Interplay between the $d$- and $\pi$-electron systems in magnetic torque of the layered organic conductor $\kappa$-(BETS)${}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$

In the organic charge transfer salt $\kappa$-(BETS)${}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ the metallic conductivity is provided by itinerant $\pi$ electrons in the layers of BETS molecules, whereas magnetization is largely dominated by the localized $d$ electrons of the ${\mathrm{Mn}}^{2+}$ ions in the insulating anionic layers. We study magnetic properties of the compound in its low-temperature, Mott-insulating state by means of magnetic torque technique. The complex behavior of the torque can be consistently explained by the coexistence of two weakly interacting magnetic subsystems associated with paramagnetic $d$-electron spins and antiferromagnetically ordered $\pi$-electron spins, respectively. Based on the experimental data, we determine the principal axes of magnetization of the ${\mathrm{Mn}}^{2+}$ sublattice and propose a qualitative model for the $\pi$-electron spin arrangement, implying an important role of the Dzyaloshinskii-Moriya interaction.

Figure 1

Field dependence of the magnetic torque of $\kappa$-(BETS)${}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ measured at $T=1.5$ K for the rotation axis parallel to directions: (a) $\left[0\overline{1}0\right]$, (b) [001], (c) $\left[0\overline{1}1\right]$, and (d) perpendicular to $\left[0\overline{1}1\right]$ in the $bc$ plane. Numbers to the right of the curves indicate the polar angle $\theta$ between the field direction and $a^{*}$, the normal to the crystallographic $bc$ plane.

Figure 2

A closeup of the steplike features (kinks) in the $H$ dependence of ${\tau }_c$ (a), ${\tau }_d$ (b), and ${\tau }_{\perp d}$ (c). The curves are shifted along the vertical axis for clarity.

Figure 3

Angular dependence of the torque at 1.5 K, 15 T for the field rotated around $\left[0\overline{1}0\right]$ (squares), [001] (circles), $\left[0\overline{1}1\right]$ (up-triangles), and perpendicular to $\left[0\overline{1}1\right]$ in the $bc$ plane (down-triangles). Solid lines: Fits to the data using Eq. (7).

Figure 4

Angle dependence of the position of the kink feature in the field-dependent torque ${\tau }_c$ (squares), ${\tau }_d$ (circles), and ${\tau }_{\perp d}$ (triangles).

Figure 5

Presentation of the $({\mathit{M}}_1,{\mathit{M}}_2)$ sublattice moments in the basis of ferromagnetic ${\mathit{M}}_F$ and staggered ${\mathit{M}}_S$ magnetization vectors.

Figure 6

Arrangement of the AF sublattice moments: (a) at zero field and (b) above the critical field of the SR transition $H_{\mathrm{kink}}$.