Bcc iron is known as an archetypal collinear ferromagnet. Deviations from this fundamental magnetic structure are expected for ultrathin Fe films as a consequence of symmetry breaking at surfaces or interfaces as well as of magnetoelastic effects originating from the misfit between the Fe film and the substrate. Recently, we have shown that, under certain conditions, collinear ferromagnetism becomes less favourable than a spin-spiral-like, vertically-inhomogeneous magnetisation state. This happens at the spin reorientation transition (SRT) in Fe films grown on W(110).

The SRT in the case of Fe/W(110) consists in the switching of spontaneous magnetisation during the film growth from the [10] to the [001] in-plane direction as the iron film approaches the critical thickness, dc. Our thickness-induced SRT was monitored in situ using grazing incidence nuclear resonant scattering (NRS) of synchrotron radiation [1]. The numerical analysis of the NRS data indicated that a non-collinear magnetisation structure is formed in the vicinity of the critical thickness, with a strong surface magnetisation pinning along the [10] direction. With increasing thickness, the transition is initiated at the bottom atomic layers, neighbouring with the tungsten substrate, and finally is completed at the surface layer.

The measurements were done at the Nuclear Resonance beamline ID18 [2]. 57Fe was deposited on a freshly cleaned W(110) surface. Directly during the preparation, the NRS time spectra were collected in thickness steps corresponding to a fraction of a Fe monolayer. The deposition of Fe was not interrupted from the beginning up to the completion of the SRT process, and the spectra were accumulated in situ during the film growth. The fitted time spectra are shown in Figure 18 for selected Fe film thicknesses. A regular beat structure that is exemplified in Figure 18a reflects, according to the theoretical fits, the uniform magnetisation state along the [10] direction, with the hyperfine magnetic field close to bulk Fe (BHF = 32.9 T). Such a state persists up to a thickness of about δ = 51 Å. The spectra for coverages larger than 56 Å (see Figure 18f) can be easily fitted assuming a homogeneous magnetisation, but now parallel to the [001] direction. It is clear that the SRT process is not abrupt but extends over a relatively large thickness range of δ ~ 6 Å, corresponding to three monolayers. The most unique and also challenging time spectra to fit were those accumulated during the progress of the SRT (Figure 18,b-e). The two most commonly considered ways of the magnetisation transition from [10] to [001]: (i) coherent rotation and (ii) coexistence of [001] and [10] magnetised domains, assuming a homogeneous magnetisation depth profile across the Fe(110) films, produced distinctly different spectra, however, neither of them could satisfactorily fit the experimental data in the transition.

Fig. 18: Time spectra of nuclear resonant scattering in the vicinity of the SRT accumulated during continuous Fe evaporation. The corresponding Fe thicknesses d are labelled on the right.

The successful fits (Figure 18) could be obtained when the distribution of the magnetisation directions was modelled by dividing the film with thickness d into five sub-layers of equal thicknesses. For each sub-layer, an in-plane orientation of the hyperfine magnetic field (sub-layer magnetisation N) was defined by the angle φN with respect to the [10] in-plane direction. The orientation of the sub-layer magnetisations 1–5, as derived from the analysis of the NRS data, is shown schematically in Figure 19 for various thicknesses d of the Fe film. The onset of the transition was noticed for a thickness of 51.6 Å. The SRT from [10] to [001] is initiated at the deepest layers, which switch first, while the magnetisation of the remaining sub-layers forms a fan-like structure. With increasing thickness, the magnetisation of the subsequent sub-layers rotates, and finally the transition is completed by the top-most surface layers.

Fig. 19: Magnetisation structure of the epitaxial Fe/W(110) film during the thickness-induced SRT as derived from NRS measurements using a five sublayer model. The sublayer magnetisation vectors are labelled as  1–5.

Our studies clearly show that a non-collinear, exotic magnetic phase of epitaxial Fe films is stabilised at the vicinity of a critical SRT thickness.


Principal publication and authors

T. Ślęzak (a), M. Ślęzak (a), M. Zając (a.b), K. Freindl (a,c), A. Kozioł-Rachwał (a), K. Matlak (a), N. Spiridis (c), D. Wilgocka-Ślęzak (c), E. Partyka-Jankowska (d), M. Rennhofer (d,e), A.I. Chumakov (b), S. Stankov (b), R. Rüffer (b) and J. Korecki (a,c), Phys. Rev. Lett. 105, 027206 (2010).
(a) AGH University of Science and Technology, Kraków (Poland)
(b) ESRF
(c) Polish Academy of Sciences (Poland)
(d) University of Vienna (Austria)
(e) AIT - Austrian Institute of Technology (Austria)


[1] E. Gerdau and H. DeWaard, Hyperfine Interact. 123-124 (1999).
[2] R. Rüffer and A.I. Chumakov, Hyperfine Interact. 97-98, 589 (1996).