Acknowledgments

The author would like to express sincere thanks to Professor Yasutomo J. Uemura, who, not only introduced me $\mu$SR investigations of the spin systems in this thesis, but also gave me precious hints through the discussions. I learned from him that qualitative understanding of a physical system is much more important than quantitative discussions. I would also like to thank Professor Graeme M. Luke, who carefully read my thesis manuscript and gave me relevant remarks, which improved the physics and the logics in this thesis.

I would also like to express a sincere gratitude to Professor Kanetada Nagamine, who introduced me the $\mu$SR technique five years ago. At KEK/UTMSL muon facility, I learned from him that muon is effective not only as a microscopic probes to spin systems, but also in wider fields, such as nuclear, atomic and geophysical researches. At KEK/UTMSL muon facility, Professor Kusuo Nishiyama educated me on the technical aspects of $\mu$SR measurements, as well as theoretical basics of spin relaxation in solids. I would like to thank him for these points.

The measurements presented in this thesis is based on collaborations. The group lead by Professor Shinichi Uchida at the University of Tokyo provided the specimen of (Y_2-xCa_x)Ba(Ni_1-yMg_y)O₅. I would like to express special thanks to two Professors Shinichi Uchida and Hidenori Takagi, as well as to their students Mr. Kenjirou Kiyono and Mr. Shigeki Miyasaka, for giving us the opportunity to measure their specimens.

The spin ladder materials Sr_n-1Cu_n+1O_2n were provided from the group lead by Professor Mikio Takano at Kyoto University. I would like to express sincere thanks to him and Dr. Masaki Azuma for preparing the specimens, which took their several weeks to accumulate the amount necessary for the $\mu$SR measurements. Also, I would like to thank Professor Takano for having invited me to an informal meeting at his group in February of 1995. The discussions at the meeting was quite educational for me.

The group lead by Professor Kunimitsu Uchinokura at the University of Tokyo provided us the spin Peierls materials (Cu_1-xZn_x)GeO₃. I would like to thank him, Dr. Masashi Hase and Mr. Yoshitaka Sasago for their sample preparations. It should be mentioned that they kindly invented a new sinter technique of the powder samples, so that our measurements using the dilution refrigerator became possible. For the Cu(Ge_1-ySi_y)O₃ crystals, we owe to the group lead by Professor A. Revcolevschi at Université Paris-Sud. I would like to thank them for the collaboration.

Being a beam experiment, the $\mu$SR measurements themselves involved many collaborators. I would like to thank the former members of the Columbia University $\mu$SR group, Drs. Liang Ping Le, Amit Keren and Weidong Wu, as well as the current members, Ben Nachumi, Oleg Tchernyshyov, Mike Larkin and Jack Merrin for taking the shift works during the beam time. Among them, I would also like to thank especially Ben Nachumi and Mike Larkin for their critical reading of and comments to this thesis. I also would like to thank to the exciting discussions with the TRIUMF people, Professors Jess Brewer and Rob Kiefl, Drs. Syd Kreizman, Kim Chow, Bessam Hitti, Andrew MacFarlane and Sarah Dunsiger. Some of the measurements in this thesis became possible owing to the recent technical developments at TRIUMF, which are contribution of these TRIUMF people and the $\mu$SR technician/engineers Kieth Hoyle, Curtis Ballard and Mel Good, as well as other technical staffs of TRIUMF.