Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases of $\lambda$-(BETS)$_{2}$FeCl$_{4}$

TL;DR

This study uses proton NMR to analyze local magnetic fields in $ ext{λ}$(BETS)$_{2}$FeCl$_{4}$, revealing how magnetic phases influence proton spectra and confirming Fe$^{3+}$ ions' dominant role in local magnetism.

Contribution

First detailed NMR analysis of local magnetic fields in $ ext{λ}$(BETS)$_{2}$FeCl$_{4}$ across magnetic phase transition, modeling dipolar fields with a modified Brillouin function.

Findings

Spectral peaks broaden and shift at low temperatures.

Discontinuous changes in linewidth and shift at the phase transition.

Good fit of data with a mean field model using J₀ = -1.7 K.

Abstract

Measurements of the $^{1}$H-NMR spectrum of a small ($\sim$ 4 $\mu$g) single crystal of the organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$ are reported with an applied magnetic field $\bf{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 Fe$^{3+}$ ion moments (spin $S_{\rm{d}}$ = 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 Fe$^{3+}$ 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 Fe$^{3+}$ 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.