Magnetic Ordering Process in Geometrically Frustrated Systems

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The results of the research on geometrically frustrated systems are as follow.1) Magnetic ordering process in CsNiCl_3-type compoundsNMR spectra of CsFeBr_3 and RbFeBr_3 turn to powder patterns in certain temperature range just below T_N indicating randomness of mutual phases of adjacent magnetic layrs with 120ﾟ structure. But it is not yet known whether the randomness is static or dynamic. It was also found that NMR signal in the intermediate temperature phase of CsCoBr_3 due to fluctuations of the internal field caused by domain wall solitons which was revealed to survive down to the low temperature phase. The magnetic structure at high magnetic fields of CsNiCl_3 was identified to be as predicted bu theory.2) Crystalline field splitting and magnetic phase diagram in FeI_2An electronic transition associated with the crystalline field splitting was observed around 100K by ESR and muSR in FeI_2. Measurements of magnetization on FeI_2 in intense magnetic fields revealed the absence of t … More he tricritical point speculated previously which may be related to the first order transition at T_N.3) Impurity effect on spin symmetry in CuFeO_2Al doping to CuFeO_2 was found to change the Ising spin system to a Heisenberg system, but the experimental results concerning the crossover concentration as well as the coexistence of both systems are not yet consistent with each other.4) Ground state of three dimensional competing system with corner shared tetrahedronsRemarkable spin fluctuations probed by muon spin relaxation survive down to 0.1K in SrCr_8Ga_4O_<19> with Kagome lattice composed of corner shared tetrahedrons, whereas neutron spin echo revealed the spin freezing below 3.5K consistently with the field cooling effect observed in the magnetic susceptibility. Similarly spin freezing was observed below 2K by means of muSR in Y (Sc) Mn2 although no anomaly in the relaxation time of NMR has been detected by NMR down to 0.07K.Thus in strongly frustrated systems with corner shared tetrahedronsan spins freeze at low temperatures. The spin freezin in the geometrically frusatrated systems has different features from that in random spin systems. The elucidation of the difference is a future problem. Less