Origin of the High Néel Temperature in ${\mathrm{SrTcO}}_3$

We investigate the origin of the high Néel temperature recently found in ${\mathrm{Tc}}^{4+}$ perovskites. The electronic structure in the magnetic state of ${\mathrm{SrTcO}}_3$ and its $3d$ analogue ${\mathrm{SrMnO}}_3$ is calculated within a framework combining band-structure and many-body methods. In agreement with experiment, for ${\mathrm{SrTcO}}_3$ a smaller magnetic moment and 4 times larger Néel temperature are found. We show that this is because the Tc compound lies on the verge of the itinerant-to-localized transition, while the Mn compound lies deeper into the localized side. For ${\mathrm{SrTcO}}_3$ we predict that the Néel temperature depends weakly on applied pressure, in clear violation of Bloch’s rule, signaling the complete breakdown of the localized picture.

Figure 1

Solid lines: Temperature dependence of ${\mathrm{SrTcO}}_3$ (filled circles) and ${\mathrm{SrMnO}}_3$ (squares) “Wannier” magnetic moments in orbitals ${\psi }_m$ (see discussion in the text). Note that the $x$ axis has been rescaled due to systematic overestimation of $T_N$ in DMFT [21]. Open circles: Results obtained for a hypothetical compound having the ${\mathrm{SrMnO}}_3$ band structure but artificially reduced interactions yielding a maximal Néel temperature.

Figure 2

The total and orbitally resolved LDA paramagnetic (PM) DOS for (a) ${\mathrm{SrTcO}}_3$ and (b) ${\mathrm{SrMnO}}_3$. Inset: Predicted spin-resolved $\mathrm{LDA}+\mathrm{DMFT}$ DOS.

Figure 3

The Néel temperatures as a function of coupling $g\equiv \mathcal{U}/{\mathcal{U}}_c$, with $\mathcal{U}\equiv U+2J$ and ${\mathcal{U}}_c=(U+2J{)}_c$ the value at which the MIT in the paramagnetic state occurs. $T_N$ are normalized to the maximum Néel temperature $T_N^{*}$ found when varying $\mathcal{U}$. The gray line is for the single-band Hubbard model (DMFT calculation of Ref. 22). The full circle and square indicate the data at the physical value of interaction for the band structure of the Tc and Mn compound, respectively. The empty circle denotes data for the Mn compound but artificially reduced interaction; see Fig. 1. Inset: The dependence of $T_N$ on $J/U$ at few $g$.