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Description
We report β-NMR spin-lattice relaxation of 8Li+ implanted at 25 keV into polycrystal samples of the high entropy oxides (Co1/5 Cu1/5Mg1/5 Ni1/5Zn1/5 )O with the rocksalt (RS) structure and (Co1/5 Cr1/5Fe1/5Mn1/5Ni1/5 )3 O4 with the spinel (Sp) structure in a high applied longitudinal field of 5.65 T. Both are magnetic materials with a high degree of cation disorder. In the RS, occupation of the cation site is random, while the octahedral and tetrahedral sites of the Sp have preferential occupations. In the RS compound, we find fast relaxation with a rate that increases towards the antiferromagnetic transition ∼ 120 K. Below this, about 2/3 of the asymmetry is wiped out. Upon further cooling, below about 50 K, the relaxation of the remaining asymmetry becomes slow enough to be measured, and it continues to slow towards zero. This is qualitatively consistent with the LF-μSR data of Frandsen et al.[1] with a relaxation rate about 3 orders of magnitude slower. We find no evidence for additional diffusive relaxation up to 300 K indicating that the ionic conductivity of the lithiated compound[2] is not due to motion of the isolated interstitial. We also find fast relaxation in the Sp compound which orders above room temperature, so our data is confined to the magnetic state. We discuss the relaxation in terms of the magnetic excitation spectrum.
[1] B. Frandsen et al., Phys. Rev. Materials 4, 074405 (2020).
[2] D. Bérardan et al., J. Mater. Chem. A 4, 9536 (2016).
| wam@chem.ubc.ca | |
| Funding Agency | NSERC Canada |
