Proton spin lattice relaxation in low dimensional ferromagnetic copper halides (abstract)

¹H spin lattice relaxation times have been measured as functions of temperature and frequency in powder samples of the two dimensional ferromagnetic compound (CH₃NH₃)₂CuCl₄ and in single crystals of the one dimensional ferromagnets (C₆H₁₁NH₃)CuB₃ (CHAB), (C₆H₁₁NH₃)CuCl₃ (CHAC), and (C₄H₁₂N)CuCl₃ (TMCuC). Sample temperatures were varied between 4.2 and 298 K, and NMR frequencies ranging from 12.6 to 54.0 MHz were used. Widths and shapes of the lines, typically several hundred Gauss broad at low temperatures, were recorded. The dependence of T₁ upon magnetic field orientation was measured for the one dimensional (1D) single crystal samples. Each compound showed basically two temperature regimes of different spin lattice relaxation behavior, separated by a narrow transition temperature region. From 4.2 K, T₁ in the compounds decreased strongly as the temperature was raised, a behavior expected for second order Raman processes [K. M. Kopinga, A. M. C. Tinus, W. J. M. de Jonge, and G. C. de Vries, Phys. Rev. B 36, 5398 (1987)]. At the transition temperature region the decrease of T₁ ceased, and T₁ began to increase weakly and quasilinearly to 300 K. In the three 1D compounds, the transition regions occurred well below temperatures corresponding to 1D exchange interaction strengths in CHAC (70 K), CHAB (55 K), and TMCuC (30 K), and also above the compounds' 3D ordering temperatures (1.5 K and below). We noted a correlation between the T₁ transition temperatures and temperatures at which spin dimensionality “crossovers” are observed in magnetic susceptibilities, going from Heisenberg to non Heisenberg behavior as the temperature is decreased. The latter occur at approximately 10 K in CHAC. TMCuC, which has the most isotropic J tensor of these compounds and also the lowest weak strong T₁ transition, does not show a spin dimensionality crossover in susceptibility down to 2 K, but based on our NMR results one would be expected at or below this temperature. Further theoretical work appears to be necessary in order to elucidate the role of magnons and solitons in the transition behavior of the temperature dependence of T₁.