Photoemission study of the spin-density-wave state in thin films of Cr

Abstract

Angle-resolved photoemission (PE) was used to characterize the spin-density wave (SDW) state in thin films of Cr grown on W(110). The PE data were analyzed using results of local spin density approximation layerKorringa-Kohn-Rostoker calculations. It is shown that the incommensurate SDW can be monitored and important parameters of SDW-related interactions, such as coupling strength and energy of collective magnetic excitations, can be determined from the dispersion of the renormalized electronic bands close to the Fermi energy. The developed approach can readily be applied to other SDW systems including magnetic multilayer structures.Bulk Cr is an almost unique material revealing itinerant antiferromagnetism with a spin-density wave (SDW) ground state at room temperature. 1 At the Néel temperature T N = 311 K chromium exhibits a transition from a paramagnetic (bcc lattice) to an antiferromagnetic (AF) order (sc of CsCl type) that is modulated by the incommensurate SDW along the ͗100͘ directions. 1-3 Thereby the periods of the AF arrangement and the SDW oscillations amount to 2 and ϳ21 monolayers (ML) of Cr, respectively. It is widely accepted that the AF order (although this order is often referred to as the commensurate SDW, everywhere in the following we will use the term SDW to denote the incommensurate SDW) is caused by a nesting of the Fermi surface (FS) sheets around the ⌫ and the H points of the Brillouin zone (BZ) of bcc Cr, while the nature of the incommensurate SDW is still the subject of extensive debates. 4 The SDW in Cr is accompanied by a strain wave and a charge-density wave (CDW) with half the period of the SDW 5 as well as by a series of collective excitations including spin waves (magnons) and phonons. 1 Electron interactions particularly with the magnetic excitations lead to renormalization of the electronic structure of the ground state. Although a number of attempts were made to study the renormalization of the electronic bands in some Cr systems 1,6 the subject requires further investigations.A detailed understanding of the SDW and the SDWrelated phenomena in Cr is of great importance, since the above short-and long-range magnetic modulations give much reason to use Cr as spacers in magnetic multilayer structures providing giant magnetoresistance, spin-valve effect and applications in magnetic sensor technology. 7 One of the mostly investigated systems Fe/ Cr/ Fe͑100͒ shows that the ferromagnetic or AF type of coupling between Fe layers varies with thickness of Cr spacer following the short period, whereas the strength of the coupling changes with the long period of oscillations. [8][9][10][11] The description of the SDW in thin films is complicated by the fact that the boundary conditions at the interfaces have to be properly considered. In the density-functional theory (DFT) study of Fe/ Cr/ Fe͑100͒ by Niklasson et al. 12 mainly AF order was found for Cr spacers with thicknesses Ͻ10 ML. For thicker layers, various branches of sometimes coexisting SDWs, which differ from each other by the n...

Surface states and spin density wave periodicity in Cr(110) films

“…Similar to Mo(110) and W(110) [16,18,20], the Fermi contour of this band undergoes a topological transition upon hydrogen adsorption. We compare our measured dispersion relations to existing calculations [21,22] and find that some bands are well-described by theory, while others are not. …”

“…Two recent first-principles calculations of the electronic structure of clean Cr(110) slabs can distinguish surface-localized states long the¯ →S direction [21,22]. In one of these, Schiller et al [22], include more chromium layers and distinguish the surface bands more completely, so we compare our results to that calculation.…”

“…As discussed previously [14], angular scans at this photon energy probe along a spherical surface through the electron and hole octahedra of the bulk chromium Fermi surface. As indicated in figure 2, the¯ →S line projects states from a plane passing through the flat faces of the -centered electron octahedral segment of the bulk chromium Fermi surface [14,[22][23][24][25]. Along 100 -directions, these flat faces are well-nested to the faces of H-centered hole octahedral segments, and this nesting drives the incommensurate bulk chromium SDW [26]- [28].…”

“…In one of these, Schiller et al [22], include more chromium layers and distinguish the surface bands more completely, so we compare our results to that calculation. The calculation assumed a commensurate SDW phase, which corresponds to a simple cubic bulk unit cell, though results are plotted in the bcc SBZ.…”

“…Also included are calculated surface band dispersions [22] (red lines), and a projection of the bulk band structure of Papaconstantopoulos (gray) [25]. While the surface calculation predicts the dispersion relations of band A and the feature SR near the zone center at the Fermi level fairly well, the calculation does not correctly place SR or S 1 + S 2 on clean Cr(110).…”