X.N. Lin

Competing ground states in triple-layered Sr4Ru3O10_ Verging on itinerant ferromagnetism with critical fluctuations

The triple-layered

Colossal magnetoresistance by avoiding a ferromagnetic state in the Mott system Ca3Ru2O7.

{\mathrm{Sr}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}

High-temperature weak ferromagnetism on the verge of a metallic state: Impact of dilute Sr doping on BaIrO 3

is characterized by an unexpectedly strong quasi-two dimensional characteristic, and a sharp metamagnetic transition and ferromagnetic behavior occurring within the basal plane and along the c axis, respectively. The interplane resistivity at magnetic field B, up to 32 T, exhibits very low-frequency quantum oscillations associated with the spin polarized state when B is parallel to the c axis, and a large magnetoresistivity accompanied by critical fluctuations governed by the metamagnetic transition when B is perpendicular to the c axis. The complex behavior evidenced in magnetization, specific heat, and resistivity presented is not characteristic of any obvious ground states, and points to a unusual state that shows a delicate balance between fluctuations and order. Implications of these results are discussed.

Field-tuned collapse of an orbitally ordered and spin-polarized state: Colossal magnetoresistance in the bilayered ruthenate Ca3Ru2O7

Transport and magnetic studies of Ca3Ru2O7 for temperatures ranging from 0.4 to 56 K and magnetic fields B up to 45 T lead to strikingly different behavior when the field is applied along the different crystal axes. A ferromagnetic (FM) state with full spin polarization is achieved for the B//a axis, but colossal magnetoresistance is realized only for the B//b axis. For the B//c axis, Shubnikov-de Haas oscillations are observed and followed by a less resistive state than that for B//a. Hence, in contrast with standard colossal magnetoresistive materials, the FM phase is the least favorable for electron hopping. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom.

Orbitally driven behaviour: Mott transition, quantum oscillations and colossal magnetoresistance in bilayered Ca3Ru2O7

The 5d-electron based BaIrO3 is a nonmetallic weak ferromagnet with a Curie temperature at Tc=175 K. Its largely extended orbitals generate strong electron-lattice coupling, and magnetism and electronic structure are thus critically linked to the lattice degree of freedom. Here we report results of our transport and magnetic study on slightly Sr doped BaIrO3. It is found that dilute Sr-doping drastically suppresses Tc, and instantaneously leads to a nonmetal-metal transition at high temperatures. All results highlight the instability of the ground state and the subtle relation between magnetic ordering and electron mobility. It is clear that BaIrO3 along with very few other systems represents a class of materials where the magnetic and transport properties can effectively be tuned by slight alterations in lattice parameters.

Violation of the Mott–Ioffe–Regel limit: high-temperature resistivity of itinerant magnets Srn+1RunO3n+1 (n=2,3,∞) and CaRuO3

Ca3Ru2O7 with a Mott-like transition at 48 K and a Neel temperature at 56 K features different in-plane anisotropies of the magnetization and magnetoresistance. Applying a magnetic field along the magnetic easy- axis 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 4 d-based electron system. Evidence of a density wave is also presented. DOI: 10.1103/PhysRevB.69.014404 PACS number~s!: 75.47.Gk, 71.70.Ej The physics of magnetoresistance has generated enor- mous interest in recent years. While this quantum mechani- cal phenomenon is in general associated with the spin scat- tering process of conduction electrons, the origins of various kinds of magnetoresistance are vastly different. The giant magnetoresistance observed in magnetic metallic multiplayer structures can be qualitatively explained using the two cur- rent model, corresponding to up-spin and down-spin electrons. 1 Tunneling magnetoresistance, often seen in mag- netic tunnel junctions separated by an insulating spacer layer, is a consequence of spin-polarization. On the other hand, colossal magnetoresistance ~CMR!, seen only in the mixed- valence manganites so far, originates from a metal-insulator transition in the vicinity of the Curie temperature driven pri- marily by double exchange due to the hopping of eg elec- trons of Mn 31 ions and the Jahn-Teller effect. 2 The novelty of the bilayered Ca3Ru2O7 , as presented in this paper, is that the colossal magnetoresistivity is a result of the collapse of the orbitally ordered state that is realized by demolishing the spin-polarized state . This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detri- mental to the colossal magnetoresistence in this 4 d-based electron system. Indeed, the electron kinetic energy hinges on the spin-orbital-lattice coupling in such a way that apply- ing magnetic field, B, along the magnetic easy axis ~a axis! precipitates a spin-polarized state via a first-order metamag- netic transition, but does not lead to a full suppression of the Mott state, whereas applying B along the magnetic hard axis ~b axis! does, giving rise to a resistivity reduction of three orders of magnitude. Our previous work indicated the puz- zling anisotropic behavior observed in the field dependence

Observation of oscillatory magnetoresistance periodic in 1B and B in Ca3 Ru2 O7

We report recent transport and thermodynamic experiments over a wide range of temperatures for the Mott-like system Ca3Ru2O7 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 behaviour. 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 behaviour.

Borderline magnetism in Sr4Ru3O10: Impact of La and Ca doping on itinerant ferromagnetism and metamagnetism

Abstract Srn+1RunO3n+1 represents a class of layered materials whose physical properties are a strong function of the number of Ru–O layers per unit cell, n. These materials along with the paramagnetic CaRuO3 shares two major characteristics, namely, robust Fermi liquid behavior at low temperatures and anomalous transport behavior featured by linear temperature dependence of resistivity at high temperature. There is no crossover separating such two fundamentally different states. In this paper, we report results of our study that systematically addresses anisotropy and temperature dependence of resistivity as a function of n for the entire Srn+1RunO3n+1 series and CaRuO3 and for a wide temperature range of 1.7 K K . It is found that the anomalous transport behavior correlates with magnetic susceptibility and that the scattering mechanism(s) driving the anomalous transport behavior becomes stronger with reducing dimensionality.

Itinerant ferromagnetism to insulating antiferromagnetism: A magnetic and transport study of single crystal SrRu 1 − x Mn x O 3 ( 0 ⩽ x 0.60 )

We report magnetoresistance oscillations in high magnetic fields, B, up to 45 T and over a wide range of temperature in the Mott-like system Ca3Ru2O7. For B rotating within the ac-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T≤1.5 K in the presence of metamagnetism. These oscillations are highly angular dependent and intimately correlated with the spin-polarization of the ferromagnetic state. For B||[110], oscillations are also observed but periodic in B (rather than 1/B) which persist up to 15 K. While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the t2g-orbitals that makes the extremal cross-section of the FS field-dependent.

Highly anisotropic magnetism in Cr-doped perovskite ruthenates

An investigation of La and Ca doped Sr4Ru3O10, featuring a coexistence of interlayer ferromagnetism and intralayer metamagnetism, is presented. La doping readily changes magnetism between ferromagnetism and metamagnetism by tuning the density of states. It also results in different Curie temperatures for the c-axis and the basal plane, highlighting a rare spin-orbit coupling with the crystal field states. In contrast, Ca doping enhances the c-axis ferromagnetism and the magnetic anisotropy. La doping also induces a dimensional crossover in the interlayer transport whereas Ca doping exhibits a tunneling magnetoresistance and an extraordinary T 3/2 dependence of the resistivity. The drastic changes caused by the doping demonstrate a rare borderline magnetism that is delicately linked to the interplay of the density of states and spin-orbit coupling.