Ferromagnetic - spin glass transition induced by pressure in the geometrically frustrated pyrochlore (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$

TL;DR

This study investigates how pressure influences the magnetic states in (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$ pyrochlores, revealing a transition from ferromagnetic to spin-glass behavior and providing insights into spin correlations and fluctuations.

Contribution

It demonstrates pressure-induced suppression of ferromagnetic order and recovery of spin-glass state in a geometrically frustrated pyrochlore, elucidating the microscopic spin dynamics involved.

Findings

Substitution of La induces ferromagnetism from a spin-glass state.

Pressure suppresses long-range order, reverting to spin-glass behavior.

Muon spin relaxation reveals a second dynamical transition below T$_C$.

Abstract

We have studied (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$ pyrochlores by neutron diffraction and $\mu$SR at ambient and under applied pressure. Substitution of Tb for La expands the lattice and induces a change from a spin-glass like state ({\itshape x}=0) to a non collinear ferromagnet ({\itshape x}=0.2). In the ferromagnetic structure, the Tb moments orient close to their local anisotropy axis as for an ordered spin ice, while the Mo moments orient close to the net moment. The $\mu$SR dynamical relaxation rate shows a cusp-like peak at the Curie temperature T$_C$ and a broad anomaly at T$^\star$$<$T$_C$, suggesting a second transition of local or dynamical nature. Under pressure, the long range order breaks down and a spin glass-like state is recovered. The whole set of data provide a microscopic picture of the spin correlations and fluctuations in the region of the ferromagnetic-spin glass threshold.