Short-range magnetic behavior in manganites La_0.93K_0.07Mn_{1- x} Cu _x O₃ (0.0 ⩽ x ⩽ 0.09) above the Curie temperature

Abstract Manganites La0.93K0.07Mn1−x Cu x O3 ( 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 d ( Mn,Cu)-O Å and the bond angle θ ( Mn,Cu)- O 1 -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 M T at low magnetic field H=0 .01T were performed in the range 5 < T < 300 K under field coolingand zero field cooling conditions. All the samples exhibited a second-order paramagnetic–ferromagnetic (FM) transition at Curie temperature, T C , in the range between 199 and 285 K. The inverse susceptibility, χ − 1 ( T ) exhibits a linear Curie-Weiss (C-W) behavior for T_{{\text{CW}}}^{\text{*}}$?> T > T CW * , while for T C < T < T CW * , it shows a deviation from the linear behavior predicted by the Heisenberg model. The mentioned deviation of χ − 1 T means that a short ferromagnetic state formation is present even for {T_{\text{C}}}$?> T > T C , which were characterized by the experimental effective magnetic moment, μ eff exp . A null spontaneous magnetization, m S , above T 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 C < T < T CW * region, i.e., μ eff exp ( T ) = 2.3 C ( T ) μ B . Finally, the critical coefficient values, β and γ , 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.