Theoretical estimates for proton-NMR spin-lattice relaxation rates of heterometallic spin rings

TL;DR

This paper provides theoretical estimates for proton-NMR spin-lattice relaxation rates in heterometallic chromium wheels, analyzing experimental susceptibility data and modeling relaxation behavior under magnetic fields to understand magnetic interactions.

Contribution

It introduces a simple isotropic Heisenberg model for MCr7 wheels and highlights unique exchange interactions for FeCr7, along with modeling relaxation rates at level crossings.

Findings

FeCr7 requires a different iron-chromium exchange parameter.

CuCr7 and NiCr7 show reduced relaxation rates at specific level crossings.

The model explains the magnetic relaxation behavior observed in NMR experiments.

Abstract

Heterometallic molecular chromium wheels are fascinating new magnetic materials. We reexamine the available experimental susceptibility data on MCr7 wheels in terms of a simple isotropic Heisenberg Hamiltonian for M=Fe, Ni, Cu, and Zn and find in that FeCr7 needs to be described with an iron-chromium exchange that is different from all other cases. In a second step we model the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in Nuclear Magnetic Resonance (NMR) experiments. It appears that CuCr7 and NiCr7 show an unexpectedly reduced relaxation rate at certain level crossings.