  Short-range magnetic correlations can significantly increase the thermopower
of magnetic semiconductors, representing a noteworthy development in the
decades-long effort to develop high-performance thermoelectric materials. Here,
we reveal the nature of the thermopower-enhancing magnetic correlations in the
antiferromagnetic semiconductor MnTe. Using magnetic pair distribution function
analysis of neutron scattering data, we obtain a detailed, real-space view of
robust, nanometer-scale, antiferromagnetic correlations that persist into the
paramagnetic phase above the N\'eel temperature $T_{\mathrm{N}}$ = 307 K. The
magnetic correlation length in the paramagnetic state is significantly longer
along the crystallographic $c$ axis than within the $ab$ plane, pointing to
anisotropic magnetic interactions. Ab initio calculations of the spin-spin
correlations using density functional theory in the disordered local moment
approach reproduce this result with quantitative accuracy. These findings
constitute the first real-space picture of short-range spin correlations in a
magnetically enhanced thermoelectric and inform future efforts to optimize
thermoelectric performance by magnetic means.

Materials Science