Abstract
We discuss the nature of ferromagnetism in ultrathin films of magnetic ions, here regarded as two-dimensional Heisenberg ferromagnets subject to uniaxial anisotropy with the easy axis normal to the film. We show that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy. Renormalization-group scaling relations for the transition temperature and the temperature variation of the correlation length are obtained. Implications of these results are discussed.
Highlights
We discuss the nature of ferromagnetism in uItrathin films of magnetic ions, here regarded as two-dimensional Heisenberg ferromagnets subject to uniaxial anisotropy with the easy axis normal to the film
We show that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy
The nature of the magnetism in such ultrathin films is of fundamental interest, since it may differ from that realized in bulk single crystals by virtue of their reduced dimensionality
Summary
The recent Mossbauer study of monolayer Fe films described by Koon et al 2 shows that at and considerably below room temperature, large-amplitude, slow-spin fluctuations are present. These are evident as broadening of the spectra; resolvable structure appears only below room temperature. These results are consistent with the absence of long-range ferromagnetic order in the spin system. This question was answered in the affirmative by Khokhlachev, within a mean-field approach This issue was discussed by Pelcovits and Nelson, within the framework of a renormalizationgroup treatment that outlines the nature of the phase transition, upon assuming that a 6xed point exists. [K(ao)/Jao2] [T(ao)/J] 2iiN are invariant under changes of ao
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