Compensation of the Effective Field in the Field-Induced Superconductor
ƒÈ-(BETS)₂FeBr₄

The organic charge-transfer-salts containing magnetic ions have been attracting strong interest since field-induced superconductivity (FISC) was discovered in ƒÈ-(BETS)₂FeBr₄ [1]. At zero magnetic field, this material shows simultaneous antiferromagnetic (AF) order of localized moments (spin 5/2) on Fe³⁺ ions and metal-insulator transition of ƒÎ-conduction electrons on BETS molecules. A magnetic field destabilizes both the antiferromagnetic order and the insulating state. The FISC appears at high fields between 18 and 41 tesla [2]. This was suggested to be due to the Jaccrino-Peter (JP) mechanism [3]: if there is antiferromagnetic coupling between Fe spins and ƒÎ-electrons, superconductivity is expected when the external field compensates the exchange field on ƒÎ-electrons from Fe spins, minimizing the spin-Zeeman pair breaking effect.


The field-induced superconductivity has been observed also in the ƒÈ-type polymorph ƒÈ-(BETS)₂FeBr₄ for a much lower range of the field between 10 and 15 T [4] below 300 mK. Here we report the frequency shift of the ⁷⁷Se NMR in ƒÈ-(BETS)₂FeBr₄ to measure the spin polarization of the ƒÎ-electrons. Our results demonstrate that the polarization of ƒÎ-electrons at low temperatures vanishes for the field near 12 T, which is the central field for FISC, hence support the JP mechanism.




Fig. 1. (a) Crystal structure of ƒÈ-(BETS)₂FeBr₄ (b)Structure of BETS molecules.




Fig. 2. ⁷⁷Se NMR spectra at T = 1.5 K for the field along the b-axis (a) and the c-axis (b). The horizontal axes show the frequency shift from the resonance in diamagnetic materials.


The crystal structure of ƒÈ-(BETS)₂FeBr₄ consists of alternating stacks of the conducting BETS layers and the insulating FeBr₄ magnetic layers (Fig. 1a). The four inequivalent Se sites in a molecule (Fig. 1b) are resolved in the NMR spectra for a single crystal at T = 1.5 K (Fig. 2). The frequency shift ƒÂƒË has two contribution, ƒÂƒË = ƒÁ_NA^ƒÎ_i,ƒ¿m_ƒÎ + ƒÁ_N(A^dip_i,ƒ¿ + B_i)m_d: the first term represents the contribution from the ƒÎ-spin polarization m_ƒÎ through the anisotropic hyperfine coupling A^ƒÎ_i,ƒ¿, while the second term is due to the Fe magnetization m_d through the direct dipolar coupling A^dip_i,ƒ¿ and the transferred hyperfine coupling B_i. (ƒÁ_N is the nuclear gyromagnetic ratio.) Since A^dip_i,ƒ¿ can be calculated and m_d is completely saturated to 5ƒÊ_B for the fields of our measurements, the dipolar field can be subtracted from the measured shift. The remaining shift ƒÂƒË' = ƒÂƒË - ƒÁ_NA^dip_i,ƒ¿m_d = ƒÁ_NA^ƒÎ_i,ƒ¿m_ƒÎ + ƒÁ_NB_im_d is plotted in Fig. 3 against the external field for various resonance lines.


The linear relation between ƒÂƒË' and H_ext indicates validity of the mean field expression m_ƒÎ = ƒÔ(H_ext + Jm_d) where Jm_d is the exchange field from Fe spins. If the compensation of the effective field occurs, m_ƒÎ should vanish and ƒÂƒË' should become isotropic at the compensating field. Thus the value of the compensation field is known from the crossing of lines in Fig. 3 for H // b and H // c corresponding to the same sites. The plot in Fig. 3 indeed shows that such crossing occurs in the field range 10 - 12 T, supporting the JP mechanism. We also measured temperature dependence of the shift at a constant field. The analysis of the data lead to similar values of the exchange coupling between the Fe moments and the ƒÎ-electrons. The consistent results from the two independent set of data provide convincing evidence for the JP mechanism of FISC in ƒÈ-(BETS)₂FeBr₄.




Fig. 2. Field dependence of the frequency shift of various resonance lines for H // b and H // c.


References
[1] S. Uji et al., Nature 410, 908 (2001).
[2] L. Balicas et al., Phys. Rev. Lett. 87, 067002 (2001).
[3] V. Jaccarino and M. Peter, Phys. Rev. Lett. 7, 290 (1962).
[4] T. Konoike et al., Phys. Rev. B 70, 094515 (2004).
[5] S. Fujiyama et al., Phys. Rev. Lett. 96, 217001 (2006).