The itinerant antiferromagnetism in Cr has attracted considerable attention during the last 40 years. Its low-temperature behaviour is associated with the existence of spin-density-waves (SDWs) and charge-density-waves (CDWs) incommensurate with the bcc lattice periodicity of Cr. Whereas the SDW behaviour in bulk Cr is now well established[1], there are a good number of gaps in our understanding of its nature in Cr thin films and heterostructures [2]. The present research addresses Cr/V layered heterostructures where the SDW state is affected by the hybridisation between very similar Fermi surfaces of chromium and vanadium. We provide direct evidence for the polarisation and propagation direction of the SDWs and CDWs in Cr films being conditioned by the V/Cr interface effects.

The investigations were done from a series of Cr(2000 Å), Cr(2000 Å)/V(14 Å), and V(14 Å)/Cr(2000 Å)/V(14 Å) layered heterostructures grown under the same conditions on MgO(001) substrates. The SDW and CDW behaviour was determined by combined X-ray and neutron scattering studies. The neutron measurements were conducted at D10 (ILL) and UNIDAS (FZ Jülich) instruments, the X-ray measurements were conducted at the ID20 beamline. We have measured the temperature dependence of satellite peaks around the (011) and (002) Bragg reflections corresponding to the CDW positions. Figure 115 shows a typical example with the temperature dependence of the CDW satellite peaks measured in the K direction on both sides of the (011) Cr fundamental peak in the Cr(2000 Å)/V(14 Å) heterostructure. In this case the SDW and CDW propagate in the film plane. As evident from the picture, the satellites positions move smoothly towards the (011) peak as temperature increases, while the intensity decreases continuously with temperature, which are the hallmarks for a phase transition from the low temperature SDW to a high temperature paramagnetic phase.


Fig. 115: Temperature dependence of CDW satellite reflections in Cr/V film along the K direction around the Cr(011) reflection taken with polarisation analysis of the scattered beam. The fundamental Cr(011) peak is removed from the figure since its intensity is many orders of magnitude higher than the intensity of the satellite reflections.


The synchrotron and neutron scattering results on the SDW behaviour within the Cr/V heterostructures are summarised schematically in Figure 116. In thin Cr/V films, the SDW is confined and therefore the spontaneous formation of the SDW due to the Fermi surface nesting competes with hybridisation effects of adjacent vanadium layers, with interfacial structures (order and disorder), and with dimensionality effects (film thickness). The SDW propagation in the plain Cr(2000 Å) film on MgO(001) is conditioned mainly by the dimensionality effects and an in-plane epitaxial tensile stress from the substrate. The SDW and CDW propagate here in the film plane. The Cr spins are aligned in the film plane at low temperatures (longitudinal phase) and out-of-plane at higher temperatures (transverse phase). The Néel temperature for the SDWs is estimated to be about 300 K that is close to the bulk value. The addition of a neighbouring V layers initiates new mechanisms competing with the ones above. We found that the addition of one neighbouring thin V layer in the Cr/V heterostructures leads to a change in the polarisation of the transverse SDW from the out-of-plane to in-plane direction. The addition of the second V neighbouring layer changes the SDW propagation direction, such that the SDW in the V/Cr/V systems propagates out-of-plane.



Fig. 116: Schematical pictures of the SDW behaviour in Cr, Cr/V and V/Cr/V heterostructures as determined by combined X-ray and neutron experiments.


In conclusion, we have studied the proximity effect of vanadium layers on spin density waves in Cr/V heterostructures grown on MgO(001) substrates. The Cr-V interface hybridisation produces long-range re-orientational effects in Cr/V multilayers. The vanadium proximity hybridisation determines the SDW polarisation in Cr/V systems as well as the SDW propagation direction in V/Cr/V systems. To the best of our knowledge, there is no theoretical consideration of orientational effects caused by the hybridisation of Cr with neighbouring paramagnetic layers. The strong re-orientational effects observed are unexpected and need to be explained by theory.

[1] E. Fawcett, Rev. Mod. Phys. 60, 209 (1988).
[2] H. Zabel, J. Phys.: Condens. Matter 11, 9303 (1999).

Principal Publications and Authors
E. Kravtsov (a,b), A. Nefedov (a), F. Radu (a), A. Remhof (a), H. Zabel (a), R. Brucas (c), B. Hjörvarsson (c), A. Hoser (d,e), S.B. Wilkins (f,g), Phys. Rev. B 70, 054425 (2004); E. Kravtsov (a,b), A. Nefedov (a), R. Brucas (c), B. Hjörvarsson (c), A. Hoser (d,e), G. McIntyre (h), H. Zabel (a), J. Magn. Magn. Mater., in press.
(a) Institut für Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum (Germany).
(b) Institute of Metal Physics, Ekaterinburg (Russia).
(c) Department of Physics, Uppsala University (Sweden).
(d) Institut für Kristallographie, RWTH-Aachen (Germany).
(e) Institut für Festkörperforschung, Forschungszentrum Jülich (Germany).
(f) European Commission, JRC, Institute for Transuranium Elements, Karlsruhe (Germany).
(g) ESRF
(h) ILL