Spin Dynamics In Rare Earth Titanates R₂Ti₂O₇(R=Dy/Tb,Nd) Studied By Ac Susceptibility

Rare earth titanates R22Ti2O7 (R=Y, lanthanide) are a family of materials showing rich physical properties. Pyrochlore structure with smaller rare earth ions (Y3+, Sm3+-Lu3+) and monoclinic structure with larger rare earth ions (La3+-Nd3+) are the two common structures of R2Ti2O7. In pyrochlore R2Ti2O7, the geometrical frustration generic to the pyrochlore lattice, together with competing interactions of different energy scales, including crystal field, exchange interactions and dipolar interactions, give rise to exotic magnetic states, such as spin ice state and spin liquid state. Research efforts on the monoclinic R2Ti2O7 are mainly focused on their ferroelectric behavior, optical catalyst effects, etc, while their detailed magnetic properties are rarely investigated. In this thesis, we studied the spin dynamics of pyrochlore DyxTb2-xTi2O7 and monoclinic Nd2Ti2O7 using the ac susceptibility measurement as the main probe.In the DyxTb2-x Ti2O7 system, the two compounds at the composition boundaries are spin ice Dy2Ti2O7 and spin liquid Tb2Ti2O7, which are the two representative geometrically frustrated pyrochlores. Considering the sensitivity of the spin ice and spin liquid ground states to the external perturbations such as the external pressure and magnetic field, we investigated the DyxTb2-xTi2O7 system in which both Dy3+ and Tb3+ spins coexist, to explore the possible existence of new magnetic states. We studied the ac susceptibility of DyxTb2-xTi2O7(x∈[0,2]) in detail. In addition to the known Dy3+ single-ion peak at Ts (Ts peak), we identified a new freezing-like peak associated with Tb3+ spins (T* peak). The frequency dependence of the T* peak can be fitted by the Arrhenius law, which indicates its thermally activated nature. We proposed that T* peaks correspond to the low-lying crystal field levels of Tb3+ spins, and the crystal field scheme evolves as the composition x changes, which explains the phase diagram of T*(x). Moreover, the strong magnetic field dependence of Ts and T* peaks, especially their absence in zero field, indicates the possible existence of strong magnetoelastic coupling. The finding of the T* peak in DyxTb2-xTi2O7 demonstrates the rich dynamical magnetic behavior of such systems, and provides experimental foundations for future explorations of hybrid rare earth spin systems.The structure of R2Ti2O7 transforms into monoclinic as the ionic radius of R increases. We have investigated Nd2Ti2O7 in this category. We showed anisotropic paramagnetism arising mainly from the crystal electric field, and separated the exchange contributionθex and the crystal field contributionθCF to the total Weiss temperatureθW.Theθex orders of magnitude higher than the magnetic ordering temperature Torder suggests strong spin frustration in the system, despite the absence of a pyrochlore structure. The 4I9/2 ground state of Nd3+ spins splits into five Kramers doublets, which further split into singlets in the presence of a magnetic field. The ground state doublet dominates the magnetic specific heat of Nd2Ti2O7 at low temperatures. The zero field magnetic entropy is integrated to be Rln2, consistent with the Ising nature of the Nd3+ spins dictated by the ground state doublet. More importantly, using the ac susceptibility measurement, we found in Nd2Ti2O7 a novel field-induced slow spin relaxation in the paramagnetic state. Spin dilution effect by substituting Nd3+ with nonmagnetic La3+ indicates that the relaxation is closely related to spin correlations. The significance of the spin correlation is also revealed in the magnetic specific heat which shows that approximately 50% of the total spin entropy freezes before entering the magnetically ordered ground state, indicating the formation of correlated spin regions at higher temperatures. The spin dilution effect as well as the external pressure effect further reveal rich spin dynamics in Nd2Ti2O7. Based on these experiments, we propose that the field-induced slow spin relaxation is associated with the cooperative behavior of the correlated regions formed by partially polarized spins through spin correlations. The novel field-induced slow spin relaxation in Nd2Ti2O7 is another example of spin correlation effect in a paramagnet among rare experimental realizations, including the slow spin relaxation in high field in Dy2Ti2O7 and Tb2Ti2O7. We further investigated the Y-doped Nd2Ti2O7, which stabilizes in pyrochlore structure due to the smaller ionic radius of Y3+, and accommodates the geometrical frustration. ac susceptibility measurements show that the field-induced slow spin relaxation still persists. Therefore the field-induced slow spin relaxation is not specific to the detailed crystal field scheme. We propose that cooperative behaviors can exist not only in the so-called cooperative paramagnets, which have large ground state degeneracy as observed previously in Dy2Ti2O7 and Tb2Ti2O7, but also in many more paramagnets with strong spin correlations, while their observable microscopic effects are closely associated with the degree of frustration in the system.