The Physics and Chemistry of Quantum Magnetism

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In this project, we have performed several experimental studies on the spin, S=1 quasi-one-dimensional(Q1D) Heisenberg antiferromagnet Ni(C_5H_<14>N_2)_2N_3(PF_6)(abbreviated NDMAP) for which a field induced magnetic long range ordering (LRO)has been discovered by the present investigator and co-workers. From the temparature and magnetic field dependence of the magnetization, we got information on the spin arrangement in the ordered phase. Namely, at low temperature and high magnetic fields, spins are confined in the c plane and directed perpendicular to the external field. From electron spin resonance(ESR)measurements on NDMAP, we observed two kinds of signals at low temperature and high magnetic field, one of which corresponds to an antiferromagnetic resonance and the other to a one-dimensional excitation. This finding Provides a new knowledge towards the understanding of magnetic excitation in an S=1 Q1D antiferromagnet. We have also measured neutron scattering on deuterated single … More crystals of NDMAP and found a Bragg peak at(0, 1/2, 1/2)point associated with the long range magnetic ordering at high field and low temperature, thus confirming that the field unduced LRO really exists.In collaboration with theoretical physicists, we have analyzed the magnetic field versus temperature phase diagram of NDMAP.We calculated the staggered susceptibility of S=1 one dimensional system with a single ion snisotropy taking into account the effects of the inter-chain couplings as the form of a mean-field. The theoretical result well reproduces the experimental phase diagram obtained by us.We have measured the magnetic excitations from the ladder part of La_6Ca_8Cu_<24>O_<41> using a pulsed neutron source. We were successful in observing a full dispersion curve for the low energy excitation in the ladder. From the analysis of the data, we found that a four spin interaction with about 10% of the dominant interactions(leg and ladder) is necessary to get a reasonable result.In addition to these, we have studied several quantum magnets and obtained new results, details of which are reported in the booklet. Less