Pseudogap opening in single-crystal YMn4Al8 investigated through Mn55 and Al27 NMR measurements

Abstract

$^{55}\mathrm{Mn}$ and $^{27}\mathrm{Al}$ NMR measurements are performed on a single crystal of ${\mathrm{YMn}}_{4}{\mathrm{Al}}_{8}$, which is known as one of the rare examples of pseudogap opening in transition intermetallic compounds, to investigate the formation of this pseudogap. The NMR spectrum, Knight shift, linewidth, and spin-lattice relaxation rate $1/{T}_{1}$ are measured as functions of the temperature down to 4 K for the $c$ axis, which is parallel and perpendicular to a magnetic field of 8 T. The temperature dependencies of the Knight shifts and $1/({T}_{1}T)$ for both NMRs as well as the magnetic susceptibility show significant decrease as the temperature decreases below room temperature, indicating the opening of a pseudogap down to \ensuremath{\sim}100 K, where these data become nearly constant, confirming that the formation of the spin pseudogap is incomplete and a finite density of states remains below. By analyzing $^{55}K(T)$ and $^{27}K(T)$ thoroughly, we decompose the orbital and spin shifts for both Mn and Al sites, from which the anisotropies of the Korringa ratios are extracted. The Korringa ratios at the Mn sites are temperature dependent and highly anisotropic above 100 K, and then decrease approaching unity and become isotropic and constant below 100 K. From the temperature dependencies of $1/({T}_{1}T)$ for both NMRs, the pseudogap temperature, ${T}^{*}$ is found to be approximately 270 K. From the anisotropy of $^{55}1/({T}_{1}T)$ during the opening of the pseudogap, we observed that the antiferromagnetic spin fluctuation is anisotropic and stronger by up to $\ensuremath{\sim}4.2$ times in the basal plane perpendicular to the quasi-one-dimensional (1D) manganese chain. This significant anisotropy must originate from the 1D nature of the Mn magnetic interaction.