Magnetic Order and Excitation in Frustrated Pyrochlore 5d - Transition Metal Oxides

Feng, Erxi

Book/Dissertation / PhD Thesis

FZJ-2018-07549

Magnetic Order and Excitation in Frustrated Pyrochlore 5d - Transition Metal Oxides

Feng, E. (Corresponding author)FZJ*

2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-365-5

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / Key Technologies 187, IV, 182 S. (2018) = RWTH Aachen, Diss., 2018

Please use a persistent id in citations: http://hdl.handle.net/2128/21067

Abstract: Motivated by the recent experimental realizations of novel magnetic phases and emergent quasiparticles, such as magnetic monopoles and Majorana fermions, in frustrated and topological quantum magnets, this thesis focuses on the studies of the magnetic ground state of a complete series of pyrochlore 5d transition metal oxides, including the Ln$_{2}$Ir$_{2}$O$_{7}$ (Ln =rare-earth elements) family and Nd$_{2}$Hf$_{2}$O$_{7}$, via in-house specific heat and magnetization measurements as well as advanced neutron scattering techniques. This has allowed us to establish a global phase diagram of the magnetic ground state of Ln$_{2}$Ir$_{2}$O$_{7}$. Except for Pr$_{2}$Ir$_{2}$O$_{7}$, other pyrochlore iridates exhibit a transition at finite temperature from a paramagnetic metal to an antiferromagnetic semimetal with the “all-in-all-out” magnetic order of the Ir$^{4+}$ sublattice. The magnetic ground-state of Ln$^{3+}$ is strongly dependent on the Ln$^{3+}$ single-ion anisotropy and the molecular magnetic field generated by the surrounding Ir$^{4+}$ sublattice. For the Ln$^{3+}$ ions with an easy-axis anisotropy along the local [1 1 1] axis, which is parallel to the molecular field, an “all-in-all-out” long-range magnetic order is universally observed on the Ln$^{3+}$ sublattice. On the contrary, for the Ln$^{3+}$ ions with an easy plane anisotropy, a dynamic spin liquid phase persists down to the sub-Kelvin temperature range. Furthermore, aided by the magnetic structure refinements, sum-rule analysis of the total magnetic moments, quantitative analyses of low-temperature magnetic entropy and simulations of inelastic neutron scattering spectra, a deeper understanding on the exotic magnetic order and emergent quantum excitations in the ground state of two representative 5d pyrochlore compounds, the metallic spin-ice Pr$_{2}$Ir$_{2}$O$_{7}$ and insulating quantum spin-ice candidate Nd$_{2}$Hf$_{2}$O$_{7}$, have been obtained. Pr$_{2}$Ir$_{2}$O$_{7}$ undergoes a magnetic transition from paramagnetic state to the “2-in-2-out” long-range order of Pr$^{3+}$ below 0.76 K, signalized by a slightly dispersive gapped magnetic excitation. Nd$_{2}$Hf$_{2}$O$_{7}$ shows an “all-in-all-out” longrange order of Nd$^{3+}$ below 0.53 K. Its magnetic excitation consists of a flat band modecentered at 0.1 meV and a dispersive mode up to 0.23 meV in the ordered state. The XYZ model based on the “dipole-octupole” ground-state doublet is introduced to explain these observations. The Occurrence of exotic quantum fragmentation of magnetic moments in Nd$_{2}$Hf$_{2}$O$_{7}$ can thus be confirmed.

Keyword(s): Magnetic Materials (1st) ; Magnetism (2nd) ; Crystallography (2nd) ; Materials Science (2nd)