ESR study of La_1–xPb_xMnO₃ (0.1 ≤ x ≤ 0.5) perovskites

Phan, T. L.,Min, S. G.,Phan, M. H.,Ha, N. D.,Chau, N.,Yu, S. C. Wiley - VCH Verlag GmbH & Co. KGaA 2007 Physica status solidi. B Vol. No.

<P>Electron spin resonance (ESR) spectra of La<SUB>1–x </SUB>Pb<SUB>x </SUB>MnO<SUB>3</SUB> (0.1 ≤ x ≤ 0.5) compounds were recorded at different temperatures. Asymmetrical and distored resonance signals due to ferromagnetic correlations at temperatures T < T<SUB>min</SUB> became Lorentzian at T > T<SUB>min</SUB>, where T<SUB>min</SUB> is the temperature corresponding to the narrowest ESR linewidth. The ESR linewidth with respect to temperature, ΔH (T), for the samples was fitted to the one-phonon process, ΔH (T) = A + BT. We found that B decreased from 5.45 Oe/K for x = 0.1 to 4.61 Oe/K for x = 0.5, indicating the decrease of lattice distortions with the Pb addition. The tem- perature dependence of the ESR intensity, I (T), for the samples was described well to an expression of I (T) = I<SUB>0</SUB> exp (E<SUB>a</SUB>/k<SUB>B</SUB>T). In the high-temperature region, 1/I (T) obeyed the Curie–Weiss law. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</P>

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>

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>

Electron Spin Resonance Spectra of <tex> ${\hbox{La}}_{0.7}{\hbox{Cd}}_{0.3}({\hbox{Mn}},{\hbox{Co}}){\hbox{O}}_{3}$</tex> Perovskites

Phan, T. L.,Thanh, T. D.,Phan, M. H.,Yu, S. C. IEEE 2014 IEEE transactions on magnetics Vol.50 No.1

<P>We have prepared two perovskite samples La<SUB>0.7</SUB>Cd<SUB>0.3</SUB>MnO<SUB>3</SUB> (LCMO) and La<SUB>0.7</SUB>Cd<SUB>0.3</SUB>CoO<SUB>3</SUB> (LCCO) by using a solid-state reaction technique and studied their electron spin resonance (ESR) spectra versus temperature up to ~500 K. Experimental results reveal that as compared to LCMO, the resonant signals of LCCO are weaker and easily extinguished by thermal energy. For both samples, there exists a temperature called T<SUB>min</SUB> at which the narrowest ESR linewidth (ΔH) is obtained. At temperatures , the resonant lines associated with the ferromagnetic phase are asymmetrical. They become a symmetrical single line in the Lorentzian shape when the samples enter the paramagnetic region with T > T<SUB>min</SUB>. A detailed analysis of resonant spectra in this paramagnetic region reveals that temperature dependences of ΔH can be described by a linear function of the single-phonon process. The resonant intensity versus temperature obeys an exponent function I = I<SUB>0</SUB> exp(E<SUB>a</SUB>/k<SUB>B</SUB>T), where E<SUB>a</SUB> is the activation energy associated with decomposition of ferromagnetic clusters. Temperature dependences of the resonant position (H<SUB>r</SUB>) and the Lande factor (g) indicate the dominance of spin-spin and spin-lattice interactions in both LCMO and LCCO, but with the additional presence of the spin-orbit interaction in LCCO at high temperatures (T > 380 K). We believe that the differences in the electronic structure of e<SUB>g</SUB> and t<SUB>g</SUB> levels, as well as in the electronic spin configuration of Mn and Co ions, caused the distinct difference in the ESR spectra of LCMO and LCCO.</P>

Conventional and inverse magnetocaloric effects, and critical behaviors in Ni₄₃Mn₄₆Sn₈In₃ alloy

Thanh, T.D.,Nan, W.Z.,Nam, G.,Van, H.T.,You, T.S.,Phan, T.L.,Yu, S.C. Elsevier 2015 CURRENT APPLIED PHYSICS Vol.15 No.10

A systematic study of the conventional and inverse magnetocaloric effects, and critical behaviors in an alloy ingot of Ni<SUB>43</SUB>Mn<SUB>46</SUB>Sn<SUB>8</SUB>In<SUB>3</SUB> has been performed. Our results reveal the sample exhibiting structural and magnetic phase transitions at temperatures T<SUB>C</SUB><SUP>M</SUP> = 166 K (T<SUB>C</SUB> of the martensitic phase), T<SUB>M-A</SUB> = 260 K (the martensitic-to-austenitic phase transformation) and T<SUB>C</SUB><SUP>A</SUP> = 296 K (T<SUB>C</SUB> of the austenitic phase). The large values of refrigerant capacity (RC) around T<SUB>M-A</SUB> and T<SUB>C</SUB><SUP>A</SUP> are found to be RC<SUB>M-A</SUB> = 172.6 and RC<SUB>A</SUB> = 155.9 J kg<SUP>-1</SUP>, respectively, under an applied field change of 30 kOe. Our critical analyses near the T<SUB>C</SUB><SUP>M</SUP> and T<SUB>C</SUB><SUP>A</SUP> reveal that a coexistence of the long- and short-range ferromagnetic order in the martensitic phase, while the long-range ferromagnetic order exists in the austenitic phase. Interestingly, at around T<SUB>C</SUB><SUP>A</SUP>, the maximum magnetic entropy change (|ΔS<SUB>max</SUB>|) versus magnetic field H obeys a power law, |ΔS<SUB>max</SUB>| = a.H<SUP>n</SUP>, where the exponent n is found to be about 0.66.

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>

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>

Ferromagnetic Order in Rapidly Cooled Nd-Fe-Co-Al Alloy Ribbons

Phan, T. L.,Zhang, Y. D.,Dan, N. H.,Thang, D. D.,Thanh, T. D.,Zhang, P.,Yu, S. C. IEEE 2013 IEEE transactions on magnetics Vol.49 No.7

<P>We have studied the magnetic properties of Nd<SUB>45</SUB>Fe<SUB>30</SUB>Co<SUB>15</SUB>Al<SUB>10</SUB> alloy ribbons with various thicknesses of about 120 (N<SUB>1</SUB>) and 50 μm (N<SUB>2</SUB>) prepared by melt-spinning. Structural analyses based on an X-ray diffractometer and a high-resolution transmission electron microscope revealed an existence of nanocrystals with sizes of 10 ~ 20 nm surrounded by an amorphous host matrix. With decreasing ribbon thickness and nanocrystalline size, magnetic studies upon a vibrating sample magnetometer indicated the decrease of the Curie temperature (T<SUB>C</SUB>), coercive force (H<SUB>c</SUB>), and magnetic entropy change (ΔS<SUB>m</SUB>). In the ferromagnetic region, however, magnetization values determined for N<SUB>2</SUB> are greater than those determined for N<SUB>1</SUB>. These results are related to the differences in the critical exponents of N<SUB>1</SUB> (β=0.418 and γ=1.173), and N<SUB>2</SUB> (β=0.512 and γ=1.077), which are characteristic for the ferromagnetic nature existing in the alloy ribbons with different thicknesses.</P>

Influence of Co doping on the critical behavior of La_0.7Sr_0.3Mn_1-xCo_xO₃

Phan, T.L.,Thanh, T.D.,Yu, S.C. Elsevier Sequoia 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.615 No.suppl1

We prepared single-phase rhombohedral samples La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>Mn<SUB>1-x</SUB>Co<SUB>x</SUB>O<SUB>3</SUB> (x=0, 0.06, 0.08 and 0.1) by conventional solid state reaction, and then studied in detail their magnetic and critical properties. Investigations into temperature and magnetic-field dependences of magnetization indicated a gradual decrease of the ferromagnetic-paramagnetic transition temperature (T<SUB>C</SUB>) from 360 to 281K with increasing Co-doping concentration up to 10%. Having used Banerjee's criteria, we found all the samples undergoing a second-order phase transition. The values of the critical exponent β=0.387-0.487 determined from modified Arrott plots are located in between those expected for the 3D Heisenberg and mean-field models while the values of γ=1.109-1.166 are close to those of the 3D Ising and mean-field models. These features reveal an existence of ferromagnetic short-range order and magnetic inhomogeneity in La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>Mn<SUB>1-x</SUB>Co<SUB>x</SUB>O<SUB>3</SUB>. Particularly, with increasing Co concentration, the critical exponent values trend to shift towards those of the mean-field theory. Such the results prove the Co doping favors establishing ferromagnetic long-range order.