Second-Order Phase Transition and the Magnetocaloric Effect in <inline-formula> <tex-math notation='TeX'> <tex> ${\rm La}_{{0.7}}{\rm Ca}_{0.3-{x}}{\rm Sr}_{x}{\rm MnO}_{{3}}$ </tex> </tex-math></inline-formula> Nanoparticles

Tran Dang Thanh,Phan, T. L.,Nguyen Van Chien,Do Hung Manh,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.4

<P>In this paper, we present a detailed study of the magnetocaloric effect and critical properties around the ferromagnetic-paramagnetic (FM-PM) phase transition of La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Sr<SUB>x</SUB>MnO<SUB>3</SUB> nanoparticles with x = 0.10, 0.11, and 0.12. The samples were synthesized by a combination of reactive milling and thermal processing. The average crystallite size of nanoparticles estimated from the linewidth of X-ray diffraction peaks by using the Williamson-Hall method is about 50 nm. Under a magnetic field change of 10 kOe, the maximum magnetic entropy change (|ΔS<SUB>max</SUB>|) reaches values of 1.47, 1.42, and 1.38 J·kg<SUP>-1</SUP>·K<SUP>-1</SUP> for x = 0.10, 0.11, and 0.12, respectively, at around 300 K. The refrigerant capacity is thus in between 44 and 54 J·kg<SUP>-1</SUP>. Particularly, the M<SUP>2</SUP> versus H/M curves prove that all the samples exhibit a second-order magnetic phase transition. Based on Landau's phase-transition theory and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical exponents β, y, δ, and T<SUB>C</SUB>. Here, the β values obtained are 0.397, 0.453, and 0.456 for x = 0.10, 0.11, and 0.12, respectively, which are in between those expected on the basis of the mean-field theory (β = 0.5) and value of the 3-D Heisenberg model (β = 0.365). The result proves the coexistence of shortand long-range FM interactions in La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Sr<SUB>x</SUB>MnO<SUB>3</SUB> nanoparticles. The nature of this phenomenon is discussed thoroughly.</P>

Room Temperature Magnetocaloric Effect in <tex> ${\hbox{La}}_{0.7}{\hbox{Sr}}_{0.3}{\hbox{Mn}}_{1\hbox{-}{\rm x}}{\hbox{Co}}_{\rm x}{\hbox{O}}_{3}$</tex>

Tran Dang Thanh,Le Viet Bau,Phan, T. L.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.1

<P>We have investigated the magnetic properties and magnetocaloric effect in La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>Mn<SUB>1-x</SUB>Co<SUB>x</SUB>O<SUB>3</SUB> prepared by a conventional solid-state reaction method. Magnetic measurements versus temperature revealed that the Curie temperature (T<SUB>C</SUB>) decreased gradually with increasing Co content (x); T<SUB>C</SUB> values are about 360, 310, 296 and 280 K for x =0.0, 0.06, 0.08 and 0.1, respectively. Magnetic entropy change (ΔS<SUB>m</SUB>) of the samples under a magnetic field change of 10 kOe was calculated by using isothermal magnetization data. We have found its maximum (|ΔS<SUB>max</SUB>|) achieved around T<SUB>C</SUB>, which is not changed much (~ 1.5 J·kg<SUP>-1</SUP>·K<SUP>-1</SUP>) with increasing Co-doping concentration. If combining the samples with x = 0.06, 0.08 and 0.10 for magnetic refrigeration, particularly, the temperature range can be used in between 266 K and 322 K, where |ΔS<SUB>max</SUB>| values are stable at about 0.98 J·kg<SUP>-1</SUP>·K<SUP>-1</SUP>. The relative cooling power is accordingly about 55 J·kg<SUP>-1</SUP> and comparable to that of other magnetocaloric alloys. These results suggest La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>Mn<SUB>1-x</SUB>Co<SUB>x</SUB>O<SUB>3</SUB> compounds to be a potential candidate for magnetic refrigerators at room temperature.</P>

Magnetocaloric Effect and Critical Behavior of <inline-formula> <tex-math notation='TeX'> <tex> ${\rm Ni}_{42}{\rm Ag}_{8}{\rm Mn}_{37}{\rm Sn}_{13}$ </tex> </tex-math></inline-formula> Alloys

Tran Dang Thanh,Phan, T. L.,Pham Thi Thanh,Nguyen Hai Yen,Nguyen Huy Dan,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.4

<P>This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of Ni<SUB>50</SUB>Mn<SUB>37</SUB>Sn<SUB>13</SUB> doped with 8% Ag, which were prepared by an arc-melting and rapidly quenched melt-spinning methods, respectively. Experimental results reveal that a partial replacement of Ag for Ni leads to stamping out the antiferromagnetic martensitic phase. This means that there is only the austenitic phase with a ferromagnetic-paramagnetic (FM-PM) phase-transition temperature of T<SUB>C</SUB> ≈ 295 K. Detailed studies and analyses around the phase transition region prove both samples undergoing a second-order magnetic phase transition. Basing on magnetic field dependences of magnetization, we have determined the magnetic-entropy change (ΔS<SUB>m</SUB>) of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change (|ΔS<SUB>max</SUB>|) reaches values 0.54 and 0.69 J · kg<SUP>-1</SUP> · K<SUP>-1</SUP> for the alloy ingot and ribbon, respectively. Using Landau's phase-transition theory, and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical parameters (T<SUB>C</SUB>, β, γ, and δ) in the low field range (below 10 kOe) with T<SUB>C</SUB> = 294.8 K, β = 0.469 ± 0.011, γ = 1.149 ± 0.060, and δ = 3.4 ± 0.1 for the alloy ingot, and with T<SUB>C</SUB> = 294.4 K, β = 0.449 ± 0.005, γ = 1.319 ± 0.040, and δ = 3.9 ± 0.1 for the alloy ribbon. One can see that β values fall in between those expected for the 3-D Heisenberg model (β = 0.365) and mean-field theory (β = 0.5). This indicates a coexistence of short-range and long-range FM interactions in both the samples. The nature of changes in value related to the critical parameters and maximum ΔS<SUB>m</SUB> is thoroughly discussed by means of structural analyses.</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.

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.

Magnetic and magnetocaloric properties of Ni-Ag-Mn-Sn ribbons and their composites

Thanh, Tran Dang,Duc, Nguyen Huu,Dan, Nguyen Huy,Mai, N.T.,Phan, T.L.,Oh, S.K.,Yu, Seong-Cho Elsevier 2017 Journal of Alloys and Compounds Vol.696 No.-

<P><B>Abstract</B></P> <P>In this work, we have investigated the influence of Ag-doping on the magnetic and magnetocaloric effect (MCE) of Ni<SUB>50-x</SUB>Ag<SUB>x</SUB>Mn<SUB>37</SUB>Sn<SUB>13</SUB> ribbons with <I>x</I> = 1, 2, and 4, which were prepared by a melt-spinning method. With increasing Ag concentration, a systematic decrease in the antiferromagnetic interaction and in the magnetic phase transition temperatures was observed. Analyses of the critical behavior based on the Banerjee criterion and scaling hypothesis for <I>M</I>(<I>H</I>, <I>T</I>) data near the ferromagnetic-paramagnetic transformation prove an increase of Ag favors establishing long-range ferromagnetic interactions in the austenitic phase. The temperature and magnetic field dependences of magnetic entropy change, Δ<I>S</I> <SUB>m</SUB>(<I>T</I>, <I>H</I>) were investigated via isothermal magnetization measurements. Interestingly, these samples exhibit a MCE at room-temperature with the Δ<I>S</I> <SUB>m</SUB>(<I>T</I>) curves distributed over a quite wide temperature range. To enhance the relative cooling power (RCP) value and to extend the magnetic phase transition region, we have prepared the composites in the form of the layered material samples based on ribbons obtained above. Under Δ<I>H</I> = 10 kOe, the maximum value of Δ<I>S</I> <SUB>m</SUB> (denoted as |Δ<I>S</I> <SUB>max</SUB>|) at around room-temperature is 1.08 J kg<SUP>−1</SUP> K<SUP>−1</SUP>, corresponding to RCP = 51.8 J kg<SUP>−1</SUP>, which is about 10% higher than that obtained from a separate sample. Additionally, we also pointed out that the dependences of |Δ<I>S</I> <SUB>max</SUB>| on Δ<I>H</I> at around room-temperature for samples obey a power law, |Δ<I>S</I> <SUB>max</SUB>| = <I>a</I> × Δ<I>H</I> <SUP>n</SUP>, and all the Δ<I>S</I> <SUB>m</SUB>(<I>T</I>, <I>H</I>) data obey completely a universal master curve.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An increase of Ag in Ni<SUB>1-x</SUB>Ag<SUB>x</SUB>Mn<SUB>37</SUB>Sn<SUB>13</SUB> ribbons favors establishing long-range FM order in the austenitic phase. </LI> <LI> High magnetic entropy change, wide operative temperature range, and high RCP value around room-temperature. </LI> <LI> The composite of Ni<SUB>1-x</SUB>Ag<SUB>x</SUB>Mn<SUB>37</SUB>Sn<SUB>13</SUB> ribbons exhibits higher RCP value and extends the magnetic phase transition region. </LI> <LI> Field dependence of |Δ<I>S</I> <SUB>max</SUB>| can be expressed by the power law. </LI> <LI> All the Δ<I>S</I> <SUB>m</SUB>(<I>T, H</I>) data around room temperature are followed a universal master curve. </LI> </UL> </P>

Ferromagnetic Resonance of Soft Magnetic CoFeAlO Thin Films

V. S. Dang,T.L. Phan,N.D. Ha,C.O.Kim,S.C. Yu 한국자기학회 2007 한국자기학회 학술연구발표회 논문개요집 Vol.- No.-

Magnetic and magnetocaloric properties of Sm_1−xCa_xMnO₃ (<i>x</i> =0.88) nanoparticles

Phan, T.L.,Dang, N.T.,Ho, T.A.,Rhyee, J.S.,Shon, W.H.,Tarigan, K.,Manh, T.V. North-Holland Pub. Co 2017 Journal of magnetism and magnetic materials Vol.443 No.-

<P><B>Abstract</B></P> <P>We used the mechanical milling to prepare orthorhombic Sm<SUB>0.12</SUB>Ca<SUB>0.88</SUB>MnO<SUB>3</SUB> samples with the average crystallite size (<I>d</I>) changing from 100 to 139nm. Their magnetic and magnetocaloric properties were then studied upon magnetization data versus the temperature and magnetic field, <I>M</I>(<I>T</I>, <I>H</I>). The results revealed the samples undergoing the ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature <I>T</I> <SUB>C</SUB> ≈110K. Around this transition, the magnetic-entropy change (Δ<I>S</I> <SUB>m</SUB>) reaches the maxima. The maximum |Δ<I>S</I> <SUB>m</SUB>| values are about 2–4J·kg<SUP>−1</SUP>·K<SUP>−1</SUP>, corresponding to relative cooling power of 35–60J·kg<SUP>−1</SUP>, for an applied field <I>H</I> =30kOe. The assessments based on Banerjee’s criteria and constructing a universal curve for |Δ<I>S</I> <SUB>m</SUB>(<I>T</I>, <I>H</I>)| data indicate the samples having the nature of a second-order phase transition. Also, the detailed analyses based on the Curie-Weiss law and magnetic-order exponent prove the existence of the Griffiths phase and magnetic inhomogeneity in the samples. These properties would be changed by changing <I>d</I>.</P>

Unusual Critical Behavior in La_1.2Sr_1.8Mn₂O₇ Single Crystal

Thanh, Tran Dang,Xuan Hau, Kieu,Huyen Yen, Pham Duc,Manh, T. V.,Yu, S. C.,Phan, T. L.,Telegin, A.,Telegin, S.,Naumov, S. IEEE 2018 IEEE transactions on magnetics Vol.54 No.11

<P>In this paper, we present a detailed analysis on the critical behavior of La<SUB>1.2</SUB>Sr<SUB>1.8</SUB>Mn<SUB>2</SUB>O<SUB>7</SUB> single crystal via isothermal magnetization measured at different temperatures around the paramagnetic–ferromagnetic phase transition at <TEX>$T_{C} = 85$</TEX> K. Using the Landau–Lifshitz coefficients, the Arrott plots ( <TEX>$H/M = a(T) + b(T)M^{2}$</TEX>) of sample have been analyzed. It showed that a(T) changed from positive to negative values at different temperatures in the field ranges of <TEX>$H = 0$</TEX>–10, 10–30, and 30–50 kOe, indicating that the critical behavior could not be described with a single model under different applied fields. Through the modified Arrott plots method, the Kouvel–Fisher method, and the critical isotherm analysis, we determined the values of the critical exponents for La<SUB>1.2</SUB>Sr<SUB>1.8</SUB>Mn<SUB>2</SUB>O<SUB>7</SUB> around its magnetic phase transition over different magnetic field ranges. The critical exponent <TEX>$\beta $</TEX> value is found to be 0.501, 0.417, and 0.371 under field ranges of <TEX>$H = 0$</TEX>–10, 10–30, and 30–50 kOe, respectively. This means that the <TEX>$\beta $</TEX> value depends strongly on the strength of the applied field, shifting from the value approaching that of the mean field model ( <TEX>$\beta = 0.5$</TEX>) to the 3-D-Heisenbeg model ( <TEX>$\beta = 0.365$</TEX>). Meanwhile, its <TEX>$\gamma $</TEX> value is quite stable ( <TEX>$\gamma =0.973$</TEX>–1.074), almost independent on the choice of field fitting range. In addition, using the reduced temperature <TEX>$\varepsilon = (T-T_{C}$</TEX>)/ <TEX>$T_{C}$</TEX> and the obtained critical exponents, almost <TEX>$M(H, T$</TEX>) data measured near <TEX>$T_{C}$</TEX> obey the scaling equation <TEX>$M(H, \varepsilon) = \varepsilon ^{\boldsymbol {\beta }}f_{\pm }(H/\varepsilon ^{\boldsymbol {\beta +\gamma }}$</TEX>), where <TEX>$f_{+}$</TEX> and <TEX>$f_{-}$</TEX> are regular analytic functions corresponding to data at <TEX>$T > T_{C}$</TEX> and <TEX>$T < T_{C}$</TEX>, respectively.</P>

Electrical and Magnetotransport Properties of <inline-formula> <tex-math notation='TeX'> <tex> ${\rm La}_{{0.7}}{\rm Ca}_{{0.3}}{\rm Mn}_{{1-x}}{\rm Co}_{{x}}{\rm O}_{3}$ </tex> </tex-math></inline-formula>

Tran Dang Thanh,Phan, T. L.,Phung Quoc Thanh,Hoang Nam Nhat,Duong Anh Tuan,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.6

<P>This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of La0.7Ca0.3Mn1-xCoxO3(x = 0.09-0.17) prepared by solid-state reaction. Magnetic measurements versus temperature revealed a gradual decrease of the magnetization (M) and Curie temperature (T-C) with increasing Co concentration (x). The T-C values vary from 194 to 159 K as changing x from 0.09 to 0.17, respectively. H/M versus M-2 performances around T-C prove the x = 0.09 sample undergoing a first-order magnetic phase transition (FOMT) while the samples with x >= 0.11 undergo a second-order magnetic phase transition (SOMT). The other with x = 0.10 is considered as a threshold concentration of the FOMT-SOMT transformation. Considering temperature dependences of resistivity, rho(T), in the presence and absence of the magnetic field, the samples (excepting for x = 0.17) exhibit a metal-insulator transition at T (P) = 60-160 K, which shifts toward lower temperatures with increasing x. In the metallic-ferromagnetic region, the rho(T) data are well fitted to a power function rho(T) = rho(0) + rho(2) T-2 + rho(4.5) T-4.5. This indicates electron-electron and electron-magnon scattering processes are dominant at temperatures T < T (P). In addition, the conduction data at temperatures T > theta(D)/2 (theta(D) is the Debye temperature) and T (P) < T < theta(D)/2 obey the small-polaron and variable-range hopping models, respectively. The values of activation energy E-p, and density of states at the Fermi level N(E-F) were accordingly determined. Here, N(E-F) increases while E-p decreases when an external magnetic field is applied. We also have found that N(E-F) increases when materials transfer from the FOMT to the SOMT, and N(E-F) value becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.10).</P>