| Popis: |
The dynamical behaviour of the S = 1 2 Heisenberg antiferromagnetic linear-chain system in single crystals CuSeO 4 .5H 2 O is investigated through proton spin-lattice relaxation experiments. This insulator contains besides the linear-chain system with exchange interaction J/k =-0.85 K, a paramagnetic system with θ=0.05 K. The dynamic properties of both magnetic systems have been studied extensively as a function of field (5–15 kOe) and temperature (0.3–20 K) and prove to be quite different. The contributions of these two systems to the relaxation of different protons are separated. The use of single crystals allows a study of T 1 as a function of the direction of the external field. Moreover the large dipolar fields at the proton sites due to the presence of the paramagnetic system, provide the unique opportunity for a chain system to study the relaxation times of different singular resonance lines. The spin-lattice relaxation of protons that interact dominantly with the paramagnetic system is governed by the spectral density of the auto-correlation function of the z component of the paramagnetic electron spins. The relaxation of protons close to the ALC system, however, proves to be governed by the spectral densities of the auto-correlation function of both the parallel and perpendicular component of the nearest ALC electron spin, φ z 0 (ω n ) and φ + 0 (ω n ). A study of the angular dependence of T 1 shows that these spectral densities of the ALC system exhibit different field and temperature dependencies. Particularly the difference in the temperature dependence is striking: φ z 0 (ω n ) decreases towards lower temperatures, whereas φ + 0 (ω n ) is temperature independent. |
