Structural, electrical, magnetic, and thermal studies of Cr-doped La _0.7 Ca_0.3 Mn_1-x Cr_xO₃ (0≤x≤1) manganites

We report detailed structural, electrical, magnetic, and specific heat studies on La_0.7 Ca_0.3 Mn_1-x Cr_x O₃ manganites. Rietveld analysis of fitted and observed x-ray diffraction patterns exhibited the single-phase nature of all the studied materials, which crystallize in P_bn_m space group. Successive substitution of Cr at Mn-site in La_0.7 Ca_0.3 Mn_1-x Cr_x O₃ manganites increases the electrical resistivity and decreases the characteristic insulator-metal transition temperature (TIM) of the parent compound along with a humplike feature for higher Cr-content (x>0.06) samples. The hump structure basically signifies the onset of antiferromagnetic (AFM) interactions as inferred by both the magnetic and infrared (IR) spectroscopy studies. The systematic suppression of FM state results in a spin glass (SG)-like behavior. IR studies revealed that the vibration mode at 413 cm-1 being associated with internal bending of MnO₆ octahedra, becomes softer, indicating an increase in distortion and hence the possible SG behavior. The critical exponents (α, Β, and γ) are calculated from the heat capacity (CP) data near the TIM / TFM. The same exhibited variations of their values with doping. In particular, the value of Β increases from 0.37 (x=0.0) to 0.43 (x=0.04), clearly indicating the coexistence of both long and short range magnetic orders, i.e., tendency toward SG state for Cr-doped samples. On the basis of present results, it is suggested that Cr dilutes double-exchange based FM and rather promotes the AFM based superexchange interactions via Cr³⁺ / Mn⁴⁺ ions. Substitution of Cr systematically destroys both the metallic state and long range FM order.