Paramagnetic spin correlations in CaFe₂As₂ single crystals

Magnetic correlations in the paramagnetic phase of CaFe₂As ₂ (TN =172K) have been examined by means of inelastic neutron scattering from 180 K (∼1.05 TN) up to 300 K (1.8 TN). Despite the first-order nature of the magnetic ordering, strong but short-ranged antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wave vector QAFM of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (ω>60meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt-doped superconducting BaFe2 As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-ranged and anisotropic spin correlations with overdamped dynamics.