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Abstract:
The level crossing mechanism between the ground and the first
excited state of Na:Fe-6 antiferromagnetically coupled iron
rings is studied by torque magnetometry down to 40 mK and in
magnetic fields up to 28 T. The step width at the crossing
field B. assumes a finite value at the lowest temperatures.
This fact is ascribed to the presence of level anticrossing,
not expected for a ring with axial, i.e. S-6 point group,
symmetry. Assuming a reduced symmetry, we revised the model
Hamiltonian of such a spin system by introducing a
Dzyaloshinsky-Moriya (DM) term and we show, by exact
diagonalization, that DM term can account for the mixing of
states with different parity. In particular, analytical as well
numerical analysis show that the introduction of the DM term
may contribute to the broadening of the torque step as well as
for the finite energy gap at B, observed by heat capacity in a
similar ring Li:Fe-6 as previously reported [9].
