Effect of the internal pressure due to the A-site cations on the giant magnetoresistance and related properties of doped rare earth manganates, Ln_1−xA_xMnO₃ (Ln=La,Nd,Gd,Y; A=Ca,Sr,Ba,Pb)

Abstract

Based on an extensive study of a large variety of rare earth manganates (rare earth=La, Nd, Gd, or Y) with Ca, Sr, Ba, and Pb as the substituents, as well as of LaMnO₃ samples with varying Mn⁴⁺ content, it is shown that the ferromagnetic transition temperature, T_c, as well as the insulator-metal (I-M) transition temperature, T_p, increases with the weighted average radius of the A-cations, <r_A>, showing a maximum at 1.23±0.01 Å. The essentially linear region (<r_A>=1.18-1.24 Å) demarcates the ferromagnetic metal and paramagnetic insulator regimes. The peak resistivity at the I-M transition, as well as the magnitude of the magnetoresistance (MR), decreases with increasing <r_A>. Only below a <r_A> of 1.23±0.01 Å is the MR greater than 50%.