Magnetic field tuning of low-energy spin dynamics in the single-atomic magnet Li2(Li1−xFex)N

Abstract

We present a systematic ${}^{57}\mathrm{Fe}$ M\"ossbauer study on highly diluted Fe centers in ${\mathrm{Li}}_{2}({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{N}$ single crystals as a function of temperature and magnetic field applied transverse and longitudinal with respect to the single-ion anisotropy axis. Below 30 K, the Fe centers exhibit a giant magnetic hyperfine field of ${\overline{B}}_{A}=70.25(2)\phantom{\rule{4pt}{0ex}}\mathrm{T}$ parallel to the axis of strongest electric field gradient ${\overline{V}}_{zz}=\ensuremath{-}154.0(1)\phantom{\rule{4pt}{0ex}}\mathrm{V}/{\AA{}}^{2}$. Fluctuations of the magnetic hyperfine field are observed between 50 and 300 K and described by the Blume two-level relaxation model. From the temperature dependence of the fluctuation rate, an Orbach spin-lattice relaxation process is deduced. An Arrhenius analysis yields a single thermal activation barrier of ${\overline{E}}_{A}=570(6)\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and an attempt frequency ${\overline{\ensuremath{\nu}}}_{0}=309(10)\phantom{\rule{4pt}{0ex}}\mathrm{GHz}$. M\"ossbauer spectroscopy studies with applied transverse magnetic fields up to 5 T reveal a large increase of the fluctuation rate by more than one order of magnitude. In longitudinal magnetic fields, a splitting of the fluctuation rate into two branches is observed consistent with a Zeeman induced modification of the energy levels. The experimental observations are qualitatively reproduced by a single-ion effective spin Hamiltonian analysis assuming a ${\mathrm{Fe}}^{1+}\phantom{\rule{4pt}{0ex}}{d}^{7}$ charge state with the unquenched orbital moment and a $J=7/2$ ground state. It is demonstrated that a weak axial single-ion anisotropy $D$ of the order of a few Kelvin can cause a two orders of magnitude larger energy barrier for longitudinal spin fluctuations.