Ferromagnetism versus slow paramagnetic relaxation in Fe-doped Li3 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 polycrystalline Li2(Li1-xFex)N with x=0 and x≈0.30. Magnetic hysteresis emerges at temperatures below T≈50K with coercivity fields of up to μ0H=11.6T at T=2K and magnetic anisotropy energies of 310 K (27 meV). The ac susceptibility is strongly frequency-dependent (f=10-10000 Hz) and reveals an effective energy barrier for spin reversal of ΔE≈1100K (90 meV). The relaxation times follow Arrhenius behavior for T>25K. For T<10K, however, the relaxation times of τ≈1010 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 25J molFe-1 K-1, which significantly exceeds Rln2, the value expected for the entropy of a ground-state doublet. Thermal expansion and magnetostriction indicate a weak magnetoelastic coupling in accordance with slow relaxation of the magnetization. The classification of Li2(Li1-xFex)N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.