Magnetic and transport properties of the pyrochlore iridates (Y1−xPrx)2Ir2O7 : Role of f−d exchange interaction and d−p orbital hybridization (original) (raw)
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Giant Magnetoresistance Effect in the Metal–Insulator Transition of Pyrochlore Oxide Nd2Ir2O7
Journal of the Physical Society of Japan, 2013
We investigated the magnetoresistance (MR) effect of the pyrochlore oxide Nd 2 Ir 2 O 7 , which shows a metal-insulator transition at T MI = 33 K. A small positive MR effect was observed in the metallic state above T MI , while a large negative MR effect was observed in the insulating state below T MI. MR effects ([ρ(B = 0) − ρ(B)]/ρ(B)) exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed the crossover from the insulating state to a state with a small or partial band gap in a field up to 56 T. Furthermore, from the MR effect in Eu 2 Ir 2 O 7 (T MI = 120 K) and Gd 2 Ir 2 O 7 (T MI = 127 K), we revealed that the large negative MR effect of the pyrochlore iridate Ln 2 Ir 2 O 7 depends on the magnetism of the lanthanide Ln 3+ ion. The d-f interaction plays a significant role in the large negative MR effect in the insulating state. KEYWORDS: magnetoresistance, pyrochlore oxides, Nd 2 Ir 2 O 7 , metal-insulator transition, Eu 2 Ir 2 O 7 , Gd 2 Ir 2 O 7
Magnetism and magnetic order in the pyrochlore iridates in the insulator-to-metal crossover region
Journal of Physics: Conference Series, 2014
We present resistivity, magnetization, and zero field muon spin relaxation (µSR) data for the pyrochlore iridate materials Nd2−xCaxIr2O7 (x = 0, 0.06, and 0.10) and Sm2Ir2O7. While Nd2Ir2O7 (Nd227) is weakly conducting, Sm2Ir2O7 (Sm227) has a slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at TM = 105K and 137K, respectively. However, zero-field µSR measurements show that long-range magnetic order of the Ir 4+ sublattice sets in at much lower temperatures (TLRO ∼ 8K for Nd227 and 70K for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at TM in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd2−xCaxIr2O7 samples, but the muon precession is fully suppressed for both.
Effect of carrier concentration on magnetism and magnetic order in the pyrochlore iridates
We present resistivity, magnetization, and zero field muon spin relaxation ($\mu$SR) data for the pyrochlore iridate materials Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ ($x = 0, 0.06$, and 0.100.100.10) and Sm$_2$Ir$_2$O$_7$. While Nd$_{2}$Ir$_{2}$O$_{7}$ (Nd227) is weakly conducting, Sm$_{2}$Ir$_{2}$O$_{7}$ (Sm227) has slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at TM=105KT_{M} = 105 KTM=105K and 137K137 K137K, respectively. However, zero-field mu\mumuSR measurements show that long-range magnetic order of the Ir$^{4+}$ sublattice sets in at much lower temperatures ($T_{LRO} \sim 8 K$ for Nd227 and 70K70 K70K for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at TMT_{M}TM in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ samples, but the muon precession is fully suppressed for both.
Possible scale invariant linear magnetoresistance in pyrochlore iridates Bi2Ir2O7
New Journal of Physics, 2019
We report the observation of a linear magnetoresistance in single crystals and epitaxial thin films of the pyrochlore iridate Bi2Ir2O7. The linear magnetoresistance is positive and isotropic at low temperatures, without any sign of saturation up to 35 T. As temperature increases, the linear field dependence gradually evolves to a quadratic field dependence. The temperature and field dependence of magnetoresistance of Bi2Ir2O7 bears strikingly resemblance to the scale invariant magnetoresistance observed in the strange metal phase in high Tc cuprates. However, the residual resistivity of Bi2Ir2O7 is more than two orders of magnitude higher than the curpates. Our results suggest that the correlation between linear magnetoresistance and quantum fluctuations may exist beyond high temperature superconductors.
Anisotropy-Tuned Magnetic Order in Pyrochlore Iridates
Physical review letters, 2015
The magnetic behavior of polycrystalline samples of Er_{2}Ir_{2}O_{7} and Tb_{2}Ir_{2}O_{7} pyrochlores is studied by magnetization measurements and neutron diffraction. Both compounds undergo a magnetic transition at 140 and 130 K, respectively, associated with an ordering of the Ir sublattice, signaled by thermomagnetic hysteresis. In Tb_{2}Ir_{2}O_{7}, we show that the Ir molecular field leads the Tb magnetic moments to order below 40 K in the all-in-all-out magnetic arrangement. No sign of magnetic long-range order on the Er sublattice is evidenced in Er_{2}Ir_{2}O_{7} down to 0.6 K where a spin freezing is detected. These contrasting behaviors result from the competition between the Ir molecular field and the different single-ion anisotropy of the rare-earth elements on which it is acting. Additionally, this strongly supports the all-in-all-out iridium magnetic order.
Physical Review B
We report on the evolution of the thermal metal-insulator transition in polycrystalline samples of Nd2Ir2O7 upon hole-doping via substitution of Ca 2+ for Nd 3+. Ca substitution mediates a fillingcontrolled Mott-like transition with minimal resolvable structural changes and without altering site symmetry. Local structure confirms that Ca substitution does not result in local chemical phase separation, and absorption spectroscopy establishes that Ir cations maintain a spin-orbit entangled electronic configuration. The metal-insulator transition coincides with antiferromagnetic ordering on the Ir sublattice for all measured samples, and both decrease in onset temperature with Ca content. Weak low-temperature upturns in susceptibility and resistivity for samples with high Ca content suggest that Nd sublattice antiferromagnetism continues to couple to carriers in the metallic regime.
Magnetic order and the electronic ground state in the pyrochlore iridate Nd_{2}Ir_{2}O_{7}
Physical Review B, 2012
We report a combined muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport study of the electronic properties of the pyrochlore iridate Nd 2 Ir 2 O 7 . We observe the onset of strongly hysteretic behavior in the temperature dependent magnetization below 120 K, and an abrupt increase in the temperature dependent resistivity below 8 K. Zero field muon spin relaxation measurements show that the hysteretic magnetization is driven by a transition to a magnetically disordered state, and that below 8 K a complex magnetically ordered ground state sets in, as evidenced by the onset of heavily damped spontaneous muon precession. Our measurements point toward the absence of a true metal-to-insulator phase transition in this material and suggest that Nd 2 Ir 2 O 7 lies either within or on the metallic side of the boundary of the Dirac semimetal regime within its topological phase diagram.
Physical Review B, 2017
The interplay between spin-orbit coupling (SOC) and electron correlation (U) is considered for many exotic phenomena in iridium oxides. We have investigated the evolution of structural, magnetic and electronic properties in pyrochlore iridate Y2Ir2−xRuxO7 where the substitution of Ru has been aimed to tune this interplay. The Ru substitution does not introduce any structural phase transition, however, we do observe an evolution of lattice parameters with the doping level x. X-ray photoemission spectroscopy (XPS) study indicates Ru adopts charge state of Ru 4+ and replaces the Ir 4+ accordingly. Magnetization data reveal both the onset of magnetic irreversibility and the magnetic moment decreases with progressive substitution of Ru. These materials show nonequilibrium low temperature magnetic state as revealed by magnetic relaxation data. Interestingly, we find magnetic relaxation rate increases with substitution of Ru. The electrical resistivity shows an insulating behavior in whole temperature range, however, resistivity decreases with substitution of Ru. Nature of electronic conduction has been found to follow power-law behavior for all the materials.
Magnetic order in the pyrochlore iridates A_{2}Ir_{2}O_{7} (A = Y, Yb)
Physical Review B, 2012
We present results from muon spin relaxation/rotation, magnetization, neutron scattering and transport measurements on polycrystalline samples of the pyrochlore iridates Y 2 Ir 2 O 7 (Y-227) and Yb 2 Ir 2 O 7 (Yb-227). Well-defined spontaneous oscillations of the muon asymmetry are observed together with hysteretic behavior in magnetization below 130 K in Yb-227, indicative of commensurate long-range magnetic order. Similar oscillations are observed in Y-227 below 150 K; however the onset of hysteretic magnetization at T = 190 K indicates a transition to an intermediate state lacking long-range order as observed in Nd-227. Our results also show that insulating members of the iridate family have nearly identical magnetic ground states, and that the presence of magnetic A-site species does not play any significant role in altering the ground state properties.
Anisotropic Magnetoresistance in Antiferromagnetic Sr 2 IrO 4
We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic Mott insulator Sr 2 IrO 4. The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature reveal negative magnetoresistances (up to 28%) for modest magnetic fields (250 mT) applied within the IrO 2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of magneto-resistance shows a crossover from fourfold to twofold symmetry in response to an increasing magnetic field with angular variations in resistance from 1% to 14%. We tentatively attribute the fourfold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of antiferromagnetic-coupled moments in Sr 2 IrO 4. The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys or oxides (0.1%–0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order, and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also to better harness the power of spintronics in a more technically favorable fashion.