Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition (original) (raw)
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We report recent transport and thermodynamic experiments over a wide range of temperatures for the Mott-like system Ca 3 Ru 2 O 7 at high magnetic fields, B (≤ 30 T). This work reveals a rich and highly anisotropic phase diagram, where applying B along the a-, b-, and c-axis leads to vastly different behavior. A fully spin-polarized state via a first order metamagnetic transition is obtained for B ≥ 6 T and B||a, and colossal magnetoresistance is seen for B||b, and quantum oscillations in the resistivity are observed for B||c, respectively. The interplay of the lattice, orbital and spin degrees of freedom are believed to give rise to this strongly anisotropic behavior.
Physical Review B, 2004
Ca 3 Ru 2 O 7 with a Mott-like transition at 48 K and a Néel temperature at 56 K features different in-plane anisotropies of the magnetization and magnetoresistance. Applying a magnetic field along the magnetic easyaxis precipitates a spin-polarized state via a first-order metamagnetic transition, but does not lead to a full suppression of the Mott state, whereas applying a magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of an orbitally ordered and spin-polarized state. This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detrimental to the colossal magnetoresistence in this 4d-based electron system. Evidence of a density wave is also presented.
Physical Review B, 2003
We report magnetic and inter-plane transport properties of Ca 3 Ru 2 O 7 at high magnetic fields and low temperatures. Ca 3 Ru 2 O 7 with a bilayered orthorhombic structure is a Mott-like system with a narrow charge gap of 0.1eV. Of a host of unusual physical phenomena revealed in this study, a few are particularly intriguing: (1) a collapse of the c-axis lattice parameter at a metal-nonmetal transition, T MI (=48 K), and a rapid increase of T MI with low uniaxial pressure applied along the c-axis; (2) quantum oscillations in the gapped, nonmetallic state for 20 mK<T<6.5 K; (3) tunneling colossal magnetoresistance, which yields a precipitate drop in resistivity by as much as three orders of magnitude; (4) different in-plane anisotropies of the colossal magnetoresistance and magnetization. All results appear to indicate a highly anisotropic ground state and a critical role of coupling between lattice and magnetism. The implication of these phenomena is discussed. PACS numbers: 72.15.Gd, 75.30. Vn, 75.50.Ee Layered ruthenium oxides or ruthenates as a new class of correlated electron systems is characterized by the coexistence of different kinds of order and various magnetic and electronic transitions that are generally abrupt and anisotropic. These materials have increasingly drawn attention in recent years [1], but the 4d-electron based ruthenates are still by and large an uncharted territory rich with novel physical phenomena that very often deviate from those of semiconductors, metals or even 3delectron transition metal oxides. Here the rich and intriguing physical properties observed in the bilayered Ca 3 Ru 2 O 7 provide another striking example that defies conventional notions.
Destruction of an orbitally ordered and spin-polarized state: Colossal magnetoresistance in Ca3Ru2O7
Journal of Electronic Materials, 2004
The Ca 3 Ru 2 O 7 with a Mott-like transition at 48 K and a Neel temperature at 56 K features different in-plane anisotropies of magnetization and magnetoresistance. Applying the magnetic field along the magnetic easy axis precipitates a spin-polarized state via a first-order metamagnetic transition but does not lead to full suppression of the Mott state, whereas applying a magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of a novel, orbitally ordered and spin-polarized state. This new phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detrimental to the colossal magnetoresistance (CMR) in this 4d-based electron system. Evidence for a density wave is also presented.
Physical Review B, 2004
We report the high-field magnetoresistivity, magnetization, and magnetostriction data of a bilayered ruthenate Ca 3 Ru 2 O 7 grown by a floating-zone method. The samples used in this study show metallic inplane conduction, but nonmetallic interplane conduction, below 30 K; these results are suggestive of a highly twodimensional metallic ground state. We demonstrate here the existence of two types of field-induced metamagnetic transitions at 6 and 15 T, accompanied by the colossal magnetoresistance effect in the interplane conduction ͓ c ͑20 T͒ / c ͑0 T͒ Ͻ 10 −3 ͔. Interestingly, the higher-field transition is accompanied by large inplane lattice shrinkage that is sufficient to cause orbital polarization in nearly threefold t 2g orbitals. The lattice change due to the magnetic field coincides with the discontinuity at 48 K observed in the thermal contraction data, suggesting that the high-temperature crystal structure is restored by the application of a magnetic field. In this paper, we will discuss this anomalous coupling between spin, charge, and lattice in Ca 3 Ru 2 O 7 in terms of structural distortions.
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Physical review, 1995
Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic Geld have been investigated for prototypical double-exchange ferromagnets, namely, crystals of Lai Sr MnOs (0& x &0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature T&, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as-Ap/p=C(M/M,) for M/M, 0.3, where M, is the saturated magnetization. The coeKcient t strongly depends on x, i.e. , C 4 near the compositional insulator-metal phase boundary (x, 0.17), but decreases down to 1 for x)0.4, indicating the critical change of the electronic state.
Two-phase scenario for the metal-insulator transition in colossal magnetoresistance manganites
Physical Review B, 2001
Recent experiments indicate the coexistence of localized and itinerant charge carriers close to the metalinsulator transition in the ferromagnetic phase of colossal magnetoresistive manganese perovskites. For a theoretical description of the colossal magnetoresistance transition we propose a two-phase model of competing insulating polaronic and ferromagnetic metallic phases with equal hole densities. We find that the subtle balance between these two phases with distinctly different electronic properties can be readily influenced by varying physical parameters, producing various ''colossal'' effects, such as the large magnetization and conductivity changes in the vicinity of the transition temperature.
Physical Review Letters, 2012
Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudo-dipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudo-dipolar interactions, may emerge in small charge-gap iridates.
Tunneling magnetoresistance and quantum oscillations in bilayered Ca 3 Ru 2 O 7
Physical Review B, 2003
We report the interplane resistivity, c , at high magnetic fields B, with different orientations together with structural and magnetic properties of bilayered Ca 3 Ru 2 O 7 , a Mott-like system with a gap of 0.1 eV. A wide array of conventionally unanticipated phenomena revealed in this work includes ͑1͒ a collapse of the c-axis lattice parameter at a metal-nonmetal transition, ͑2͒ quantum oscillations in c in the gapped, nonmetallic state for the Bʈc axis, ͑3͒ interplane tunneling magnetoresistivity for the Bʈa or b axis, and yet conspicuously different anisotropies of the colossal magnetoresistivity and magnetization, and ͑4͒ a non-Fermi-liquid behavior in a metallic state fully recovered in high magnetic fields. The implications of the coexistence of these conflicting phenomena are discussed.