Magnetic properties of heavy rare-earth A2Ir2O7 pyrochlore iridates

The magnetic order is supposed to be a long-range all-in-all-out (AIAO) order, however short-range correlations between Ir$^{4+}$ magnetic moments have also been proposed [3]. Very recently, neutron diffraction experiments on Yb$_2$Ir$_2$O$_7$ and more importantly Lu$_2$Ir$_2$O$_7$ revealed several magnetic peaks consistent with the AIAO magnetic structure of the iridium sublattice [5]. Although significant attention has been dedicated to the rare-earth iridate pyrochlores in last years, our knowledge on these systems is still in an early stage and further experimental and theoretical effort is to be devoted to the topic.
The present study is focused on a systematic investigation of magnetic properties of the heavy rare-earth A$_2$Ir$_2$O$_7$ pyrochlore oxides with A = Er – Lu, including, the so-far-unreported, Tm$_2$Ir$_2$O$_7$. The iridates were investigated by means of specific heat, magnetization and ac-susceptibility measurements at temperatures down to 0.4 K. The magnetic ordering of the iridium sublattice at high temperature for all investigated analogues was confirmed by an anomaly in specific heat, bifurcation of zero-field-cooled and field-cooled magnetization and an anomaly in ac-susceptibility data. A$_2$Ir$_2$O$_7$ with A being a magnetic ion revealed, moreover, the low-temperature anomalies ascribed to magnetic ordering (rather short-range) of A$^{3+}$ magnetic moments. The measured physical properties and their development with A were discussed in respect with crystal field schemes, either determined from inelastic neutron scattering data or scaled utilizing previous results. Dynamical response of the system indicates rather short-range ordering of both magnetic sublattices, which is however at least in the case of A = Yb and Lu in disagreement with a previous neutron diffraction study [5]. Different time-scales could play a role, assuming the Ir moments fluctuate slow enough to appear static to neutrons, and fast enough to induce a frequency dependence of the ac-susceptibility signal. The experimental data are supported by DFT calculations.

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