Al2O3 11 02 film, c and d the 2 nm Fe/25 nm Cr 10 / 50 nm Nb/Al2O3 11 02 multilayer. The spectra on the left correspond to the out-of- plane geometry; the spectra on the right to the in-plane geometry. The main contribution arising from the chromium incommensurate SDW has been explicitly shown. A minor contribution is due to a volume fraction in a commensurate SDW ordering. perpendicular Fig. 2 a and in-plane geometry Fig. 2 b we observe the second and the first harmonic, respectively. This immediately proves that the Cr hyperfine field is ori- ented along the 100 direction normal to the Cr layer. In contrast, the PAC time spectra for the 2 nm Fe/25 nm Cr 10 multilayer do not correspond to an out-of-plane spin orienta- tion. The precession pattern obtained in the perpendicular geometry Fig. 2 c is approached using the first harmonic FIG. 3. Temperature dependence of the PAC time spectra for only, while the in-plane geometry Fig. 2 d leads to a su- the 250 nm Cr/ 50 nm Nb/Al2O3 11 02 film in the in-plane geom- perposition of the single and double frequency. Therefore, etry measured at a 77 K, b 250 K, c 325 K, and d 400 K. The the hyperfine field in the 2 nm Fe/25 nm Cr 10 multilayer is thin lines represent the incommensurate SDW chromium contribu- oriented in plane. This result, in conjunction with the evi- tion. dence from neutron-diffraction experiments for perpendicu- lar spin orientation in the Cr thin film and in-plane spin chromium8 and thus indicates the presence of the transverse spin-density-wave phase, in agreement with the neutron- orientation in the Fe/Cr multilayer, explicitly proves the col- diffraction experiments. Therefore, the type of magnetic or- linearity of the hyperfine field with the local magnetization dering of the spin-density wave AF of the matrix. We thus prove that perturbed-angular- 1 or AF2 can thus be deduced from the hyperfine field value. correlation spectroscopy unequivocally determines the spin The spectra measured at different temperatures on the polarization of the SDW. 250-nm-thick Cr film are shown in Fig. 3. The main contri- The interaction frequency in the chromium thin film for bution to the spectra is explicitly shown. Clearly, the pro- the majority contribution at 77 K is 14.3 2 MHz, which nounced slow precession pattern due to the SDW observed at agrees closely with the value observed at 77 K in bulk low temperatures disappears at 325 K. At high temperatures, chromium8 and in Fe/Cr multilayers.4 This hyperfine field we observe a rather slow decay corresponding to probes in value thus corresponds to the longitudinal spin-density-wave paramagnetic bcc chromium with a weak nuclear electric structure. Comparison with the neutron diffraction data fur- quadrupole interaction due to defects or strain in the crystal. ther establishes the fact that indeed an enhanced hyperfine Therefore we do not observe an incommensurate- field represents a longitudinal spin-density-wave phase. The commensurate transition at 330 K. On the contrary, above frequency measured at 77 K in the present 325 K the majority of the probes is in an environment with 2 nm Fe/25 nm Cr 10 multilayer is, however, markedly re- vanishing moments indicating that the majority volume frac- duced and has a value of 12.1 2 MHz. This hyperfine field tion becomes paramagnetic, consistent with the resistivity value is comparable with the value obtained at 250 K in bulk measurement Fig. 1 . RAPID COMMUNICATIONS R5578 J. MEERSSCHAUT et al. 57 In the PAC time spectrum obtained at 400 K, a minority rection of the Q vector itself, the polarization of the spin- fraction is clearly visible as the rapid precession superim- density wave can be determined from the hyperfine field posed on the slow signal. This high-frequency modulation value. The present PAC experiments agree with the neutron- corresponds to a commensurate SDW fraction which is defi- scattering data and indeed confirm the longitudinal incom- nitely also present at lower temperatures Fig. 3 . We obtain mensurate spin-density-wave ordering with the spins out of from the f ISDW and fCSDW fractions in Eq. 1 , independent plane for the single-layer 250 nm Cr film, while for the Fe/Cr of the magnetic moments and temperature, the relative vol- multilayer a transverse SDW with the spins in plane is ob- ume fractions of 85 5 % incommensurate and 15 5 % com- served. This consistency proves that the radioactive probes mensurate chromium. The minority fraction reflects a sub- used in perturbed-angular-correlation spectroscopy indeed stantially enhanced Cr magnetic hyperfine field of 21.1 2 sense the intrinsic chromium magnetism. By virtue of this MHz corresponding to a chromium magnetic moment result we further support the existence of a longitudinal 0.90(2) B . In the temperature interval studied, the hy- perfine field due to the minority fraction remains nearly un- SDW with single Q normal to the layers for Cr thickness changed. above 5.0 nm in Fe/Cr superlattices, MBE grown on MgO, The commensurate antiferromagnetic structure found here as reported earlier.4 Interestingly, in the 2 nm Fe/ has earlier been identified by neutron diffraction7 and shown 25 nm Cr]10 multilayer grown for neutron diffraction experi- to be present in Cr films to a volume fraction depending on ments, a transversal SDW with in-plane Cr spins is present. thickness and temperature. In single crystalline bulk chro- In addition, we have recognized the presence of the commen- mium this phase is not present, whereas in strained polycrys- surate antiferromagnetic phase as an enhanced hyperfine talline chromium, the neutron diffraction experiments of Ref. field. Further complementary study therefore should aim to 2 could not distinguish between the AF explain the difference in both types of Fe/Cr multilayers. 0 or the AF2 phases. We have shown that perturbed angular correlation spectros- copy is definitely sensitive to the commensurate antiferro- The authors are very much indebted to Professor H. Zabel magnetic phase, but this phase was not disclosed in our and Dr. A. Schreyer Institut fušr Experimentalphysik/ Fe/Cr multilayers grown on MgO studied previously.4 Fur- Festkošrperphysik, Ruhr-Universitašt Bochum for their col- ther investigations are necessary to clarify the origin of this laboration by bringing the samples for the present experi- commensurate phase apparently present in the samples ments to our disposal and for helpful discussions. We thank grown for neutron diffraction experiments. Dr. K. Freitag Institut fušr Strahlen- und Kernphysik, Uni- In conclusion, we have shown that PAC experiments de- versitašt Bonn for his kind cooperation in the ion implanta- termine the magnetic state of chromium and particularly the tions. This work was financially supported by the FWO/ Cr spin orientation. Although PAC is insensitive to the di- G.013795 and GOA/94.2 projects. 1 E. Fawcett, Rev. Mod. Phys. 60, 209 1988 ; E. Fawcett, H. L. 8 R. Venegas, P. Peretto, G. N. Rao, and L. Trabut, Phys. Rev. B Alberts, V. Yu. Galkin, D. R. Noakes, and J. V. Yakhmi, ibid. 21, 3851 1980 . 66, 25 1994 . 9 P. Sonntag, P. Bošdeker, T. Thurston, and H. Zabel, Phys. Rev. B 2 G. E. Bacon and N. Cowlam, J. Phys. C 2, 238 1969 . 52, 7363 1995 . 3 10 B. Swinnen, J. Meersschaut, J. Dekoster, and M. Rots, J. Magn. P. Bošdeker, P. Sonntag, A. Schreyer, J. Borchers, K. Hamacher, Magn. Mater. 140-144, 543 1995 . H. Kaiser, and H. Zabel, Physica B 234-236, 464 1997 ; P. 4 Bošdeker, P. Sonntag, A. Schreyer, H. 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