In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped La 05 Ca 05 MnO 3 (LCMO) synthesized by chemical route, having particle size down to an average diameter of 15 nm It was observed that the size reduction leads to change in crystal structure, and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples The structural change mainly affects the orthorhombic distortion of the lattice By making comparison to observed crystal structure data under hydrostatic pressure, it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction This not only changes the structure but also causes the room temperature structure to freeze The size reduction also does not allow the long supercell modulation needed for the charge ordering, characteristic of this half-doped manganite, to set in The magnetic and transport measurements also show that the charge ordering (CO) does not occur when the size is reduced below a critical size Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic-type conductivity Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals

Crystal structure and physical properties of half-doped manganite nanocrystals of less than 100-nm size

Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally, the opposite effect has been recently observed-ferromagnetism appears in some nanosized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half doping, n = 0.5, using open boundary conditions to simulate the surface of either bulk or nanosized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.

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Ferromagnetic tendency at the surface of CE-type charge-ordered manganites

Recent experiments showed that the robust charge ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.

http://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/P191.pdf

Surface phase separation in nanosized charge-ordered manganites

La(0.4)Ca(0.6)MnO(3) nanoparticles of grain size as small as similar to 20 nm are prepared and their magnetic behaviors are investigated in order to understand the size effect of the charge ordering in manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic phase separation due to the competition between ferromagnetic state and charge-ordered state as well as the surface spin disordering is responsible for the spin-glass-like state at low temperature. (C) 2007 American Institute of Physics.

Charge-order breaking and ferromagnetism in La 0.4 Ca 0.6 MnO 3 nanoparticles

Perovskite manganites exhibit a broad range of structural, electronic, and magnetic properties, which are widely investigated since the discovery of the colossal magnetoresistance effect in 1994. As compared to the parent perovskite manganite oxides, rare earth-doped perovskite manganite oxides with a chemical composition of LnxA1-xMnO3 (where Ln represents rare earth metal elements such as La, Pr, Nd, A is divalent alkaline earth metal elements such as Ca, Sr, Ba) exhibit much diverse electrical properties due to that the rare earth doping leads to a change of valence states of manganese which plays a core role in the transport properties. There is not only the technological importance but also the need to understand the fundamental mechanisms behind the unusual magnetic and transport properties that attract enormous attention. Nowadays, with the rapid development of electronic devices toward integration and miniaturization, the feature sizes of the microelectronic devices based on rare earth-doped perovskite manganite are down-scaled into nanoscale dimensions. At nanoscale, various finite size effects in rare earth-doped perovskite manganite oxide nanostructures will lead to more interesting novel properties of this system. In recent years, much progress has been achieved on the rare earth-doped perovskite manganite oxide nanostructures after considerable experimental and theoretical efforts. This paper gives an overview of the state of art in the studies on the fabrication, structural characterization, physical properties, and functional applications of rare earth-doped perovskite manganite oxide nanostructures. Our review first starts with the short introduction of the research histories and the remarkable discoveries in the rare earth-doped perovskite manganites. In the second part, different methods for fabricating rare earth-doped perovskite manganite oxide nanostructures are summarized. Next, structural characterization and multifunctional properties of the rare earth-doped perovskite manganite oxide nanostructures are in-depth reviewed. In the following, potential applications of rare earth-doped perovskite manganite oxide nanostructures in the fields of magnetic memory devices and magnetic sensors, spintronic devices, solid oxide fuel cells, magnetic refrigeration, biomedicine, and catalysts are highlighted. Finally, this review concludes with some perspectives and challenges for the future researches of rare earth-doped perovskite manganite oxide nanostructures.

/pdf/research-progress-in-rare-earth-doped-perovskite-manganite-5gf6cyvokp.pdf

Research Progress in Rare Earth-Doped Perovskite Manganite Oxide Nanostructures

Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ nanoparticles having average particle size of 30 nm and 45 nm. Our measurements suggest that the formation of charge ordered state is largely affected due to lowering of particle size, but the ferromagnetic transition temperature ($T_{C}$) remains unaffected.

Modification of the Charge ordering in Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ Nanoparticles

Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr1∕2Sr1∕2MnO3 nanoparticles having an average particle size of 30 and 45nm. Our measurements suggest that the formation of a charge-ordered state is largely affected due to lowering of the particle size, but the ferromagnetic transition temperature (TC) remains unaffected.

Modification of the charge ordering in Pr1/2Sr1/2MnO3 nanoparticles

A gradual transformation from the polycrystalline to nanocrystalline and amorphous phase is studied by mechanical milling of bulk La0.67Ca0.33MnO3 sample. The particle size is not monotonically decreasing with the increase of milling time. The grain growth kinetics appears at higher milling time, where the material shows the properties of amorphous ferromagnet.

Chandan Majumdar

Papers

Modification of the Charge ordering in Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ Nanoparticles

Modification of the charge ordering in Pr1/2Sr1/2MnO3 nanoparticles

Structural And Magnetic Transition In Mechanically Milled La0.67Ca0.33MnO3

The Modification of Charge ordering in Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ Nanoparticles