Antiferromagnetic $d$-Electron Exchange via a Spin-Singlet $\pi$-Electron Ground State in an Organic Conductor

Electron spin resonance reveals the spin behavior of conduction ($\pi$) and localized ($d$) electrons in $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_{2}M{\mathrm{Cl}}_4$ ($M=\mathrm{Fe}$, Ga). Both the ${\mathrm{Ga}}^{3+}(S=0)$ and ${\mathrm{Fe}}^{3+}(S=5/2)$ compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the $\pi$ electron system of the donor molecules. The behavior is consistent with charge ordering in $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_{2}M{\mathrm{Cl}}_4$ at the metal-insulator transition. At 5 K, the ${\mathrm{Fe}}^{3+}$ compound orders antiferromagnetically, even though the $\pi$ electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.

Figure 1

(a) Crystal structure of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_{2}M{\mathrm{Cl}}_4$ ($M=\mathrm{Fe}$,Ga) viewed down the $a$ axis. The donors are stacked in the $ac$ plane [20, 21]. Inset: Chemical formula of the BDA-TTP donor molecule. The axes correspond to the principal axes of the molecule. (b) ESR signals of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{GaCl}}_4$ at room temperature at 240 GHz at 9 ° intervals. (c) Complete angular dependence of the $g$ value.

Figure 2

(a) Left axis: Resistance ratio of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{GaCl}}_4$. Right axis: $\chi$ from dc SQUID measurements. (b) $\chi$ and (c) $g$ value from ESR along the $b$ axis.

Figure 3

(a) Left axis: Resistance ratio of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$ near $T_{\mathrm{MI}}$. Right axis: Magnetic susceptibility of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$. Dotted line: Curie-Weiss law; solid line: Bonner-Fisher fit (see text). Inset: Magnified view of the susceptibility around the peak. (b) $g$ values of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$ at 240 GHz along the $a$, $b$, and $c$ axes. Inset: $g$-value change along $b$ at $T_{\mathrm{MI}}$. (c) Linewidths of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$ at 240 GHz along the $a$, $b$, and $c$ axes. Inset: Linewidth along $b$ at $T_{\mathrm{MI}}$.

Figure 4

Temperature dependent Raman signals: (a) $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{GaCl}}_4$; (b) $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$.

Figure 5

Frequency dependence of $\beta \mathrm{\text{−}}(\mathrm{BDA}\mathrm{\text{−}}\mathrm{TTP}{)}_2{\mathrm{FeCl}}_4$ at 1.5 K along the $a$, $b$, and $c$ axes. Inset: Magnetic field direction dependence of the AFM cantilever (torque) signal at 2.5 ° intervals.