Low dimensional molecular magnets investigated by NMR

Abstract

The molecular magnets synthesized by metal-organic matter show interesting physical phenomena induced from properties such as the weak exchange interaction, the well defined dimensionality and controllable lattice structure. The CuPzN(Pz=pyrazine=$N_2 C_4 H_4$), which is one of the molecular magnets, is considered as an ideal one-dimensional antiferromagnetic Heisenberg spin-1/2 chain system. Tomonaga-Luttinger-Liquid(TLL) theory describing the one dimensional system predicts that NMR relaxation time 1/$T_1$ has a power law behavior as a function of temperature. In order to test the TLL theory, NMR relaxation time 1/$T_1$ was measured in CuPzN. NMR 1/T_1 shows the power law behavior at H = 7 T in this experiment. We also studied another Cu-pyz-based sample $[CuCl(pyz)_2]BF_4$, which is quasi two-dimensional weakly anisotropic spin-1/2 Heisenberg square lattice antiferromagnet. This material shows a crossover phenomena from the 2D Heisenberg model to 2D XY(Ising) model . We determined the crossover temperature $T_{CO}$ by using the susceptibility measurement. NMR experiment was also carried to determine critical temperature and universality class. This material shown $T_{CO}<$T_N$, which is different from other weakly anisotropic antiferromagnets. Critical temperature $T_N$ increases and critical exponent $\beta$ of the magnetization decreases with increasing magnetic field.