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

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.

Structural Characterization of CVD-grown ZnO Nanocombs

The-Long PHAN,Yuekui Sun,R. Vincent 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.1

We present the growth of ZnO nanocombs by using chemical vapor deposition (CVD) under atmospheric pressure. Two types of single- and double-side teethed nanocombs have been obtained. Structural analyses based on electron microscopy reveal the diameter and the period of teeth to be about 50 nm and 75 - 100 nm, respectively, while the length of the comb ribbons is 7 - 10 μm. For the single-teethed type, the entire nanocomb is a single crystal with the teeth grown along the [0001] direction, and the comb ribbon grown along the [10-10] direction. For double-side teethed nanocombs, they are twin crystals with twin boundaries of {10-13}. The comb ribbons grow along the directions of <10-11> while the teeth grow along the [0001] direction. With the results obtained, we believe that the growth of nanocombs follows the vapour-solid process. Studies of the optical properties basing on photoluminescence and Raman spectroscopy have indicated that the ZnO nanocombs have good crystal quality.

Critical behavior of Y-doped Nd_0.7Sr_0.3MnO₃ manganites exhibiting the tricritical point and large magnetocaloric effect

Phan, The-Long,Ho, T.A.,Thang, P.D.,Tran, Q.T.,Thanh, T.D.,Phuc, N.X.,Phan, M.H.,Huy, B.T.,Yu, S.C. Elsevier 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.615 No.-

<P><B>Abstract</B></P> <P>We have determined the values of critical exponents of two polycrystalline samples (Nd<SUB>1−</SUB> <I> <SUB>x</SUB> </I>Y<I> <SUB>x</SUB> </I>)<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> (<I>x </I>=0 and 0.07) from the magnetization data versus temperature and magnetic field, <I>M</I>(<I>H</I>, <I>T</I>), to learn about their magnetic and magnetocaloric (MC) properties. The results reveal the samples exhibiting the crossover of first-order and second-order phase transitions, where the exponent values <I>β </I>=0.271 and <I>γ </I>=0.922 for <I>x </I>=0, and <I>β </I>=0.234–0.236 and <I>γ </I>=1.044–1.063 for <I>x </I>=0.07 determined by using modified Arrott plots and static-scaling hypothesis are close to those expected for the tricritical mean-field theory (<I>β </I>=0.25 and <I>γ </I>=1.0). Particularly, the <I>T</I> <SUB>C</SUB> of <I>x </I>=0 and 0.07 can be any value in the temperature ranges of 240–255K and 170–278K, respectively, depending on the magnitude of applied magnetic field and determination techniques. Around the <I>T</I> <SUB>C</SUB>, studying the MC effect of the samples has revealed a large magnetic-entropy change (Δ<I>S</I> <SUB>m</SUB>) up to ∼8J/kgK for the applied field interval Δ<I>H </I>=50kOe, corresponding to refrigerant capacity values of 200–245J/kg. These phenomena are related to the crossover nature and the persisting of FM/anti-FM interactions even above the <I>T</I> <SUB>C</SUB>, as further confirmed by electron-spin-resonance data, Curie–Weiss law-based analyses, and an exponential parameter characteristic of magnetic order <I>n </I>=dLn|Δ<I>S</I> <SUB>m</SUB>|/dLn<I>H</I>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Tricritical point in Y-doped Nd<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> manganites. </LI> <LI> A large magnetic-entropy change. </LI> <LI> Magnetic inhomogeneity and phase separation. </LI> </UL> </P>

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>.

Short-range magnetic order in La1-xBaxCoO3 cobaltites

Phan The Long,Dimitar N. Petrov,J. Ćwikd,N.T. Dang,Viet Dongquoc 한국물리학회 2018 Current Applied Physics Vol.18 No.11

Magnetization versus temperature and magnetic-field measurements, M(T, Ha), have been carried out to study the magnetic and critical properties of polycrystalline La1-xBaxCoO3 (x=0.3 and 0.5) cobaltites. These compounds with the density of ∼6.2 g/cm3 crystallized in the R3c rhombohedral and Pm3m cubic structures, respectively. With an applied field Ha=200 Oe, M(T) data have revealed that the samples with x=0.3 and 0.5 exhibit the ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature points TC=202 and 157 K, respectively. At 4.2 K, the saturation magnetization (Msat) decreases from 35.9 emu/g for x=0.3–26.1 emu/g for x=0.5. Particularly, the critical-behavior analyses in the vicinity of TC reveal all samples undergoing a second-order phase transition, with critical exponent values (β=0.328 and γ=1.251 for x=0.3, and β=0.331 and γ=1.246 for x=0.5) close to those expected for the 3D Ising model. This proves short-range magnetic order existing in La1-xBaxCoO3. We believe that magnetic inhomogeneities due to the mixture of holerich FM regions (confined in the trivalent-cobalt hole-poor anti-FM matrix) and uniaxial anisotropy prevent longrange order in La1-xBaxCoO3.

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.

Crossover from first-order to second-order phase transitions and magnetocaloric effect in La_0.7Ca_0.3Mn_0.91Ni_0.09O₃

Phan, The-Long,Zhang, P.,Thanh, T. D.,Yu, S. C. American Institute of Physics 2014 Journal of Applied Physics Vol.115 No.17

We have prepared La0.7Ca0.3Mn0.91Ni0.09O3 and then studied its critical behavior and magnetocaloric effect. Analyzing temperature and field dependences of magnetization around the ferromagnetic-paramagnetic transition reveals the sample undergoing the second-order magnetic phase transition with the critical parameters T-C approximate to 199.4K, beta = 0.171 +/- 0.006, and gamma = 0.976 +/- 0.012. A considerable difference of these critical exponents compared with those expected for the standard models is due to the sample exhibiting the crossover property (tricriticality); its exponent values are more close to those expected for the tricritical mean-field theory with beta = 0.25 and gamma = 1. Under the field 40 kOe, the maximum magnetic entropy change (-Delta S-max) around T-C is about 7.1 J center dot kg(-1)center dot K-1, corresponding to a refrigerant capacity RC approximate to 170 J/kg. Particularly, its magnetic-field dependence obeys a power law vertical bar Delta S-max vertical bar infinity H-n, where n = 0.55 is quite far from the value calculated from the relation n = 1 + (beta - 1)/(beta + gamma). (C) 2014 AIP Publishing LLC.