Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12

Abstract

We examine quantitatively the temperature dependence of the linewidths and line shifts in electron paramagnetic resonance experiments on single crystals of the single-molecule magnets Fe${}_8$ and Mn${}_{12}$, at fixed frequency, with an applied magnetic field along the easy axis. We include inter-molecular spin-spin interactions (dipolar and exchange) and distributions in both the uniaxial anisotropy parameter $D$ and the Landé $g$-factor. The temperature dependence of the linewidths and the line shifts are mainly caused by the spin-spin interactions. For Fe${}_8$ and Mn${}_{12}$, the temperature dependence of the calculated line shifts and linewidths agrees well with the trends of the experimental data. The linewidths for Fe${}_8$ reveal a stronger temperature dependence than those for Mn${}_{12}$, because for Mn${}_{12}$ a much wider distribution in $D$ overshadows the temperature dependence of the spin-spin interactions. For Fe${}_8$, the line-shift analysis suggests two competing interactions: a weak ferromagnetic exchange coupling between neighboring molecules and a longer-ranged dipolar interaction. This result could have implications for ordering in Fe${}_8$ at low temperatures.

published version

References in corpus (7)

• Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets Fe8 and Mn12
• Detailed single crystal EPR lineshape measurements for the single molecule magnets Fe8Br and Mn12-ac
• Spin tunneling via dislocations in Mn-12 acetate crystals
• Dislocation-induced spin tunneling in Mn-12 acetate
• Quantum Step Heights in Hysteresis Loops of Molecular Magnets
• Linewidth of single photon transitions in Mn${}_{12}$-acetate
• Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12

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