| Research Abstract |
Recently, magnetic properties of the low-dimensional quantum spin systems with spatial structures have been of great interest. In particular, the quantization of the magnetization in the one-dimensional system is a new important problem. In the present study, we have investigated new quantum effect in KCuCl_3, TlCuCl_3 and NH_4CuCl_3 with the double chain. The exchange interactions in the double chain are described as a Heisenberg spin ladder with the diagonal interaction or an alternating chain with the next-nearest-neighbor interaction. From the recent theories, it is expected that the ground of the present systems is a spin singlet with an excitation gap. We first prepared large single crystals, and measured the magnetic susceptibilities of these three compounds. We found that the ground states of KCuCl_3 and TlCuCl_3 are spin singlets with excitation gaps as expected. We estimated the magnitude of the excitation gap by means of the high-field magnetization measurements. In order to
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investigate the magnetic excitations, we carried out the high-frequency high-field ESR measurements in KCuCl_3 and TlCuCl_3, and observed the excitations from the singlet ground states. We also performed neutron inelastic scattering measurements, and determined the dispersion relations in KCuCl_3 and TlCuCl_3.
NH_4CuCl_3 has the same crystal structure as those of KCuCl_3 and TlCuCl_3 at room temperature. However, its zero-field ground state is magnetic in contrast with those of KCuCl_3 and TlCuCl_3. The remarkable feature of the magnetization process in NH_4CuCl_3 is two plateaus at one-quarter and three-quarters of the saturation magnetization. The plateaus appear in every external field direction. Between the magnetization plateaus, there are slopes. Thus, the plateaus cannot be understood in terms of the metamagnetic transitions in the Ising-like spin systems. The origin of the plateaus should be attributed to the quantum effect. It is suggested that the plateaus are accompanied with the excitation gap. We carried out the ESR measurements, and found that there are excitation gaps in the plateau regions. To the best of the authors' knowledge, NH_4CuCl_3 shows the first example of the quantization of the magnetization in a spin-1/2 system. The discovery of the magnetization plateaus in NH_4CuCl_3 is the most important fruit in the present study. However, the microscopic mechanism of the quantum magnetization process in NH_4CuCl_3 has not not clarified. Thus, to clarify the mechanism is one of the most important future problem. Less
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