Nguyen Phuc Duong

Ultrafast magnetization dynamics of antiferromagnetic compounds

In three different antiferromagnetic (AFM) materials we show that the magnetization dynamics of AFM compounds differs noticeably from that of ferromagnetic compounds. Optical second harmonic generation and linear reflection were used to monitor with a temporal resolution of <1u2009ps the evolution of the AFM order parameter subsequent to an intense optical excitation. Comparison of the dynamical properties of the model antiferromagnet Cr2O3 with model ferro- and ferrimagnets reveals that spin–lattice relaxation as the temporally limiting thermalization process of the magnetic subsystem is more complex and inherently faster than in compounds displaying a spontaneous magnetization. The exchange-bias compound NiO exhibits an ultrafast photoinduced AFM phase transition in the course of which the AFM order parameter is switched by 90°. Properly timed sequences of pump pulses can repeatedly reorient the AFM order parameter within 10u2009ps, thus demonstrating ultrafast AFM switching. In the colossal-magnetoresistance compound Pr1−xCaxMnO3 optical pumping triggers a transition from an insulating AFM to a conducting metallic state within 230u2009fs.

Formation and magnetic properties of the RCo4Si compounds

Abstract We have succeeded in preparing RCo 4 Si compounds (R = Y, Nd, Pr, Sm, Gd, Tb, Dy, Ho and Er). All samples are single phase with the CaCu 5 structure as revealed by X-ray analysis. Si substitution drastically reduces the Curie temperature of the host RCo 5 compounds. In particular, we have observed a spontaneous magnetisation of 2.34 μ B /f.u. at 5 K for SmCo 4 Si, which is significantly lower than the value of 3.0 μ B /f.u. found for YCo 4 Si. Also the temperature dependence of the spontaneous magnetisation of SmCo 4 Si shows ferrimagnetic characteristics. These results suggest an antiparallel coupling of the 3d and 4f moments in SmCo 4 Si, which has so far not been observed in light-rare-earth—transition-metal compounds. The Co magnetocrystalline anisotropy and the crystalline electric field interaction of the compounds have been studied. The results suggest the distinctive role of Si substitution in modifying the magnetic properties of the host RCo 5 compounds.

Magnetic and Mössbauer studies of the DyFe11Mo compound

Abstract A detailed magnetic and Mossbauer study focusing on intrinsic magnetic and anisotropy properties of the DyFe 11 Mo compound is reported. The compound shows a spin reorientation phase transition at T sr =220xa0K. Anomalies in physical properties such as saturation magnetization, AC-susceptibility and hyperfine field at T sr were identified, analysed and are discussed in terms of the individual site anisotropy model.

Cation distribution in CuFe2O4 nanoparticles: Effects of Ni doping on magnetic properties

The Cu1−xNixFe2O4 nanoparticles (with xu2009=u20090, 0.3, 0.5, 0.7, and 1) were synthesized by using spray co-precipitation method at annealing temperature Tau2009=u2009900u2009°C in air for 5 h. The crystal structure, microstructure, oxidation state, and magnetic properties of the samples were characterized by using X-ray diffraction, synchrotron X-ray diffraction, scanning electron microscopy, X-ray absorption spectroscopy, and vibrating sample magnetometer. It was shown that all the samples have cubic structure. Lattice constant and grain size decrease, while the Curie temperature TC increases with increasing of Ni2+ content. A small amount of Fe2+ was found in all the samples. Cation distribution was determined by using a combination of magnetization measurements, extended X-ray absorption fine structure analysis, and Rietveld refinement from synchrotron X-ray diffraction data. It was indicated that Ni2+ ions occupy in octahedral site only, while Cu2+ ions distribute in both tetrahedral and octahedral sites. The variatio...

Influence of Structure and Oxidation State on Magnetic Properties of Sr1−xLaxFe12−xCoxO19 Nanoparticles Prepared by Sol–Gel Combustion Method

Nanoparticles of Sr1−xLaxFe12−xCoxO19 (xxa0=xa00 to 0.2) hexagonal ferrite have been synthesized by the sol–gel method using citric acid to complex the ions, followed by an autocombustion reaction after annealing at 900°C for 2xa0h in air. The crystal structure, valence state, and morphology of the samples were studied by x-ray diffraction (XRD) analysis, synchrotron x-ray diffraction (SXRD), x-ray absorption near-edge spectroscopy (XANES), and scanning electron microscopy. Rietveld refinement of XRD and SXRD patterns revealed that single-phase SrFe12O19 was obtained for all samples; the La3+ ion preferentially occupies the 2d site, whereas the Co2+ ion preferentially occupies the 2a and 4f2 sites. XANES analysis results indicated presence of Fe2+ ion in the samples with xxa0=xa00.1, 0.15, and 0.2. The particles were single domain with size in the range from 50xa0nm to 80xa0nm. The magnetic properties of the hexaferrite samples (SrFe12O19) were significantly improved as a result of the appropriate La/Co substitution. The coercivity values at room temperature increased from 6.7xa0kOe to 8xa0kOe with increasing La/Co concentration from 0 to 0.2. Maximum saturation magnetization of 73xa0emu/g was achieved at La/Co concentration of xxa0=xa00.1.

Enhancement of Magnetic Properties of La-substituted Strontium Hexaferrite Particles Prepared by Sol-gel Route

La-substituted strontium hexaferrite particles Sr1−xLaxFe12O19 (x = 0 ÷ 0.2) were prepared by using sol-gel method and subsequent calcination at temperatures ranging from 750̊ C to 1050̊ C for 2h in air. The effects of the initial La concentration and the calcination temperature on the structure, particle morphology and magnetic properties of the hexaferrite samples were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The high magnetic coercivity (iHc) of 6.6 6.7 kOe and magnetization in maximum applied magnetic field (13.5 kOe) of 68 72 emu/g were achieved with the particles of compositions x = 0.1 ÷ 0.2 and calcination temperatures between 850̊ C and 1000̊ C. The experimental data indicate that La replacement for Sr improves the crystallinity and inhibits the grain growth of the samples in a wide range of calcination temperature. These factors are important for production of high-coercivity hexaferrites by sol-gel technology.

Single Phase Formation, Cation Distribution, and Magnetic Characterization of Coprecipitated Nickel-Zinc Ferrites

X-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry were used to investigate the structural, morphological, and magnetic properties of Ni1-xZnxFe2O4 nanosubmicron powders (xxa0=xa00–0.8). The samples were prepared by a coprecipitation method at temperatures between 600 and 1100 degree Celsius for five hours. The lattice constant, average size of coherent scattering regions, and cation distribution were analyzed by Rietveld refinement of the X-ray diffraction measurements. Scanning electron micrographs showed that the particle size increased from the nano- to the submicron scale as the zinc content increased from xxa0=xa00 to xxa0=xa00.8. The magnetization as functions of temperature and applied magnetic field were measured on the single-phase samples obtained at an annealing temperature of 1100 degrees Celsius. The derived saturation magnetization and Curie temperature were compared to those of the bulk counterparts reported previously and discussed based on finite-size effects and poss...X-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry were used to investigate the structural, morphological, and magnetic properties of Ni1-xZnxFe2O4 nanosubmicron powders (x = 0–0.8). The samples were prepared by a coprecipitation method at temperatures between 600 and 1100 degree Celsius for five hours. The lattice constant, average size of coherent scattering regions, and cation distribution were analyzed by Rietveld refinement of the X-ray diffraction measurements. Scanning electron micrographs showed that the particle size increased from the nano- to the submicron scale as the zinc content increased from x = 0 to x = 0.8. The magnetization as functions of temperature and applied magnetic field were measured on the single-phase samples obtained at an annealing temperature of 1100 degrees Celsius. The derived saturation magnetization and Curie temperature were compared to those of the bulk counterparts reported previously and discussed based on finite-size effects and possible nonmagnetic impurities. The influence of zinc substitution on the magnetic exchange interactions were studied based on molecular-field theory for compositions of 0 ≤x ≤ 0.4.

Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH Batteries

The effects of the substitution of nickel by gallium on the structures and the electrochemical properties of LaNi5-xGax (x = 0.1−0.5) alloys were studied systematically. The structure of the alloy was tested by X-ray diffraction (XRD) measurements. Electrochemical properties and battery parameters were measured by bipotentiostat and battery tester equipment. The results showed that when gallium is doped into alloys, the lattice of the LaNi5-xGax is slightly increased but retains the CaCu5 structure. Gallium has a low melting temperature. When gallium replaces nickel in the LaNi5 alloy, it covers material particles and reduces oxidation process, which leads to a longer lifetime and makes charge/discharge process more stable. The shapes of electrochemical impedance spectroscopy measurements of all the LaNi5-xGax samples were similar, and the value increases as the substitution of Ni by Ga increases. The cyclic voltammograms of all the LaNi5-xGax samples were similar to the one of pure LaNi5. For the same Ga-doped concentration and experimental conditions, the current density Jmax and charge quantity Q of the samples were increased cycle by cycle of charge/discharge.

Structural, magnetic, and magnetotransport properties of NiMnSb thin films deposited by flash evaporation

To date, the use of flash evaporation (FE) as a deposition technique for NiMnSb thin films has not yet been reported. In this letter, we report on NiMnSb thin films deposited on heated Si (111) substrates at 300u2009°C via FE. Investigations of the structural characteristics and magnetic and magnetotransport properties of these thin films show typical features of a half-metallic ferromagnetic semi-Heusler alloy. The origin of the film’s extraordinary magnetotransport behavior is examined under the perspective of spin-order levels attached to a grain-grain boundary-type structure.

Molecular-field Molecular-field Study on Sn Substituted Yttrium Iron Garnet

Samples with the compositions Y2.9Ca0.1Fe4.9Sn0.1O12 and Y3Fe4.9Sn0.1O12 were prepared by using a sol-gel technique. The influence of the substituted non-magnetic cations with different valences in the structural and magnetic properties was studied. X-ray diffraction and field-emission electron scanning microscope techniques were used to study the crystal structure and morphology. Magnetization curves in fields up to 10 kOe and in temperature range from 80 K to 570 K were measured by means of a vibrating sample magnetometer. Saturation magnetization as a function of temperature of the two samples was analyzed based on the molecular-field theory from that models for site occupancy and valence states of cations in the crystal structures were derived.