Structural and magnetic ordering in La_0.5Ca_0.5−xBa_xMnO₃(0 <x≤ 0.5) manganite

Abstract

We report a structural, magnetic and transport study in La(0.5)Ca(0.5-x)Ba(x)MnO(3) (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements. The samples crystallize in an orthorhombic structure with Pnma 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, ⟨r(A)⟩ and cation size disorder (σ(2)). Consequently, the nature of magnetic ordering undergoes changes as a function of x. At low temperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. At x = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. For x≥0.2, the ferromagnetic phase is stabilized. The signature of orbital ordering in addition to spin and charge ordering is observed in all samples with x≤0.15. T(C), T(CO) and T(N) remain nearly constant for x≤0.2. The transport studies reveal that for samples x≤0.2, adiabatic hopping of small polarons governs the transport behaviour above T(CO) (∼160 K). A parallel combination of small polaron hopping and variable range hopping mechanisms is found to describe resistivity data below T(CO). At x = 0.2, an insulator to metal transition is observed as temperature is lowered, and at x = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data follows the Bloch-Grüneisen model together with contributions from high frequency optical phonons and magnetic scattering. The experimentally established phase diagram from the present study is compared with theoretical studies in the intermediate electron-phonon interaction regime.

Influence of B-site disorder in La0.5Ca0.5Mn1 −xBxO3(B = Fe, Ru, Al and Ga) manganites

“…The d-values and the relative intensities of these superlattice reflections are in agreement with those reported previously in half-doped manganites and attributed to A-type antiferromagnetic ordering [9,75,76]. Rietveld refinement of the diffraction data at 5 K shows that these superlattice reflections correspond to A-type antiferromagnetic ordering.…”

“…The magnetic moment in this A-type antiferromagnetic phase is 2.64 (6) μ B , oriented ferromagnetically in the ac plane and antiferromagnetically along the b axis. The suppression of the CE-type and emergence of the A-type antiferromagnetic state observed in these systems is similar to that observed in previously reported neutron diffraction studies on A-site doped compounds [9,8]. On increasing the Al doping to 7%, both the CE-type antiferromagnetic and ferromagnetic phases are suppressed, while the superlattice reflections corresponding to an A-type antiferromagnetic state are still visible below T N ≈ 125 K. The obtained value of magnetic moment at 5 K is 3.12(6) μ B .…”

“…Generally, two sources of substitutional disorder have been described in charge ordered manganites: A-site and B-site disorder. The magnetic and transport properties in several systems are observed to be systematically influenced by structural distortions and lattice disorder associated with the average A-site ionic radii ( r A ) and variance (σ 2 ) due to size mismatch between the A-site cations [6][7][8][9][10]. The B-site substitution, as against A-site, induces disorder directly in the Mn-O network, leading to much stronger impact on the magnetic and transport properties.…”

The structural and magnetic ordering in La0.5−Nd Ca0.5MnO3 (0.1≤x≤0.5) manganites

“…However, in these studies the change in /r A S and s 2 were significantly large [20][21][22][23][24][25][26]. In the present study we have investigated the influence of introducing very small disorder ð $ 10 À4 Þ in La 0.5 Ca 0.5 MnO 3 compound.…”

The Structural and Magnetic ordering in $La{}_{0.5-x}Nd_{x}Ca_{0.5}MnO_{3}$ (0.1 \ensuremath{\le} x \ensuremath{\le} 0.5) Manganites

“…The effect of A -site disorder on the nature of magnetic ordering in charge ordered manganites has been reported previously. However, in these studies the change in <r A > and σ 2 were significantly large [20,21,22,23,24,25,26]. In the present study we have investigated the influence of introducing very small disorder (∼ 10 −4 ) in La 0.5 Ca 0.5 M nO 3 compound.…”