44 research outputs found
Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12
We study theoretically Electron Paramagentic Resonance (EPR) linewidths for
single crystals of the molecular magnets Fe and Mn as functions of
energy eigenstates , frequency, and temperature when a magnetic field
along the easy axis is swept at fixed excitation frequency. This work was
motivated by recent EPR experiments. To calculate the linewidths, we use
density-matrix equations, including dipolar interactions and distributions of
the uniaxial anisotropy parameter and the Land\'{e} factor. Our
calculated linewidths agree well with the experimental data. We also examine
the lineshapes of the EPR spectra due to local rotations of the magnetic
anisotropy axes caused by defects in samples. Our preliminary results predict
that this effect leads to asymmetry in the EPR spectra.Comment: 2001 MMM conferenc
DFT calculation of the intermolecular exchange interaction in the magnetic Mn dimer
The dimeric form of the single-molecule magnet
[MnOCl(OCEt)(py)] recently revealed interesting
phenomena: no quantum tunneling at zero field and tunneling before magnetic
field reversal. This is attributed to substantial antiferromagnetic exchange
interaction between different monomers. The intermolecular exchange
interaction, electronic structure and magnetic properties of this molecular
magnet are calculated using density-functional theory within
generalized-gradient approximation. Calculations are in good agreement with
experiment.Comment: 4 page
Magnetic Quantum Tunneling: Insights from Simple Molecule-Based Magnets
This article takes a broad view of the understanding of magnetic bistability
and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on
three families of relatively simple, low-nuclearity transition metal clusters:
spin S = 4 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III)
complexes are related by the fact that they contain triangular Mn3 units in
which the exchange may be switched from antiferromagnetic to ferromagnetic
without significantly altering the coordination around the Mn(III) centers,
thereby leaving the single-ion physics more-or-less unaltered. This allows for
a detailed and systematic study of the way in which the individual-ion
anisotropies project onto the molecular spin ground state in otherwise
identical low- and high-spin molecules, thus providing unique insights into the
key factors that control the quantum dynamics of SMMs, namely: (i) the height
of the kinetic barrier to magnetization relaxation; and (ii) the transverse
interactions that cause tunneling through this barrier. Numerical calculations
are supported by an unprecedented experimental data set (17 different
compounds), including very detailed spectroscopic information obtained from
high-frequency electron paramagnetic resonance and low-temperature hysteresis
measurements. Diagonalization of the multi-spin Hamiltonian matrix is necessary
in order to fully capture the interplay between exchange and local anisotropy,
and the resultant spin-state mixing which ultimately gives rise to the
tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and
Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of
the molecules highlighted in this study proves to be of crucial importance.Comment: 32 pages, incl. 6 figure
Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12
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 and Mn, 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 and the Land\'{e} -factor. The temperature
dependence of the linewidths and the line shifts are mainly caused by the
spin-spin interactions. For Fe and Mn, the temperature dependence of
the calculated line shifts and linewidths agrees well with the trends of the
experimental data. The linewidths for Fe reveal a stronger temperature
dependence than those for Mn, because for Mn a much wider
distribution in overshadows the temperature dependence of the spin-spin
interactions. For Fe, 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 at low temperatures.Comment: published versio
Detailed single crystal EPR lineshape measurements for the single molecule magnets Fe8Br and Mn12-ac
It is shown that our multi-high-frequency (40-200 GHz) resonant cavity
technique yields distortion-free high field EPR spectra for single crystal
samples of the uniaxial and biaxial spin S = 10 single molecule magnets (SMMs)
[Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O and [Fe8O2(OH)12(tacn)6]Br8.9H2O. The
observed lineshapes exhibit a pronounced dependence on temperature, magnetic
field, and the spin quantum numbers (Ms values) associated with the levels
involved in the transitions. Measurements at many frequencies allow us to
separate various contributions to the EPR linewidths, including significant
D-strain, g-strain and broadening due to the random dipolar fields of
neighboring molecules. We also identify asymmetry in some of the EPR lineshapes
for Fe8, and a previously unobserved fine structure to some of the EPR lines
for both the Fe8 and Mn12 systems. These findings prove relevant to the
mechanism of quantum tunneling of magnetization in these SMMs.Comment: Phys. Rev. B, accepted with minor revision