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arXiv · 2003.00089

Charge Density Modulation and Defect Ordering in Na$_x$MnBi$_y$ magnetic semimetal

Aaron Wegner·Despina Louca·Keith Taddei·Joerg Neuefeind
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Abstract

The I-Mn-V antiferromagnet, NaMnBi, develops a very large positive magnetoresistance (MR) up to 10,000% at 2 K and 9 T in crystals showing a semiconductor-to-metal transition (SMT). In the absence of an SMT, a modest (20%) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears giving rise to new Bragg peaks due to charge and defect ordering in a checkerboard pattern, with two kinds of modulation vectors, $q_1$=($\frac23$, 0, 1) and $q_2$=($\frac23, \frac13, \frac12$). This constitutes a superlattice transition ($T_s$) that lowers the symmetry from the high temperature centrosymmetric P4/nmm to the non-centrosymmetric P$\overline4$m2. In crystals with a large MR, a close to room temperature $T_s$ is observed with $q_1$ appearing first, followed by $q_2$. In crystals with low MR however, $T_s$ is much lower and only $q_1$ is observed. The charge modulation and spin fluctuations may both contribute to the enhancement of MR.

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Aaron Wegner, Despina Louca, Keith Taddei, Joerg Neuefeind. 2020-02-28. Charge Density Modulation and Defect Ordering in Na$_x$MnBi$_y$ magnetic semimetal. https://doi.org/10.1103/physrevb.102.020403

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