Abstract
Comparative studies of the structural, magnetic, and optical properties of the sol–gel synthesized Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) nanoparticles were carried out focusing on the effect of the A-site average cation size ⟨rA⟩. Rietveld refinements of x-ray diffraction data demonstrate all nanoparticles crystallize in an orthorhombic crystal structure (Pnma space group). Their unit cell volumes and the Mn–O–Mn bond angle decreased with reducing ⟨rA⟩, whereas the Mn–O bond length increased. The morphologies of nanoparticles evolved from spherical to irregular shapes, and their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching mode of the Mn–O bond near 600 cm−1, and the softening of this phonon mode as reducing ⟨rA⟩ is ascribed to the elongation of the Mn–O bond length. X-ray photoelectron spectroscopy reveals the mixed Mn3+ and Mn4+ cations with a content ratio of Mn3+/Mn4+ = 2:1, divalent Ca cations, and trivalent rare earth Ln cations in all nanoparticles and oxygen element existing as lattice oxygen and chemically absorbed oxygen. The La0.67Ca0.33MnO3 nanoparticles exhibited ferromagnetic behavior, whereas Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles displayed antiferromagnetic behavior and strong exchange bias effect. Temperature dependence of dc magnetizations suggests the spin-glass behavior established in the La0.67Ca0.33MnO3 nanoparticles, while magnetic cluster-glass behavior formed in the Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles, in which the charge-ordered and antiferromagnetic phases were completely suppressed. Electronic bandgaps of the nanoparticles were about 1.55 eV–1.66 eV, which was ascribed to the electronic charge transfer between two eg bands of the Mn cation with up-spins and down-spins separated by Hund’s coupling energy.
Highlights
The applied magnetic field with several tesla (∼6 T) leads to a resistance drop in a few orders of magnitude in the LCMO system, which is called the colossal magnetoresistance (CMR) effect.9–11. Another example is the electronic phase separation (EPS), which states that the electronic phases of perovskite manganite oxides are intrinsically inhomogeneous and they exhibit a strong tendency toward phase separation because of the phase competition between the FM metallic phase and the charge-ordered insulating phase
The energy dispersive analysis of the x-ray (EDAX) spectra demonstrate that the atomic ratios of Ln:Ca:Mn elements in the synthesized nanoparticles are close to the nominal value
XPS spectra demonstrated that Mn cations existed in dual chemical states of Mn3+ and Mn4+ with the Mn3+/Mn4+ content ratio of 2:1, rare earth elements Ln (= La, Pr, Nd, and Sm) showed a trivalent state, the Ca element showed a stable divalent state, and oxygen existed in the forms of lattice oxygen and chemical absorbed oxygen
Summary
The applied magnetic field with several tesla (∼6 T) leads to a resistance drop in a few orders of magnitude in the LCMO system (such as bulks and thin films), which is called the CMR effect.. The applied magnetic field with several tesla (∼6 T) leads to a resistance drop in a few orders of magnitude in the LCMO system (such as bulks and thin films), which is called the CMR effect.9–11 Another example is the electronic phase separation (EPS), which states that the electronic phases of perovskite manganite oxides are intrinsically inhomogeneous and they exhibit a strong tendency toward phase separation because of the phase competition between the FM metallic phase and the charge-ordered insulating phase.. In the Pr-doped LCMO system, the magnetocaloric (MC) effect and transverse Kerr effect were observed, and the EPS region involving FM and CO AFM domains was as large as the sub-micrometer size (∼0.2 μm). In the Gd-doped LCMO system, the system is found to undergo successive transitions from the state of long-range FM ordering to the state of clusterspin glass, and to the AFM states. An enhanced magnetic phase inhomogeneity was observed in the Dy-doped LCMO system.
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