Showing papers by "Tran Dang Thanh published in 2014"

Crossover from first-order to second-order phase transitions and magnetocaloric effect in La0.7Ca0.3Mn0.91Ni0.09O3

Abstract: We have prepared La0.7Ca0.3Mn0.91Ni0.09O3 and then studied its critical behavior and magnetocaloric effect. Analyzing temperature and field dependences of magnetization around the ferromagnetic-paramagnetic transition reveals the sample undergoing the second-order magnetic phase transition with the critical parameters TC ≈ 199.4 K, β = 0.171 ± 0.006, and γ = 0.976 ± 0.012. A considerable difference of these critical exponents compared with those expected for the standard models is due to the sample exhibiting the crossover property (tricriticality); its exponent values are more close to those expected for the tricritical mean-field theory with β = 0.25 and γ = 1. Under the field 40 kOe, the maximum magnetic entropy change (−ΔSmax) around TC is about 7.1 J·kg−1·K−1, corresponding to a refrigerant capacity RC ≈ 170 J/kg. Particularly, its magnetic-field dependence obeys a power law |ΔSmax| ∝ Hn, where n = 0.55 is quite far from the value calculated from the relation n = 1 + (β − 1)/(β + γ).

Coexistence of considerable inter-particle interactions and spin-glass behavior in La0.7Ca0.3MnO3 nanoparticles

Abstract: We have studied the magnetic and spin-glass (SG) properties of La0.7Ca0.3MnO3 single-crystalline nanoparticles, which were prepared by the mechanical milling method with different milling times (tm). Analyzing the susceptibility data in the paramagnetic region indicates both ferromagnetic (FM) and anti-FM interactions coexisting in nanoparticles. Additionally, there is a peak associated with the freezing temperature (Tf) appearing on the real part curve of the ac susceptibility, χ′(T). The Tf value increases with increasing frequency as expected for SG systems. The SG behavior was also checked by using the criterion parameter c = ΔTf/TfΔ(log10f), and the power law τ = τ0(T/Tg − 1)−zν. The obtained values of c ≈ 5 × 10−2, τ0 ≈ 10−5 s and zν ≈ 2–3 are consistent with those expected for SG-like systems, suggesting an existence of a SG phase transition at Tg below Tf, which decreases with decreasing ⟨D⟩. Basing on ln(f) versus Tf data, and the Neel-Arrhenius model [ln(f) = ln(f0) - Ea/kBT] and Vogel–Fulcher l...

Critical behavior of the ferromagnetic-paramagnetic phase transition in Fe90−xNixZr10 alloy ribbons

Abstract: This work presents a detailed study on the critical behavior of the ferromagnetic-paramagnetic (FM-PM) phase transition in Fe90−xNixZr10 (x = 0 and 5) alloy ribbons. Basing on field dependences of magnetization (M-H), M2 versus H/M plots prove the alloys exhibiting a second-order magnetic phase transition. To investigate the nature of the FM-PM phase transition at TC = 245 and 306 K for x = 0 and 5, respectively, we performed a critical-exponent study. The values of critical components β, γ, and δ determined by using the modified Arrott plots, Kouvel-Fisher (KF), and critical isotherm analyses agree with each other. For x = 0, the critical parameters β = 0.365 ± 0.013 and γ = 1.615 ± 0.033 are obtained by modified Arrott plots while β = 0.368 ± 0.008 and γ = 1.612 ± 0.016 are obtained by the KF method. These values are close to those expected for the 3D-Heisenberg model, revealing short-range FM interactions in Fe90Zr10. Meanwhile, for x = 5, the values of the critical parameters β = 0.423 ± 0.008 and γ =...

Influences of the first-to-second order magnetic phase transformation on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds

Abstract: We studied the influence of the magnetic phase transition on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds with x = 0.05, 0.075, and 0.10. Our experimental results demonstrated the ferromagnetic-paramagnetic and metal-insulator transitions taking place at temperatures TC = 265–300 K and TMI = 280–310 K, respectively, which increase with increasing Ba-doping content. While the x = 0.05 sample undergoes a first-order magnetic phase transition (FOMT), x = 0.10 undergoes a second-order magnetic phase transition (SOMT). The other sample with x = 0.075 is considered as a threshold concentration of the FOMT-SOMT transformation. Further, ρ(T) data in different temperature regions were fitted to different models. The activation energy Ep and density of states at the Fermi level N(EF) were accordingly determined. Notably, N(EF) increases while Ep decreases in the case of applying an external field. We also have found that N(EF) increases when materials transfer from the FOMT to the SOMT, which becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.075).

Influence of Mn Doping on the Crystal Structure, and Optical and Magnetic Properties of ${\rm SrTiO}_{3}$ Compounds

Abstract: This paper presents the influence of Mn doping on the structural characterization, and optical and magnetic properties of ${\rm SrTi}_{1-x}{\rm Mn}_{x}{\rm O}_{3}(x=0.0\hbox{--}0.1)$ materials prepared by a solid-state reaction method. The detailed analyses of X-ray diffraction patterns indicate an incorporation of Mn dopants into Ti sites of the ${\rm SrTiO}_{3}$ host lattice. There is a cubic to tetragonal transformation, which takes place at a threshold concentration $x\approx 0.04$ . The optical absorption spectra show a rapid increase in the absorption coefficient. The bandgap energy $(E_{g})$ related to the direct electron transition decreases with increasing Mn concentration: $E_{g}$ decreases from 3.15 eV for $x=0$ to 1.28 eV for $x=0.10$ . From this point of view, the ${\rm SrTi}_{1-x}{\rm Mn}_{x}{\rm O}_{3}$ materials are considered as promising materials for photocatalytic applications. Interestingly, while the samples with $x=0.0\hbox{--}0.02$ are diamagnetic, the others with $x=0.04\hbox{--}0.10$ exhibit weak ferromagnetism. The ferromagnetic order increases with increasing Mn concentration. Based on the results of structural and optical analyses, the nature of magnetism in the samples is explained thoroughly.

Critical Behavior and Magnetocaloric Effect of Pr 1- x Sr x CoO 3

Abstract: The modified Arrott plot method was used to analyze magnetic-field dependences of magnetization, M(H), for two polycrystalline samples Pr_1-xSr_xCoO₃ with x = 0.4 and 0.5. The analyses determined the values of critical parameters T_C ≈ 204 K, β = 0.654 ± 0.016, y = 1.11 ± 0.009, and δ = 2.70 ± 0.009 for x = 0.4, and T_C ≈ 218 K, β = 0.495 ± 0.004, y = 0.991 ± 0.003, and δ = 2.86 ± 0.009 for x = 0.5. With these critical values, the M(H) data around the T_C of the samples fall into two universal branches of a scaling function M(H, ε) = |ε|^βf_±(H/|ε|^β+y), where ε = (T-T_C)/T_C, and f₊ for T > T_C and f_ for T <; T_C. Here, the T_C increase with the increasing Sr content is ascribed to an increase of ferromagnetic (FM) double-exchange interaction pairs of Co³⁺-Co⁴⁺, and to the β change from 0.654 (for x = 0.4) to 0.495 (for x = 0.5). This corresponds to the change in FM interactions from short-range to nearly long-range FM orders. Based on the M(H) data, we also determined the magnetic-entropy change, which reaches a maximum value (|ΔS_max|) around the T_C; |ΔS_max| ≈ 1.9 and 2.2 J/kg · K for x = 0.4 and 0.5, respectively, for a magnetic field change ΔH = 50 kOe. The relative cooling power thus increases from 52 J/kg (for x = 0.4) to 84 J/kg (for x = 0.5). In particular, field dependences of the magnetic-entropy change (ΔS_m) obey a power function |ΔS_m (H)| ∝ Hⁿ, where the magnetic-ordering parameter (n) slightly decreases from 0.75 for x = 0.4 to 0.70 for x = 0.5.

An Effective Route to Control the Magnetic-Phase Transition and Magnetocaloric Effect of La 0.7 Ca 0.3 MnO 3 Nanoparticles

Abstract: This paper points out that the magnetic-phase transition and magnetocaloric effect of La0.7Ca0.3MnO3 (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 TC, 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 Mn3+-Mn4+ ferromagnetic interactions in LCMO NPs weaker than that in the as-prepared sample.

Dielectric resonance effect with negative permittivity in a La1.5Sr0.5NiO4+δ ceramic

Abstract: A polycrystalline sample of La1.5Sr0.5NiO4+δ was prepared by using a solid-state reaction. X-ray diffraction proved the sample to be a single phase with a tetragonal structure (space group: I4/mmm). By using an iodometric titration method to determine the non-stoichiometric oxygen concentration (δ) in La1.5Sr0.5NiO4+δ , we found δ = −0.017, which corresponds to a doping level of n h = x + 2δ = 0.466. Also, a strong increase of the magnetization in the M(T) curve at temperatures below the spin-ordering temperature (T SO ~ 100 K) was observed. The M(H) curves show very small magnetic moments, which proves the weak ferromagnetic nature of La1.5Sr0.5NiO4+δ . The dependences of the dielectric constant on the frequency and the temperature, e(ω, T) = e′(ω, T) + ie″(ω, T), was investigated in the frequency range of 1–13 MHz. At temperatures around room temperature, the maximum of the real part (e′) was higher than 105. Particularly, an abnormal dependence of the permittivity on frequency was observed. Depending on temperature, a dielectric resonance was observed at about 500 kHz or 8 MHz. Interestingly, we observed the dielectric-resonance effect with a negative permittivity. Such a feature is very similar to that observed in left-handed materials. The fitting of the experimental data for the dielectric constant at frequencies around the resonance frequency to the equations associated with an equivalent RLC series circuit proves that La1.5Sr0.5NiO4+δ belongs to the class of multiferroic materials.

Structural Characterization and Magnetic and Electrical Properties of La0.7Ca0.3MnO3–La1.5Sr0.5NiO4 Nanocomposites

Abstract: Nanocomposite samples of (1 − x)La0.7Ca0.3MnO3 + xLa1.5Sr0.5NiO4 (x = 0 to 0.3) were synthesized by a combination of the mechanical milling and solid-state reaction methods. X-ray diffraction analyses and magnetic measurements indicated that no reaction occurred between La0.7Ca0.3MnO3 (LCMO) and La1.5Sr0.5NiO4 (LSNO). The Curie temperature (TC) was almost independent of x, while the metal–insulator transition temperature (TMI) shifted from 251 K for x = 0.0 to 65 K for x = 0.2. The samples with x ≥ 0.25 exhibited insulating behavior in the temperature range from 30 K to 300 K. Addition of LSNO substantially increased the resistivity of the composites. This is attributed to enhanced magnetic disorder at LCMO grain boundaries due to the addition of LSNO. The temperature dependence of the resistivity, ρ(T), could be described by the phenomenological percolation model of phase segregation. Fitting the experimental ρ(T) data in the temperature range of 30 K to 300 K indicated that the activation energy of the composites increases as a function of the LSNO doping concentration (x).

Influence of Cr Doping on the Critical Behavior of Amorphous Alloy Ribbons Fe 78– x Cr x Si 4 Nb 5 B 12 Cu 1

Abstract: 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 78-x Cr x Si 4 Nb 5 B 12 Cu 1 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 78-x Cr x Si 4 Nb 5 B 12 Cu 1 reduces the T C 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 C 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.

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

Abstract: We have prepared two perovskite samples La0.7Cd0.3MnO3 (LCMO) and La0.7Cd0.3CoO3 (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 Tmin 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 > Tmin. 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 = I0 exp(Ea/kBT), where Ea is the activation energy associated with decomposition of ferromagnetic clusters. Temperature dependences of the resonant position (Hr) 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 eg and tg 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.

Second-Order Phase Transition and the Magnetocaloric Effect in ${\rm La}_{{0.7}}{\rm Ca}_{0.3-{x}}{\rm Sr}_{x}{\rm MnO}_{{3}}$ Nanoparticles

Abstract: In this paper, we present a detailed study of the magnetocaloric effect and critical properties around the ferromagnetic-paramagnetic (FM–PM) phase transition of ${\rm La}_{0.7}{\rm Ca}_{0.3-x}{\rm Sr}_{x}{\rm MnO}_{3}$ 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 $(\vert\Delta S_{\rm max}\vert)$ reaches values of 1.47, 1.42, and 1.38 ${\rm J}\cdot{\rm kg}^{-1}\cdot{\rm K}^{-1}$ for $x=0.10$ , 0.11, and 0.12, respectively, at around 300 K. The refrigerant capacity is thus in between 44 and 54 ${\rm J}\cdot{\rm kg}^{-1}$ . Particularly, the $M^{2}$ 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 $\beta$ , $\gamma$ , $\delta$ , and $T_{C}$ . Here, the $\beta$ 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 $(\beta=0.5)$ and value of the 3-D Heisenberg model $(\beta=0.365)$ . The result proves the coexistence of short- and long-range FM interactions in ${\rm La}_{0.7}{\rm Ca}_{0.3-x}{\rm Sr}_{x}{\rm MnO}_{3}$ nanoparticles. The nature of this phenomenon is discussed thoroughly.

Magnetocaloric Effect and Critical Behavior of ${\rm Ni}_{42}{\rm Ag}_{8}{\rm Mn}_{37}{\rm Sn}_{13}$ Alloys

Abstract: This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of ${\rm Ni}_{50}{\rm Mn}_{37}{\rm Sn}_{13}$ 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 $T_{\rm C}\approx 295~{\rm K}$ . 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 $(\Delta S_{m})$ of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change $(\vert \Delta S_{\rm max}\vert)$ reaches values 0.54 and 0.69 ${\rm J}\cdot~{\rm kg}^{-1}\cdot{\rm K}^{-1}$ 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 ( $T_{\rm C}$ , $\beta$ , $\gamma$ , and $\delta$ ) in the low field range (below 10 kOe) with $T_{\rm C}=294.8~{\rm K}$ , $\beta=0.469\pm 0.011$ , $\gamma=1.149\pm 0.060$ , and $\delta=3.4\pm 0.1$ for the alloy ingot, and with $T_{\rm C}=294.4~{\rm K}$ , $\beta=0.449\pm 0.005$ , $\gamma=1.319\pm 0.040$ , and $\delta=3.9\pm 0.1$ for the alloy ribbon. One can see that $\beta$ values fall in between those expected for the 3-D Heisenberg model $(\beta=0.365)$ and mean-field theory $(\beta=0.5)$ . 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 $\Delta S_{m}$ is thoroughly discussed by means of structural analyses.

Large Magnetocaloric Effect Around Room Temperature in Double-Exchange Ferromagnets Pr 1-x Sr x MnO 3 With Short-Range Interactions

Abstract: We present a detailed study on a large magnetocaloric effect at room temperature in polycrystalline ceramic samples of Pr_1-xSr_xMnO₃ (x = 0.3, 0.4, and 0.45), exhibiting a second-order phase transition. The experimental results demonstrate the replacement of Sr²⁺ for Pr³⁺ in Pr_1-xSr_xMnO₃, which leads to the Curie temperature (TC) increasing from 258 K for x = 0.3 to 301 K for x = 0.45. In particular, the maximum values of magnetic entropy change (|ΔS_max|) in the field H = 10 kOe are about 2.36-3.33 J·kg ^-1·K ^-1, corresponding to the refrigerant capacity RC = 50 - 53 J·kg ^-1 for x = 0.3-0.45. Based on the modified Arrott-plot method and the isothermal magnetization data around TC, we obtained the values of the critical exponents (β, γ, and δ) and TC for all the samples. Here, the values β = 0.370-0.397 are quite close to those expected for the 3-D Heisenberg model. This proves that ferromagnetic short-range interactions are present in Pr_1-xSr_xMnO₃ compounds. In addition, magnetic-field dependences of |ΔS_max| obey the power law, |ΔS_max| ~ Hn, where the field exponent n ≈ 0.6 is close to the value calculated from the relation n = 1 + (β - 1)/(β + γ).

Protecting FPGA-based Partially Reconfigurable Embedded Systems and IP Cores from Remote Update

Abstract: Intellectual Property (IP) core remote update via the Internet is an advantage of FPGA-based devices. Whereby the system designer or user can change or renew a part of his equipment. However, the problem of communication in an untrusted environme nt can lead to significant damage: The risks for the IP cores of designers or damaging devices or losing the properties of the user. There have been already many proposals to overcome these problems, for example, using symmetric encryption techniques for bitstream, bitstream version to prevent replay attacks. This paper presents a framework, which include a protocol and authentication and encryption algorithms for protecting IP cores and partially reconfigurable embedded systems based on FPGA. Experimental results and analysis show that the proposed technique reduces resource overhead, increase the flexibility of the system, and it is robust against attacks.

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

Abstract: We have investigated the magnetic properties and magnetocaloric effect in La0.7Sr0.3Mn1-xCoxO3 prepared by a conventional solid-state reaction method. Magnetic measurements versus temperature revealed that the Curie temperature (TC) decreased gradually with increasing Co content (x); TC values are about 360, 310, 296 and 280 K for x =0.0, 0.06, 0.08 and 0.1, respectively. Magnetic entropy change (ΔSm) of the samples under a magnetic field change of 10 kOe was calculated by using isothermal magnetization data. We have found its maximum (|ΔSmax|) achieved around TC, which is not changed much (~ 1.5 J·kg-1·K-1) 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 |ΔSmax| values are stable at about 0.98 J·kg-1·K-1. The relative cooling power is accordingly about 55 J·kg-1 and comparable to that of other magnetocaloric alloys. These results suggest La0.7Sr0.3Mn1-xCoxO3 compounds to be a potential candidate for magnetic refrigerators at room temperature.

Inhomogeneous Ferromagnetism and Spin-Glass-Like Behavior in $({\rm Nd}_{1-x}{\rm Y}_{x})_{0.7}{\rm Sr}_{0.3}{\rm MnO}_{3}$ With $x=0.21{-}0.35$

Abstract: The magnetic properties of polycrystalline ceramic samples $({\rm Nd}_{1-x}{\rm Y}_{x})_{0.7}{\rm Sr}_{0.3}{\rm MnO}_{3}$ 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, $H_{\rm ex}$ ; $T_{B}\to T_{g}$ (the SG phase-transition temperature) as $H_{\rm ex}\to 0$ . 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, $\chi^{\prime}(T)$ , it shows a frequency-dependent peak at the SG freezing temperature $(T_{f})$ ; $T_{f}\to T_{g}$ as $f\to 0$ . Dynamics of this process were analyzed by means of the slowing down scaling law, $\tau/\tau_{0}\propto (T_{f}/T_{g}-1)^{-zv}$ , 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 $\tau_{0}=10^{-21}{-}10^{-15}~{\rm s}$ . These values are different from those expected for canonical SG systems with $zv={10}$ and $\tau_{0}=10^{-13}~{\rm s}$ , revealing the cluster-SG behavior of $({\rm Nd}_{1-x}{\rm Y}_{x})_{0.7}{\rm Sr}_{0.3}{\rm MnO}_{3}$ 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.

Magnetic properties and magnetocaloric effect at room temperature of Ni50−xAgxMn37Sn13 alloys

Abstract: In this work, we present a detailed study of the magnetic properties and the magnetocaloric effect at room temperature of Ni50−xAgxMn37Sn13 alloys with x = 1, 2, and 4, which were prepared by using an arc-melting method. Experimental results reveal that a partial replacement of Ag for Ni leads to a decrease in the anti-FM phase in the alloys. In addition, the martensitic-austenitic phase transition shifts towards lower temperature and is broaded. The Curie temperature (TCA) for the austenitic phase also shifts toward to lower temperature, but not by much. The Curie temperature was found to be 308, 305, and 298 K for x = 1, 2, and 4, respectively. The magnetic entropy change (ΔSm) of the samples was calculated by using isothermal magnetization data. Under an applied magnetic field change of 10 kOe, the maximum value of ΔSm (|ΔSmax|) was achieved at around room temperature and did not change much (~0.8 J·kg−1·K−1) with increasing Ag-doping concentration. Particularly, the M2 vs. H/M curves prove that all the samples exhibited a second-order magnetic phase transition. Based on Landau’s phase-transition theory and careful analyses of the magnetic data around the TCA, we have determined the critical parameters β, γ, δ, and TC. The results show that the β values are located between those expected for the 3D-Heisenberg model (β = 0.365) and mean-field theory (β = 0.5). Such a result proves the coexistence of short-range and long-range ferromagnetic interactions in Ni50−xAgxMn37Sn13 alloys. The nature of the changes in the critical parameters and the |ΔSmax| is thoroughly discussed by means of structural analyses.

Electrical and Magnetotransport Properties of ${\rm La}_{{0.7}}{\rm Ca}_{{0.3}}{\rm Mn}_{{1-x}}{\rm Co}_{{x}}{\rm O}_{3}$

Abstract: This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of ${\rm La}_{0.7}{\rm Ca}_{0.3}{\rm Mn}_{1-x}{\rm Co}_{x}{\rm O}_{3}(x={0.09}{-}0.17)$ 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 $x\geq 0.11$ 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 $x={\rm 0.17}$ ) exhibit a metal–insulator transition at $T_{P}=60{-}160~{\rm K}$ , 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 . In addition, the conduction data at temperatures $T>\theta_{D}/2$ ( $\theta_{D}$ is the Debye temperature) and $T_{P} 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., $x={\rm 0.10}$ ).

Influence of the Change in Oxidation Number of Mn on Magnetic Properties of ${\rm BaTi}_{{1}-{x}}{\rm Mn}_{x}{\rm O}_{3}$

Abstract: We prepared BaTi1-xMnxO3 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 BaTi1-xMnxO3 with x = 0.05, where the Mn2+/Mn3+ ratio is about one for the sample annealed at 700°C. These Mn ions prefer locating at the Ti site in the tetragonal BaTiO3 host lattice. In particular, there is a change in the oxidation number of Mn2+ → Mn3+ when the annealing temperature changes from 700°C to 900°C. Their magnetic properties are accordingly changed. Meanwhile, annealing pure BaTiO3 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 BaTiO3 annealed in an Ar ambient is associated with exchange interactions of Mn2+ ions mediated by oxygen vacancies rather than associated with Mn3+ ions.

Critical behavior and exponent parameters of the austenitic phase in Ni 50- x Pr x Mn 37 Sn 13 alloys with x = 1

Abstract: 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 ≈ 299 K. Using the modified Arrott plots, asymptotic relations, and a universal scaling law, we determined the values of the critical exponents β = 0.501 ± 0.009 and γ = 1.045 ± 0.006. These values are very close to those expected for the mean-field theory with β = 0.5 and γ = 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 (∆S max ). Its magnetic-field dependences can be described by using a power law |∆S max | ∝ H n , where n = 0.687 is close to the value 0.677 calculated from the theoretical relation n = 1 + (β − 1)/(β + γ). We believe that the doping of a suitable Pr amount in Ni50−x Pr x Mn37Sn13 (x ≈ 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.

Magnetic Properties and Magnetocaloric Effect in Pb-Doped ${\rm La}_{0.9}{\rm Dy}_{0.1}{\rm MnO}_{3}$ Manganites

Abstract: We have studied the magnetic properties and magnetocaloric effect of rhombohedral $({\rm La}_{0.9}{\rm Dy}_{0.1})_{1\hbox{-}x}{\rm Pb}_{x}{\rm MnO}_{3}$ ( $x={0.1}$ , 0.2, and 0.3) fabricated by solid-state reaction. Thermomagnetization curve reveals that the increase of Pb doping in $({\rm La}_{0.9}{\rm Dy}_{0.1})_{1\hbox{-}x}{\rm Pb}_{x}{\rm MnO}_{3}$ shifts the ferromagnetic (FM)–paramagnetic (PM) phase-transition temperature $(T_{C})$ toward room temperature; the $T_{C}$ values determined for $x={0.1}$ , 0.2, and 0.3 are about 172, 249, and 322 K, respectively. Based on magnetic-field dependencies of magnetization, $M(H)$ , the magnetic entropy changes $(\Delta S_{m})$ of the samples were calculated. Under an applied magnetic field of $H={10}~{\rm kOe}$ , the $\vert\Delta S_{\rm max}\vert$ value slightly increases from 0.74 ${\rm J}/{\rm kg}\cdot~{\rm K}$ for $x={\rm 0.1}$ to about 1.1 ${\rm J}/{\rm kg}\cdot~{\rm K}$ for $x={\rm 0.2}$ and 0.3. These results correspond to a relative cooling power of ${\sim}{\rm 50}~{\rm J}/{\rm kg}$ , which is comparable with some perovskite manganites, indicating the applicability of $({\rm La}_{0.9}{\rm Dy}_{0.1})_{1\hbox{-}x}{\rm Pb}_{x}{\rm MnO}_{3}$ in magnetic refrigeration devices working around room temperature. Particularly, we find that the variation of $\vert\Delta S_{\rm max}\vert$ versus the magnetic field $H$ can be described by a power law $\vert\Delta S_{\rm max}\vert\propto H^{n}$ , where the magnetic-ordering parameter $(n)$ decreases from 0.861 for $x={0.1}{-}0.834$ for $x={\rm 0.3}$ . A deviation of the $n$ values estimated from the mean-field theory $(n=2/3)$ demonstrates an existence of short-range FM order in the samples.