Abstract:
Electron transport and magnetic properties of three series of manganates of the formula (La(1-x)Ln(x))(0.7)Ca0.3MnO3 with Ln = Nd, Gd and Y, wherein only the average A-site cation radius (r(A)) and associated disorder vary, without affecting the Mn4+ /Mn3+ ratio, have been investigated in an effort to understand the nature of phase separation. All three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic T-c decreasing with increasing 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 less than or equal to, x(c)) show insulator-metal transitions, while the compositions with x > x(c) are insulating. The magnetic and electrical resistivity behaviour of these manganates is consistent with the occurrence of phase separation in the compositions around x, corresponding to a critical average radius of the A-site cation, (r(A)(c)), of 1.18 Angstrom. Both T-c and T-IM increase linearly when (r(A)) > (r(A)(c)) or x less than or equal to x(c) as expected of a homogeneous ferromagnetic phase. Both T-c and T-M decrease linearly with the A-site cation size disorder as measured by the variance sigma(2). Thus, an increase in sigma(2) favours 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) conform to the variable-range hopping mechanism. More interestingly, when x > x(c) the real part of dielectric constant (epsilon') reaches a high value (10(4) - 10(6)) at ordinary temperatures dropping to a very small (similar to500) value below a certain temperature, the value of which decreases with decreasing frequency.
