Route to high Néel temperatures in 4$d$ and 5$d$ transition metal oxides

There are very few magnetic members among the 4$d$ and 5$d$ transition metal oxides. In the present work, we examine the recent observation of a high Néel temperature $T_N$ in the 4$d$ oxides SrTcO${}_3$ and CaTcO${}_3$. Considering a multiband Hubbard model, we find that $T_N$ is larger in the limit of a large bandwidth and vanishingly small intra-atomic exchange interaction strength, contrary to our conventional understanding of magnetism. This is traced to specific aspects of the $d^3$ configuration at the transition metal site and the study reveals additional examples with high $T_N$.

Figure 1

SrTcO${}_3$ (a) crystal structure (b) A (c) C, and (d) G-type antiferromagnetic configurations. Large green, medium gray, and small red spheres in (a) are Sr, Tc, and O atoms, respectively. Black arrows in (b)–(d) denote the spins on the Tc sites for different antiferromagnetic configurations.

Figure 2

(a) The ab-initio and (b) tight-binding band dispersions for SrTcO${}_3$ along different symmetry directions. The parameters entering the tight-binding Hamiltonian are determined by a least-square error minimization fit of the ab initio band dispersions.

Figure 3

Partial density of states of Tc $d$ and O $p$ states in SrTcO${}_3$ for (a) nonmagnetic solution of a multiband Hubbard model with $\Delta$ $=$ 2.0 eV, $U$ $=$ 0.6 eV; (b) G-AFM spin configuration with $\Delta$ $=$ 2.0 eV, $U$ $=$ 0.6 eV; (c) G-AFM spin configuration with $\Delta$ $=$ 2.0 eV, $U$ $=$ 2.0 eV for majority [upper part of (b) and (c)] and minority [lower part of (b) and (c)] spins. The variations of band gap as well as magnetic moment on Tc with $U$ are plotted in (d) for $\Delta$ $=$ 2.0 eV.

Figure 4

(a) The variation of $J_1$ with $U$ for $\Delta$ $=$ 0, 2, 4 eV calculated within a multiband Hubbard model. The inset shows the corresponding variation of $J_2$. (b) $J_0$ as a function of $U$ for $J_h$ $=$ 0.3, 0.8 for SrTcO${}_3$ and 0.8 eV for SrMnO${}_3$. (c) Contour plot of $J_0$ in the $U$-$\Delta$ plane for SrTcO${}_3$ with $J_h$ $=$ 0.1 eV. The dotted line indicates the position of maxima of $J_0$.

Figure 5

The variation of the magnetic moment as a function of temperature (T) for SrTcO${}_3$ and SrMnO${}_3$ calculated within a Heisenberg model with nearest neighbor ($J_1$) and next-nearest-neighbor ($J_2$) exchange interactions. The experimental variations (solid circle) for the saturation magnetic moment from Ref. 27 are provided for SrTcO${}_3$.

Figure 6

Majority and minority spin partial density of states for Tc $d$ and O $p$ calculated for ferromagnetic configuration with $\Delta =2$ eV and $U$ varied from 2.4 to 2.9 eV calculated within a multiband model for SrTcO${}_3$. The band gap is just opening near $U=2.6$ eV and is clearly visible at $U=2.9$ eV.

Figure 7

Total DOS and majority and minority spin Tc $d$ partial dos for antiferromagnetic configuration of (a) Sr${}_2$TcO${}_4$, (b) Ca${}_2$TcO${}_4$, and (c) Sr${}_3$Tc${}_2$O${}_7$. The zero in energy denotes $E_F$ in every case.

Figure 8

(a) Antiferromagnetic spin arrangement in Li${}_2$TcO${}_3$. (b) Total DOS and Tc $d$ partial dos for majority and minority spin channels.

Figure 9

(a) The variation in the magnetic stabilization energy for G-AFM and C-AFM configurations plotted as a function of the percentage decrease in each lattice constant. (b) Tc $d$ and O $p$ contributions to the density of states for G-AFM configurations for different isotropic compressions of the lattice constants. At a 15% decrease of all the lattice constants, the G-AFM solution turns nonmagnetic.