OBSERVATION OF THE DYNAMIC BEHAVIOR OF THE ANTIFERROMAGNETIC-FERROMAGNETIC PHASE-TRANSITION IN THE ONE-DIMENSIONAL SPIN-1-2 ANTI-FERROMAGNET ALPHA-CUNSAL

Proton spin lattice relaxation rates in the spin-1/2 Heisenberg antiferromagnet α-CuNSal have been measured in applied fields up to 125 kOe in the temperature range 1-4 K. All inequivalent protons in the unit cell can be identified. The strong coupling of protons close to the antiferromagnetic (AF) chain provides an opportunity to use 1H spin lattice relaxation rates, T1-1, to probe the dynamics of the spin system through the antiferromagnetic-ferromagnetic (AF-F) phase transition as the critical field, Hc, is exceeded. The AF-F phase transition is observed at Hc=4J/geμB(J/kB=3.04 K) as a strong enhancement of T-11. For fields above Hc, the proton T-11 decreases over 4 orders of magnitude. The effect of increasing temperature is to broaden the peak in T -11, and move it to lower field. The uniform magnetization of the AF spin system, as measured by proton resonance shifts, is in excellent agreement with calculations by Bonner and Fisher. The behavior of the relaxation rate can be understood in terms of the fermion representation.