Slow spin relaxation in the Heisenberg antiferromagnet Gd₂(fum)₃(H₂O)₄·3H₂O induced by magnetic field

The spin relaxation of Gd₂(fum)₃(H₂O)₄·3H₂O, an S = 7/2 Heisenberg antiferromagnet was investigated by ac-susceptibility measurements at temperatures from 2 to 30 K, frequencies from 100 Hz to 10 kHz, and magnetic fields up to 3 T. It was found, that the magnetic field induces thermally activated relaxation over an energy barrier E_B/k_B approx 55 K. Subsequent investigation of the specific heat, magnetic entropy, magnetization, and shape of the electron-spin-resonance line yielded consistent values of the g-factor g = 2.0, single-ion anisotropy D/k_B approx 0.23 K, and exchange interaction J/k_B approx 0.12 K. The knowledge of these characteristic parameters allows to discuss the origin of the observed relaxation behavior. In particular, resonance trapping of low-energy phonons, recently proposed in Ni₁₀ magnetic molecules [1] will be considered.