Presence of 3d Quadrupole Moment in LaTiO3 Studied by 47,49Ti NMR

Ti NMR spectra of LaTiO3 are reexamined and the orbital state of this compound is discussed. The NMR spectra of LaTiO3 taken at 1.5 K under zero external field indicate a large nuclear quadrupole splitting. This splitting is ascribed to the presence of the rather large quadrupole moment of 3d electrons at Ti sites, suggesting that the orbital liquid model proposed for LaTiO3 is inappropriate. The NMR spectra are well explained by the orbital ordering model expressed approximately as $1/\sqrt{3}(d_{xy}+d_{yz}+d_{zx})$ originating from a crystal field effect. It is also shown that most of the orbital moment is quenched.Comment: 4 pages, 3 fugures; to appear in Phys. Rev. Let

Disorder Induced Ferromagnetism in CaRuO3

The magnetic ground state of perovskite structure CaRuO3 has been enigmatic for decades. Here we show that paramagnetic CaRuO3 can be made ferromagnetic by very small amounts of partial substitution of Ru by Ti. Magnetic hysteresis loops are observed at 5 K for as little as 2% Ti substitution. Ti is non-magnetic and isovalent with Ru, indicating that the primary effect of the substitution is the disruption of the magnetic ground state of CaRuO3 through disorder. The data suggest that CaRuO3 is poised at a critical point between ferromagnetic and paramagnetic ground states

Scaling of the anomalous Hall effect in Sr1−x_{1-x}Cax_xRuO3_3

The anomalous Hall effect (AHE) of ferromagnetic thin films of Sr$_{1-x}$Ca$_{x}$RuO$_3$ (0 $\leq x \leq$ 0.4) is studied as a function of $x$ and temperature $T$. As $x$ increases, both the transition temperature $T_c$ and the magnetization $M$ are reduced and vanish near $x \sim$ 0.7. For all compositions, the transverse resistivity $\rho_{H}$ varies non-monotonously with $T$, and even changes sign, thus violating the conventional expression $\rho_{H}=R_o B + 4\pi R_s M(T)$ ($B$ is the magnetic induction, while $R_o$ and $R_s$ are the ordinary and anomalous Hall coefficients). From the rather complicated data of $\rho_H$, we find a scaling behavior of the transverse conductivity $\sigma_{xy}$ with $M(T)$, which is well reproduced by the first-principles band calculation assuming the intrinsic origin of the AHE.Comment: REVTeX 4 style; 5 pages, 3 figures; revised 23/2 and accepted for publicatio

Novel critical exponent of magnetization curves near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La)

We report a novel critical exponent delta=3/2 of magnetization curves M=H^{1/delta} near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La), which the mean field theory of the Ginzburg-Landau-Wilson type fails to reproduce. The effect of dirty ferromagnetic spin fluctuations might be a key.Comment: 4 pages, 5 figure

G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm

The origin of the antiferromagnetic order and puzzling properties of LaTiO_3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually lifted by the La cations in the GdFeO_3-type structure, which generates a crystal field with nearly trigonal symmetry. This allows the description of the low-energy structure of LaTiO_3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO_3 with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out that through generating the R crystal field, the GdFeO_3-type distortion has a universal relevance in determining the orbital-spin structure of the perovskite compounds in competition with the Jahn-Teller mechanism, which has been overlooked in the literature. Since the GdFeO_3-type distortion is a universal phenomenon as is seen in a large number of perovskite compounds, this mechanism may also play important roles in other compounds of this type.Comment: 20 pages, 15 figure

Uniaxial-Pressure induced Ferromagnetism of Enhanced Paramagnetic Sr3Ru2O7

We report a uniaxial pressure-dependence of magnetism in layered perovskite strontium ruthenate Sr3Ru2O7. By applying a relatively small uniaxial pressure, greater than 0.1 GPa normal to the RuO2 layer, ferromagnetic ordering manifests below 80 K from the enhanced-paramagnet. Magnetization at 1 kOe and 2 K becomes 100 times larger than that under ambient condition. Uniaxial pressure dependence of Curie temperature T_C suggests the first order magnetic transition. Origin of this uniaxial-pressure induced ferromagnetism is discussed in terms of the rotation of RuO6 octahedra within the RuO2 plane.Comment: 8 pages, 3 figures. to be published in Journal of the Physical Society of Japan, vol.73, No.5 (2004

Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a Structural Transition

We report a first-order phase transition at T_M=357 K in single crystal Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at TM and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.Comment: Accepted for publication in Phys. Rev. B (Rapid Communications

Investigation of the ferromagnetic transition in the correlated 4d perovskites SrRu1−x_{1-x}Rhx_xO3_3

The solid-solution SrRu$_{1-x}$Rh$_x$O$_3$ ($0\le x \le1$) is a variable-electron-configuration system forming in the nearly-cubic-perovskite basis, ranging from the ferromagnetic 4$d^4$ to the enhanced paramagnetic 4$d^5$. Polycrystalline single-phase samples were obtained over the whole composition range by a high-pressure-heating technique, followed by measurements of magnetic susceptibility, magnetization, specific heat, thermopower, and electrical resistivity. The ferromagnetic order in long range is gradually suppressed by the Rh substitution and vanishes at $x \sim 0.6$. The electronic term of specific-heat shows unusual behavior near the critical Rh concentration; the feature does not match even qualitatively with what was reported for the related perovskites (Sr,Ca)RuO$_3$. Furthermore, another anomaly in the specific heat was observed at $x \sim 0.9$.Comment: Accepted for publication in PR

Phase separation and suppression of critical dynamics at quantum transitions of itinerant magnets: MnSi and (Sr1−x_{1-x}Cax_{x})RuO3_{3}

Quantum phase transitions (QPTs) have been studied extensively in correlated electron systems. Characterization of magnetism at QPTs has, however, been limited by the volume-integrated feature of neutron and magnetization measurements and by pressure uncertainties in NMR studies using powderized specimens. Overcoming these limitations, we performed muon spin relaxation ($\mu$SR) measurements which have a unique sensitivity to volume fractions of magnetically ordered and paramagnetic regions, and studied QPTs from itinerant heli/ferro magnet to paramagnet in MnSi (single-crystal; varying pressure) and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$ (ceramic specimens; varying $x$). Our results provide the first clear evidence that both cases are associated with spontaneous phase separation and suppression of dynamic critical behavior, revealed a slow but dynamic character of the ``partial order'' diffuse spin correlations in MnSi above the critical pressure, and, combined with other known results in heavy-fermion and cuprate systems, suggest a possibility that a majority of QPTs involve first-order transitions and/or phase separation.Comment: 11 pages, 4 figures, 21 authors, to appear in Nature Physic

Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -

Orbital degree of freedom of electrons and its interplay with spin, charge and lattice degrees of freedom are one of the central issues in colossal magnetoresistive manganites. The orbital degree of freedom has until recently remained hidden, since it does not couple directly to most of experimental probes. Development of synchrotron light sources has changed the situation; by the resonant x-ray scattering (RXS) technique the orbital ordering has successfully been observed . In this article, we review progress in the recent studies of RXS in manganites. We start with a detailed review of the RXS experiments applied to the orbital ordered manganites and other correlated electron systems. We derive the scattering cross section of RXS where the tensor character of the atomic scattering factor (ASF) with respect to the x-ray polarization is stressed. Microscopic mechanisms of the anisotropic tensor character of ASF is introduced and numerical results of ASF and the scattering intensity are presented. The azimuthal angle scan is a unique experimental method to identify RXS from the orbital degree of freedom. A theory of the azimuthal angle and polarization dependence of the RXS intensity is presented. The theoretical results show good agreement with the experiments in manganites. Apart from the microscopic description of ASF, a theoretical framework of RXS to relate directly to the 3d orbital is presented. The scattering cross section is represented by the correlation function of the pseudo-spin operator for the orbital degree of freedom. A theory is extended to the resonant inelastic x-ray scattering and methods to observe excitations of the orbital degree of freedom are proposed.Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phy