${}^1$H-NMR spin-echo measurements of the static and dynamic spin properties in $\hat{I}»$-(BETS)${}_2$FeCl${}_4$

Abstract

${}^1$H-NMR spin-echo measurements of the spin-echo decay $M(2\ddot{I}„)$ with a decay rate 1/$T_2$ and the frequency shift $\hat{I}”\hat{I}½/\hat{I}{½}_0$ under applied magnetic field $\mathbf{B}$${}_0$ = 9 T along the a-axis over a temperature range 2.0$-$180 K are reported for a single crystal of the organic conductor $\hat{I}»$-(BETS)${}_2$FeCl${}_4$. It provides the spin dynamic and static properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) states as well as across the PM$-$AFI phase transition. A large slow beat structure in the spin-echo decay is observed with a typical beat frequency of $f$ $\sim$ 7 kHz and it varies across the spectrum. Its origin is attributed to the ${}^1$H$-$${}^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 $\ddot{I}€-$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 $\hat{I}»$-(BETS)${}_2$FeCl${}_4$ is driven magnetically and first order.

9 pages, 10 figures, resubmitted to Phys. Rev. B in response to comments of Editor and reviewers on March 23, 2007