Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets

A theoretical study has been carried out to analyse the available results from the inelastic neutron scattering experiment performed on a quasi-two dimensional spin- \hbox{$\frac{1}{2}$} 1 2 ferromagnetic material K<Subscript>2</Subscript>CuF<Subscript>4</Subscript>. Our formalism is based on a conventional semi-classical like treatment involving a model of an ideal gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg ferromagnet on a square lattice. The results for dynamical structure functions for our model corresponding to spin- \hbox{$\frac{1}{2}$} 1 2 , show occurence of negative values in a large range of energy transfer even encompassing the experimental range, when convoluted with a realistic spectral window function. This result indicates failure of the conventional theoretical framework to be applicable to the experimental situation corresponding to low spin systems. A full quantum formalism seems essential for treating such systems. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2012