Showing papers by "Pham Thanh Phong published in 2014"

Structural and magnetic study of La0.7Sr0.3MnO3 nanoparticles and AC magnetic heating characteristics for hyperthermia applications

Abstract: We investigated structural and magnetic properties and alternating current magnetic heating characteristics of La 0.7 Sr 0.3 MnO 3 nanoparticles with respect to the possible application for magnetic hyperthermia treatments. Using Rietveld Profile refinement of powder X-ray diffraction data, the hexagonal structure has been observed. The particle sizes varied from 20 to 50 nm as the annealing temperature increases from 700 to 900 °C. The hysteresis loop is not observed and the good fit of Langevin function with magnetization data reveals the superparamagnetic nature at room temperature for all samples. Characteristic magnetic parameters of the particles including saturation magnetization in the temperature range 10–300 K, an effective anisotropy constant and a magnetocrystalline anisotropy constant have been determined. The Specific Absorption Rate for 15 mg/mL sample concentration was measured in alternating magnetic fields of 50–80 Oe at a fixed frequency of 236 kHz. In addition, the intrinsic loss power (ILP) has been calculated from SAR values. It is believed that La 0.7 Sr 0.3 MnO 3 nanoparticles with a high ILP will be useful for the in situ hyperthermia treatment of cancer.

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...

Magnetic Properties of Fe 3 O 4 Nanoparticles Synthesized by Coprecipitation Method

Abstract: In this paper, we elucidate several specific magnetic properties of Fe 3 O 4nanoparticles synthesized by coprecipitation method. The characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) showed the particles to be of spinel structure and spherical shapes whose diameter could be controlled in the range from 14 to 22 nm simply by adjusting the precursor salts concentration and coprecipitation temperature. Magnetic properties of the Fe 3 O 4 nanoparticles measured by using vibration sample magnetometer (VSM) indicated the saturation magnetization and blocking temperature to increase with the particles size. Fe 3 O 4 nanoparticles with crystal size smaller than 22 nm exhibits superparamagnetic behavior at room temperatures. Characteristic magnetic parameters of the particles including saturation magnetization, effective anisotropy constant, and magnetocrystalline anisotropy constant have been determined. The observed decrease of saturation magnetization was explained on the base of core-shell model. A simple analysis indicated that the shell thickness decreases with an increase in particle size.

Magnetic Surface Effects and Magnetoresistance in Manganite-based Composite Nanoparticles

Abstract: The effect of Ag doping on the magnetization and the magneto transport of (1−x)La0.7Ca0.3MnO3/xAg composite nanoparticles have been studied. The introduction of Ag into the grain boundaries forms a second conducting network and improves conductivity and increases the metal–insulator transition temperature. The magnetoresistance of composites in the whole studied temperature range is enhanced by increasing the Ag content. The enhanced extrinsic magnetoresistance can be related with the spin dependent scattering of spin-polarized electrons at the interfaces between the La0.7Ca0.3MnO3 grains and silver granules; whereas the monotonic increase in the intrinsic magnetoresistance has been attributed to such factors like: decrease in electrical resistivity due to the disorder reduction, magnetic heterogeneity, and growth of spin clusters. Both high field magnetoresistance (HFMR) and low field magnetoresistance (LFMR) have been calculated as a function of temperature. The characteristic of HFMR and LFMR turns out to be substantially different: While the LFMR decreases rapidly with temperature, the HFMR is increased slightly with temperature. We propose that for the Ag-doped nanomanganites, the HFMR originates from the weakened ferromagnetic spin interaction at the grain boundaries. In addition, it is shown that these composites become magnetically harder when reducing the Ag dopant. It is believed that the phenomena are due to the effect of Ag on the degree of the spin disorder at the grain boundaries. Implications of these finding improvements of the field response of the magnetic surface of nanoparticles are discussed.

Magnetic Properties of Annealed ${\rm Fe}_{65}{\rm Co}_{35}$ Powders Prepared By Mechanical Alloying

Abstract: Fe-Co alloy powders were prepared by mechanical alloying of the elemental Fe and Co powders in air and subsequently annealed at various temperatures. Structural and magnetic characteristics of the annealed powders were studied in detail as a function of annealing temperatures by using an X-ray diffractometer (XRD), a field emission scanning electron microscope, a vibrating sample magnetometer, and a physical property measurement system. The XRD results showed an existence of the nanocrystals with sizes of 15–50 nm. The magnetic studies indicated a strong increase of magnetization and a sharp decrease of coercivity as annealing temperature increased. Both the effect of the oxidation on the magnetic properties as well as magnetization stability of the annealed samples will be discussed.