H1-NMR spin-echo measurements of the spin dynamic properties inλ−(BETS)2FeCl4

$^{1}\mathrm{H}$-NMR spin-echo measurements of the spin-echo decay $M(2\ensuremath{\tau})$ with a decay rate $1∕{T}_{2}$ under applied magnetic field ${\mathbf{B}}_{0}=9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ along the $a$ axis over the temperature $(T)$ range of $2.0\char21{}180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are reported for a single crystal of the organic conductor $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}\mathrm{Fe}{\mathrm{Cl}}_{4}$. It provides the spin dynamic 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 ${f}_{B}\ensuremath{\sim}7\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ that varies across the spectrum. Its origin is attributed to the interactions between protons that are very strongly detuned by the large inhomogeneous field on a microscopic distance scale that is generated by the ${\mathrm{Fe}}^{3+}$ moments (spin ${S}_{d}=5∕2$). A simple phenomenological model provides an excellent fit to the data. The values of $1∕{T}_{2}$ in the PM phase are consistent with a $T$-independent contribution from the proton-proton dipole interaction plus the proton spin-lattice relaxation rate $(1∕{T}_{1})$ [W. G. Clark et al., Appl. Magn. Reson. 27, 279 (2004)], which has a significant contribution only above $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At the PM-AFI transition $(3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, there is a discontinuous drop in $1∕{T}_{2}$ by $\ensuremath{\sim}34%$, indicating that the transition is first order, consistent with prior reports. Two possible main contributions to this drop are discussed. They are based upon the change in the local magnetic field caused by the change in the orientation of the ${\mathrm{Fe}}^{3+}$ moments at the transition.