Nguyen Hai Yen

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...

Detection of the pesticide by functionalised quantum dots as fluorescence–based biosensor

In this paper, we present the new results of biosensor that is made from the surface–modified quantum dots with acetylcholinesterase enzymes (AChE) for optical detection of the pesticides. The quantum dots (QDs) mentioned in this study are CdTe, CdSe/ZnS and CdSe/ZnSe/ZnS - the thick shell QDs are totally new. The results pointed out that all of the quantum dots are fit for the role of transducers in biosensor. In the biosensor, the QD– streptavidine - AChE is used as the substrate for the detection of pesticide. The pesticides used in this work are Parathion Methyl (PM) and Acetamiprid. The acetylthiocholine (ATCh) is used as an indicator of the activity of the AChE enzyme. Alternatively, the organophosphorus (OP) pesticides are the inhibitor for the AChE enzymes. Therefore, the mixture of the pesticide and ATCh is used for the goal of the specification of pesticide. We can detect pesticides by the change in PL intensity of QDs biosensor, with the content ranges from 0.05 ppb to 10 ppb.

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}

This paper reveals that a partial replacement of Fe by Ni (5%) and/or Ag (2%) in amorphous Fe_85-xAg_xNi₅Zr₁₀ (x = 0 and 2) alloys leads to the shift of T toward room temperature (RT) (T_C = 308 and 303 K for x = 0 and 2, respectively). Basing on isothermal magnetization data, the magnetic entropy change (ΔS_m) of the amorphous alloys was calculated. Their maximum ΔS_m values (|ΔS_max|) achieve just around RT, corresponding to the ferromagnetic-paramagnetic (FM-PM) phase transition of the amorphous phase. Under a magnetic field change of 10 kOe, |ΔS_max| values are ~1 J · kg^-1 · K^-1. We have also used different methods, such as the modified Arrott plots, the Kouvel-Fisher method, and critical isotherm analysis to study their critical property around the FM-PM phase transition. Our results suggest that the amorphous Fe_85-xAg_xNi₅Zr₁₀ alloys exhibit a second-order magnetic phase transition with the critical exponents of β = 0.389-0.396 and y = 1.279-1.334 close to those expected for the 3-D Heisenberg model. This proves the existence of short-range FM interactions in these alloys.

Alloys

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.

Magnetic Properties and Large Magnetocaloric Effect in Amorphous Fe-Ag-Ni-Zr for Room-Temperature Magnetic Refrigeration

The electronic phase diagram of strongly correlated systems with disorder is constructed using the typical-medium theory. For half-filled system, the combination of the linearized dynamical mean field theory and equation of motion approach allows to derive the explicit equations determining the boundary between the correlated metal, Mott insulator, and Anderson insulator phases. Our phase diagram is consistent with those obtained by the more sophisticated methods.

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

The electronic properties of strongly correlated systems with binary type of disorder are investigated using the coherent potential approximation. For half-filled system, two transitions from a band insulator via a metallic state to a Mott insulator are found with increasing the correlation strength of only one of the constituents. Our phase diagram is consistent with those obtained by the dynamical mean field theory.

Constructing Electronic Phase Diagram for the Half-filled Hubbard Model with Disorder

In order to search for new structures and compositions of quantum dots, suppress the blinking photoluminescence (random changes between high emission state ( on) and low emission status ( off ) under continuous photo- excitation ) and serve the application purposes in bio- medical and in optoelectronic devices , we have studied the fabrication of new alloy quantum dots ( QDs ). In this paper, we present new results on alloy core / shell quantum dots, with changed alloy shell composition, that was CdZnSe/ZnSe x S 1-x a ML with x ( x = 0 , 0.2, 0.4, 0.5 , 0.6, 0.8) and the shell thickness in monolayer (ML) ( a = 2 , 4 , 6 ) . The emission spectra and the intensity change according to the composition of the alloy shell. The full width a half maximum ( FWHM ) of the emission spectra of quantum dots CdZnSe is 25.5 nm. Covered with a shell layer, the emission intensity of the CdZnSe core increases along with the shell thickness. For comparison purpose, two different shell materials have been used, which are ZnS and ZnSeS alloy. With the same shell thickness, the emission wavelengths and intensity of the QDs change when the shell’s composition changes.The photoluminescence (PL) decay and the PL blinking of the alloy QDs was studied. It was shown that the alloy QDs spent by the nanocrystal in the ON state ranged typically between 20 and 40 %, and was dependent on the core composition. Detailed discussions on the experiment results are presented.