Environmental factors influencing EPR in Mn12-Ac and Fe8Br

Abstract

A multi-high-frequency (40–200 GHz) resonant cavity perturbation technique yields distortion-free high-field EPR spectra for oriented single crystal samples of the uniaxial and biaxial spin S=10 single molecule magnets (SMMs) Mn12-Ac and Fe8Br. We examine quantitatively the temperature dependence of the EPR linewidths and line shifts for fixed frequency measurements with an applied magnetic field along the easy axis. Simulations of the obtained experimental data take into account various environmental couplings, including intermolecular spin–spin interactions (dipolar and exchange), as well as distributions in the zero-field crystal field parameters. The temperature dependence of the linewidths and the line shifts are mainly caused by spin–spin interactions. For Fe8Br and Mn12-Ac, the calculated line shifts and linewidths agree well with the observed experimental trends. The linewidths for Fe8Br reveal a stronger temperature dependence than those for Mn12-Ac because, for the latter, a much wider distribution in D overshadows the temperature dependence of the spin–spin interactions. For Fe8Br, the line-shift analysis suggests two competing interactions: a weak effective ferromagnetic exchange coupling between neighboring molecules, and a longer-range antiferromagnetic dipolar interaction. For Mn12-Ac, a pronounced modulation of the EPR lineshapes for transverse applied fields suggests the possibility of a solvent-disorder-induced transverse anisotropy, as has recently been proposed by other groups. These findings could have implications for the mechanism of quantum tunneling of magnetization in both of these SMMs.