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Register a new userDouble metal-insulator transitions and MR in La(0.67)Ca(0.33)Mn(1-x)Ru(x)O(3) (x</=0.10): A qualitative understanding in light of possible magnetic phase separation
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L. Seetha Lakshmi
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This paper is in continuation of our previous work on the structural, electrical and magnetic properties of Ru doped La(0.67)Ca(0.33)MnO(3) compounds (Ref.: L.Seetha Lakshmi et.al, J. Magn. Magn. Mater. 257, 195 (2003)). Here we report the results of magnetotransport measurements on La(0.67)Ca(0.33)Mn(1-x)Ru(x)O(3) (0<x< 0.1) compounds in the light of proposed magnetic phase separation.
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In this paper, we examine the possible influence of extrinsic factors on the electrical and magnetotransport of La(0.67)Ca(0.33)Mn(1-x)Ru(x)O(3) (x < 0.10). These results not only exclude the extrinsic factors, but establishes the fact that the metal transitions both exhibiting MR is intrinsic to Ru substituted La(0.67)Ca(0.33)MnO(3) and the system. These results substantiate our hypothesis that Ru substituted system undergoes a magnetic phase separation involving the co-existence of two ferromagnetic-metallic phases in its ground state.
We report thermal-expansion, lattice-constant, and specific-heat data of the series La_1-xA_xCoO_3 for 0<= x <= 0.30 with A = Ca, Sr, and Ba. For the undoped compound LaCoO_3 the thermal-expansion coefficient alpha(T) exhibits a pronounced maximum around T=50K caused by a temperature-driven spin-state transition from a low-spin state of the Co^{3+$ ions at low towards a higher spin state at higher temperatures. The partial substitution of the La^{3+} ions by divalent Ca^{2+}, Sr^{2+}, or Ba^{2+} ions causes drastic changes in the macroscopic properties of LaCoO3. The large maximum in alpha(T) is suppressed and completely vanishes for x> 0.12. For A = Ca three different anomalies develop in alpha(T) with further increasing x, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data we identify several phase transitions as a function of the doping concentration x and temperature. From these data we propose an extended phase diagram for La_1-xCa_xCoO_3.
We report the Sr substitution effect in an antiferromagnetic insulator LaMnAsO. The Sr doping limit is $xsim$ 0.10 under the synthesis conditions, as revealed by x-ray diffractions indicate. Upon Sr doping, the room-temperature resistivity drops by five orders of magnitude down to $sim$0.01 $Omegacdot$cm, and the temperature dependence of resistivity shows essentially metallic behavior for $xgeq$0.08. Hall and Seebeck measurements confirm consistently that the insulator-to-metal transition is due to hole doping. Strikingly, the room-temperature Seebeck coefficient for the metallic samples is as high as $sim240 mu$V/K, making the system as a possible candidate for thermoelectric applications.
We study magnetic and multiferroic behavior in Ca$_3$Co$_{2-x}$Mn$_{x}$O$_6$ ($x sim$0.97) by high-field measurements of magnetization ($M$), magnetostriction ($L$($H$)/$L$), electric polarization ($P$), and magnetocaloric effect. This study also gives insight into the zero and low magnetic field magnetic structure and magnetoelectric coupling mechanisms. We measured $M$ and $Delta$$L$/$L$ up to pulsed magnetic fields of 92 T, and determined the saturation moment and field. On the controversial topic of the spin states of Co$^{2+}$ and Mn$^{4+}$ ions, we find evidence for $S$ = 3/2 spins for both ions with no magnetic field-induced spin-state crossovers. Our data also indicate that Mn$^{4+}$ spins are quasi-isotropic and develop components in the $ab$-plane in applied magnetic fields of 10 T. These spins cant until saturation at 85 T whereas the Ising Co$^{2+}$ spins saturate by 25 T. Furthermore, our results imply that mechanism for suppression of electric polarization with magnetic fields near 10 T is flopping of the Mn$^{4+}$ spins into the $ab$-plane, indicating that appropriate models must include the coexistence of Ising and quasi-isotropic spins.
We report an unusual nearly ferromagnetic, heavy-mass state with a surprisingly large Wilson ratio $R_{textrm{w}}$ (e.g., $R_{textrm{w}}sim$ 700 for $x =$ 0.2) in double layered ruthenates (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ru$_{2}$O$_{7}$ with 0.08 $< x <$ 0.4. This state does not evolve into a long-range ferromagnetically ordered state despite considerably strong ferromagnetic correlations, but freezes into a cluster-spin-glass at low temperatures. In addition, evidence of non-Fermi liquid behavior is observed as the spin freezing temperature of the cluster-spin-glass approaches zero near $x approx$ 0.1. We discuss the origin of this unique magnetic state from the Fermi surface information probed by Hall effect measurements.
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