Coexistence of Ferromagnetism and Anti-ferromagnetism in Sm_{0.58}Sr_{0.42}MnO_{3} Nanoparticles Prepared by Mechanical Milling

판더롱 한국물리학회 2014 새물리 Vol.64 No.5

We have studied carefully the magnetic properties of Sm_{0.58}Sr_{0.42}MnO_{3} nanoparticles with an average crystallite size of 32 nm. Experimental results indicate the coexistence of ferromagnetic (FM) and anti-ferromagnetic (AFM) interactions even at temperatures above the Curie temperature, T_C. Particularly, the zero-field-cooled magnetization-versus-temperature curve exhibits a cusp at T_{a} (< T_{C}), which is shifted towards lower temperatures with increasing applied field. Compared to conventional spin-glass (SG) systems, and the De Almeida-Thouless (AT) line, we find that the reduced temperature θ_{a}=T_{a} / T_{p} can scale with the field H as θ_{a}=1-(C_{AT} / T_{p})· H^{n}. The best fits are C_{AT} = 3.23 and n = 0.32 for T_{p}/T_{g} the SG transition temperature, and C_{AT} = 51.3 and n = 0.06 for T_{p} = T_{C}. In our case, the obtained n value is much smaller than the value n = 2/3 predicted by theoretical models for a conventional SG system. This reflects the existence of a cluster-SG behavior and a magnetic imhomogeneity in Sm$_{0.58}Sr_{0.42}MnO_{3} nanoparticles. We believe that the observed phenomena are related to additional oxygen-vacancy defects (or Mn^{3+} ions), as indicated by X-ray absorption studies.

Long-range Ferromagnetic Order in the La0.7Pb0.3MnO3 Perovskite Manganite

판더롱 한국물리학회 2010 새물리 Vol.60 No.6

We have studied the critical behavior of the La0.7Pb0.3MnO3 perovskite manganite around its paramagnetic-to-ferromagnetic phase transition temperature. Using the Arrott technique to analyze the magnetic isotherms M(H), we have found the critical parameters to be TC = 346.29 K, β= 0.502 ± 0.007, γ = 1.063 ± 0.013, and δ = 3.12 ± 0.01. In comparison to the values given by standard models, the critical exponents β,γ, and δ in our case are close to those expected from the mean-field theory. This proves the existence of a long-range ferromagnetic order in La0.7Pb0.3MnO3.

Optical Properties of ZnO Nanoparticles Prepared by Mechanical Ball Milling

판더롱 한국물리학회 2012 새물리 Vol.62 No.5

ZnO nanoparticles were fabricated from commercial micro-sized ZnO powders by mechanical ball milling. By varying the milling time (t_m) from 1 to 24 hrs, we obtained nanocrystalline samples with particle sizes of 15-150 nm. Their optical properties were then investigated at room temperature by using Raman scattering (RS) and photoluminescence (PL) spectrometers. Their PL spectra had two emission regions, are centered at about 380 nm (UV emission) and the other at 626 nm (visible emission). The intensity of the visible emission increased strongly with increasing t_m due to an increase in the number of surface defects, but the intensity of the UV emission decreased. The peak position of both emissions was independent of t_m. A study of the lattice dynamics indicated that the peak positions of the standard vibration modes, such as E_2(H), E_2(H)-E_1(L) and LO, were lightly shifted towards lower frequencies in comparison to those of the as-received ZnO sample. Concurrently, vibration intensity changed remarkably due to the presence of defects. The obtained results were discussed in detail.

Magnetic Properties and Magnetocaloric Effect of Ti-doped La0.7Sr0.3MnO3

판더롱 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.3

We have studied the influence of Ti doping on the magnetic properties and the magnetocaloric effect of La_0.7Sr_0.3MnO₃ prepared by using solid-state reaction. The temperature dependence of magnetization reveals that the presence of the Ti dopant strongly reduces the Curie temperature (<i>T_C</i>) to a value of ∼235 K. The magnetic-entropy change (Δ<i>S_m</i>) has been assessed by means of isothermal-magnetization data versus the magnetic field, M-H curves, recorded at different temperatures. Around <i>T_C</i>, -Δ<i>S_m</i> reaches maximum values of 1.1, 2.0, and 2.7 J/(kg·K) for applied fields of 10, 20, and 30 kOe, respectively. A relative cooling power (RCP) of 135 J/kg is, thus, achieved for an applied field of 30 kOe. Detailed analyses of the M-H curves around TC indicate that the sample exhibits a second-order magnetic phase transition, where the variations of the saturation magnetization (Ms) and the inverse initial susceptibility (χ⁻¹₀) versus temperature obey asymptotic relations. Using the Arrott-Noakes method, we have obtained the critical exponents β = 0.425 ± 0.016, γ = 1.017 ± 0.055, and δ = 3.393 ± 0.039. These exponents are in good agreement with the magnetic equation of state. The β value determined in our case is between those expected for the mean-field (β = 0.5) and the 3D Heisenberge (β = 0.365) models, demonstrating the existence of ferromagnetic clusters.

An Effective Route to Prepare La0.7Ca0.3MnO3 Nanoparticles: Structural Characterization and Magnetic Properties

판더롱 한국물리학회 2013 새물리 Vol.63 No.5

This work presents a top-down approach based on mechanical milling to fabricate La0.7Ca0.3MnO3 nanoparticles (LCMO NPs) from a bulk polycrystalline sample. By changing the milling time (tm), we fabricated nanocrystalline samples with average crystallite sizes ranging from 12 to 40 nm. Structural analyses based on X-ray diffraction reveal that the samples exhibit a single phase with an orthorhombic structure, where the lattice parameters vary as a function of tm. This influences directly the magnetic properties (characterized by magnetic parameters of the Curie temperature, magnetization, and coercivity) of the samples. Interestingly, while the bulk sample undergoes a first-order magnetic phase transition, all LCMO NPs exhibit a second-order nature. This is due to the presence of lattice defects created by high-energy mechanical milling.

Influence of Annealing Conditions on the Microstructural, Luminescent, and Magnetic Properties of PLD-grown ZnO Films

판더롱,유성초 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.4

As-prepared ZnO thin films on Si-wafer substrates were annealed in vacuum and in an oxygen ambient at 700 ℃. Structure analyses basing on x-ray diffraction (XRD) and transmission electron microscope (TEM) revealed that the films were formed by nanocolumns developed along the c-axis perpendicular to the substrate, with a high density of threading defects (TDs). The TD density was about 3.7 × 1010/cm2 for the as-prepared film and 9.2 × 109/cm2 for the annealed ones. Magnetic measurement demonstrated that the as-prepared films exhibited weak-ferromagnetic (FM) behavior at room temperature. The FM order increased remarkably for the film annealed in vacuum, but decreased for the film annealed in an oxygen ambient. Concerning their photoluminescence spectra,the annealing led to an increased intensity of the ultraviolet emission and caused a blue shift of the visible emissions. By means of these data, we suggest that the enhancement of the FM order for the film annealed in vacuum is related to FM interactions induced by defect clusters, where a complex of an oxygen vacancy and a zinc interstitial could play an important role.

Optical Prperties of Mn-Doped ZnO Nanowires

판더롱,Yue-Kui Sun,Roger Vincent,유성초,David Cherns,Nguyen Xuan Nghia 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.5

We used chemical vapor deposition to prepare ZnO nanowires. After that, the synthesized nanowires were doped with Mn by using thermal diffusion. The optical properties of the samples were investigated by means of using photoluminescence (PL) and Raman scattering (RS) spec- troscopy. The presence of Mn in the ZnO nanowires changed in the RS intensity at wavenumbers between 500 and 600 cm..1 and resulted in generation of longitudinal optical (LO) phonons at higher orders. The recorded PL spectra revealed the dependences of the PL intensity and the emission peak position on the Mn doping. Our results also indicated that the incorporation of a small amount of Mn into ZnO nanowires did not modify the wurtzite structure, but improved signifiantly the luminescence effciency of the ZnO nanowires.

Electron Spin Resonance Study of Mn-doped Metal Oxides Annealed at Different Temperatures

판더롱,P. Zhang,H. D. Tran,유성초 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.5

We investigated the influence of the annealing temperature (Tan) on the electron spin resonance (ESR) spectra of some Mn-doped metal oxides. Samples of MgO, CaO, BaO, SrO, PbO, CuO, NiO, ZnO, Y2O3, Dy2O3, TiO2, and ZrO2 doped with 2.0-at.% Mn were annealed at temperatures of 400 - 1400 ℃. Their ESR spectra were found to depend strongly on the Tan and the oxide host lattices. PbO:Mn, Y2O3:Mn, and Dy2O3:Mn did not show any ESR signal with changing Tan. Meanwhile, the ESR spectra of MgO:Mn, CaO:Mn, BaO:Mn, SrO:Mn, CuO:Mn, ZnO:Mn, NiO:Mn, TiO2:Mn, and ZrO2:Mn revealed a remarkable change in shape at a temperature Ttr between 500 and 800 ℃. At temperatures Tan ∼ Ttr, MgO:Mn, CaO:Mn, ZnO:Mn exhibited Mn2+ hyperfine lines while SrO:Mn, BaO:Mn, CuO:Mn, NiO:Mn, TiO2:Mn, and ZrO2:Mn exhibited symmetrical and/or asymmetrical lines. However, most samples showed no ESR signal for Tan < Ttr. The nature of these phenomena was discussed.

Critical behavior and magnetic entropy change in La_0.7Ca_0.3Mn_0.9Zn_0.1O_3 perovskite manganite

판더롱,P.Q. Thanh,N.H. Sinh,K.W. Lee,유성초 한국물리학회 2011 Current Applied Physics Vol.11 No.3

We studied the critical behavior and magnetic entropy change in a perovskite-manganite compound of La_0.7Ca_0.3Mn_0.9Zn_0.1O_3 around its Curie temperature of T_C = 206.75 K. Experimental results revealed that the sample exhibited the second-order magnetic phase transition with the exponents β = 0.474 and γ = 1.152 close to those expected from the mean-field theory (β = 0.5 and γ = 1.0). In the vicinity of T_C,the magnetic entropy change DSM reached maximum values of 1.1, 1.7, and 2.7 J/kg K under magnetic-field variations of 10, 20, and 35 kOe, respectively. These ΔS_M values are much lower than those reported previously on the parent compound of La_0.7Ca_0.3MnO_3. The nature of this phenomenon is discussed by means of the characteristics of the magnetic phase transition, and critical exponents.

Influences of Heat Treatment on the Optical Properties of ZnO Nanorods

판더롱 한국물리학회 2011 새물리 Vol.61 No.6

ZnO nanorods grown by using an aqueous solution method were annealed in an oxygen ambient at various temperatures in the range of T_a =300-1000℃. Their optical properties were then studied by using photoluminescence and Raman scattering spectrometers. The experimental results revealed that the peak positions of the ultraviolet emission and of the vibrational-phonon modes were independent of T_a, indicating the stability of the ZnO nanorods in the wurtzite structure. However, the intensity of visible emissions changed strongly as a function of T_a. This is assigned to lattice defects and/or impurities generated from precursors during the crystal growth and heat treatment. Particularly,annealing temperatures greater than 300℃ lead to a decomposition and escape of impurities from the ZnO nanorods, thus improving their crystal quality.