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Tola, P.S.,Kim, H.S.,Kim, D.H.,Phan, T.L.,Rhyee, J.S.,Shon, W.H.,Yang, D.S.,Manh, D.H.,Lee, B.W. Elsevier 2017 The Journal of physics and chemistry of solids Vol.111 No.-
<P><B>Abstract</B></P> <P>The crystal and electronic structures and the magnetic and magnetocaloric properties of off-stoichiometric LaMnO<SUB>3</SUB> nanoparticles (NPs) with various particle sizes <I>D</I> = 20–100 nm were studied. The Rietveld refinement revealed that all NPs were crystallized in the rhombohedral structure, with varied structural parameters dependent on <I>D</I>. Magnetization (<I>M</I>) measurements indicated a considerable difference between zero-field-cooled and field-cooled magnetizations at temperatures below ferromagnetic-paramagnetic (FM-PM) phase transition, particularly for the samples with <I>D</I> = 25–40 nm. These results are ascribed to spin-glass-like behaviors and magnetic inhomogeneity. We also found the possibility of tuning the FM-PM phase transition temperature (<I>T</I> <SUB>C</SUB>) from 77 to 262 K, which is dependent on both <I>D</I> and <I>W</I> (the <I>e</I> <SUB>g</SUB>-electron bandwidth). Under an applied field of <I>H</I> = 50 kOe, the absolute maximum magnetic entropy change that achieved around <I>T</I> <SUB>C</SUB> can be improved from 4.02 J kg<SUP>−1</SUP> K<SUP>−1</SUP> for <I>D</I> = 40 nm to 6.36 J⋅ kg<SUP>−1</SUP>⋅ K<SUP>−1</SUP> for <I>D</I> = 100 nm, corresponding to the relative-cooling-power values of 241–245 J⋅ kg<SUP>−1</SUP>. We also analyzed the data of <I>M</I> and magnetic entropy change based on theoretical models to further understand the magnetic property and phase-transition type of the NP samples.</P>
Mn-doped (Ba, Y)Fe12O19 hexaferrites: Crystal structure and oxidation states of Mn and Fe
Thang P.D.,Ho T.A.,Dang N.T.,Lee B.W.,Phan T.L.,Manh T.V.,Kim D.H.,Yang D.S. 한국물리학회 2020 Current Applied Physics Vol.20 No.11
We have fabricated Ba0.95Y0.05Fe12-xMnxO19 samples with large Mn-doping amounts of x = 4 and 6, using the mechanical milling and heat treatment. X-ray diffraction analysis indicated the samples crystallized in the M-type hexaferrite structure. The Mn doping caused the modification, shift and broadening of some characteristic phonon-vibration modes, which were recorded by Raman spectroscopy. This is due to an incorporation of Mn ions into the M-type structure that disorders the periodic lattice and changes symmetry. Basing on X-ray absorption spectroscopy, we have found Fe in all samples stable with an oxidation state 3+ (Fe3+). Though Mn2+ and Mn3+ ions coexist, the concentration of Mn2+ in x = 4 is larger than that in x = 6. The analysis of Fouriertransform spectra have demonstrated the replacement of Mn2+,3+ ions for Fe3+ in the M-type structure. The sites of Mn2+,3+ ions in this structure have been discussed.
Magnetic and Magnetocaloric Properties of Zn1−x Co x Fe2O4 Nanoparticles
Phan, T. L.,Tran, N.,Kim, D. H.,Dang, N. T.,Manh, D. H.,Bach, T. N.,Liu, C. L.,Lee, B. W. Springer Science + Business Media 2017 Journal of electronic materials Vol.46 No.7
<P>We have prepared Zn1-x Co (x) Fe2O4 nanoparticles (NPs) by using a hydrothermal method, and then studied their structural and magnetic properties. The analyses of x-ray diffraction (XRD) patterns and Raman scattering spectra reveal that the samples crystallized mainly in a cubic-spinel structure with the lattice parameter a ae 8.4 . Averaged crystallite sizes determined from the XRD linewidth are about 16-22 nm, close to the particle sizes of 19-28 nm determined from scanning electron microscopy images. Magnetization measurements versus temperature, M(T), in the field H = 100 Oe indicate that the ferromagnetic-paramagnetic (FM-PM) phase transition temperature (T (C)) of Zn1-x Co (x) Fe2O4 NPs increases from 606 K for x = 0 to similar to 823 K for x = 1. The features of the M(T) curves also indicate magnetic inhomogeneity in the samples, and their magnetic property is unstable versus temperature. This is ascribed to the changes in the structural characterization and/or concentration of magnetic ions situated at the A and B sites in the spinel lattice. At room temperature, we found that both the saturation magnetization (M (s)) and coercivity (H (c)) increase with increasing Co content, with M (s) = 59-70 emu/g and H (c) = 100-1100 Oe. These results reflect that the Co doping into ZnFe2O4 NPs greatly improves their magnetic property, making them more useful for practical applications. Additionally, we also assess magnetic interactions and the magnetocaloric effect in the samples based on analyzing initial magnetization data, M(H), recorded at temperatures around T (C).</P>
Phan, T. L.,Thanh, T. D.,Ho, T. A.,Manh, T. V.,Tran, Q. T.,Lampen, P.,Phan, M. H.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.11
<P>This paper points out that the magnetic-phase transition and magnetocaloric effect of La<SUB>0.7</SUB>Ca<SUB>0.3</SUB>MnO<SUB>3</SUB> (LCMO) can be easily controlled by using the mechanical milling method. Changing the milling time from 5 to 30 min, we have obtained LCMO nanoparticles (NPs) with average crystallite sizes (d, determined by the Williamson-Hall method) ranging from 100 to 45 nm. The magnetic studies (based on a superconducting quantum interference device) and simple analyses (based on Banerjee's criteria) prove the magnetic-phase transformation from the first-order to the second-order, which takes place at a threshold value of d located in the range 60-70 nm. Compared with the as-prepared LCMO sample (a first-order magnetic phase transition), though the d decrease reduces the values of the T<SUB>C</SUB>, magnetization, magnetic-entropy change, and refrigerant capacity, but the width of the magnetic phase transition is increased remarkably. This widens the working range of LCMO NPs in magnetic refrigeration applications. We believe that the presence of surface-related effects, lattice strain, and distortions leads to Mn<SUP>3+</SUP>-Mn<SUP>4+</SUP> ferromagnetic interactions in LCMO NPs weaker than that in the as-prepared sample.</P>
Tran, N.,Kim, D.H.,Phan, T.L.,Dang, N.T.,Bach, T.N.,Manh, D.H.,Lee, B.W. North-Holland 2018 Physica. B, Condensed matter Vol.532 No.-
<P><B>Abstract</B></P> <P>Our studies on the crystal characterization and magnetic property of CoFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles (NPs) point out their instability in a specific temperature range. While as-prepared NPs exhibit single phase in a cubic spinel structure, annealing at temperatures <I>T</I>=673–1273K leads to the development of an impurity phase of Fe<SUB>2</SUB>O<SUB>3</SUB>. Interestingly, annealing at higher temperatures re-creates the single phase of NPs. This strongly influences their magnetic property. The magnetic inhomogeneity and/or multiple phase exist in as-prepared NPs and in those annealed below 1273K, better magnetic property is found in the samples with annealing temperature (<I>T</I> <SUB>an</SUB>) higher than 1273K. Ferromagnetic-paramagnetic phase transition temperatures of these samples are located around 815–850K, and are less dependent on <I>T</I> <SUB>an</SUB>. At room temperature, their saturation magnetization is located in the range of 41–55emu/g, while the coercivity can be changed from 600 to ~3200Oe. These results are related to microstructures, structural phases, and exchange interactions between Fe and Co ions situated in the <I>A</I> and <I>B</I> sites of the spinel structure, which are modified by heat treatment.</P>
Phan, T. L.,Manh, T. V.,Ho, T. A.,Yu, S. C.,Dan, N. H.,Yen, N. H.,Thanh, T. D. Korean Physical Society 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.11
We fabricated Huesler alloy ingots of Ni50-x Pr (x) Mn37Sn13 with x = 0 - 5 by using an arcmelting method. Crystalline-structural analyses revealed the coexistence of austenitic and martensitic phases in the samples with x = 0 and 1, in which the volume fraction of the austenitic phase for x = 1 was higher than that for x = 0. With higher Pr concentrations, x > 1, Pr- and Ni3Sn-related secondary phases, which reduced the magnetic order of the alloys, were formed. Thus, only the sample with x = 1 was more suitable for studying the critical behavior. Based on Landau's phase-transition theory and Banerjee's criteria, we found that this sample undergoes a second-order magnetic phase transition (SOMT) at a temperature around the Curie temperature T (C) a parts per thousand 299 K. Using the modified Arrott plots, asymptotic relations, and a universal scaling law, we determined the values of the critical exponents beta = 0.501 +/- 0.009 and gamma = 1.045 +/- 0.006. These values are very close to those expected for the mean-field theory with beta = 0.5 and gamma = 1, proving the existence of long-range ferromagnetic (FM) order in the sample with x = 1. Particularly, around at temperature T (C) , the magnetic-entropy change reaches the maximum value (a dagger S (max) ). Its magnetic-field dependences can be described by using a power law |a dagger S (max) | ae H (n) , where n = 0.687 is close to the value 0.677 calculated from the theoretical relation n = 1 + (beta - 1)/(beta + gamma). We believe that the doping of a suitable Pr amount in Ni50-x Pr (x) Mn37Sn13 (x a parts per thousand 1) promotes the formation of the austenitic phase and results in long-range FM order. However, the persistence of the martensitic phase and secondary phases favors short-range FM order and thus decreases the FM order in Ni50-x Pr (x) Mn37Sn13.
Interacting Superparamagnetism in La0.7Sr0.3MnO3 Nanoparticles
Le Van Hong,D. H. Manh,Ha M. Nguyen,N. X. Phuc,Y. D. Yao 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.5
ThemagneticorderofLa0:7Sr0:3MnO3nanoparticles(NPs)fabricatedinaSPEXD800milwas systematicalystudied. TheLa0:7Sr0:3MnO3 nanocrystalsgrowfromthemiledconstituentoxides during the miling proceses. The magnetization data obtained by using a SQUID magnetometer show the NPs as a superparamagnet in terms of anhysteretic curves near rom temperature. Un- overlaping of the scaled M(Hext;T)=Ms vs. Hext=T plots and the dc susceptibility obeying the Curie-Weis behavior rather than the Curie law at high temperatures provide evidence that the NPs are interacting superparamagnetic ensembles. A mean-eld corection to the Langevin func- tion L([Hext+M]=kBT) worked wel for the magnetic ordering of the NPs. By means of the Langevinting,thediameteroftheNPwasestimatedtobelowerthan15nm,dependingonthe miling time. The saturation magnetization of NPs varied from 48.5 em/g to 19 emu/g, with the highervaluecorespondingtoalargerparticlesize. Acore-shelstructureoftheNPwasadopted, with the NP having the core-shel magneticaly-eective mas density.