H. D. Chinh

Hanoi University of Science and Technology, Hà Nội, Ha Nội, Vietnam

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Publications (13)17 Total impact

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    ABSTRACT: Bilayered perovskite manganites of the composition La1.25Sr1.75Mn2O7 (LSMO) were synthesized successfully by the sol–gel method. The effects of pH concentration, calcination temperature, and time on the phase formation, structure, and magnetic properties of the material were systematically investigated. X-ray diffraction and Rietveld refinement analysis revealed that the single phase of LSMO was only obtained in samples sintered at a temperature as high as 1250 °C. The best magnetic properties were achieved in samples with ({rm pH} = 7) , calcinated at 1450 °C for 10 h. Our studies show that the pH concentration, calcination temperature, and time are important factors in tailoring the magnetic properties of double-layered perovskite manganites.
    IEEE Transactions on Magnetics 06/2014; 50(6):1-4. DOI:10.1109/TMAG.2014.2308482 · 1.21 Impact Factor
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    ABSTRACT: SrFe12O19/La1�xCaxMnO3 (SFO/LCMO) composites with x¼0.1 and 0.5 were synthesized via a two stage combined citrate precursor sol-gel and hydrothermal method. The structure and composition of the samples were refined from the X-ray diffraction patterns. The morphologies of the composites were investigated using transmission and scanning electron microscopies, which revealed micron-sized hexagonal platelets of SFO with LCMO particles with average diameters of 110–130 nm formed at the surface. An investigation of the temperature and field dependence of the magnetization found that the SFO phase with a ferrimagnetic characteristic up to 720K dominates the magnetic properties of the composite samples. The SFO/LCMO composite samples showed a reduction in saturation magnetization and coercivity when compared with pure SFO. However, low temperature hysteresis loops recorded after cooling in applied fields up to 5 T revealed the absence of exchange bias in the SFO/LCMO composite, ruling out the possibility of significant interfacial magnetic coupling between SFO and LCMO.
    Journal of Applied Physics 09/2013; 114(12):123901. DOI:10.1063/1.4821971 · 2.21 Impact Factor
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    ABSTRACT: Mixed-valent manganites of the form R1-xMxMnO3 (R=La, Pr, Nd, Sm and M=Sr, Ca, Ba, Pb) are of interest as low-cost materials for potential application in the area of active magnetic refrigeration (AMR). An important parameter to optimize for AMR is the refrigerant capacity (RC), which depends on both the magnitude and breadth of the magnetic entropy change peak. Reducing the dimensions of a system to the nanoscale has the potential to enhance the RC by broadening a transition, but can also lead to a drop in entropy change. In this study, we contrast the impact of size reduction on the magnetic and magnetocaloric properties of single-phase La0.4Ca0.6MnO3 (LCMO) and phase-separated La0.35Pr0.275Ca0.375MnO3 (LPCMO). Nanoparticles of LCMO and LPCMO were prepared by a sol-gel method; single crystals were grown in an optical floating zone furnace. XRD, SEM, and TEM were used to characterize the samples and DC magnetometry measurements were performed using a Quantum Design VSM. We find that size reduction negatively impacts both magnetization and the magnetocaloric properties in LCMO, while enhancing RC and entropy change simultaneously in LPCMO.
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    ABSTRACT: Bulk manganites of the form La5/8−yPryCa3/8MnO3 (LPCMO) exhibit a complex phase diagram due to coexisting charge-ordered antiferromagnetic (CO/AFM), charge-disordered paramagnetic (PM), and ferromagnetic (FM) phases. Because phase separation in LPCMO occurs on the microscale, reducing particle size to below this characteristic length is expected to have a strong impact on the magnetic properties of the system. Through a comparative study of the magnetic and magnetocaloric properties of single-crystalline (bulk) and nanocrystalline LPCMO (y = 3/8) we show that the AFM, CO, and FM transitions seen in the single crystal can also be observed in the large particle sizes (400 and 150 nm), while only a single PM to FM transition is found for the small particles (55 nm). Magnetic and magnetocaloric measurements reveal that decreasing particle size affects the balance of competing phases in LPCMO and narrows the range of fields over which PM, FM, and CO phases coexist. The FM volume fraction increases with size reduction, until CO is suppressed below some critical size, ∼100 nm. With size reduction, the saturation magnetization and field sensitivity first increase as long-range CO is inhibited, then decrease as surface effects become increasingly important. The trend that the FM phase is stabilized on the nanoscale is contrasted with the stabilization of the charge-disordered PM phase occurring on the microscale, demonstrating that in terms of the characteristic phase separation length, a few microns and several hundred nanometers represent very different regimes in LPCMO.
    Physical review. B, Condensed matter 01/2012; 86(6). DOI:10.1103/PhysRevB.86.064420 · 3.66 Impact Factor
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    ABSTRACT: Bulk manganites La5/8-yPryCa3/8MnO3 (LPCMO) exhibit a complex phase diagram due to coexisting and competing charge-ordered (CO) and ferromagnetic (FM) phases. Of particular interest is the CO phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. We report systematic studies of the influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline LPCMO (y=3/8) synthesized by sol-gel method. The nanocrystalline samples with mean particle sizes of 30 nm, 150 nm, and 250 nm were structurally characterized by XRD, SEM, and TEM. Magnetic and magnetocaloric measurements were conducted using a Quantum Design PPMS. We find that the 150 nm and 250 nm samples exhibit features similar to their bulk counterpart. However, the case is very different for the 30 nm sample where only a paramagnetic to ferromagnetic transition occurs. Size reduction has been found to suppress the CO phase, decrease the magnetization, and strongly modify the magnetocaloric effect in LPCMO.
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    ABSTRACT: We demonstrate the possibility of enhancing both the magnetocaloric effect (MCE) and refrigerant capacity (RC) in nanostructured mixed phase manganites. A comparative study of the magnetic and magnetocaloric properties of La0.35Pr0.275Ca0.375MnO3 in single crystalline and nanocrystalline forms is presented. While the conventional trend is reduction of magnetization and MCE with nanostructuring, we show that the opposite is true in the case of mixed phase manganites. The charge-ordered state is largely suppressed and ferromagnetic order is established in the nanocrystalline sample with an average particle size of 50 nm. Consequently, a strong enhancement of MCE and RC and a strong reduction of thermal and field hysteresis losses are achieved in the nanocrystalline sample. This finding opens up a way of exploring magnetic refrigerant materials at the nanometer scale for active magnetic refrigerators.
    Applied Physics Letters 12/2010; 97(24):242506-242506-3. DOI:10.1063/1.3526380 · 3.52 Impact Factor
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    ABSTRACT: La5/8Pr3/8-xCaxMnO3 (LPCMO) manganites exhibit a complex phase diagram due to coexisting and competing magnetic and electronic phases. Of particular interest is the charge-ordered (CO) phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. Our systematic magnetic, transverse susceptibility (TS) and magnetocaloric studies on LPCMO nanocrystalline and thin film materials reveal that the long-range CO is largely suppressed and the ferromagnetic (FM) order is established in these systems. The nanocrystalline samples (˜50nm in diameter) were prepared by sol-gel method, while the thin films (˜70 nm in thickness) were epitaxially grown on LaAlO3 and SrLaGaO4 substrates. The magnetic anisotropy of the FM phase and the metamagnetic transition are probed by TS experiments. These results point to the importance of the particle-size and strain effects on the CO and FM states in low-dimensional LPCMO systems.
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    ABSTRACT: The temperature dependence of the electrical resistivity in (1 − x) La0.7Ca0.3MnO3 + xAg composites has been investigated between 5 and 300 K for H = 0 and 3T. Ag addition has increased the conductivity of this system. Curie temperature (TC) is almost independent of Ag content and is ~ 260 K for all the samples, while the metal–insulator transition temperature (TMI) increases with increasing content of Ag. The ρ–T of all samples fit well with the phenomenological percolation approach, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. In addition, all the samples clearly reveal the unusual low temperature resistivity minimum. Analysis of our results in terms of the model based on charge carrier tunneling between the anti-ferromagnetically coupled grains shows excellent agreement with the experiment data for both H = 0 and 3T.
    Materials Letters 06/2009; 63(11):899-902. DOI:10.1016/j.matlet.2009.01.058 · 2.27 Impact Factor
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    Journal- Korean Physical Society 05/2008; 52(5):1327-. DOI:10.3938/jkps.52.1327 · 0.43 Impact Factor
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    P. Q. Thanh, H. N. Nhat, H. D. Chinh
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    ABSTRACT: The excellent agreement with experimental data has been achieved in fitting the resistivity of doped manganate–ruthenates for whole temperature range. The analysis interpreted the resistivity in terms of percolation of carriers through the system of grain boundaries, having been assumed as the conductive fractal medium. The percolative conduction regime has been shown substantial for the K-doped ruthenates [H.N. Nhat, H.D. Chinh and M.H. Phan, Solid State Commun. 139 (2006) 456], and we confirm here that this approach also correctly discusses the unusual semiconductor-like behaviours of the doped manganate–ruthenates.
    Journal of Magnetism and Magnetic Materials 03/2007; 310(2). DOI:10.1016/j.jmmm.2006.11.031 · 2.00 Impact Factor
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    H.N. Nhat, H.D. Chinh, M.H. Phan
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    ABSTRACT: Percolation theory has been involved to explain the temperature dependence of conductivity in the K-doped perovskite ruthenates and to estimate the resistivity of grain boundary in the percolative conduction regime. Using the two-layer simple effective medium model [A. Gupta, G.Q. Gong, G. Xiao, P.R. Duncombe, P. Lecoeur, P. Trouilloud, Y.Y. Wang, V.P. Dravis, J.Z. Sun, Phys. Rev. B 54 (1996) R15629] and assuming the scaling property of grain boundary system, we have obtained the new formula for grain boundary resistivity, which contains important factors for the grain size, boundary thickness, and boundary fractal dimension. The numerical results for the system A0.5K0.5RuO3 (A=La, Y, Nd, Pr) are in very good agreement with the experiment. Importantly, it reveals that the percolative conduction plays a significant role in ceramic compounds containing polycrystalline grains and grain boundaries.
    Solid State Communications 01/2006; DOI:10.1016/j.ssc.2006.07.011 · 1.70 Impact Factor