Ferromagnetism or slow paramagnetic relaxation in Fe-doped Li${}_3$N?

Abstract

We report on isothermal magnetization, Mössbauer spectroscopy, and magnetostriction as well as temperature-dependent alternating-current (ac) susceptibility, specific heat, and thermal expansion of single crystalline and polycrstalline Li${}_2$(Li${}_{1-x}$Fe${}_x$)N with $x=0$ and $x\approx 0.30$. Magnetic hysteresis emerges at temperatures below $T\approx 50$K with coercivity fields of up to ${\mu }_{0}H=11.6$T at $T=2$K and magnetic anisotropy energies of $310$K ($27$meV). The ac susceptibility is strongly frequency dependent ($f=10$--$10,000$Hz) and reveals an effective energy barrier for spin reversal of $\Delta E\approx 1100$K. The relaxation times follow Arrhenius behavior for $T>25$K. For $T<10$K, however, the relaxation times of $\tau \approx 10^{10}$s are only weakly temperature-dependent indicating the relevance of a quantum tunneling process instead of thermal excitations. The magnetic entropy amounts to more than $25$J mol${}_{\mathrm{Fe}}^{-1}$K${}^{-1}$ which significantly exceeds $R$ln2, the value expected for the entropy of a ground state doublet. Thermal expansion and magnetostriction indicate a weak magneto-elastic coupling in accordance with slow relaxation of the magnetization. The classification of Li${}_2$(Li${}_{1-x}$Fe${}_x$)N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.

12 pages, 10 figures