Ultrahin iron films are possible candidates for future magnetic recording media. For epitaxial iron films grown on Cu(100) stabilization of the face-centered cubic (fcc) ã-phase (which is non magnetic, more precisely, paramagnetic) can be achieved at room temperature. This is in contrast to bulk iron for which the fcc phase is stable only above 1184 K, whereas below this temperature the body-centered cubic (bcc) á-phase (which is ferromagnetic) is thermodynamically stable. The thickness of these nonmagnetic fcc iron films is for pure iron limited to 5 to 10 monolayers, corresponding to about 0.9–1.8 nm. Films with a thickness below and above this range will exhibit a spontaneous structural transition to a strained or relaxed body-centered structure respectively.
We have shown by STM (Scanning tunneling microscopy) with atomic resolution [1] that very few nano-sized nucleation centers of the bcc phase in such 5-10 ML thick fcc epitaxial Fe/Cu(100) films are present from the beginning on. This few martensitic nuclei, in which the transition to the bcc phase has already taken place, typically have a needle-like shape of very small lateral width (typically about 2 nm).
We could show that in this fcc ultra thin Fe layers the phase transition from the mainly fcc (face centered cubic) phase to the bcc (body centered cubic) phase can be induced and promoted by keV ion beam irradiation.
Since iron is ferromagnetic only if it is bcc or at least strained bcc but never if it is fcc, this ion induced transition can be also seen by measuring the magnetic moment. This was done by using SMOKE (Surface Magneto-Optical Kerr Effect) in situ.
Examples to form magnetic nanostructures with the resolution given by the dimension of the ion beam will be discussed in this talk.
[1] [A. Biedermann et al. Phys. Rev. Lett. 86 (2001), pp. 464–467
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