Extreme Field Sensitivity of Magnetic Tunneling in Fe-Doped ${\mathrm{Li}}_3\mathrm{N}$

The magnetic properties of dilute ${\mathrm{Li}}_2({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x)\mathrm{N}$ with $x\sim 0.001$ are dominated by the spin of single, isolated Fe atoms. Below $T=10\mathrm{K}$ the spin-relaxation times become temperature independent indicating a crossover from thermal excitations to the quantum tunneling regime. We report on a strong increase of the spin-flip probability in transverse magnetic fields that proves the resonant character of this tunneling process. Longitudinal fields, on the other hand, lift the ground-state degeneracy and destroy the tunneling condition. An increase of the relaxation time by 4 orders of magnitude in applied fields of only a few milliTesla reveals exceptionally sharp tunneling resonances. ${\mathrm{Li}}_2({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x)\mathrm{N}$ represents a comparatively simple and clean model system that opens the possibility to study quantum tunneling of the magnetization at liquid helium temperatures.

Figure 1

Spin reversal in transverse fields for ${\mathrm{Li}}_2({\mathrm{Li}}_{0.994}{\mathrm{Fe}}_{0.006})\mathrm{N}$ ($M\parallel H_z\parallel c$). Shown are sections of isothermal $M\text{−}H$ half loops. (a) The step in $M$ at $H_z\approx 0$ roughly doubles its size in a transverse field of ${\mu }_0{H}_x=30\mathrm{mT}$. A full $M\text{−}H$ loop is given as inset. Compared to an applied transverse field, the effects of temperature and sweep rate on the step-size are small as shown in (b) and (c), respectively.

Figure 2

Spin reversal in ${\mathrm{Li}}_2({\mathrm{Li}}_{0.999}{\mathrm{Fe}}_{0.001})\mathrm{N}$ in $H\approx 0$ ($M\parallel c$). Time dependence of the magnetization after ramping the applied field from ${\mu }_{0}H=7\mathrm{T}$ to (a) $-2.5\mathrm{mT}$ and (b) 0.0(1) mT. (c) Relaxation times $\tau$ were determined by fitting a stretched exponential function to $M(t)$ and are shown in the form of an Arrhenius plot ($T\le 16\mathrm{K}$). The solid lines are fits to the equation given in the plot. For $T>20\mathrm{K}$, $\tau$ was determined from ac susceptibility (dotted line).

Figure 3

Time-dependent magnetization of ${\mathrm{Li}}_2({\mathrm{Li}}_{0.999}{\mathrm{Fe}}_{0.001})\mathrm{N}$ as response to a longitudinal applied field ($H\parallel c$). (a) Increasing the applied field to above $\sim 0.5\mathrm{mT}$ leads to decreasing magnetization in the tunneling regime. (b) At higher temperatures the magnetization shows the expected monotonic increase with $H$. The lines are fits to a stretched exponential function. (c) Field-dependent relaxation times $\tau (H)$ at different temperatures (lines are guides to the eye).