This thesis reports experimental investigations of three types of spin systems, namely, (1) spin-ladder materials Srn-1Cun+1O2n, (2) a Haldane material Y2BaNiO5 and (3) a spin-Peierls compound CuGeO3. The common feature of these spin systems is the absence of conventional Néel order due to quantum mechanical effects; the ground state structures are characterized by singlet-pair formations of spins, as introduced in Chapter 1. In this thesis, the muon spin relaxation (SR) method is the main experimental technique. Therefore in Chapter 2, the SR technique is introduced, followed by a brief introduction to spin relaxation theories in solids (Chapter 3). Most of the content of these two chapters is important for the understanding of the subsequent chapters, in which the experimental results are presented.
In Chapter 4, the magnetism of the spin-ladder materials Srn-1Cun+1O2n is discussed. With the SR technique, it was found that magnetic behavior of these spin ladder cuprates strongly depends on the width of the ladder. The SR spectra from the ladder materials provide a good experimental example of the spin relaxation theories introduced in Chapter 3. The content of this chapter has been published as Ref. .
Chapter 5 presents the SR results of a Haldane material Y2BaNiO5. A related vacancy-doped system, Y2Ba(Ni1-yMgy)O5, and charge-doped system, (Y2-xCax)BaNiO5, were also investigated. It was found that vacancy doping and charge doping lead to completely different ground states. In the charge-doped compounds especially, unconventional spin dynamics were observed in the milli-Kelvin regime. Most of the results in this chapter have been published in Ref. [2,3].
In Chapter 6, SR results of an inorganic spin-Peierls material CuGeO3 and two types of doped compounds [(Cu1-xZnx)GeO3 and Cu(Ge1-ySiy)O3] are reported. It was found that these two types of doping result in a magnetically ordered state; it was clearly Néel order in the Si-doped system.
Concluding remarks are given in the last chapter, Chapter 7.