arXiv subjects

L. E. DeLong

Publications and source records attributed to L. E. DeLong.

12 recordsLinked to original sources

arXiv2018

Observation of a pressure-induced transition from interlayer ferromagnetism to intralayer antiferromagnetism in Sr4Ru3O10

Sr4Ru3O10 is a Ruddlesden-Popper compound with triple Ru-O perovskite layers separated by Sr-O alkali layers. This compound presents a rare coexistence of interlayer (c-axis) ferromagnetism and intralayer (basal-plane) metamagnetism at ambient pressure. Here we report the observation of pressure-induced, intralayer itinerant antiferromagnetism arising from the interlayer ferromagnetism. The application of modest hydrostatic pressure generates an anisotropy that causes a flattening and a tilting of RuO6 octahedra. All magnetic and transport results from this study indicate these lattice distortions diminish the c-axis ferromagnetism and basal-plane metamagnetism, and induce a basal-plane antiferromagnetic state. The unusually large magnetoelastic coupling and pressure tunability of Sr4Ru3O10 makes it a unique model system for studies of itinerant magnetism.

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arXiv2015

Ground state tuning of the metal-insulator transition by compositional variations in BaIr1-xRuxO3(0<x<1)

BaIrO3 is a magnetic insulator driven by the spin-orbit interaction (SOI), whereas BaRuO3 is a paramagnet and exhibits a crossover from a metallic to an insulating regime. Our investigation of structural, magnetic, transport and thermal properties reveals that substitution of Ru4+ (4d4) ions for Ir5+ (5d5) ions in BaIrO3 reduces the magnitudes of the SOI and a monoclinic structural distortion, and rebalances the competition between the SOC and the lattice degrees freedom to generate a rich phase diagram for BaIr1-xRuxO3 (0< x <1). There are two major effects of Ru additions: (1) Light Ru doping (0 < x < 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature TN and the electrical resistivity, which exhibits a crossover behavior from a metallic to an insulating state near TN. (2) Heavier Ru doping (0.41< x < 0.9) induces a robust metallic state with a strong spin frustration generated by competing antiferromagnetic and ferromagnetic interactions.

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arXiv2015

Coexisting charge and magnetic orders in the dimer-chain iridate Ba5AlIr2O11

We have synthesized and studied single-crystal Ba5AlIr2O11 that features dimer chains of two inequivalent octahedra occupied by tetravalent and pentavalent ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes a subtle structural phase transition near 210 K and a magnetic transition at 4.5 K; the latter transition is surprisingly resistant to applied magnetic fields up to 12 T, but sensitive to modest applied pressure. All results indicate that the phase transition at 210 K signals an enhanced charge order that induces electrical dipoles and strong dielectric response near 210 K. It is clear that the strong covalency and spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state. The behavior of Ba5AlIr2O11 therefore provides unique insights into the physics of SOI along with strong covalency in competition with double exchange interactions of comparable strength.

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arXiv2013

Novel magnetism of Ir5+ ions in the double perovskite Sr2YIrO6

We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5+(5d4) ions which are anticipated to have Jeff=0 singlet ground states in the strong spin-orbit coupling (SOC) limit, and geometric frustration in a face centered cubic structure formed by the Ir5+ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong non-cubic crystal fields and intermediate-strength SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates.

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arXiv2013

Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7

Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure, PC = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles θ. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the Jeff = 1/2 insulator.

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arXiv2012

Strong magnetic instability in correlated metal Bi2Ir2O7

The interplay of spin-orbit interactions and electronic correlations dominates the physical properties of pyrochlore iridates, R2Ir2O7 (R = Y, rare earth element), which are typically magnetic insulators. We report an experimental/theoretical study of single-crystal Bi2Ir2O7 where substitutions of Bi for R sensitively tips the balance between competing interactions so as to favor a metallic state with a strongly exchange enhanced paramagnetism. The ground state is characterized by the following features: (1) A divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T- and T3-terms of the specific heat; (3) a conspicuously large Wilson ratio R_W \approx 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi2Ir2O7.

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arXiv2010

Giant Magnetoelectric Effect in Antiferromagnetic BaMnO3-δand Its Derivatives

Hexagonal perovskite 15R-BaMnO2.99 with a ratio of cubic to hexagonal layers of 1/5 in the unit cell is an antiferromagnetic insulator that orders at a Néel temperature TN = 220 K. Here we report structural, magnetic, dielectric and thermal properties of single crystal BaMnO2.99 and its derivatives BaMn0.97Li0.03O3 and Ba0.97K0.03MnO3. The central findings of this work are: (1) these materials possess a usually large, high-temperature magnetoelectric effect that amplifies the dielectric constant by more than an order of magnitude near their respective Néel temperature; (2) Li and K doping can readily vary the ratio of cubic to hexagonal layers and cause drastic changes in dielectric and magnetic properties; in particular, a mere 3% Li substitution for Mn significantly weakens the magnetic anisotropy and relaxes the lattice; consequently, the dielectric constant for both the a- and c-axis sharply rises to 2500 near the Néel temperature. This lattice softening is also accompanied by weak polarization. These findings provide a new paradigm for developing novel, high-temperature magnetoelectric materials that may eventually contribute to technology.

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arXiv2009

Pressure-Induced Insulating State in Ba1-xRExIrO3 (RE = Gd, Eu) Single Crystals

BaIrO3 is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. Since pressure generally increases orbital overlap and broadens energy bands, a pressure-induced insulating state is not commonplace. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO3.

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arXiv2009

Giant Magneto-electric Effect in the Novel Mott Insulator, Sr2IrO4

Our magnetic, electrical, and thermal measurements on single-crystals of the novel Mott insulator, Sr2IrO4, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state whose signatures are: (1) a strongly enhanced electric permittivity that peaks near a newly observed magnetic anomaly at 100 K, (2) a large (~100%) magneto-dielectric shift that occurs near a metamagnetic transition, and (3) magnetic and electric polarization hysteresis. The GME and electric polarization hinge on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.

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arXiv2008

Decisive Influence of Cation Size on the Magnetic Groundstate and Non-Fermi Liquid Behavior of ARuO3 (A = Ca, Sr)

We report calorimetric, magnetic and electric transport properties of single-crystal CaRuO3 and SrRuO3 as a function of temperature T and applied magnetic field B. We find that CaRuO3 is a non-Fermi-liquid metal near a magnetic instability, as characterized by the following properties: (1) the heat capacity C(T,B) ~ -T log T is readily enhanced in low applied fields, and exhibits a Schottky peak at 2.3 K that exhibits field dependence when T is reduced; (2) the magnetic susceptibility diverges as T^-x at low temperatures with 1/2 < x < 1, depending on the applied field; and (3) the electrical resistivity exhibits a T3/2 dependence over the range 1.7 < T < 24 K. No Shubnikov-de Haas oscillations are discerned at T = 0.65 K for applied fields up to 45 T. These properties, which sharply contrast those of the itinerant ferromagnet SrRuO3, indicate CaRuO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point.

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arXiv2007

Ca3(Ru1-xCrx)2O7: A new paradigm for spin valves

The spin valve effect is a quantum phenomenon so far only realized in multilayer thin films or heterostructures. Here we report a strong spin valve effect existing in bulk single crystals of Ca3(Ru1-xCrx)2O7 having an anisotropic, bilayered crystal structure. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices.

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arXiv2007

Non-Fermi-liquid behavior in nearly ferromagnetic metallic SrIrO3 single crystals

We report transport and thermodynamic properties of single-crystal SrIrO3 as a function of temperature T and applied magnetic field H. We find that SrIrO3 is a non-Fermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small ordered moment but no evidence for long-range order down to 1.7 K; (2) strongly enhanced magnetic susceptibility that diverges as T or T1/2 at low temperatures, depending on the applied field; (3) heat capacity C(T,H) ~ -Tlog T that is readily amplified by low applied fields; (4) a strikingly large Wilson ratio at T< 4K; and (5) a T3/2-dependence of electrical resistivity over the range 1.7 < T < 120 K. A phase diagram based on the data implies SrIrO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T = 0 and H = 0.23 T).

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