Structural and magnetic ordering inLa0.5Ca0.5−xBaxMnO3 (0 < x ≤ 0.5) manganite

Abstract

We report a structural, magnetic and transport study inLa0.5Ca0.5−xBaxMnO3 (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements.The samples crystallize in an orthorhombic structure withPnma space group and no structural transition is observed with Ba doping.Substituting Ca with Ba leads to an increase in average A-site ionic radii,⟨rA⟩ and cationsize disorder (σ2). Consequently, the nature of magnetic ordering undergoes changes as a function ofx. At lowtemperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. Atx = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. Forx≥0.2, the ferromagnetic phase is stabilized. The signature of orbital orderingin addition to spin and charge ordering is observed in all samples withx≤0.15.TC,TCO andTN remain nearlyconstant for x≤0.2. The transport studies reveal that for samplesx≤0.2, adiabatic hopping of small polarons governs the transport behaviour aboveTCO (∼160 K). A parallel combination of small polaron hopping and variablerange hopping mechanisms is found to describe resistivity data belowTCO. Atx = 0.2, an insulator to metal transition is observed as temperature is lowered, and atx = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data followsthe Bloch–Grüneisen model together with contributions from high frequency optical phononsand magnetic scattering. The experimentally established phase diagram from the presentstudy is compared with theoretical studies in the intermediate electron–phonon interactionregime.