The neutron diffraction measurements of the nano-size RMnO3 (R=Pr, Nd) manganites have been performed. The obtained results indicate that these compounds crystallize in the orthorhombic crystal structure described by the space group Pnma. Low temperature data indicate both the absence of a long-range magnetic order down to 1.5 K in the PrMnO3 sample annealed at 800 degrees C and the CxFy magnetic structure with a very low value of the Mn magnetic moment as compared with bulk sample in the sample annealed at 900 degrees C. In the NdMnO3 samples annealed at 850 degrees C and 900 degrees C the magnetic order is described by the CxFy mode in the Mn sublattice and the F-y mode in the Nd sublattice. The values of the Mn and Nd magnetic moments are lower that those in a bulk sample. Based on the model proposed in Lopez-Quintela et al. [1] and Balcells et al. [2] the thickness of nonmagnetic shell was determined to be 7.1(4) nm in PrMnO3 and 4.6(5) nm in both the NdMnO3 compounds. (C) 2013 Elsevier B.V. All rights reserved.

X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize REMnO3 (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of synthesized nanoparticles (from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering. The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rare-earth sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of Mn sublattice and paramagnetic Curie temperatures as well as the magnetic moment values for the nanosize samples was found to be smaller than those for polycrystalline sample.

W. Ba»ela, M. Dul, V. Dyakonov, Š. Gondek, A. Hoser, J.-U. Hoffmann, B. Penc , A. Szytuaa ∗, Z. Kravchenko, I. Nosalev and A. Zarzycki Institute of Physics, Technical University of Cracow, Podchor¡»ych 1, 30-084 Kraków, Poland Institute of Physics, PAS, al. Lotników 32/46, 02-668 Warszawa, Poland A.A. Gaakin Donetsk Physico-Technical Institute, NANU, R. Luxemburg 72, 83114 Donetsk, Ukraine AGH University of Science and Technology, Faculty of Physics and Applied Computer Science,

X-ray diffraction and magnetic measurements of polycrystalline and nanosize TbMnO3 manganites were performed. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature. Nanosize manganites were synthesized via the sol-gel method at different (800 and 900 °C) temperatures. The average size of the synthesized nanoparticles (from 45 to 70 nm) was estimated by x-ray diffraction and low-temperature adsorption of argon. Information about the evolution of properties of TbMnO3 with changing grain size, temperature, and magnetic field was obtained. Crystal structure parameters of nano-samples change slightly with changing nanoparticle size. Peculiarities of magnetic ordering in polycrystalline and nanosize TbMnO3 were compared. Magnetization and the Nèel temperature corresponding to antiferromagnetic ordering of the Tb3+ sublattice decrease as the particle size is reduced. Inverse magnetic susceptibility of the nanoparticle samples deviates from the Curie–Weiss law below 50 K; this is related to the magnetic ordering of Mn3+ moments. Some peculiar anomalities related to the magnetic ordering of Tb3+ and Mn3+ sublattices were noticed in specific heat values of the nanosize samples.

This paper reports on investigations of magnetic properties, crystal and magnetic structures on TbMnO3 prepared in various ways, namely, as conventional polycrystalline sample and two nano particle specimens (synthesized with a sol gel method at temperatures of 800 and 850 degrees C). The X-ray and neutron diffraction data confirm the orthorhombic crystal structure (space group Pbnm, No. 62) without noticeable differences of the lattice parameters for poly- and nanocompounds. For the polycrystalline sample, a subsequent ordering of the Mn and Tb sublattices with decreasing temperature was observed. Namely, the Mn sublattice exhibits a modulated magnetic structure with the propagation vector k = (0, k(x), 0) in between 41-5 K. Below T = 21 K, a change from a collinear (A(y) mode) into non-collinear (A(y)G(z) mode) structure was evidenced. Further decreasing of temperature below 10 K results in magnetic ordering of the Tb sublattice (modulated G(x)A(y)F(z) mode). For nanoparticle compounds, magnetic ordering in the Mn and Tb sublattices is described by propagation vector k = (0, k(y), 0), with k(y) components higher than observed for polycrystalline sample. The magnetic ordering in the Mn sublattice is described by a collinear A(y) mode down to 1.6 K where the Tb moment becomes ordered (G(x)A(y) mode). The observed broadening of the Bragg peaks connected to the Tb sublattice suggests the cluster-like character of its magnetic structure.

The magnetic properties including magnetic structure of poly and nano samples of TbMnO3 are determined. All the samples investigated are antiferromagnets. In these samples the Mn ad Tb moments order antiferromagnetically at different temperatures and form modulated magnetic structure described by the propagation vector k=(k(x),0,0) with different value of k(x) for the Mn and Tb sublattices. Comparison of the data for poly and nano samples indicates the decrease of the moment and increase of the k(x) component of propagation vector in the nano specimens. The wide Bragg peak related to the Tb sublattice suggests that the magnetic order has the claster-like character. The magnetic moments value in both sublattices is smaller, whereas the k(x) values are larger for nano samples.

X-ray powder diffraction, magnetization, transport and magnetic resonance measurements of nanosize La0.7Sr0.3MnO3 (LCMO) manganites have been performed. The nanosize manganites were synthesized with a co-precipitation method at different (600, 700, 800 and 1000°C) temperatures. The crystal structure of the nanopowders obtained was determined to be perovskite-like with a rhombohedral distortion (the space group R3¯c). The average size of synthesized nanoparticles (from 17 to 88nm) was estimated using the X-ray diffraction and low temperature adsorption of argon methods. All the nanosize manganites show ferromagnetic-like ordering. Both the Curie temperature and magnetization decrease with reducing the particle size. The decrease of magnetization is due to the disordered surface shell of particles. The disordered surface layer is a source of the surface anisotropy and is responsible for the increase of coercivity. Temperature dependences of the magnetic resonance spectra parameters have allowed obtaining information on dynamics of magnetic properties in the nanoparticle systems. The resistivity was established to become higher by reducing the particles’ size and increases to a great extent in nanoparticles with the smallest average size at low temperatures. The magnetic entropy was shown to be smaller for the small particles. Using the temperature dependence of magnetic entropy the relative cooling power of the nanosize samples studied was evaluated.

Magnetic and transport thermal measurements of nanosize (La0.7Sr0.3)0.9Mn1.1O3 manganite are reported. The nanoparticles are synthesized with use of the co-precipitation method at different (800, 900, and 950°C) temperatures. Their crystal structure is determined to be perovskite-like with a rhombohedral distortion (the space group R3¯c). The phase composition and specific surface nanopowders are determined. The average size of synthesized nanoparticles (from 40to100nm) is estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with decreasing particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the framework of the double exchange model. The decrease of the magnetization with decreasing particle size is due to the increasing surface contribution to the magnetization. The magnetic entropy is shown to increase with increasing applied magnetic field and to be smaller for the small particles. The resistivity is found to become higher with decreasing particles size at any temperatures.