Distribution Of Magnetic Ions Research Articles

Effects of Ce–Dy rare earths co-doping on various features of Ni–Co spinel ferrite microspheres prepared via hydrothermal approach

The effects of Ce–Dy co-doping on the crystal structure, optical, dielectric, magnetic properties, and hyperfine interactions of Ni–Co spinel ferrite microspheres synthesized hydrothermally have been studied. A series of ferrites with the general formula Ni0.5Co0.5CexDyxFe2-2xO4 were synthesized with x values ranging from 0.00 to 0.10. The phase, crystallinity, and morphology of ferrite microspheres were analyzed by X-ray powder diffractometry (XRD), scanning and transmission electron microscopes (SEM and TEM), respectively. The structural analyses of the synthesized ferrite microspheres confirmed their high purity and cubic crystalline phase. The Diffuse reflectance spectroscopic (DRS) measurements were presented to calculate direct optical energy band gaps (Eg) and is found in the range 1.63 eV - 1.84 eV. 57Fe Mossbauer spectroscopy showed that the hyperfine magnetic field of tetrahedral (A) and octahedral (B) sites decreased with the substitution of Dy3+-Ce3+ ions that preferrentially occupy the B site. The impact of the rare-earth content (x) on the magnetic features of the prepared NiCo ferrite microspheres was investigated by analyzing M-H loops, which showed soft ferrimagnetism. The magnetic features illustrate a great impact of the incorporation of Ce3+-Dy3+ ions within the NiCo ferrite structure. The saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) increased gradually with increasing Ce–Dy content. At x = 0.04, Ms, Mr, and Hc attain maximum values of about 31.2 emu/g, 11.5 emu/g, and 512.4 Oe, respectively. The Bohr magneton (nB) and magneto-crystalline anisotropy constant (Keff) were also determined and evaluated with correlation to other magnetic parameters. Further increase in Ce3+-Dy3+ content (i.e., x ≥ 0.06) was found to decrease Ms, Mr, and Hc values. The variations in magnetic parameters (Ms, Mr, and Hc) were largely caused by the surface spins effect, the variations in crystallite/particle size, the distribution of magnetic ions into the different sublattices, the evolutions of magneto-crystalline anisotropy, and the variations in the magnetic moment (nB). The squareness ratios were found to be lower than the predicted theoretical value of 0.5 for various samples, indicating that the prepared Ce–Dy substituted NiCo ferrite microspheres are composed of NPs with single-magnetic domain (SMD). Temperature and frequency-dependent electrical and dielectric measurements have been done to estimate the ac/dc conductivity, dielectric constant, and tangent loss values for all the samples. The ac conductivity measurements confirmed the power-law rules, largely dependent on Ce–Dy content. Impedance analysis stated that the conduction mechanisms in all samples are mainly due to the grains-grain boundaries. The dielectric constant of NiCo ferrite microspheres give rise to normal dielectric distribution, with the frequency depending strongly on the Ce–Dy content. The observed variation in tangential loss with frequency can be attributed to the conduction mechanism in ferrites, like Koop's phenomenological model.

Structural, optical and magnetic properties of Ni1−xZnxO/Ni nanocomposite

Ni1−xZnxO/Ni nanocomposite samples synthesized by wet chemical method were characterized by X-ray powder diffraction, Fourier Transform Infrared (FTIR) spectrometer, UV–Vis spectrometer and Nine Tesla Physical Property Measurement System (PPMS)-based vibrating sample magnetometer (VSM). XRD spectra contained peaks due to NiO and Ni. Rietveld refinement of XRD data indicated the change in lattice parameter is more than twice in NiO than in Ni suggests that Zn goes more favorably into NiO than in Ni. The band gap of NiO in Ni1−xZnxO/Ni samples increased from 3.58 to 3.81 eV with increasing x from 0 to 0.07 and thereafter the band gap slightly decreases. NiO/Ni nanocomposites show strong room temperature ferromagnetism. The ferromagnetic interaction further enhanced with 1% of Zn doping into NiO in NiO/Ni nanocomposite. Beyond this doping concentration, the magnetic interaction does not show any systematic variation. The observation of room temperature ferromagnetism in nanocomposite samples is explained by combining the effect of bound magnetic polaron and the magnetization of system with the distribution of magnetic ions as meagerly dispersed in large clusters. Our study indicated that Ni0.99Zn0.01O/Ni nanocomposite sample could be useful for magnetic recording device application.

Role of indium in controlling the magnetic properties of bulk and nano magnetic systems

ABSTRACTIn the present ferrites (soft & hard) series, Saturation Magnetization, increases only at lowers concentration of Indium. The cation distribution from Mössbauer analysis indicates that Indium ion at lower concentration (10 to 20%) goes to tetrahedral site only, whereas higher concentration it goes octahedral sites also. Magnetization has been explained on the basis of super exchange hyperfine interactions as well as cation distribution of magnetic ions as per the Neel's two sub lattice model of ferromagnetism. In all the chosen ferrite series magnetic behavior is consistent with substitution of dopant (In3+ ions). Ferromagnetic results are further authenticated by Mössbauer Spectroscopic analysis.

Interdiffusion Processes in High-Coercivity RF-Sputtered Alnico Thin Films on Si Substrates

Alnico V thin film samples with the thickness of 100 nm were prepared by radio-frequency sputtering on Si substrates with and without a SiO2 layer. Heat treatment of the as-deposited thin films in above ambient pressure in an Ar atmosphere at a temperature range of 600–900°C, followed by quenching and/or slow cooling, leads to higher coercivity values compared to bulk Alnico magnets. Annealing at 800°C followed by quenching results in the highest coercivity reported here of 1.8 kOe. The formation of several triangular-shaped features along the interface between the substrate and the film were observed. A high-resolution transmission electron microscope analysis showed these to be formed via interdiffusion of Fe, Co and Ni atoms into the Si substrate. These features show a large difference in lattice parameters compared with the magnetically soft bulk Fe-Co-Si alloys, and a heterogeneous or layered magnetic ion distribution inside these features could be the origin of the high coercivity observed in the heat-treated films.

Magnetic Effects in Films of Mn12-Acetate

Pulsed laser deposition has been optimized to deposit Mn12-acetate films. Extensive DC and AC susceptibility measurements on these films reveal the presence of two peaks due to (i) a single molecule spin blocked species below ∼3 K as in crystalline Mn12-acetate, and (ii) a magnetic species at 28 K due to significant additional interactions (below ∼40 K). Lattice deformation and random distribution of magnetic ions resulting from laser ablation may account for the presence of a new species with enhanced magnetic anisotropy leading to a higher transition temperature. Results suggest that the high temperature species stabilizes the spin blocked species and the two-species system may display an exchange bias effect.

Mediating distribution of magnetic Co ions by Cr-codoping in (Co,Cr): ZnO thin films

The control over the distribution of magnetic ions in a host is crucial for the functionality of magnetically doped semiconductors. Herein, (Co,Cr)-codoped ZnO shows a possibility of Cr-codoping engineering in mediating the distribution of magnetic Co ions via manipulating the charge state of the Co ions. The x-ray absorption fine structure analyses at Co K-edge indicate that a secondary phase of metallic Co clusters is formed in the Zn0.92Co0.08O film. However, Cr-codoping suppresses the formation of Co clusters, so that all doped Co ions occupy the substitutional sites in ZnO. The ability of Cr in mediating Co distribution, as revealed by first-principles calculations, arises from the strong hybridization between the Co 3d states and the donor band induced by substitutional Cr ions, which facilitates the charge transfer from the donor band to the Co 3d states, changes the charge state of Co ions and modifies the electrostatic interactions among Co ions.

Subtle interplay between native point defects and magnetism in ZnO:Co

Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.

Isolation and aggregation of substituent Co in ZnO:Co diluted magnetic semiconductors

Distribution of magnetic ions in a semiconducting host is critical for the functionality ofdiluted magnetic semiconductors. By investigating the temperature- and field-dependentmagnetization of single-phase polycrystalline ZnO:Co oxides, the substitution of Co at theZn site is found not to occur randomly but Co ions appear to have a tendency foraggregation via oxygen with an antiferromagnetic coupling, in contrast to paramagneticisolated free Co. The experimental findings are justified through first-principles densityfunctional calculations based on the generalized gradient approximation. It suggests thatCo dopants in ZnO:Co have a tendency towards staying close to each other along theab plane.

Controlled Aggregation of Magnetic Ions in a Semiconductor: An Experimental Demonstration

The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by the growth rate and doping with shallow impurities.

Ferromagnetic resonance in epitaxial(In0.53Ga0.47)1−xMnxAs: Angle- and temperature-dependent studies

Ferromagnetic resonance was investigated in MBE grown $\mathrm{InGaMnAs}$ epilayers. Strong azimuthal (in-plane) and polar (out-of-plane) anisotropy was observed, and described reasonably with Magnetic-Anisotropy-Energy (MAE) model taking into account both magnetocrystalline and demagnetization contributions. The different symmetries of mixed $(\text{cubic}+\text{uniaxial})$ anisotropy revealed by nominally the same epilayers were successfully interpreted in terms of MAE model parameters. This difference was attributed to the possible different ordering in distribution of magnetic ions, originating from subtle changes of the growth process.

Structure and microstructure of SmxY2−xO3

SmxY2−xO3 solid solutions are semimagnetic semiconducting materials with wide energy gaps. Magnetic properties of these materials are coupled with their structural and microstructural parameters. Evidently, it is of interest to investigate these parameters in order to explain magnetic properties of diluted magnets. All mixed sesquioxides SmxY2−xO3 (x = 0.2, 0.5, 0.7, 1.0, 1.4 and 1.7) crystallize in C-M2O3 crystaltype in space group Ia3 with Y3+ and Sm3+ cations in 24(d) and 8(b) Wyckoff positions, respectively. The X-ray diffraction data were used to establish the distribution of magnetic ions over crystallographic positions 8(b) and 24(d). A random distribution was found. The same data were used for evaluation of the crystal size, microstrain and paracrystallinity. The concentration dependence of these values was concluded. Results show that unit cell parameters obey Vegard's rule. That fact leads to indirect information about well mixing of Y2O3 and Sm2O3. Agreement of the unit cell parameters with Vegard's rule is connected with the difference between cationic radii and electronegativity. The Sm3+O2−Sm3+ bond geometry, important for the superexchange interaction, strongly depends on Sm3+ concentration. By comparing difference of unit cell parameters of the constitutive components Y2O3 and Sm2O3 and values of microstrains of their solid solutions it can be concluded that clusters are randomly formed.

産地の異なるクロム鉄鉱の粉砕性・磁性と鉱物学的性質との関係

Mineralogical properties of eight kinds of chromites from various localities, and their effects on the grindability and magnetic property were investigated. The results obtained are summarized as follows.(1) Mineralogical property: There are great differences in the chemical composition (for example 37.3-62.6% of Cr2O3 content), lattice constant (8.21-8.30Å), specific gravity (4.09-4.53), crystallinity, and surface structure in the chromites investigated.(2) Hardness and grindability: Vicker's hardness becomes higher, as the Cr2O3 content of chromite decreases. The adequate correlation is not observed between the hardness and grindability. The lower the homogeneity of the surface and internal structure is, the higher the grindability of chromite becomes.(3) Magnetic property: All chromites investigated show the paramagnetic property at room temperature and their susceptibilities have the apparent positive correlation with Fe2+ content of chromite. Some chromites have the relatively large spontaneous magnetization at liquid nitrogen temperature. It is supposed that the distribution of magnetic ions is not uniform in the crystal structure and so the ferrimagnetism is generated in some higher concentrated parts.

The Néel Theory of Ferrimagnetism

The Néel theory of ferrimagnetism is a molecular-field theory of magnetic ordering in systems which contain nonequivalent substructures of magnetic ions. The two principal results of the theory are (a) the curve of reciprocal susceptibility vs temperature above the Curie temperature is an hyperbola, and (b) the curve of spontaneous magnetization vs temperature below the Curie temperature may have any one of a number of unusual forms. The exact form depends on the distribution of magnetic ions between the substructures and on the relative values of the molecular field coefficients. The agreement between the predictions of the Néel theory and some experimental observations on ferrites is discussed briefly.