Inhomogeneous Ferromagnetism and Spin-Glass-Like Behavior in <inline-formula> <tex-math notation='TeX'> <tex> $({\rm Nd}_{1-x}{\rm Y}_{x})_{0.7}{\rm Sr}_{0.3}{\rm MnO}_{3}$ </tex> </tex-math></inline-formula> With <inline-formula> <tex-math notation='TeX

Phan, T. L.,Nguyen, V. D.,Ho, T. A.,Khiem, N. V.,Thanh, T. D.,Phuc, N. X.,Thang, P. D.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.6

<P>The magnetic properties of polycrystalline ceramic samples (Nd1-xYx)(0.7)Sr0.3MnO3 with x = 0.21 - 0.35 were studied by means of dc magnetization and ac susceptibility measurements. Experimental results reveal a strong decrease of the ferromagnetic (FM)-paramagnetic phase-transition temperature (T-C) from 97 to 65 K as increasing x from 0.21 to 0.35, respectively. There is magnetic inhomogeneity associated with short-range FM order. Particularly, the samples undergo a spin-glass (SG) phase transition at the so-called blocking temperature (T-B) below T-C, which shifts toward lower temperatures with increasing the applied field, Hex; T-B -> T-g (the SG phase-transition temperature) as H-ex -> 0. The existence of the SG behavior in these samples was also confirmed by frequency (f) dependences of the ac susceptibility. For the in-phase/real component, chi' (T), it shows a frequency-dependent peak at the SG freezing temperature (T-f); T-f -> T-g as f -> 0. Dynamics of this process were analyzed by means of the slowing down scaling law, tau/tau(0) proportional to (T-f/T-g - 1)(-zv), where tau(0) and zv are the characteristic time and critical exponent, respectively. Fitting the experimental T-f(f) data to the scaling law gave the results of zv = 10.1-12.3 and tau(0) = 10(-21)-10(-15)s. These values are different from those expected for canonical SG systems with zv = 10 and tau(0) = 10(-13)s, revealing the cluster-SG behavior of (Nd1-xYx)(0.7)Sr0.3MnO3 samples. Notably, the increase in Y content leads to the shift of tau(0) and zv values toward those of canonical SG systems, which is ascribed to an expansion of SG clusters.</P>

Magnetic and magnetocaloric properties in La_0.7Ca_0.3-xNa_xMnO₃ exhibiting first-order and second-order magnetic phase transitions

Ho, T.A.,Dang, N.T.,Phan, T.L.,Yang, D.S.,Lee, B.W.,Yu, S.C. Elsevier Sequoia 2016 JOURNAL OF ALLOYS AND COMPOUNDS Vol.676 No.-

Polycrystalline orthorhombic samples La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Na<SUB>x</SUB>MnO<SUB>3</SUB> (x = 0-0.09) were prepared by solid-state reaction. The study of magnetic properties revealed that the ferromagnetic-paramagnetic (FM-PM) transition temperature (T<SUB>C</SUB>) increases from 255 to about 271 K with increasing Na-doping content (x) from 0 to 0.09, respectively. Around the T<SUB>C</SUB>, we have found the samples showing a large magnetocaloric (MC) effect with maximum values of magnetic entropy change (|ΔS<SUB>max</SUB>|) of 7-8 J kg<SUP>-1</SUP> K<SUP>-1</SUP> and relative cooling power RCP = 232-236 J/kg for the samples x = 0.03-0.09 in a magnetic-field interval ΔH = 40 kOe. Detailed analyses of isothermal magnetization data M(T, H) based on Banerjee's criteria indicated a first-to-second-order magnetic-phase transformation taking place at a threshold Na-doping concentration x<SUB>c</SUB> ~ 0.06. This could also be observed clearly from the feature of entropy universal curves. An assessment of the magnetic-ordering exponent N = dLn|ΔS<SUB>m</SUB>|/dLnH demonstrates an existence of short-range magnetic order in the samples. We believe that the changes of the magnetic properties and MC effect in La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Na<SUB>x</SUB>MnO<SUB>3</SUB> caused by Na doping are related to the changes in the structural parameters and Mn<SUP>4+</SUP>/Mn<SUP>3+</SUP> ratio, which are confirmed by the geometrical and electronic analyses based on X-ray diffraction and X-ray absorption fine structure.

Influence of Cr Doping on the Critical Behavior of Amorphous Alloy Ribbons Fe_{78–<i>x</i>}Cr_<i>x</i>Si₄Nb₅B₁₂Cu₁

Phan, T. L.,Thanh, P. Q.,Chau, N.,Huu, C. X.,Ngo, D.-T,Ho, T. A.,Thanh, T. D.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.11

<P>Though many previous works focused on studying Cr-doped Fe-Si-Nb-B-Cu amorphous alloys, magnetic-interaction mechanisms in these materials have not been carefully investigated yet. Dealing with these issues, we have prepared the amorphous alloy ribbons Fe<SUB>78-x</SUB>Cr<SUB>x</SUB>Si<SUB>4</SUB>Nb<SUB>5</SUB>B<SUB>12</SUB>Cu<SUB>1</SUB> with x= 1, 3, and 6, and then studied their magnetic and critical properties. Magnetization versus temperature and magnetic-field measurements, MHT, performed on a vibrating sample magnetometer reveal that the Cr-content increase in Fe<SUB>78-x</SUB>Cr<SUB>x</SUB>Si<SUB>4</SUB>Nb<SUB>5</SUB>B<SUB>12</SUB>Cu<SUB>1</SUB> reduces the T<SUB>C</SUB> from 430 K for x= 1 to about 322 K (for x= 6). This indicates the decline of ferromagnetic (FM) exchange interactions between Fe atoms when there is the presence of Cr atoms. We have also analyzed the M(H) data at the temperatures in the vicinity of the T<SUB>C</SUB> using the modified Arrott plot method and the scaling hypothesis, and determined the values of the critical exponents β = 0.367-0.376 and γ = 1.315-1.338. These values are close to those expected for the 3-D Heisenberg model with β = 0.365 and γ = 1.336, proving the existence of short-range FM order in the amorphous alloy ribbons.</P>

Influence of the Change in Oxidation Number of Mn on Magnetic Properties of <inline-formula> <tex-math notation='TeX'> <tex> ${\rm BaTi}_{{1}-{x}}{\rm Mn}_{x}{\rm O}_{3}$ </tex> </tex-math></inline-formula>

Phan, T. L.,Thanh, T. D.,Ho, T. A.,Thang, P. D.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.4

<P>We prepared BaTi<SUB>1-x</SUB>Mn<SUB>x</SUB>O<SUB>3</SUB> samples (x = 0 and 0.05) by standard solid-state reaction at 700°C and 900°C in an Ar ambient to change oxidation number of Mn dopants. The fabricated samples were then studied structural characterization and electronic structures by means of X-ray diffraction and absorption, and Raman scattering and electron-spin-resonance spectrometers. We found oxidation numbers 2+ and 3+ of Mn ions coexisting in BaTi<SUB>1-x</SUB>Mn<SUB>x</SUB>O<SUB>3</SUB> with x = 0.05, where the Mn<SUP>2+</SUP>/Mn<SUP>3+</SUP> ratio is about one for the sample annealed at 700°C. These Mn ions prefer locating at the Ti site in the tetragonal BaTiO<SUB>3</SUB> host lattice. In particular, there is a change in the oxidation number of Mn<SUP>2+</SUP> → Mn<SUP>3+</SUP> when the annealing temperature changes from 700°C to 900°C. Their magnetic properties are accordingly changed. Meanwhile, annealing pure BaTiO<SUB>3</SUB> in an Ar ambient at 700°C and 900°C does not lead to ferromagnetic (FM) order; they are almost diamagnetic. With the obtained results, we believe that FM order in Mn-doped BaTiO<SUB>3</SUB> annealed in an Ar ambient is associated with exchange interactions of Mn<SUP>2+</SUP> ions mediated by oxygen vacancies rather than associated with Mn<SUP>3+</SUP> ions.</P>

First-to-second-order magnetic-phase transformation in La_0.7Ca_0.3-xBa_xMnO₃ exhibiting large magnetocaloric effect

Phan, T.L.,Dang, N.T.,Ho, T.A.,Manh, T.V.,Thanh, T.D.,Jung, C.U.,Lee, B.W.,Le, A.T.,Phan, A.D.,Yu, S.C. Elsevier Sequoia 2016 Journal of alloys and compounds Vol.657 No.-

We have prepared polycrystalline samples La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Ba<SUB>x</SUB>MnO<SUB>3</SUB> (x = 0, 0.025, 0.05, 0.075 and 0.1) by solid-state reaction, and then studied their magnetic properties and magnetocaloric (MC) effect based on magnetization versus temperature and magnetic-field (M-H-T) measurements. Experimental results reveal the easiness in tuning the Curie temperature (T<SUB>C</SUB>) from 260 to about 300 K by increasing Ba-doping concentration (x) from 0 to 0.1. Under an applied field H = 50 kOe, maximum magnetic-entropy changes around T<SUB>C</SUB> of the samples can be tuned in the range between 6 and 11 J kg<SUP>-1</SUP> K<SUP>-1</SUP>, corresponding to refrigerant-capacity values ranging from 190 to 250 J kg<SUP>-1</SUP>. These values are comparable to those of some conventional MC materials, and reveal the applicability of La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Ba<SUB>x</SUB>MnO<SUB>3</SUB> materials in magnetic refrigeration. Analyses of the critical behavior based on the Banerjee criteria, Arrott plots and scaling hypothesis for M-H-T data prove a magnetic-phase separation when Ba-doping concentration changes. In the doping region x = 0.05-0.075, the samples exhibits the crossover of first- and second-order phase transitions with the values of critical exponents β and γ close to those expected for the tricritical mean-field theory. The samples with x < 0.05 and x > 0.075 exhibit first- and second-order transitions, respectively. More detailed analyses related to the Griffiths singularity, the critical behavior for different magnetic-field intervals started from 10 kOe, and the magnetic-ordering parameter n = dLn|ΔS<SUB>m</SUB>|/dLnH (where ΔS<SUB>m</SUB> is the magnetic-entropy change) demonstrate magnetic inhomogeneities and multicritical phenomena existing in the samples.

An Effective Route to Control the Magnetic-Phase Transition and Magnetocaloric Effect of La_0.7Ca_0.3MnO₃ Nanoparticles

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>

Image watermarking using a dynamically weighted fuzzy c-means algorithm

Kang, M.,Ho, L.T.,Kim, Y.,Kim, C.H.,Kim, J.-M. SPIE - THE INT SOC FOR OPTICAL ENGINEERING 2011 Optical engineering Vol.50 No.10

Temperature Dependent Microwave Dielectric Properties for (Ba,Sr)TiO3 Graded Thin Films on MgO (001) Substrate Grown by Pulsed Laser Deposition

S. E. Moon,Bae Ho Park,E.-K Kim,H.-C. Ryu,H.-Y. Lee,J. Park,K.-H. Park,K.-Y. Kang,M.-H. Kwak,S.-J. Lee,Y.-T. Kim 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.II

Oriented (Ba$_{0.6}$,Sr$_{0.4}$)TiO$_3$ (BST60) and graded (Ba,Sr)TiO$_3$ (BST) films were deposited on MgO (001) single crystals by using a pulsed laser deposition method. Structural properties of the films were investigated by using an X-ray diffractometer. The dielectric properties of the films were investigated under a dc bias field of 0 $-$ 20 V in the temperature region from 173 to 393 K by using interdigital capacitors (IDC) fabricated by photolithography and an etching process. The temperature-dependent small-signal dielectric properties of the films were measured with a Keithley 590 C-V meter. The measured temperature-dependent tunability for an IDC device based on BST60/MgO, BT/$\cdot\cdot$/ST/MgO and ST/$\cdot\cdot$/BT/MgO multilayer structure showed interesting data. The trends of the dielectric properties of the IDC may be due to the fact that they are affected by Curie temperature, lattice constant, strain/stress, {\it etc.}

Influence of Pb Doping on the Magnetocaloric Effect and Critical Behavior of (La0.9Dy0.1)0.8Pb0.2MnO3

Ho, T. A.,Phan, T. L.,Thang, P. D.,Yu, S. C. Springer Science + Business Media 2016 Journal of electronic materials Vol.45 No.5

<P>We used the modified Arrott plot method to analyze the magnetic field dependence of the magnetization, M(H), for a perovskite-type manganite (La0.9Dy0.1)(0.8)Pb0.2MnO3, and determined the critical parameter values T (C) = 248.4 K, beta = 0.484 +/- A 0.002, gamma = 0.961 +/- A 0.012, and delta = 2.90 +/- A 0.01. With these exponent values, the M(H) data around T (C) fall on two universal branches of a scaling function M(H,epsilon) = |epsilon| (beta) f (+/-)(H/|epsilon| (beta + gamma) ), where epsilon = (T - T (C))/T (C) is the reduced temperature, f (+) for T > T (C) and f (-) for T < T (C). Comparing with theoretical models, the exponent values obtained in our work are quite close to those expected from mean-field theory (with beta = 0.5, gamma = 1, and delta = 3). This reveals nearly long-range ferromagnetic order existing in (La0.9Dy0.1)(0.8) Pb0.2MnO3. From the M(H) data, we also found that, around the ferromagnetic-paramagnetic phase-transition temperature (T (C)), the magnetic entropy change reaches a maximum value (|delta S (max)|) of about 1.1 J kg(-1) K-1 for H = 10 kOe, corresponding to relative cooling power (RCP) of 50 J/kg. In addition to the above investigation, the temperature dependence of the magnetization, remanent magnetization (M (r)), and coercivity (H (c)) were recorded to learn about the physical processes taking place in (La0.9Dy0.1)(0.8)Pb0.2MnO3.</P>

Applications of Laser-Fabricated Plasma Structures in Laser-Wakefield Accelerators, X-ray Lasers and Plasma Nonlinear Optics

C.T. Hsieh,M.W. Lin,C.L. Chang,Y.C. Ho,S.Y. Chen,J. Wang,M.C. Chou,J.Y. Lin,C.H. Pai,P.H. Lin,L.C. Tai,S.H. Chen,G.Y. Tsaur,C.C. Kuo,T.Y. Chien 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.6

A general method for fabricating transient plasma structures with high-intensity laser pulses is developed to gain fine control over laser-plasma interactions. These structures have been used as programmable photonic devices in the development of laser-wakefield accelerators, soft X-ray lasers and plasma nonlinear optics driven by multi-terawatt laser pulses. Plasma ramps are used to control electron injection in laser-wakefield accelerators, plasma waveguides are used to enhance the efficiency of soft X-ray lasers by orders of magnitude and periodic plasma structures are used to achieve quasi-phase matching in relativistic harmonic generation. By scanning the interaction length with the same plasma-fabrication method, tomographic measurements are carried out to resolve the injection/acceleration process in laser-wakefield accelerators and amplification processes in X-ray lasers and relativistic harmonic generation. A theoretical analysis and a computer simulation are also carried out to provide insightful pictures of these processes. These research works show that by controlling plasma structures with optical fabrication methods, laser-plasma interaction can be engineered to expand and enrich the frontier of high-field physics.