Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Magnetic correlations in the high temperature paramagnetic phase of CaFe$_{2}$As$_{2}$ ($T_N=172$ K) have been examined by means of inelastic neutron scattering from $180$ K up to 300 K. Despite the first-order nature of the magnetic ordering, strong but short-ranged anti-ferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wavevector $\mathbf{Q}_{AFM}$ of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (above 60 meV). The weak $L$ dependence of the scattering implies rather weak interlayer coupling corresponding to nearly two-dimensional fluctuations above $T_N$. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. These experimental features can be adequately reproduced by a scattering model $S(\mathbf{Q},\hbar\omega$) that describes short-ranged aniostropic spin correlations with overdamped spin dynamics.