Short-range magnetic behavior in manganites La_0.93K_0.07Mn_1-xCu_xO₃ (0.0 ⩽ x ⩽ 0.09) above the Curie temperature

Manganites La_0.93K_0.07Mn_1−xCu_xO₃ $(0.0⩽x⩽0.09),$ prepared by the solid state reaction method at high temperature, were studied structurally and magnetically. The unit cell parameters, as well as bond length ${\text{d}}_{(\text{Mn,Cu)-O}}\left(\text{Å}\right)$ and the bond angle ${\theta }_{(\text{Mn,Cu)-}{\text{O}}_{\text{1}}\text{-Mn}}$, were determined from the Rietveld refinement of the x-ray diffraction patterns. The Fourier-transform infrared spectroscopy analysis shows that Cu²⁺ substitution induces variations in the vibration modes of the MnO₆ octahedra. Magnetization vs. temperature $\text{M}\left(T\right)$ at low magnetic field $H=0.01T$ were performed in the range $5<T<300K$ under field coolingand zero field cooling conditions. All the samples exhibited a second-order paramagnetic–ferromagnetic (FM) transition at Curie temperature, $T_{\text{C}}$, in the range between 199 and 285 K. The inverse susceptibility, ${\chi }^{-1}\left(T\right)$ exhibits a linear Curie-Weiss (C-W) behavior for $T>T_{\text{CW}}^{\text{*}}$, while for $T_{\text{C}}<T<T_{\text{CW}}^{\text{*}}$, it shows a deviation from the linear behavior predicted by the Heisenberg model. The mentioned deviation of ${\chi }^{-1}\left(T\right)$ means that a short ferromagnetic state formation is present even for $T>T_{\text{C}}$, which were characterized by the experimental effective magnetic moment, ${\mu }_{\text{eff}}^{\text{exp}}.$ A null spontaneous magnetization, ${\text{m}}_{\text{S}},$ above $T_{\text{C}}$ was evaluated for all samples by using the Kouvel-Fisher method. This work evaluates the short-range FM clusters by means of an extension of the C-W approach to the $T_{\text{C}}<T<T_{\text{CW}}^{\text{*}}$ region, i.e., ${\mu }_{\text{eff}}^{\text{exp}}\left(T\right)=2.3\sqrt{\text{C}\left(T\right)}{\mu }_{\text{B}}$. Finally, the critical coefficient values, $\beta$ and $\gamma$, showed that the 3D Heisenberg model fits adequately the $x=0.0$ and $x=0.03$ samples, while the 3D Ising model fits the $x=0.09$ sample. (paper)