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Register a new user$^{1}$H-NMR spin-echo measurements of the static and dynamic spin properties in $lambda$-(BETS)$_{2}$FeCl$_{4}$
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$^{1}$H-NMR spin-echo measurements of the spin-echo decay $M(2tau)$ with a decay rate 1/$T_{2}$ and the frequency shift $Delta u/ u_{0}$ under applied magnetic field $mathbf{B}$$_{0}$ = 9 T along the a-axis over a temperature range 2.0$-$180 K are reported for a single crystal of the organic conductor $lambda$-(BETS)$_{2}$FeCl$_{4}$. It provides the spin dynamic and static properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) states as well as across the PM$-$AFI phase transition. A large slow beat structure in the spin-echo decay is observed with a typical beat frequency of $f$ $sim$ 7 kHz and it varies across the spectrum. Its origin is attributed to the $^{1}$H$-$$^{1}$H dipole interactions rather than to the much larger dipolar field contribution from the Fe$^{3+}$ electrons (spin $S$ = 5/2). A simple phenomenological model provides an excellent fit to the data. The dominant $^{1}$H-NMR frequency shift comes from the dipolar field from the 3d Fe$^{3+}$ ions, and the Fe$^{3+}$ $-$ Fe$^{3+}$ exchange interactions ($J_{0}$) ($J_{0} $ includes the d$-$d exchange interactions through the $pi-$electrons) have a substantial effect to the local field at the proton sites expecially at low temperatures. A good fit is obtained with $J_{0}$ = - 1.7 K. The data of the spin-echo decay rate 1/$T_{2}$ indicates that there is a significant change in the slow fluctuations of the local magnetic field at the $^{1}$H-sites on traversing the PM to AFI phase. This evidence supports earlier reports that the PM$-$AFI phase transition in $lambda$-(BETS)$_{2}% $FeCl$_{4} $ is driven magnetically and first order.
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We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $lambda$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the spins textbf{s} of the BETS $pi$ conduction electrons and the Fe localized $d$ spins textbf{S}. The absolute value of the Knight shift textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative spin polarization of the $pi$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas textit{et al}. Phys. Rev. Lett. textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.
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