Electronic phase separation in the rare-earth manganates (La_1−xLn_x)_0.7Ca_0.3MnO₃ (Ln = Nd, Gd and Y)

Abstract

Electron transport and magnetic properties of three series of manganates of the formula (La 1-x Ln x ) 0.7 Ca 0.3 MnO 3 with Ln = Nd, Gd and Y, wherein only the average A-site cation radius < r A > and associated disorder vary without affecting the Mn 4+ /Mn 3+ ratio, have been investigated to understand the nature of phase separation. All the three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic T c decreasing with increasing in x up to a critical value of x, x c (x c = 0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x > x c , the magnetic moments are considerably smaller showing a small increase around T M , the value of T M decreasing slightly with increase in x or decrease in < r A >. The ferromagnetic compositions (x ≤ x c ) show insulator-metal (IM) transitions, while the compositions with x > x c are insulating. The magnetic and electrical resistivity behavior of these manganates is consistent with the occurrence of phase separation in the compositions around x c , corresponding to a critical average radius of the A-site cation, < r A c >, of 1.18 Å. Both T c and T IM increase linearly when < r A > > < r A c > or x ≤ x c as expected of a homogenous ferromagnetic phase. Both T c and T M decrease linearly with the A-site cation size disorder at the A-site as measured by 2 the variance σ 2 . Thus, an increase in σ 2 favors the insulating AFM state. Percolative conduction is observed in the compositions with < r A > > < r A c >. Electron transport properties in the insulating regime for x > x c conforms to the variable range hopping mechanism. More interestingly, when x > x c , the real part of dielectric constant (ε')reaches a high value (10 4 -10 6 ) at ordinary temperatures dropping to a very small (~500) value below a certain temperature, the value of which decreases with decreasing frequency. ________________________________________________________________________