Magnetic properties of rare earth zirconate pyrochlores

This thesis is about the experimental investigation of three magnetic pyrochlore compounds Nd${}_2$Zr${}_2$O${}_7$, Sm${}_2$Zr${}_2$O${}_7$ and Gd${}_2$Zr${}_2$O${}_7$. Powder and single crystal samples are synthesized and synchrotron X-ray diffraction and neutron diffraction indicate that the former two compounds crystallize in the well-ordered pyrochlore phase while Gd${}_2$Zr${}_2$O${}_7$ has some Gd/Zr antisite defects. All three compounds show anomalies at 0.4, 0.5 and 0.7 K in the bulk property measurements, respectively, but only Nd${}_2$Zr${}_2$O${}_7$ is found to develop a long-range magnetic order in its ground state. The crystal field splitting schemes of Nd${}^{3+}$ and Sm${}^{3+}$ ions are determined with inelastic neutron scattering which shows that the crystal field ground states of the two ions in pyrochlores are well-isolated dipolar-otcupolar doublets with Ising anisotropy, which is confirmed by the analyses of the susceptibility and specific heat data. The neutron diffraction reveals "all-in/all-out" magnetic order in Nd${}_2$Zr${}_2$O${}_7$ and liquid-like spin correlations in the Heisenberg antiferromagnet Gd${}_2$Zr${}_2$O${}_7$. The absence of long-range order in Gd${}_2$Zr${}_2$O${}_7$ is related to the antisite defects and the neutron diffraction pattern is analysed by the reverse Monte Carlo method, which reveals dipolar spin correlations and the local Palmer-Chalker type spin configu- ration on the tetrahedron. The muon spin relaxation spectra (above 20 mK) of Nd${}_2$Zr${}_2$O${}_7$ indicates persistent spin dynamics in the ordered state and anomalously slow paramagnetic spin dynamics. The muon spin relaxation spectra of Gd${}_2$Zr${}_2$O${}_7$ (above 1.5 K) and Sm${}_2$Zr${}_2$O${}_7$ (above 20 mK) indicate the strong dynamical nature of the system. The single crystal inelastic neutron scattering of Nd${}_2$Zr${}_2$O${}_7$ reveals spin wave excitations in the ordered state and the spectra are fitted using linear spin wave theory allowing the spin Hamiltonian to be extracted. The spin Hamiltonian provides an explanation for the pinch point pattern observed in the polarized neutron scattering data as well as the scattering sheets perpendicular to the crystallographic (110) direction shown in the unpolarized neutron scattering experiment with field along the (1-10) direction. The Hamiltonian also explains the field-induced spin-flip transition and the hysteresis in the ordered state in the AC susceptibility measurements with fields along the [111] direction. According to the spinon calculation and the inelastic neutron scattering data above the ordering temperature, Nd${}_2$Zr${}_2$O${}_7$ shows some features expected for the quantum spin ice state