In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped La0.5Ca0.5MnO3(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. 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 with 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-in. 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.
Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties, such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; thus the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.Recently, exotic ABO 3 -type perovskites with unusually small A-site cations have attracted much attention owing to the formation of LiNbO 3 (LN)-type polar structure at high pressure (HP; Supporting Information, Section S1). [3] So far, several LN-type ABO 3 oxides have been discovered as metastable quenched phases, including ZnSnO 3 , [4a] [5] ScFeO 3 , [3b] and the high-pressure polymorphs of MnMO 3 (M = Ti, Sn) [6] and FeTiO 3 , [7] which show either SHG [4] or (near) room-temperature (RT) multiferroic behavior. [3,[5][6][7] Compared with the research in HP-stabilized LN-type ABO 3 oxides, there are few studies of systems with multiple B-site cations, such as A 2 BB'O 6 , containing small A-ions.
Above-room-temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high-temperature polar magnetic oxides in the corundum-derived A2BB'O6 family, exemplified by the non-centrosymmetric (R3) Ni3TeO6-type Mn(2+)2Fe(3+)Mo(5+)O6, which shows strong ferrimagnetic ordering with TC = 337 K and demonstrates structural polarization without any ions with (n-1)d(10)ns(0), d(0), or stereoactive lone-pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni3TeO6 prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 μC cm(-2)) and non-centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials.
Polar oxides are technically of great interest but difficult to prepare. Our recent discoveries predicted that polar oxides can be synthesized in the corundum-derivative A2BB'O6 family with unusually small cations at the A-site and a d(0) electron configuration ion at B'-site. When magnetic transition-metal ions are incorporated more interesting polar magnetic oxides can form. In this work we experimentally verified this prediction and prepared LiNbO3 (LN)-type polar magnetic Zn2FeTaO6 via high pressure and temperature synthesis. The crystal structure analysis indicates highly distorted ZnO6 and (Fe/Ta)O6 octahedra, and an estimated spontaneous polarization (PS) of ∼50 μC/cm(2) along the c-axis was obtained from point charge model calculations. Zn2Fe(3+)Ta(5+)O6 has a lower magnetic transition temperature (TN ∼ 22 K) than the Mn2FeTaO6 analogue but is less conductive. The dielectric and polarization measurements indicate a potentially switchable component.
Magnetic properties of nanocrystalline La1-x MnO3+delta manganites: size effects V Markovich, I Fita, D Mogilyansky et al.Effect of size reduction on the structural and magnetic order in LaMnO3+delta (delta approx 0.03) nanocrystals: a neutron diffraction study Barnali Ghosh, V Siruguri, A K Raychaudhuri et al. Abstract. In this paper, we report an investigation of the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of La 0.67 Ca 0.33 MnO 3 using magnetic measurements and neutron diffraction. The investigation was performed on nanoparticles with crystal size down to 15 nm. The neutron data show that even down to a size of 15 nm the nanoparticles show finite spontaneous magnetization (M S ) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagneticto-paramagnetic transition temperature T C with size d and found that T C initially enhances upon size reduction, but for d < 50 nm it decreases again. The initial enhancement in T C was related to an increase in the bandwidth that occurred due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined from the neutron data. The size reduction also changes the nature of the ferromagnetic-to-paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.
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