Ferromagnetism versus slow paramagnetic relaxation in Fe-doped Li3N

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Fix, M.
Jesche, A.
Jantz, S. G.
Bräuninger, S. A.
Klauss, H.-H.
Manna, R. S.
Pietsch, I. M.
Höppe, H. A.
Canfield, Paul
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Ames National Laboratory

Ames National Laboratory is a government-owned, contractor-operated national laboratory of the U.S. Department of Energy (DOE), operated by and located on the campus of Iowa State University in Ames, Iowa.

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Physics and Astronomy
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We report on isothermal magnetization, Mossbauer spectroscopy, and magnetostriction as well as temperature-dependent alternating-current (ac) susceptibility, specific heat, and thermal expansion of single crystalline and polycrystalline Li-2(Li1-xFex)N with x = 0 and x approximate to 0.30. Magnetic hysteresis emerges at temperatures below T approximate to 50 K with coercivity fields of up to mu H-0 = 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 approximate to 1100 K (90 meV). The relaxation times follow Arrhenius behavior for T > 25 K. ForT < 10 K, however, the relaxation times of tau approximate to 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(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 magnetoelastic coupling in accordance with slow relaxation of the magnetization. The classification of Li-2(Li1-xFex)N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.

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