Magnetocaloric effect in La_1-xSr_xCoO₃ undergoing a second-order phase transition

Long, Phan The,Manh, T.V.,Ho, T.A.,Dongquoc, Viet,Zhang, P.,Yu, S.C. Elsevier 2018 CERAMICS INTERNATIONAL Vol.44 No.13

<P><B>Abstract</B></P> <P>We have prepared rhombohedral La<SUB>1-x</SUB>Sr<SUB>x</SUB>CoO<SUB>3-δ</SUB> (<I>x</I> = 0.2–0.5) compounds with a mass density <I>ρ</I> ≈ 5.5 g/cm<SUP>3</SUP>. Their magnetocaloric (MC) effect is studied via the magnetic-entropy change (Δ<I>S</I> <SUB>m</SUB>) and relative cooling power (<I>RCP</I>), which are calculated from initial magnetization data recorded at different temperatures. Results reveal that the Δ<I>S</I> <SUB>m</SUB> magnitude is maximum (Δ<I>S</I> <SUB>max</SUB>) around the ferromagnetic-paramagnetic phase transition and dependent on both the applied-field (<I>H</I>) magnitude and Sr content (<I>x</I>). For <I>H</I> = 50 kOe, |Δ<I>S</I> <SUB>max</SUB>| can be tuned in the range of 1.6–2.7 J/kg K, corresponding to <I>RCP</I> values of 89–141 J/kg. Among the studied La<SUB>1-x</SUB>Sr<SUB>x</SUB>CoO<SUB>3-δ</SUB> samples, the samples with <I>x</I> = 0.3–0.5 have the largest |Δ<I>S</I> <SUB>max</SUB>| values. If combining these samples as MC blocks in refrigeration application, the working temperature range of a cooling device could range from 204 to ~280 K, with |Δ<I>S</I> <SUB>max</SUB>| stable at ~2.6 J/kg K and <I>RCP</I> ≈ 198 J/kg. We have also assessed the phase-transition type and magnetic order and found La<SUB>1-x</SUB>Sr<SUB>x</SUB>CoO<SUB>3-δ</SUB> undergoing a second-order phase transition. Magnetic order tends to change from the long-range type to the short-range one when <I>x</I> varies from 0.2 to 0.5. This is in good agreement with the results obtained from the analysis of critical behavior.</P>

Influence of Crystallite Size on Magnetocaloric Effect and Critical Behavior La_0.7Sr_0.3Mn_0.92Co_0.08O₃ Nanoparticles

Tran Dang Thanh,Dinh Chi Linh,Le Viet Bau,Thi Anh Ho,Tien Van Manh,The-Long Phan,Seong-Cho Yu IEEE 2015 IEEE transactions on magnetics Vol.51 No.1

<P>Four samples of La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>Mn<SUB>0.92</SUB>Co<SUB>0.08</SUB>O<SUB>3</SUB> (LSMCO) with different crystallite sizes were prepared by the combination of solid-state reaction and mechanical milling methods. Based on isothermal magnetization data, M(H), temperature dependences of magnetic entropy change, ΔS<SUB>m</SUB>T, of the samples under a magnetic field change of 10 kOe were calculated. The maximum values of magnetic entropy change (|ΔS<SUB>max</SUB>|) at room temperature are in the range of 0.9-1.4 J · kg<SUP>-1</SUP> · K<SUP>-1</SUP>, corresponding to ferromagnetic (FM)-paramagnetic phase transition. In addition, M<SUP>2</SUP> versus H/M curves at temperatures around TC prove the samples exhibiting a second-order magnetic phase transition. The critical exponents β, γ, and δ were determined using the modified Arrott plot method and critical isotherm analysis. Here, these exponent values are located in between those expected for the mean-field theory and 3-D Heisenberg model. It means the coexistence of short-range and long-range FM interactions in LSMCO nanoparticles.</P>

Coexistence of short- and long-range ferromagnetic order in La_0.7Sr_0.3Mn_1−xCo_xO₃ compounds

Thanh, Tran Dang,Linh, Dinh Chi,Manh, T. V.,Ho, T. A.,Phan, The-Long,Yu, S. C. American Institute of Physics 2015 Journal of Applied Physics Vol.117 No.17

Magnetic Properties and Magnetocaloric Effect at Room Temperature of Ni50−xAgxMn37Sn13 Alloys

Tran Dang Thanh,Nguyen Thi Mai,Nguyen Huy Dan,The-Long Phan,유성초 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.10

In this work, we present a detailed study of the magnetic properties and the magnetocaloric effectat room temperature of Ni50−xAgxMn37Sn13 alloys with x = 1, 2, and 4, which were prepared byusing an arc-melting method. Experimental results reveal that a partial replacement of Ag for Nileads to a decrease in the anti-FM phase in the alloys. In addition, the martensitic-austenitic phasetransition shifts towards lower temperature and is broaded. The Curie temperature (TAC ) for theaustenitic phase also shifts toward to lower temperature, but not by much. The Curie temperaturewas found to be 308, 305, and 298 K for x = 1, 2, and 4, respectively. The magnetic entropychange (Sm) of the samples was calculated by using isothermal magnetization data. Under anapplied magnetic field change of 10 kOe, the maximum value of Sm (|Smax|) was achieved ataround room temperature and did not change much (0.8 J·kg−1·K−1) with increasing Ag-dopingconcentration. Particularly, the M2 vs. H/M curves prove that all the samples exhibited a secondordermagnetic phase transition. Based on Landau’s phase-transition theory and careful analyses ofthe magnetic data around the TAC , we have determined the critical parameters ,, , and TC. Theresults show that the values are located between those expected for the 3D-Heisenberg model (= 0.365) and mean-field theory ( = 0.5). Such a result proves the coexistence of short-range andlong-range ferromagnetic interactions in Ni50−xAgxMn37Sn13 alloys. The nature of the changes inthe critical parameters and the |Smax| is thoroughly discussed by means of structural analyses.

Influences of Ru-doping on the Magnetic Properties of Ca0.85Pr0.15Mn1−xRuxO3

The Long Phan,Y.D. Zhang,유성초,P. Q. Thanh,P. D. H. Yen 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.10

CaMnO<SUB>3</SUB> is an antiferromagnet, where the super-exchange interaction taking place between Mn<SUP>4+</SUP> ions plays an important role. The doping of a small amount of 15% Pr into the Ca site, Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>MnO<SUB>3</SUB>, leads the appearance of Mn<SUP>3+</SUP> ions, and introduces the ferromagnetic double-exchange interaction between Mn<SUP>3+</SUP> and Mn<SUP>4+</SUP> ions dominant in a narrow temperature range of 90~115 K. The ferromagnetic interaction becomes strong as Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>MnO<SUB>3</SUB> doped with 4 and 8 % Ru into the Mn site (<I>i.e.</I>, Ca<SUB>0.85</SUB>Pr<SUB>0.15</SUB>Mn<SUB>1-x</SUB>Ru<SUB>x</SUB>O<SUB>3</SUB> with <I>x</I> = 0.04 and 0.08). The Curie temperature obtained for <I>x</I> = 0.04 and 0.08 are about 135 and 180 K, respectively. While the FM interaction in the former is dominant due to Mn<SUP>3+</SUP>-Mn<SUP>4+</SUP> exchange pairs, the latter has the contribution of Ru ions. This results in the remarkable difference in the feature of their FM-paramagnetic phase transitions, and coercive field <I>H</I><SUB>c</SUB>.

Electron-spin-resonance Study of Polycrystalline Fe-doped ZnO Ceramics

Peng Zhang,The Long Phan,유성초,김준성 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.10

The influence of Fe doping on the structural and the magnetic properties of polycrystalline ceramics of Zn_1−xFe_xO (x = 0.01 - 0.30) was systematically investigated by means of X-ray diffraction (XRD), electron spin resonance (ESR), and a superconducting quantum interference device (SQUID) magnetometer. XRD patterns reveal the coexistence of two structural phases, wurtzite ZnO and spinel ZnFe₂O₄, even at x = 0.01. Both XRD and ESR data indicate that Fe ions are incorporated into the Zn sites of the ZnO host lattice and act as paramagnetic centers. The ESR spectral linewidth increases with increasing Fe concentration until x = 0.04 and then slightly decreases for x > 0.04. Compared with the variation tendency in the lattice parameters, we believe the Fe dopants in Zn_1−xFe_xO to be Fe²⁺ and Fe³⁺ ions, where Fe³⁺ dominates in the compounds with x ≤ 0.08 while Fe²⁺ dominates in the compounds with higher Fe concentrations. Exchange interactions due to Fe³⁺-Fe³⁺, Fe³⁺-Fe²⁺, and Fe²⁺-Fe²⁺ pairs are responsible for the ESR signals and the paramagnetic behavior of the samples.

Photoluminescence properties of Cr3+-doped MgAl2O4 Natural Spinel

The-Long Phan,Manh-Huong Phan,유성초,Thomas P J Han 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.1

We present, here, the results of a study of photoluminescence spectra of Cr3+-doped MgAl2O4 natural spinel in a given wavenumber range between 26,300 and 12,500 cm By using several excitation sources, we observed a new broadband, namely K, with a peak at 15,547 cm and the transition process of luminescence lines among the electronic levels of the Cr3+ ion, such as R, Ni (i = 1-4) lines. In terms of the experimental results and the conguration coordinates, we strongly propose that the K-band might originate from the electric dipole transition 2Eg ! 4A2g, in a strong crystal eld with Dq=B > 2:3. The decay-time and luminescence-rise-time curves on the zero-phonon lines and the K-band were also recorded.

Dielectric resonance effect with negative permittivity in a La1.5Sr0.5NiO4+δ ceramic

Tran Dang Thanh,Nguyen Van Dang,Le Van Hong,The-Long Phan,Seong-Cho Yu 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.10

A polycrystalline sample of La1.5Sr0.5NiO4+δ was prepared by using a solid-state reaction. Xraydiffraction proved the sample to be a single phase with a tetragonal structure (space group:I4/mmm). By using an iodometric titration method to determine the non-stoichiometric oxygenconcentration (δ) in La1.5Sr0.5NiO4+δ, we found δ = −0.017, which corresponds to a doping level ofnh = x + 2δ = 0.466. Also, a strong increase of the magnetization in theM(T) curve at temperaturesbelow the spin-ordering temperature (TSO 100 K) was observed. The M(H) curves show verysmall magnetic moments, which proves the weak ferromagnetic nature of La1.5Sr0.5NiO4+δ. Thedependences of the dielectric constant on the frequency and the temperature, "(!, T) = "0(!, T) +i"00(!, T), was investigated in the frequency range of 1 − 13 MHz. At temperatures around roomtemperature, the maximum of the real part ("0) was higher than 105. Particularly, an abnormaldependence of the permittivity on frequency was observed. Depending on temperature, a dielectricresonance was observed at about 500 kHz or 8 MHz. Interestingly, we observed the dielectricresonanceeffect with a negative permittivity. Such a feature is very similar to that observed inleft-handed materials. The fitting of the experimental data for the dielectric constant at frequenciesaround the resonance frequency to the equations associated with an equivalent RLC series circuitproves that La1.5Sr0.5NiO4+δ belongs to the class of multiferroic materials.

The Future of Adult Cardiology

Council on Long Range Planning and Development in Cooperation With the American College of Cardiology MediMedia Korea 1990 JAMA-Korea Vol.5 No.1

Influence of glycine nitrate on luminescence efficiency of Y2O3:Eu nanophosphors

The-Long Phan,Manh-Huong Phan,유성초,Van-Thai Pham,정용석 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3

Y2O3 : 5-mol%Eu nanophosphors with dierent glycine-to-metal nitrate (G/M) ratios were fabricated by using the combustion method. X-ray diraction conrmed a cubic structure for the samples investigated. Based on this, the powder grain sizes are estimated to range from 5.0 to 67.3 nm. The powder sample with G/M = 1.6:1 has the highest value of photoluminescence eciency on varying the G/M ratio. The optimum was realized in the sample annealed at 700 C for 1 hour. The luminescence lifetime of the 5D0-7F2 transition at 610 nm is estimated to be about 1.4 ms. Using the time-resolved photoluminescence technique, we observed two new spectrum bands, namely P1 and P2, from the 5D0-7F0 transition. The luminescence-decay curves tted well with an exponential function: I = I0 exp(lifetime). The eect of glycine agent on luminescence eciency in the samples investigated is discussed.