Low temperature spin dynamics and high pressure effects in frustrated pyrochlores

Frustrated pyrochlores R$_{2}$M$_{2}$O$_{7}$, where R$^{3+}$ is a rare earth and M$^{4+}$ a transition or sp metal ion, show a large variety of exotic magnetic states due to the geometrical frustration of the pyrochlore lattice, consisting of corner sharing tetrahedra for both R and M ions. Neutron scattering allows one to measure their magnetic ground state as well as the spin fluctuations, in a microscopic way. An applied pressure may change the subtle energy balance between magnetic interactions, inducing new magnetic states. In this talk, I will review recent neutron results on Terbium pyrochlores, investigated by high pressure neutron diffraction and inelastic neutron scattering. Tb$_{2}$M$_{2}$O$_{7}$ pyrochlores show respectively a spin liquid state for M=Ti [1], an ordered spin ice state for M= Sn [2], and a spin glass state with chemical order for M=Mo [3]. In Tb$_{2}$Ti$_{2}$O$_{7}$ spin liquid, where only Tb$^{3+}$ ions are magnetic, an applied pressure induces long range antiferromagnetic order due to a small distortion of the lattice and magneto elastic coupling [4]. In Tb$_{2}$Sn$_{2}$O$_{7}$, the substitution of Ti$^{4+}$ by the bigger Sn$^{4+}$ ion expands the lattice, inducing a long range ordered \textit{ferromagnetic }state, with the local structure of a spin ice [2] and unconventional spin fluctuations [2,5]. The local ground state and excited crystal field states of the Tb$^{3+}$ ion were recently investigated by inelastic neutron scattering in both compounds [6]. Tb$_{2}$Mo$_{2}$O$_{7}$, where Mo$^{4+}$ ions are also magnetic, shows an even more rich behaviour, due to the complex interaction between frustrated Tb and Mo lattices, having respectively localized and itinerant magnetism. In Tb$_{2}$Mo$_{2}$O$_{7}$ spin glass, the lattice expansion induced by Tb/La substitution yields an ordered ferromagnetic state, which transforms back to spin glass under applied pressure [7]. New data about the spin fluctuations in these compounds, as measured by inelastic neutron scattering, will be presented. The talk will be dedicated to the memory of Igor Goncharenko, a renowned high pressure and neutron physicist, who died accidentally on Nov. 4$^{th}$, 2007. \newline [1] J. S. Gardner \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 1012, (1999). \newline [2] I. Mirebeau \textit{et al.,} Phys. Rev. Lett. \textbf{94}, 246402, (2005). \newline [3] B. D. Gaulin \textit{et al.,} Phys. Rev. Lett. \textbf{69}, 3244, (1992). \newline [4] I. Mirebeau \textit{et al.} Nature \textbf{420}, 54 (2002); Phys. Rev. Lett. 93, 187204, (2004). \newline [5] F. Bert \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 117203, (2006)~; P. Dalmas de R\'{e}otier \textit{et al.}, Phys. Rev. Lett.\textbf{ 96}, 127202, (2006). \newline [6] I. Mirebeau, P. Bonville, M. Hennion, Phys. Rev. \textbf{76}, 184436, (2007). \newline [7] A. Apetrei et al, Phys. Rev. Lett. \textbf{97}, 206401, (2006).