The-Long Phan

Structural, optical and magnetic properties of polycrystalline BaTi1−xFexO3 ceramics

Polycrystalline BaTi1−xFexO3 ceramics have been prepared by conventional solid-state reaction. Their structural, optical and magnetic properties are then studied by means of x-ray diffraction (XRD), Raman scattering (RS) and absorption spectrometers, and a physical properties measurement system. Detailed analyses of XRD patterns and RS spectra reveal the phase separation of the tetragonal-hexagonal structure at a threshold concentration of x = 0.005. The increase in the Fe-doping content (x) leads to development of the hexagonal phase. Magnetic measurements prove that many BaTi1−xFexO3 samples exhibit the room-temperature ferromagnetic order, excepting the samples with x = 0.02–0.06. The ferromagnetism depends strongly on concentration of Fe impurities. The nature of this ferromagnetism is discussed by means of the results of structural analyses and optical absorption spectra.

Influence of Mn doping on structural, optical, and magnetic properties of Zn1-xMnxO nanorods

We prepared Zn1−xMnxO nanorods by thermal diffusion. These samples were then studied the structural, optical, and magnetic properties. The structural analyses basing on x-ray diffraction and transmission electron microscope revealed the absence of Mn-related secondary phases. The study of photoluminescence spectra revealed the blueshift in the UV emission when the Mn doping concentration was increased, as a consequence of the extension of the band gap energy. Besides this situation, the increase in emission intensity associated with extrinsic defects at about 680 nm also took place. Concerning the Raman scattering spectra, apart from conventional phonon modes related to the ZnO wurtize-type structure, there were some additional modes introduced by the doping. Their origin was assessed carefully. Particularly, the shift in peak position of E2(high) toward low frequencies due to the increase in the Mn doping concentration could be explained well by means of the spatial correlation model. Magnetic measuremen...

Electric potential profile of a spherical soft particle with a charged core

The electrostatic potential profile of a spherical soft particle is derived by solving the Poisson-Boltzmann equations on a spherical system both numerically and analytically. The soft particle is assumed to consist of an ion-permeable charged outer layer and a non-permeable charged core with constant charged density. The contribution of the core to the potential profile is calculated for different charges and dielectric constants. Our results show that the charged core heavily influences the local potential within the soft particle. By contrast, the potential distribution outside the particle in the salt solution is found to be weakly dependent on the core features. These findings are consistent with previous experiments showing the minor impact of the core of the MS2 virus on its overall electrical properties. Our studies also indicate that while a change in temperature from 290 K to 310 K only slightly varies the potential, the ionic strength in the range of 1-600 mM has a significant effect on the potential profile. Our studies would provide good understanding for experimental research in the field of biophysics and nanomedicine.

Local structure and paramagnetic properties of Zn1-xMnxO

Having based on x-ray absorption spectroscopy, we point out structural phase separation in polycrystalline Zn1-xMnxO ceramics prepared by solid-state reaction. The samples are single phase in the wurtzite structure as x < 0.04. A Mn-concentration increase with x ≥ 0.04 results in the formation of a secondary phase of tetragonal ZnMn2O4 spinel. The feature of x-ray absorption spectra for the Mn K edge reveals an existence of Mn2+ and Mn3+ ions in Zn1-xMnxO, which are responsible for room-temperature paramagnetism. Particularly, when Mn content is increased, there is a chemical shift of the absorption edge toward higher energies, indicating an increase in concentration of Mn3+ ions. Fourier analyses for the spectra of extended x-ray absorption fine structure (EXAFS) shows up the substitution of Mn for Zn sites in the ZnO host lattice. The parameters related to the local structure have been determined and discussed in detail.

Electronic and magnetic phase diagram of La0.5Sr0.5Co1−xFexO3(0⩽x⩽0.6) perovskites

Spin dynamics and magnetic frustration effects in La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds were systematically studied by means of resistivity, dc magnetization, ac susceptibility, and electron-spin-resonance spectra. The Fe substitution caused a metal-semiconductor transition for x⩽0.3 compositions whereas the only semiconducting state behaved for x>0.3 compositions. With increasing Fe-doping levels, the ferromagnetic–paramagnetic transition temperature, TC, decreased from 250 (x=0) to 166K (x=0.4). A spin-glass behavior was observed for x>0.4 compositions and is attributed to the frustration of random competing exchange interactions, namely, the ferromagnetic Co3+–Co4+ double-exchange interaction and the antiferromagnetic interactions such as Co–O–Fe and Fe–O–Fe. The internal dynamic properties of the samples were elucidated by the electron-paramagnetic-resonance spectra. An electronic and magnetic phase diagram of La0.5Sr0.5Co1−xFexO3 is drawn up.

Spin dynamics and spin-glass state in Fe-doped cobaltites

A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponen...

Perpendicular magnetic anisotropy and the magnetization process in CoFeB/Pd multilayer films

The perpendicular magnetic anisotropy (PMA) and dynamic magnetization-reversal process in [CoFeB t nm/Pd 1.0 nm]n(t = 0.4, 0.6, 0.8, 1.0 and 1.2 nm; n = 2 − 20) multilayer films have been studied by means of magnetic hysteresis and Kerr effect measurements. Strong and controllable PMA with an effective uniaxial anisotropy up to 7.7 × 106 Jm−3 and a saturation magnetization as low as 200 emu cm−3 are achieved. The surface/interfacial anisotropy of the CoFeB/Pd interfaces—the main contribution to the PMA—is separated from the effective uniaxial anisotropy of the films and appears to increase with the number of CoFeB/Pd bilayers. Observation of the magnetic domains during a magnetization-reversal process, using polar magneto-optical Kerr microscopy, reveals the detailed behavior of the nucleation and displacement of the domain walls.

Glassy ferromagnetism and frustration in La0.7Ba0.3Mn0.7Ti0.3O3

Glassy ferromagnetism and the magnetic frustration effect in La0.7Ba0.3Mn0.7Ti0.3O3 were investigated by means of dc magnetization, ac susceptibility, and electron-spin-resonance (ESR) measurements. As a result, the spin-glass (SG) phase-transition temperature, Tg, was observed at 27.6 K. The in-phase component of the ac susceptibility χ′(T) indicated that the SG freezing temperature, Tf, shifted to low temperatures with lowering frequency. Dynamic processes on the frequency dependence of Tf have been analyzed by the slowing-down scaling law, τ∕τ0∝(Tf∕Tg−1)−zv, where the characteristic time τ0 and an exponent zv obtained were about 3.7×10−16s and 9.66, respectively. The microscopic picture characterizing phase separation and competing interactions in the compound were observed by ESR.

Magnetic and magnetocaloric properties in La?(Fe?Co)?Si

In this paper we present the results of study on the magnetic and magnetocaloric properties of LaFe11?xCoxSi2 (x?=?0, 1, 2, 3 and 4) ribbons prepared by using melt-spinning method. The results show that the glass forming ability (GFA) of the alloy is considerably influenced by the addition of Co. With increasing Co-concentration of the alloy, Curie temperature (TC) increases from ?220?K (for x?=?0) to ?500?K (for x?=?4). The maximum magnetic entropy change |?SM|max of the alloy, which is achieved near TC, is larger than 1.2?J?kg?1?K?1 (under magnetic change ?H?=?12?kOe) for all the concentrations of Co. This has a significant meaning for magnetic refrigeration applications.

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.