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arXiv · 1712.07953

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

M. Fix·A. Jesche·S. G. Jantz·S. A. Bräuninger·H. -H. Klauss·R. S. Manna·I. M. Pietsch·H. A. Höppe·P. C. Canfield
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Abstract

We report on isothermal magnetization, M\"ossbauer 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 $\mu_0H = 11.6\,$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$^{-1}_{\rm Fe}\,$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.

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M. Fix, A. Jesche, S. G. Jantz, S. A. Bräuninger, H. -H. Klauss, R. S. Manna, I. M. Pietsch, H. A. Höppe, P. C. Canfield. 2017-12-21. Ferromagnetism or slow paramagnetic relaxation in Fe-doped Li$_3$N?. https://doi.org/10.1103/physrevb.97.064419

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