Muon Spin Relaxation Measurements
of
Singlet Ground-State Materials

Kenji Kojima

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF SCIENCE
IN THE GRADUATE SCHOOL OF SCIENCE

THE UNIVERSITY OF TOKYO

DECEMBER, 1995

COPYRIGHT © 1995 BY KENJI KOJIMA. ALL RIGHTS RESERVED

Abstract:

  This thesis reports experimental investigations of three types of spin systems, namely, (1) spin-ladder materials Sr_n-1Cu_n+1O_2n, (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 ($\mu$SR) method is the main experimental technique. Therefore in Chapter 2, the $\mu$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 Sr_n-1Cu_n+1O_2n is discussed. With the $\mu$SR technique, it was found that magnetic behavior of these spin ladder cuprates strongly depends on the width of the ladder. The $\mu$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 $\mu$SR results of a Haldane material Y₂BaNiO₅. A related vacancy-doped system, Y₂Ba(Ni_1-yMg_y)O₅, and charge-doped system, (Y_2-xCa_x)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, $\mu$SR results of an inorganic spin-Peierls material CuGeO₃ and two types of doped compounds [(Cu_1-xZn_x)GeO₃ and Cu(Ge_1-ySi_y)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.