Pham Thi Thanh

Coexistence of conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50−xSnx (x = 13 and 14) alloy ribbons

A systematic study of the conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50−xSnx (x = 13 and 14) alloy ribbons has been performed. We show that although the magnetic entropy change around the second-order ferromagnetic-paramagnetic (FM-PM) transition (ΔSm ≈ −4 J/kg K) in the austenitic phase is about five times smaller than that around the first-order martensitic-austenitic (M-A) transformation (ΔSm ≈ 22 J/kg K), the refrigerant capacity (RC) – an important figure of merit – is about two times larger for the former case (RC ≈ 160 J/kg) than for the latter case (RC ≈ 75 J/kg). This finding points to an important fact that to assess the usefulness of a magnetocaloric material, one should not only consider ΔSm but also must evaluate both ΔSm and RC. Our critical analysis near the second-order FM-PM transition reveals that Sn addition tends to drive the system, in the austenitic FM phase, from the short-range (x = 13) to long-range (x = 14) FM order.

Magnetic properties and magnetocaloric effect in Fe90−xNixZr10 alloy ribbons

We report magnetic properties and magnetocaloric (MC) effect in Fe90−xNixZr10 (x = 0, 5, 10, and 15) alloy ribbons prepared by rapid-quenching method. We found the Curie temperature (TC) of the alloy ribbons depends strongly on Ni-doping concentration (x) increasing from 245 K for (x = 0), through 306 K (for x = 5) up to TC = 403 (for x = 15). Also, the dependence of the maximum magnetic entropy change (|ΔSmax|) on Ni content was readily apparent. Indeed, for the x = 0 and 5 samples that have TC around room temperature, the |ΔSmax| values increment under magnetic field changes of 10, 20, and 40 kOe was found to be as high as 0.87, 1.76, and 3.04 J·kg−1·K−1 for x = 0, and 1.03, 1.90, and 3.26 J·kg−1·K−1 for x = 5, respectively. These values correspond to refrigerant capacity in the range of 86-334 J·kg−1 and are comparable to other known MC materials. High magnetocaloric performance in rare-earth-free non-expensive metallic alloys indicates that these materials could be competitive candidate for active mag...

Critical behavior and magnetocaloric effect of LaFe10−xBxSi3 alloy ribbons

We have studied the magnetic properties and magnetocaloric effect of LaFe10−xBxSi3 (x = 1, 2, and 3) alloy ribbons prepared by a rapidly quenching method. The partial replacement of Fe for B in LaFe10−xBxSi3 leads to a rapid decrease in the Curie temperature (TC) from 425 K for x = 1, through 310 K for x = 2, to 190 K for x = 3. Among LaFe10−xBxSi3 ribbons, only LaFe7B3Si3 (i.e., x = 3) has a dominancy of amorphous phase. The M2 versus H/M plots prove this sample exhibiting a second-order magnetic phase transition. The detailed analyses of M(H) data around TC based on the modified Arrott plot introduced critical values of TC ≈ 192 K, β = 0.354 ± 0.013, γ = 1.355 ± 0.032, and δ = 4.8 ± 0.1. These values are close to those expected for the 3D-Heisenberg model (β = 0.365 and γ = 1.336), indicating an existence of ferromagnetic short-range interactions. Concerning the magnetic entropy change (ΔSm), we have found its maximum ΔSm achieved just around TC, which are 1.04 and 1.42 J/kg K for x = 2 and 3, respectiv...

Magnetocaloric Effect in

We have studied the magnetic properties and magnetocaloric effect in bulk Ni_0.5Mn_0.5-xSn_x alloys (x = 0.2 and 0.3) prepared by arc-melting. Experimental results reveal that with increasing Sn content the ferromagnetic order slightly decreases, but the Curie temperature (T_C) increases from 333 K (for x = 0.2) to about 354 K (for x - 0.3). Based on magnetic-field dependences of magnetization (M - H curves) recorded at various temperatures, ΔS_M (T) curves of the samples under an applied field interval of 0-12.0 kOe were obtained. Maximum ΔS_M values achieved around T_C are about 1.2 and 1.1 J.kg^-1.K^-1 for x = 0.2 and 0.3, respectively. Detailed analyses of M - H curves in the vicinity of T_C, used the Arrott-Noakes method, reveal the samples undergoing the second-order phase transition with the critical exponents of β = 0.503 ± 0.021 and γ = 1.246 ± 0.013 for x = 0.2, and of β = 0.426 ± 0.016 and 7 = 1.232 ± 0.014 for x = 0.3. The difference in value of the exponents together with x-ray diffraction data will be used to explain the magnetic properties and magnetocaloric effect in Ni_0.5Mn_0.5-xSn_x.

\hbox{Ni}_{0.5}\hbox{Mn}_{0.5-{\rm x}}\hbox{Sn}_{\rm x}

In this paper, we investigated the influence of addition of Dy40Nd40Al30 nanoparticles on the structure and magnetic properties of sintered Nd-Fe-B magnets. The nanoparticles with a size smaller than 50 nm were prepared using high-energy ball milling method and then mixed with micrometer Nd2Fe14B powder before magnetic anisotropic pressing, vacuum sintering, and annealing. The structure of the magnets was thoroughly analyzed using X-ray diffraction and electron microscopy techniques. The magnetic properties of the magnets were investigated on a pulsed field magnetometer. The atoms of Dy were detected mainly at the grain boundaries and partly in the near-boundary area of the grains. On adding 2% of the nanoparticles, the coercivity of the magnets is enhanced by quite a large amount, from 12 kOe for the unadded magnets to 21 kOe for the added ones. The large increase of the coercivity is probably due to the diffusion of Dy to the Nd2Fe14B grains to form (Nd,Dy)2Fe14B phases with high magnetocrystalline anisotropy. The nanoparticles might make Dy distribute more homogeneously and diffuse to the Nd2Fe14B grains more efficiently.

Alloys

This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of Ni₅₀Mn₃₇Sn₁₃ 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_C ≈ 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_m) of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change (|ΔS_max|) reaches values 0.54 and 0.69 J · kg^-1 · K^-1 for the alloy ingot and ribbon, respectively. Using Landaus phase-transition theory, and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical parameters (T_C, β, γ, and δ) in the low field range (below 10 kOe) with T_C = 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_C = 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_m is thoroughly discussed by means of structural analyses.

Enhancing Coercivity of Sintered Nd-Fe–B Magnets by Nanoparticle Addition

This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of Ni₅₀Mn₃₇Sn₁₃ 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_C ≈ 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_m) of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change (|ΔS_max|) reaches values 0.54 and 0.69 J · kg^-1 · K^-1 for the alloy ingot and ribbon, respectively. Using Landaus phase-transition theory, and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical parameters (T_C, β, γ, and δ) in the low field range (below 10 kOe) with T_C = 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_C = 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_m is thoroughly discussed by means of structural analyses.

Magnetocaloric Effect and Critical Behavior of

This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of Ni₅₀Mn₃₇Sn₁₃ 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_C ≈ 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_m) of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change (|ΔS_max|) reaches values 0.54 and 0.69 J · kg^-1 · K^-1 for the alloy ingot and ribbon, respectively. Using Landaus phase-transition theory, and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical parameters (T_C, β, γ, and δ) in the low field range (below 10 kOe) with T_C = 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_C = 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_m is thoroughly discussed by means of structural analyses.

Magnetocaloric Effect and Critical Behavior of

Abstract The influence of composition and annealing temperature on the magnetic properties of (Nd0.5Pr0.5)6+xNb1.5Fe88.5–x–yB4+y (x = 0–6, y = 0–10) nanocomposites prepared via melt-spinning and subsequent annealing was investigated systematically to obtain their optimal compositions and fabrication conditions. The results show that each of the rare earth concentrations has an appropriate ratio of Fe/B to reveal the best performance of the material. The optimal annealing temperature decreases from 750 to 675°C when concentration of rare earth is increased from 6 to 12 at.%. Coercivity, Hc, and maximum energy product, (BH)max, of these nanocomposites can be regulated up to 14.5 kOe and 16 MGOe, respectively, by suitably choosing their composition.