${}^{77}\mathrm{Se}$ NMR measurements of the $\pi \text{−}d$ exchange field in the organic conductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$

${}^{77}\mathrm{Se}\text{−}\mathrm{NMR}$ spectrum and frequency shift measurements in the paramagnetic metal and antiferromagnetic insulating phases are reported for a small single crystal of the organic conductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$ as a function of temperature $\left(T\right)$ and field alignment for an applied magnetic field $B_0=9\mathrm{T}$. The results show that in the low $T$ limit, where the localized ${\mathrm{Fe}}^{3+}$ spins $(S_d=5∕2)$ are almost fully polarized, the conduction electrons (Se $\pi$ electrons, spin $s_{\pi }=1∕2$) in the BETS molecules experience an exchange field $\left({\mathbf{B}}_{\pi d}\right)$ from the ${\mathrm{Fe}}^{3+}$ spins with a value of $-32.7\pm 1.5\mathrm{T}$ at $5\mathrm{K}$ and $9\mathrm{T}$ aligned opposite to ${\mathbf{B}}_0$. This large negative value of ${\mathbf{B}}_{\pi d}$ is consistent with that predicted by the resistivity measurements and supports the Jaccarino-Peter internal field-compensation mechanism being responsible for the origin of field-induced superconductivity.

Figure 1

(Color online) (a) Cartoon of the interactions causing the Jaccarino-Peter (JP) compensation mechanism. (b) Sketch of the needlelike shape $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$ sample for the measurement.

Figure 2

(Color online) ${}^{77}\mathrm{Se}\text{−}\mathrm{NMR}$ absorption spectrum ${\chi }^{″}$ as a function of frequency shift at $\nu -{\nu }_0$ $({\nu }_0=72.90\mathrm{MHz})$ at different temperatures with ${\mathbf{B}}_0=9\mathrm{T}\Vert a^{\prime }$ in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$.

Figure 3

(Color online) $T$ dependence of the ${}^{77}\mathrm{Se}\text{−}\mathrm{NMR}$ linewidth (FWHM) $\Delta f$ (solid red circles) and the modified Brillouin function fit of the ${\mathrm{Fe}}^{3+}$ magnetization[19] $M_d(x_0+x^{\prime })$ (solid blue line) of $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$ at ${\mathbf{B}}_0=9\mathrm{T}\Vert a^{\prime }$. The error bars are our estimated uncertainty.

Figure 4

(Color online) (a) $T$ dependence of the ${}^{77}\mathrm{Se}\text{−}\mathrm{NMR}$ resonance frequency $\nu$ (solid red) for ${\mathbf{B}}_0=9\mathrm{T}\Vert a^{\prime }$ in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$. The solid line is a fit to the modified Brillouin function ${\mathrm{Fe}}^{3+}$ magnetization (Ref. 19) $M_d(x_0+x^{\prime })$. (b) $\nu$ vs $M_d(x_0+x^{\prime })$ above $7\mathrm{K}$ for ${\mathbf{B}}_0=9\mathrm{T}\Vert a^{\prime }$ in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$. The error bars are our estimated uncertainty.

Figure 5

(Color online) ${}^{77}\mathrm{Se}$-NMR resonance frequency $\nu$ as a function of angle $\varphi$ at several $T$ for the rotation of $B_0=9\mathrm{T}$ about the $c$ axis in $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$. The error bars are our estimated uncertainty.