Dominating Role of Carrier Localization over Griffiths Inhomogeneity and Phase Separation on Magnetoresistance in High Entropy Manganites

Abstract

The colossal magnetoresistance (CMR) and metal–insulator transitions in electronically correlated manganite oxides are well-known to be driven and improved by coexisting magneto-electronic phases of competitive origin. Such magneto-electronic inhomogeneity or the so-called phase-separated state appears when several physical interactions are simultaneously active. This makes doped manganites extremely sensitive to the parameters like the nature and extent of doping, A-site cation radius (⟨𝑟𝐴⟩$⟨r_A⟩$$⟨r_A⟩$), A-site size disorder parameter (σ²) and the associated local lattice deformation, external pressure, and electric and magnetic fields. The magneto-electronic inhomogeneity is believed to play a crucial role in the emergence of CMR. The Griffiths phase magnetic inhomogeneity possesses an undecided disposition to the emergence of CMR. Here, we report investigation on electrical and magneto-electric transport properties on a series of high entropy manganite (5A_0.2)MnO₃ of fixed hole doping (40%) with the wide variation in both ⟨𝑟𝐴⟩$⟨r_A⟩$$⟨r_A⟩$ and σ². The high-entropy manganite demonstrates a system of extremely exaggerated cationic disordering. Interestingly, among the seven high entropy samples, only (La_0.2Pr_0.2Dy_0.2Sr_0.2Ba_0.2)MnO₃ exhibits Griffiths phase behavior, which ruled out the generalization of ionic size disorder-induced genesis of the Griffiths phase. The maximum CMR of −9.10 × 10³(%) is observed in the non-Griffiths phase (La_0.2Sm_0.2Eu_0.2Sr_0.2Ba_0.2)MnO₃ compared to −4.82 × 10³(%) in the (La_0.2Pr_0.2Dy_0.2Sr_0.2Ba_0.2)MnO₃ Griffiths phase. This suggests that the Griffiths phase is not a necessary precursor for the CMR effect. The magnetic state in (5A_0.2)MnO₃ is attributed to the ferromagnetic cluster formation due to the fragmented long-range exchange pathway rather than coexisting ferromagnetic and antiferromagnetic type inhomogeneity. The carrier localization in fragmented clusters plays a primordial role in the CMR effect. The ferromagnetic cluster formation critically depends on both the ⟨𝑟𝐴⟩$⟨r_A⟩$$⟨r_A⟩$ and σ². The σ² is crucial to improve MR. The observed CMR in several (5A_0.2)MnO₃ is higher than the best-known values for conventional bulk manganites. The high temperature resistivity can be better described by a variable range hopping model.