Characterization and Magnetic Properties of Heavy Rare-Earth A₂Ir₂O₇ Pyrochlore Iridates, the Case of Tm₂Ir₂O₇

Abstract

The magnetic properties of newly prepared Tm 2 Ir 2 O 7 were investigated by means of magnetization and specific heat measurements and compared to the properties of other heavy rare-earth pyrochlore iridates. A bifurcation of zero-field-cooled and fieldcooled magnetization coinciding with the onset of the specific heat anomaly indicated the magnetic ordering of the Ir sublattice, in agreement with studies on other pyrochlore iridates. The phase diagram depicting the ordering/metal-insulator-transition temperature dependent on the rare-earth ionic radius was updated and completed. Magnetic dc susceptibilities and magnetic contributions to the specific heat of Er, Tm, and Yb analogues were well described by crystal field parameters and eigenenergies, previously reported for Er 2 Ir 2 O 7 and Yb 2 Ir 2 O 7 . The crystal field parameters for, the so-far-unstudied, Tm 2 Ir 2 O 7 were calculated by scaling the Er 2 Ir 2 O 7 parameters employing appropriate Stevens factors for Er 3+ and Tm 3+ . Iridates with the Kramers' Er 3+ and Yb 3+ ions revealed a splitting of the ground-state doublet, leading to a broad anomaly in specific heat at around 2 K and 3 K, respectively. The previously reported low value of magnetic moment in Yb 2 Ir 2 O 7 was confirmed by magnetization measurements. The nonmagnetic ground-state crystal-field singlet in Tm 2 Ir 2 O 7 dictated by Tm trigonal Wyckoff position was confirmed by both evolution of specific heat at low temperature and temperature dependence of the magnetic entropy.

The magnetic behaviour of Dy2Ir2O7 – Beyond the mean field approximation

“…The temperature of a bifurcation, TIr (= 128 K), is well in agreement with previous results ( 12), (15), and fits perfectly into the A2Ir2O7 phase diagram presented in Ref. (13). TIr marks the onset of the, so far unreported, anomaly in specific heat (vertical line in Figure 1), well in agreement with other pyrochlore iridates (13).…”

“…(13). TIr marks the onset of the, so far unreported, anomaly in specific heat (vertical line in Figure 1), well in agreement with other pyrochlore iridates (13).…”

“…Although the SIr value is higher than for other heavy A2Ir2O7 (13) and Nd2Ir2O7 (15), it is still significantly smaller than the magnetic entropy expected for Ir 4+ with S = 1/2. Similar values of estimated electronic specific heat coefficient γIr = SIr/ TIr are followed within the rareearth A2Ir2O7 series (13), indicating non-metallic state; while γIr = 9.2(2) mJmol -1 K -2 for Dy2Ir2O7 points rather to the semi-metallic ground state , similarlyas in Lu2Ir2O7 (23).…”

“…The specific heat measurement at temperatures 0.4 -300 K was performed using PPMS, Quantum design, equipped with the 3 He insert. The magnetic susceptibility in the temperature range 1.8 -330 K was measured employing superconducting quantum interference device magnetometer (MPMS7, Quantum design); the low-temperature data (down to 0.4 K) were collected using a PPMS equipped with 3 He insert and custom-made Hall probes option (13), (20), (21).…”

“…To properly analyse the anomaly we followed the approach used investigating other A2Ir2O7 analogues (13), that is, the estimation of related entropy. The measured data were fitted by a third-order polynomial (blue line in Figure 1b) at highest temperatures to roughly estimate the specific heat without magnetic contribution of Ir sublattice.…”