Proton Nmr Measurements Of The Local Magnetic Field In The Paramagnetic Metal And Antiferromagnetic Insulator Phases Of Lambda-(Bets)(2)Fecl4

Measurements of the H-1-NMR spectrum of a small (similar to 4 mu g) single crystal of the organic conductor lambda-(BETS)(2)FeCl4 are reported with an applied magnetic field B-0=9 T parallel to the a axis in the ac plane over a temperature (T) range 2.0-180 K. They provide the distribution of the static local magnetic field at the proton sites in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along with the changes that occur at the PM-AFI phase transition. The spectra have six main peaks that are significantly broadened and shifted at low T. The origin of these features is attributed to the large dipolar field from the 3d Fe3+ ion moments (spin S-d=5/2). Their amplitude and T dependence are modeled using a modified Brillouin function that includes a mean field approximation for the total exchange interaction (J(0)) between one Fe3+ ion and its two nearest neighbors. A good fit is obtained using J(0)=-1.7 K. At temperatures below the PM-AFI transition temperature T-MI=3.5 K, an extra peak appears on the high frequency side of the spectrum and the details of the spectrum become smeared. Also, the rms linewidth and the frequency shift of the spectral distribution are discontinuous, consistent with the transition being first order. These measurements verify that the dominant local magnetic field contribution is from the Fe3+ ions and indicate that there is a significant change in the static local magnetic field distribution at the proton sites on traversing the PM to AFI phase transition.