  We present a systematic 57Fe-Moessbauer study on highly diluted Fe centers in
Li2(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 E_A
= 70.25(2)T parallel to the axis of strongest electric field gradient Vzz =
-154.0(1) V/A2. Fluctuations of the magnetic hyperfine field are observed
between 50K and 300K and described by the Blume two-level relaxation model.
From the temperature dependence of the uctuation rate an Orbach spin-lattice
relaxation process is deduced. An Arrhenius analysis yields a single thermal
activation barrier of E_A = 570(6)K and an attempt frequency nu_0 = 309(10)
GHz. Moessbauer spectroscopy studies with applied transverse magnetic fields up
to 5T reveal a large increase of the uctuation rate by more than one order of
magnitude. In longitudinal magnetic fields a splitting of the uctuation rate
into two branches is observed consistent with a Zeeman induced modifcation of
the energy levels. The experimental observations are qualitatively reproduced
by a single-ion effective spin Hamiltonian analysis assuming a Fe1+ d7 charge
state with 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.
