Magnetoresistivity in the clustered state of ${\mathrm{La}}_{0.7-x}{\mathrm{Y}}_x{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ manganites

We have performed a detailed characterization of both magnetic and transport properties of ${\mathrm{La}}_{0.7-x}{\mathrm{Y}}_x{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ ($x=0$, 0.05, 0.10, and 0.15) compounds. These measurements provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered. The most notable is an intermediate regime called the clustered state, $T_C<T<T^{*}$, of phase coexistence involving the presence of both ferromagnetic (FM) and antiferromagnetic (AF) clusters and remnants of the high-temperature paramagnetic (PM) phase. The results further indicate a strong connection between spin cluster formation and the colossal magnetoresistivity effect. This magnetically clustered regime was observed to be altered by both magnetic field and disorder. The metal-insulator transition temperature $T_{\mathrm{MI}}$ and the width of the peak of $\rho \left(T\right)$ were found to change linearly with increasing disorder (Y content) and application of magnetic field with negative and positive derivatives, respectively. This indicates that disorder and application of magnetic fields $\left(H\right)$ would cause the same effect but in an opposite way. We argue that correlated disorder effects influence both the shape and the magnitude of $\rho$ versus $T$ curves, as predicted theoretically. We have also inferred, from the computed excess of conductivity (or resistivity decrease) $\Delta \rho$ under decreasing $T$ and application of $H$, that the electronic conduction process in the clustered state is described by a hopping mechanism. This result suggests two contributions to the electrical conductivity in the clustered state: (1) a semiconductinglike contribution and (2) hopping of carriers between FM metallic clusters.

Figure 1

Temperature dependence of the dc magnetization taken under 0.5 kOe. $T_C=260$, 230, 165, and 90 K for $x=0$, 0.05, 0.10, and 0.15, respectively. $T_C$ is defined as the maximum inflection of $M$ vs $T$. Lines are drawn as guides to the eye.

Figure 2

Temperature dependence of the coercive field obtained from $M\left(H\right)$ measurements for the sample with $x=0.15$. The inset shows an expanded view of hysteresis loops taken at 120 K. Lines are guides to the eyes.

Figure 3

Temperature dependence of the electrical resistivity for ${\mathrm{La}}_{0.7-x}{\mathrm{Y}}_x{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$. $T_{\mathrm{MI}}=265$, 240, 175, and 110 K for $x=0$, 0.05, 0.10, and 0.15, respectively. All curves were taken during increasing temperature. The inset shows $\rho$ vs $T$ for $x=0.15$ taken during increasing and decreasing $T$.

Figure 4

Temperature dependence of the electrical resistivity under applied magnetic fields for $x=0$ (a), 0.10 (b), and 0.15 (c). The inset shows the $\mathrm{MR}=[\rho (T,H=0)-\rho (T,H=3\mathrm{T})]∕\rho (T,H=0)$ vs $T$ for the sample with $x=0.15$.

Figure 5

$\Delta T_{\mathrm{MI}}$ vs $T_{\mathrm{MI}}$ of each sample (increasing disorder). The inset shows $\Delta T_{\mathrm{MI}}$ for the sample with $x=0.15$ as a function of $T_{\mathrm{MI}}$ which was now varied by increasing $H$.

Figure 6

Polaron model fitting for the sample with $x=0.15$. The inset indicates that the polaron model fits very well the electrical resistivity data of the sample with $x=0$. The lines are guides to the eye.

Figure 7

$\rho$ vs $T$ for the sample with $x=0.15$ in the insulating regime at several applied magnetic fields. The ${\rho }_{\mathrm{ext}}\left(T\right)$ curve corresponds to the extrapolated low-$T$ contribution of the polaron model fitted from 350 K down to 260 K. The excess of conductivity, defined as $\Delta \rho (T,H)={\rho }_{\mathrm{ext}}\left(T\right)-\rho (T,H)$, is also indicated.

Figure 8

Temperature dependence of the $\Delta \rho$ plotted by $\ln \left(\Delta \rho \right)$ vs $T^{-1∕2}$ in the intermediate range of temperature for $H=0$, 1.5, 3.0, and 5.0 T.