Kondo-like Behavior in Lightly Gd-Doped Manganite CaMnO3

Abstract

Manganese oxides (manganites) are among the most studied materials in condensed matter physics due to the famous colossal magnetoresistance and very rich phase diagrams characterized by strong competition between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating phases. One of the key questions that remains open even after more than thirty years of intensive research is the exact conductivity mechanism in insulating as well as in metallic phases and its relation to the corresponding magnetic structure. In order to shed more light on this problem, here, we report magnetotransport measurements on sintered nanocrystalline samples of the very poorly explored manganites Ca1-xGdxMnO3 with x=0.05 and x=0.10, in the temperature range 2-300 K, and in magnetic fields up to 16 T. Our results indicate that both compounds at low temperatures exhibit metallic behavior with a peculiar resistivity upturn and a large negative magnetoresistance. We argue that such behavior is consistent with a Kondo-like scattering on Gd impurities coupled with the percolation of FM metallic regions within insulating AFM matrix.

Keywords: Kondo-like scattering; magnetotransport; manganites; percolation; resistivity upturn.

Figure 1

Splitting of the atomic Mn 3d levels into high-energy $e_g$ and low-energy $t_{2g}$ orbitals in the crystal field created by octahedral surrounding of O atoms. In the case of ${\mathrm{Mn}}^{3+}$, there is additional splitting caused by Jahn–Teller elongation distortion of the ${\mathrm{MnO}}_6$ octahedron [5] (see text). Valence electrons are shown as red arrows, Mn atoms as red circles, and O atoms as green circles.

Figure 2

Temperature dependence of resistivity for the ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{MnO}}_3$ samples with $x=0.05$ (cyan line) and $x=0.10$ (pink line). For comparison, the resistivity curve for the parent compound ${\mathrm{CaMnO}}_3$ from ref. [50] is also shown (black line).

Figure 3

Temperature dependence of resistivity in applied magnetic field for the ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{MnO}}_3$ samples with (a) $x=0.10$ and (b) $x=0.05$. Discrete color gradation from light pink (cyan) to dark pink (cyan) corresponds to the increasing field strengths: 0, 1, 5, 8, 10, 12, and 16 T for $x=0.10$ ($x=0.05$), respectively.

Figure 4

Temperature dependence of resistivity derivative $\mathrm{d}\rho /\mathrm{d}T$ in the maximum magnetic field ${\mu }_{0}H=16\mathrm{T}$ for the ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{MnO}}_3$ samples with (a) $x=0.10$ and (b) $x=0.05$. Here, ${\mu }_0$ is the vacuum permeability. Characteristic temperatures of $T_N$, $T_{IM}$, $T_{inf}$, and $T_{min}$ are indicated by black arrows. The T-dependence of $\mathrm{d}\rho /\mathrm{d}T$ at all fields is shown in the insets, where different field strengths of 0, 1, 5, 8, 10, 12, and 16 T correspond to the discrete color gradation from light pink (cyan) to dark pink (cyan) for $x=0.10$ ($x=0.05$), respectively.

Figure 5

Fitting of the measured resistivity of the ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{MnO}}_3$ samples with (a) $x=0.10$ and (b) $x=0.05$ to the simple Kondo model described by Equation (1) in the maximum magnetic field ${\mu }_{0}H=16\mathrm{T}$. The fitting procedure at all fields is shown in the insets, where different field strengths of 0, 1, 5, 8, 10, 12, and 16 T correspond to the discrete color gradation from light pink (cyan) to dark pink (cyan) for $x=0.10$ ($x=0.05$), respectively. The measured data are shown as empty symbols, and the corresponding fits are given as black lines. To avoid non-physical behavior, the fits of $\rho \left(T\right)$ for $x=0.10$ at 0 and 1 T, as well as the fits of $\rho \left(T\right)$ for $x=0.05$ at all fields, were performed in such a way that the parameter q in Equation (1) was forced to be zero (see text).