Proton spinhyphen;lattice relaxation in lowhyphen;dimensional ferromagnetic copper halides (abstract)

摘要

1H spinhyphen;lattice relaxation times have been measured as functions of temperature and frequency in powder samples of the twohyphen;dimensional ferromagnetic compound (CH3NH3)2CuCl4and in single crystals of the onehyphen;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 ofT1upon magnetic field orientation was measured for the onehyphen;dimensional (1D) single crystal samples. Each compound showed basically two temperature regimes of different spinhyphen;lattice relaxation behavior, separated by a narrow transition temperature region. From 4.2 K,T1in the compounds decreased strongly as the temperature was raised, a behavior expected for secondhyphen;order Raman processes lsqb;K. M. Kopinga, A. M. C. Tinus, W. J. M. de Jonge, and G. C. de Vries, Phys. Rev. B36, 5398 (1987)rsqb;. At the transition temperature region the decrease ofT1ceased, andT1began 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 (sim;70 K), CHAB (sim;55 K), and TMCuC (sim;30 K), and also above the compoundsrsquo; 3D ordering temperatures (sim;1.5 K and below). We noted a correlation between theT1transition temperatures and temperatures at which spin dimensionality lsquo;lsquo;crossoversrsquo;rsquo; are observed in magnetic susceptibilities, going from Heisenberg to nonhyphen;Heisenberg behavior as the temperature is decreased. The latter occur at approximately 10 K in CHAC. TMCuC, which has the most isotropicJtensor of these compounds and also the lowest weakhyphen;strongT1transition, 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 ofT1.