Tuning Frustration in Rare Earth Pyrochlores by Platinum Substitution

A successful mechanism for exploring the rich physics of rare earth pyrochlores, R$_2$B$_2$O$_7$, is to substitute the non-magnetic B-site. Varying the ionic radius of the B-site induces an internal chemical pressure. Some rare earths are robust to substitutions; for example, the holmium-based pyrochlores all exhibit a dipolar spin ice state. In the case of other rare earths such as ytterbium, the ground states are remarkably fragile to chemical pressure. In this talk, I will introduce two materials with a new non-magnetic B-site: platinum. The ionic radius of platinum is comparable to that of titanium, which occupies the B-site in the most well-studied family of pyrochlores. Thus, platinum does not induce a strong chemical pressure on the lattice. Nevertheless, using Gd$_2$Pt$_2$O$_7$ and Er$_2$Pt$_2$O$_7$ as examples, I will show that platinum does affect a dramatic change on the magnetic properties. We trace this effect to platinum's empty $e_g$ orbitals, which mediate superexchange pathways not available in other rare earth pyrochlores. In Gd$_2$Pt$_2$O$_7$, this results in a striking 160\% enhancement of $T_N$ as compared to other Gd-based pyrochlores. In Er$_2$Pt$_2$O$_7$, the ordering temperature is strongly suppressed and the ground state is altered.