Ferromagnetism versus slow paramagnetic relaxation in Fe-doped Li 3 N
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 Li 2 ( Li 1-x Fe x ) N with x = 0 and x ≈ 0.30 . Magnetic hysteresis emerges at temperatures below T ≈ 50 K with coercivity fields of up to μ 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 Δ E ≈ 1100 K (90 meV). The relaxation times follow Arrhenius behavior for T > 25 K . For T < 10 K , however, the relaxation times of τ ≈ 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 -1 Fe K -1 , which significantly exceeds R ln 2 , 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 Li 2 ( Li 1-x Fe x ) N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.