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We use transport and inelastic neutron scattering measurements to investigatesingle crystals of iron pnictide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03$),which exhibit a tetragonal-to-orthorhombic structural transition at $T_s$ andstripe antiferromagnetic order at $T_N$ ($T_s\geq T_N$).  Using a tunable uniaxial pressure device, we detwin the crystals and studytheir transport and spin excitation properties at antiferromagnetic wave vector$S_1(1,0)$ and its 90$^\circ$ rotated wave vector $S_2(0,1)$ under differentpressure conditions. We find that uniaxial pressure necessary to detwin andmaintain single domain orthorhombic antiferromagnetic phase ofBaFe$_{2-x}$Ni$_{x}$As$_{2}$ induces resistivity and spin excitation anisotropyat temperatures above zero pressure $T_s$. In uniaxial pressure-free detwinnedsample, spin excitation anisotropy between $S_1(1,0)$ and $S_2(0,1)$ firstappear in the paramagnetic orthorhombic phase below $T_s$. These results areconsistent with predictions of spin nematic theory, suggesting the absence ofstructural or nematic phase transition above $T_s$ in iron pnictides.

Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe$_{2-x}$Ni$_x$As$_2$