5. Competing Interactions and Suppression of Ferromagnetism on La site Substitution in La0.65A0.35Mn0.95Fe0.05O3 Compounds

TL;DR

This study investigates how different A-site cation substitutions in La0.65A0.35Mn0.95Fe0.05O3 affect magnetic and transport properties, revealing complex interactions including ferromagnetism, antiferromagnetism, and spin glass behavior.

Contribution

It provides new insights into how cation size and Fe doping influence magnetic phases and transition temperatures in manganite compounds.

Findings

Pb doping results in insulating ferromagnetism with spin glass behavior.

Maximum Tc and magnetic moment observed for Sr doping due to ionic size compatibility.

Size mismatch causes deviation in Mn-O-Mn angle, weakening ferromagnetic double exchange.

Abstract

We have studied the magnetic and transport behavior of doped La0.65 A0.35 Mn0.95Fe0.05 O3 (A= Ca, Sr, Pb, Ba) compounds. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition except the Pb doped sample which is insulating and ferromagnetic in the entire temperature range. The simultaneous occurrence of ferromagnetism and insulating behavior in Pb doped compound is most likely due to the presence of FM clusters separated by Fe and Mn ions that are coupled AFM and hence prevent the current from crossing the inter-domain region. The magnetization and Tc are decreased by decreasing the Cation size on La site. We observed that (for a fixed Fe content of 5%) the transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of Tc and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, Ba). The decrease in the transition temperature and magnetic moment is due to the deviation of Mn-O-Mn angle from 1800 caused by the size mismatch of the A site cations. This deviation leads to weakening of the FM double exchange (DE). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, AC susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.