Narrow Results

We investigate the Casimir pressure between two parallel plates made of magnetic materials at nonzero temperature. It is shown that for real magnetodielectric materials only the magnetic properties of ferromagnets can influence the Casimir pressure. This influence is accomplished through the contribution of the zero-frequency term of the Lifshitz formula. The possibility of the Casimir repulsion through the vacuum gap is analyzed depending on the model used for the description of the dielectric properties of the metal plates.

We report a theoretical analysis for the experimental specific heat C(T) data of the perovskite manganites LaMnO_3+δ, with δ=0.11, 0.15 and 0.26, in the temperature domain 4≤T≤10 K. Calculations of C(T) have been made within the two-component scheme: one is the Fermionic and the other is Bosonic (phonon or magnon) contribution. Lattice specific heat is well estimated from the Debye temperature for lanthanum manganites with different δ obtained following an overlap repulsive potential. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations. Later on, following double exchange mechanism the role of magnons is assessed toward specific heat and is found that at low temperatures, specific heat shows almost T^3/2 dependence on the temperature. We note that the lattice specific heat is smaller for δ=0.11 when compared with that of magnon specific heat below 6 K, while the lattice contribution is larger with the magnon contribution for δ=0.15 and 0.26. It is further noticed that in the ferromagnetic phase, deduced electronic specific heat is smaller in comparison with reported large electronic term in low temperature domain. The present investigations allow us to stress that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in pure LaMnO_3+δ (δ=0.11, 0.15 and 0.26). These findings express that the large Coulomb interaction U suppresses the double occupancies of e_g electrons and enhanced electronic specific heat, while there is a decrease of T^3/2-term with δ from 0.26 to 0.11. The present numerical analysis of specific heat shows similar results as those revealed from experiments.

Sr₂CoMoO₆ and Sr₂CoMoO_6-δ ceramics have been obtained by solid state reaction in air and in a H₂/N₂ gas mixture. At room temperature, as shown by X-ray diffraction, both compounds have a tetragonal perovskite-like structure with doublet unit-cell parameters, space group I4/m, with a = 5.5575, c = 7.9342 in Sr₂CoMoO₆ and a = 5.5683 and c = 7.9395 in Sr₂CoMoO_0.95. The latter contains small traces of Co and SrCoO_2.61, with an almost expansion. The magnetic susceptibility indicates that Sr₂ CoMoO₆ is an antiferromagnet with T_N = 37 K. The magnetization was found to be the same in both samples, while the electric properties, originating from the oxygen content, are drastically affected.

Ni${}_{0.65-x}$Zn${}_{0.35}$Mg${}_x$Fe₂O₄ ($0\le x\le 0.2$) with lower compositions of magnesium were synthesized by the sol–gel method and sintered at 1080${}^{\circ }$C for 4 h. X-ray diffraction (XRD) patterns suggest the single phase nature of the spinel-type ferrite and increase in lattice parameter for increasing magnesium content. Surface morphology reveals that grain size increases with increasing magnesium concentration. Magnesium substitution in Ni–Zn ferrites showed influence on the interatomic distance for tetrahedral A sites and octahedral B sites calculated using the standard formula. Conductivity along with dielectric measurements gives additional information on the transport mechanism for magnesium substitution in Ni–Zn ferrites calculated at lower frequencies. Both magnetic loss and dielectric loss were investigated at lower frequency to understand the losses associated with domain wall contribution for magnesium-substituted Ni–Zn ferrites.

Vibrational, mechanical, thermodynamical properties and thermal conductivities of RE₂Ti₂O₇ (RE = Sm, Gd, Dy, Ho, Er and Yb) pyrochlores have been calculated using a proposed eight-parameter bond-bending force constant model. The main outcome of present calculation is that the first neighbor interaction (Ti–O) is stronger than the second neighbor interactions (RE–O). This means that the bonding between Ti and O is more ionic than the one between RE and O. It is also found that the bond strength of RE–O and the bulk modulus decrease in the sequence Sm $>$ Gd $>$ Dy $>$ Ho $>$ Er $>$ Yb. The bulk moduli and Young’s moduli of RE₂Ti₂O₇ also decrease when RE changes from Sm to Yb.

There has been a steadily increasing demand for magnetic materials in devices and charging systems over recent decades. The advent of in-road inductive power transfer (IPT) charging systems in the near future will massively increase the demand. Better, nonbrittle and affordable magnetic materials are required so that vehicles can run over charging pads built into the road without destroying them. This will require innovative solutions and new magnetic material sources to meet these needs in an economically viable way. Here, we have scoped New Zealand’s (NZ) natural magnetic materials and their ability to be included as a viable magnetic material for our roads. We have investigated the magnetic permeabilities of NZ natural magnetic mineral deposits for IPT application in roads.

In this work, powder $({\mathrm{\text{La}}}_{0.78}{\mathrm{\text{Ba}}}_{0.22}{\mathrm{\text{MnO}}}_3)$ samples (purity of 99.99 % and density of ${\mathrm{\text{2.6 g/cm}}}^3)$ were used. The crystal structure of ${\mathrm{\text{La}}}_{0.63}{\mathrm{\text{Ba}}}_{0.37}{\mathrm{\text{MnO}}}_3$ compound was studied by X-ray diffraction method. It was determined that the crystal structure of this compound consists of two phases at room-temperature and under normal condition. These structure phases correspond to the cubic symmetry with Pm-3m space group and rhombohedral symmetry with the R-3c space group. Thermo Gravimetric (TG), Differential Scanning Calorimetry (DSC), Differential Thermo Gravimetric (DTG) and Differential Thermal Analysis (DTA) of ${\mathrm{\text{La}}}_{0.63}{\mathrm{\text{Ba}}}_{0.37}{\mathrm{\text{MnO}}}_3$ compound were carried out in a high-temperature range of $30\le T\le 890^{\circ }\mathrm{\text{C}}$. It has been found that at high-temperatures, two-phase transition occurs in this compound. The value of thermodynamic parameters was found out for each phase transit.

The effect of Co–Ga paired substitution on the superconducting properties of YBa₂Cu₃O_7-δ (Y-123) has been investigated by X-ray diffraction, ac susceptibility, dc resistivity and oxygen content measurements. We report in this paper the results of our studies on the paired substitution of a magnetic and non-magnetic ion at Cu site in Y-123, while keeping the total dopant concentration fixed. The simultaneous substitution of Co and Ga at Cu site shows variation in the transition temperature (T_c), oxygen content and hole concentration as a function of change in the balance of magnetic (Co) and non-magnetic (Ga) concentration. Orthorhombicity (D), given as (b-a)/a, also varies as a function of increasing dopant concentration. The variation in T_c due to Co–Ga substitution is discussed in the light of dopant valency and hole filling mechanism.

Magnetic BaFe₁₂O₁₉ nanowires have been prepared by a hydrothermal process. The nanowires with diameters ~ 15 nm and lengths ~ 2 μm are clearly visible in Transmission Electron Microscopy (TEM) image. The physical properties of the BaFe₁₂O₁₉ nanowires were further characterized by X-ray diffraction (XRD) and magnetization measurement. The results of the magnetization measurement show that the sample displays ferromagnetic properties at room temperature and its saturation magnetization (Ms) reaches 13.36 emu/g. Annealing treatment of the sample in air at 800°C leads to the increasing of the saturation magnetization (Ms: 65.7 emu/g). It is suggested that the oxygen vacancies should be responsible for the low saturation magnetization, which is also supported by the magnetic property (Ms: 21.6 emu/g) of the sample annealed at 800°C in argon shield.

Magnetic behavior and structural data of the U(T_x Al_1-x)₂ compounds, where T=Co, Ni and Mn, were reported. Magnetic measurements were performed in the temperature range 4.2–800 K and magnetic fields up to 7 T. The gradual replacement of Al by Co, Ni and Mn ions leads to the decrease of the effective magnetic moments per uranium ion, and of the paramagnetic Curie temperatures in absolute magnitude. This composition evolution of the two magnetic parameters of the U(T_xAl_1-x)₂ compounds suggests a gradual suppression of the spin fluctuations with replacing the Al ions by T ions.

In this paper, a statistical theory of N-soliton systems with antisymmetric wave functions is presented. As such, the wave functions of the soliton waves have equal but opposite amplitudes. In the wave function representation, the governing energy and continuity conditions are found to be linear, admitting a linear superposition of the soliton waves functions. This property is used to form the combined wave function for the system hence to calculate its total (macroscopic) energy. The statistical theory is applied to model phase transitions in ferromagnetic materials, and used for the case of common ferromagnetic substances, such as iron (²⁶Fe), cobalt (²⁷Co) and nickel (²⁸Ni). The estimated first phase transitions are found to correspond to the respective Curie temperatures of these substances. Based on the energy calculations, the general hysteresis behavior of ferromagnetic materials is derived as a consequence of the model. The statistical theory is useful in the study of ferromagnetic phase transitions, for estimating the Curie point temperature, and an exact determination of the heat capacity of magnetic materials.

In this study, single phase cobalt disulfide $({\mathrm{\text{CoS}}}_2)$ was synthesized by temperature-controlled solid state hybrid microwave heating. The structure, composition and morphology of the obtained samples were studied using X-ray diffraction (XRD), scanning electron spectroscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and high resolution transmission electron microscopy (HRTEM), respectively. The loose ${\mathrm{\text{CoS}}}_2$ polycrystalline precursor was then hot pressed to dense bulk sample. The subsequent transport and magnetic properties measurements reveal the ferromagnetic Curie temperature at the magnetic transition near 128 K. These results suggest that the magnetic transition in ${\mathrm{\text{CoS}}}_2$ is susceptible to the preparation conditions and the microstructure of the samples.

In this study, Heusler alloy ${\mathrm{\text{Ni}}}_{50}{\mathrm{\text{Mn}}}_{37}{\mathrm{\text{Sb}}}_{13}$ has been prepared by solid state hybrid microwave heating within 40 min. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses reveal that the obtained samples are of single phase cubic structure and homogeneous. Temperature-dependence of electrical resistivity and magnetization measurements confirm the austenite to martensitic transition of the first-order nature with the absence of the exchange bias (EB), which is verified by the magnetic hysteresis loops. Compared with the samples prepared by conventional arc-melting method, the sample synthesized by fast microwave heating has lower martensitic transition temperature, smooth magnetization change in the martensitic region and disappeared EB, implying strongly suppressed antiferromagnetic (AFM) interaction. These results suggest that tuning the microstructure via different synthesis method can influence significantly the martensitic transition and magnetic interactions in the Ni–Mn–Sb system.

The structural, electronic as well as the magnetic properties of the Co₂CrX (X$=$Al, Ga and In) full-Heusler alloy have been studied using first-principles calculations performed in the framework of density functional theory (DFT) within the generalized gradient approximation (GGA). It was taken into account both possible L2₁ structures (i.e. Hg₂CuTi- and Cu₂MnAl-type). Basically, for all compounds, the Cu₂MnAl-type structure is energetically more stable than Hg₂CuTi-type structure at the equilibrium volume. The electronic structure calculations for Co₂CrAl reveal that half-metallic (HM) character in Cu₂MnAl-type structure, Co₂CrGa show nearly HM behavior and Co₂CrIn has a metallic character. The predicted total magnetic moment is $3{\mu }_{\mathrm{\text{B}}}$ for Co₂CrX (X$=$Al, Ga) which is in good convergence with the Slater–Pauling (SP) rule.

The geometrical, electronic and magnetic properties of the Zn${}_{0.75}$Mo${}_{0.25}$M (M=S, Se and Te) have been studied by spin-polarized first-principles calculation. The optimized lattice constants of 5.535, 5.836 and 6.274 Å for M=S, Se and Te are related to the atomic radius of 1.09, 1.22 and 1.42 Å for S, Se and Te atoms, respectively. The Zn${}_{0.75}$Mo${}_{0.25}$M are magnetic half-metallic (HM) with the spin-down conventional band gaps of 2.899, 2.126 and 1.840 eV, while the HM band gaps of 0.393, 0.016 and 0.294 eV for M=S, Se and Te, respectively. At the Fermi level, the less than half-filled Mo-$4d$ orbital hybridizated with the less M-$p$ orbital contributes only spin-up channel leading Zn${}_{0.75}$Mo${}_{0.25}$M an HM ferromagnetism. The tetrahedral crystal field formed by adjacent three Zn atoms and one M atom splits the spin-up channel (majority spin) of Mo-$4d$ orbital into three-fold degenerate $t_{2g}(d_{xy},d_{yz},d_{zx})$ states at the Fermi level and double degenerate $e_g$$(d_{z^2},d_{x^2-y^2})$ states below the Fermi level. The exchange splitting energies of the Zn${}_{0.75}$Mo${}_{0.25}$M are −2.611, −2.231 and −1.717 eV for M=S, Se and Te, respectively. The results provide an useful theoretical guidance for Zn${}_{0.75}$Mo${}_{0.25}$M applications in spintronic devices.

In this manuscript, the synthesis and characterization of superparamagnetic particles and their silica-coated counterparts as building blocks for magnetic photonic crystals is fully described. The advantages and disadvantages of the presented synthetic method are discussed. Preliminary results considering the presence of magnetic species within a photonic crystal are also presented. Suppression of emission of the quantum dots within photonic crystals is attributed to a decrease of the number of available photonic modes for radiative decay. The presence of materials with permanent magnetic moments within photonic crystals shows that suppression of their emission is scaled with the strength of the magnetic field.

The magnetic properties of nanocrystalline powders of Ni_0.5Zn_0.5Fe₂O₄, synthesized at low temperatures, by a combustion method, have been investigated. Different powders containing crystallites of comparable average size, obtained by varying the synthesis conditions, show a large difference in the superparamagnetic blocking temperature due to the wide difference in the size distribution. The ferrite nanoparticles show large Curie temperature enhancement up to ~ 100 K, when compared to that of the bulk.

Chain-like nickel arrays assembled from magnetic Ni spheres were successfully prepared through a facile hydrothermal process at 200°C under a 0.25 T external magnetic field. The external magnetic field is strongly believed to be the driving force of the self-assembly. The sample was highly crystalline as confirmed by the X-ray diffraction (XRD) patterns. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images show that all Ni spheres are closely interconnected to form chains, with ~ 950 nm in diameter and ~ 1 cm in length, which arrange into vertical arrays on the silicon substrate. The coercivity and remnant magnetization ratio of the sample, 670 Oe and 0.612, respectively, are substantially higher than for the sample prepared without an applied external magnetic field (68 Oe and 0.336). Such enhancements can be attributed to their novel superstructure, shape anisotropy, reduced demagnetization factor, etc. This process can be used to fabricate large arrays of uniform chains of magnetic materials and modulate their magnetic properties.

Magnetite nanoparticles are extensively studied for their applications in magnetic nanoparticle hyperthermia. However, existing methods involve invasive methods for monitoring the thermal profile while the heat generated by the magnetite nanoparticles is utilized for cancer therapy. Tumor diagnosis utilizing thermography for monitoring the thermal profile is in the early stage of development since the temperature sensitivity is influenced by various experimental factors. Magnetite nanoparticles embedded in agar matrix which mimics the human tissues and their heating characteristics were investigated using infrared thermography. The magnetite nanoparticles with an average particle size of 10nm were subjected to heating in an applied frequency of 500kHz. The influence of concentration, area and depth on the heating characteristics of the tumor phantoms were deduced from the thermography images. The parameters that influence the therapeutical sensitivity while using infrared thermography for magnetic nanoparticle hyperthermia, have been studied for potential applications in theranostics.

Chromium dioxide (CrO${}_2)$ nanoparticle arrays were fabricated on Ti nano-pit array templates by hydrothermal synthesis using CrO₃ aqueous solution as precursor. The Ti nano-pit array template was obtained by stripping TiO₂ nanotube array formed on the Ti foil. CrO₂ nanoparticles are arranged in honeycomb pattern in a large area and their surface density reaches $1.5\times 10^{10}$cm${}^{-2}$. The CrO₂ nanoparticle arrays show typical magnetic behavior, and the easy axis is parallel to the plane of Ti nano-pit array template. The influence of the concentration of CrO₃ aqueous solution on the CrO₂ nanoparticle arrays is studied and the mechanism of the formation of the CrO₂ nanoparticle arrays is briefly discussed.