Orbital domain state and finite size scaling in ferromagnetic insulating manganites

Selective substitution in orbital domains of a low doped manganite: an investigation from Griffiths phenomenon and modification of glassy features

An effort is made to study the contrast in magnetic behavior resulting from minimal disorder introduced by substitution of 2.5% Ga or Al in Mn site of La(0.9)Sr(0.1)MnO(3). It is considered that Ga or Al selectively create disorder within the orbital domains or on its walls, causing enhancement of Griffiths phase (GP) singularity for the former and disappearance of it in the latter case. It is shown that Ga replaces Mn(3+), which is considered to be concentrated within the domains, whereas Al replaces Mn(4+), which is segregated on the hole-rich walls, without causing any significant effect on structure or ferromagnetic transition temperatures. Thus, it is presumed that the effect of disorder created by Ga extends across the bulk of the domain having correlation over a similar length scale, resulting in enhancement of the GP phenomenon. In contrast, the effect of disorder created by Al remains restricted to the walls, resulting in the modification of the dynamics arising from the domain walls and suppresses the GP. Moreover, contrasting features are observed in the low temperature region of the compounds; a re-entrant spin-glass-like behavior is observed in the Ga-doped sample, while the observed characteristics for the Al-doped sample are ascribed only to modified domain wall dynamics with the absence of any glassy phase. Distinctive features in third-order susceptibility measurements reveal that the magnetic ground state of the entire series comprises of orbital domain states. These observations bring out the role of the nature of disorder on the GP phenomenon and also reconfirms the character of self-organization in low doped manganites.

Domain formation and orbital ordering transition in a doped Jahn-Teller insulator

The ground state of a double-exchange model for orbitally degenerate e(g) electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real-space Monte Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low-doping phase diagram of La1 - xCaxMnO3.

Low temperature charge and orbital textures in La0.875Sr0.125MnO3

By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.

Confined spin waves reveal an assembly of nanosize domains in ferromagneticLa1-xCaxMnO3 (x=0.17,0.2)

We report a study of spin waves in ferromagnetic La1-xCaxMnO3, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below T(C), in the quasimetallic state (T=150 K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined only in the (a,b) plane for x=0.17, and in all directions for x=0.2. They allow an estimation of the domain's size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing one to characterize the domains as "hole poor." The precursor state of the collossal magnetoresistance metallic phase appears, therefore, as an assembly of small orbital-ordered domains.