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. [1].
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.