Effect of small cation occupancy and anomalous Griffiths phase disorder in nonstoichiometric magnetic perovskites

TL;DR

This study investigates how small cation occupancy and Griffiths phase disorder affect the structural and magnetic properties of non-stoichiometric manganite perovskites, revealing non-linear Griffiths phase behavior and magnetic entropy changes.

Contribution

It provides new insights into the role of cation vacancies and substitution in tuning Griffiths phase disorder and magnetic transitions in non-stoichiometric perovskite manganites.

Findings

A non-linear Griffiths phase behavior in Sr-substituted compounds.

Identification of Mn3+-Mn4+ dimerons as the cause of Griffiths phase.

Enhanced magnetocaloric effect in Sr-substituted compounds.

Abstract

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La_(0.8-x) Sr_(0.2-y) Mn_(1+x+y) O_3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr-substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at {\mu}_0 H=2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at {\mu}_0 H=2T).