Compensation of Effective Field in the Field-Induced Superconductor $\kappa \mathrm{\text{−}}(\mathrm{\text{BETS}}{)}_2{\mathrm{FeBr}}_4$ Observed by ${}^{77}\mathrm{Se}$ NMR

We report results of ${}^{77}\mathrm{Se}$ NMR frequency shift in the normal state of the organic charge-transfer salt $\kappa \mathrm{\text{−}}(\mathrm{\text{BETS}}{)}_2{\mathrm{FeBr}}_4$ which shows magnetic field-induced superconductivity (FISC). From a simple mean-field analysis, we determined the field and the temperature dependences of the magnetization $m_{\pi }$ of the $\pi$ conduction electrons on BETS molecules. We found that the Fe spins are antiferromagnetically coupled to the $\pi$ electrons and determined the exchange field to be $J=-2.3\mathrm{T}/{\mu }_B$. The exchange field from the fully saturated Fe moments ($5{\mu }_B$) is compensated by an external field of 12 T. This is close to the central field of the FISC phase, consistent with the Jaccarino-Peter local field-compensation mechanism for FISC [Phys. Rev. Lett. 9, 290 (1962)].

Figure 1

(a) Crystal structure of $\kappa \mathrm{\text{−}}(\mathrm{\text{BETS}}{)}_2{\mathrm{FeBr}}_4$. (b) A BETS donor molecule with Se atoms.

Figure 2

${}^{77}\mathrm{Se}$ NMR spectra at $T=1.5\mathrm{K}$ for $H_{\mathrm{ext}}$ along the $b$ axis (a) and along the $c$ axis (b). The resolved four (two) peaks are denoted as $b1$, $b2$, $b3$, and $b4$ ($c1$ and $c2$).

Figure 3

$H_{\mathrm{ext}}$ dependence of $\delta {\nu }^{\prime }$ (defined in the text) at $T=1.5\mathrm{K}$. The solid (dotted) lines are the fits to straight lines for $H_{\mathrm{ext}}\parallel c$ ($H_{\mathrm{ext}}\parallel b$). The arrows show the values of $B_i{m}_d$.

Figure 4

Frequency shifts due to magnetization of $\pi$ electrons at 1.5 K.

Figure 5

Frequency shifts due to the magnetization of $\pi$ electrons are plotted against the magnetization of Fe moments $m_d\equiv B_{5/2}(5{\mu }_B{H}_{\mathrm{ext}}/k_{B}T)$ at $H_{\mathrm{ext}}=15.5\mathrm{T}$.