Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2−xNixAs2

Abstract

We use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}\phantom{\rule{0.28em}{0ex}}(x=0,0.03)$, which exhibit a tetragonal-to-orthorhombic structural transition at ${T}_{s}$ and stripe antiferromagnetic order at ${T}_{N}\phantom{\rule{0.28em}{0ex}}({T}_{s}\ensuremath{\ge}{T}_{N})$. Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector ${S}_{1}(1,0)$ and its ${90}^{\ensuremath{\circ}}$ rotated wave-vector ${S}_{2}(0,1)$ under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}$ induces resistivity and spin excitation anisotropy at temperatures above zero pressure ${T}_{s}$. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between ${S}_{1}(1,0)$ and ${S}_{2}(0,1)$ first appears in the paramagnetic orthorhombic phase below ${T}_{s}$. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above ${T}_{s}$ in iron pnictides.