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Abstract

We present a systematic Fe-57 Mossbauer study on highly diluted Fe centers in Li-2(Li1-xFex)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 (B) over bar (A) = 70.25(2) T parallel to the axis of strongest electric field gradient (V) over bar (zz) = -154.0(1) V/angstrom(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 (E) over bar (A) = 570(6) K and an attempt frequency (nu) over bar (0) = 309(10) GHz. Mossbauer 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 Fe1+ 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.