In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

The solution of electromagnetic scattering by a homogeneous prolate (or oblate) spheroidal particle with an arbitrary size and refractive index is obtained for any angle of incidence by solving Maxwell's equations under given boundary conditions. The method used is that of separating the vector wave equations in the spheroidal coordinates and expanding them in terms of the spheroidal wavefunctions. The unknown coefficients for the expansion are determined by a system of equations derived from the boundary conditions regarding the continuity of tangential components of the electric and magnetic vectors across the surface of the spheroid. The solutions both in the prolate and oblate spheroidal coordinate systems result in a same form, and the equations for the oblate spheroidal system can be obtained from those for the prolate one by replacing the prolate spheroidal wavefunctions with the oblate ones and vice versa. For an oblique incidence, the polarized incident wave is resolved into two components, the TM mode for which the magnetic vector vibrates perpendicularly to the incident plane and the TE mode for which the electric vector vibrates perpendicularly to this plane. For the incidence along the rotation axis the resultant equations are given in the form similar to the one for a sphere given by the Mie theory. The physical parameters involved are the following five quantities: the size parameter defined by the product of the semifocal distance of the spheroid and the propagation constant of the incident wave, the eccentricity, the refractive index of the spheroid relative to the surrounding medium, the incident angle between the direction of the incident wave and the rotation axis, and the angles that specify the direction of the scattered wave.

Light Scattering by a Spheroidal Particle

Computational Materials Science X

We studied the Brownian motion of isolated ellipsoidal particles in water confined to two dimensions and elucidated the effects of coupling between rotational and translational motion. By using digital video microscopy, we quantified the crossover from short-time anisotropic to long-time isotropic diffusion and directly measured probability distributions functions for displacements. We confirmed and interpreted our measurements by using Langevin theory and numerical simulations. Our theory and observations provide insights into fundamental diffusive processes, which are potentially useful for understanding transport in membranes and for understanding the motions of anisotropic macromolecules.

/pdf/brownian-motion-of-an-ellipsoid-2fpjkl3hr9.pdf

Brownian Motion of an ellipsoid

Swift heavy-ion irradiation of elemental metal nanoparticles (NPs) embedded in amorphous SiO2 induces a spherical to rodlike shape transformation with the direction of NP elongation aligned to that of the incident ion. Large, once-spherical NPs become progressively more rodlike while small NPs below a critical diameter do not elongate but dissolve in the matrix. We examine this shape transformation for ten metals under a common irradiation condition to achieve mechanistic insight into the transformation process. Subtle differences are apparent including the saturation of the elongated NP width at a minimum sustainable, metal-specific value. Elongated NPs of lesser width are unstable and subject to vaporization. Furthermore, we demonstrate the elongation process is governed by the formation of a molten ion-track in amorphous SiO2 such that upon saturation the elongated NP width never exceeds the molten ion-track diameter.

/pdf/role-of-thermodynamics-in-the-shape-transformation-of-5a5bpcw0r0.pdf

Role of thermodynamics in the shape transformation of embedded metal nanoparticles induced by swift heavy-ion irradiation.

The room-temperature photoluminescence (PL) of Tb3+ ions has been studied. The Tb ions were implanted into 200 nm thick SiO2 on Si wafers. To achieve a uniform Tb distribution, the implantations were performed at 50, 100, and 190 keV to a total dose of 8.8×1014–1.3×1016 ions/cm2, resulting in Tb concentrations of 0.18–2.7 at. %. The PL spectrum consists of sharp lines due to the Tb3+ intra-4f transitions and a broadband due to SiO2 defects. The samples were annealed at temperatures ranging from 600 to 1050 °C. Up to 900 °C, the annealing procedure improves the PL yield; at temperatures higher than 1000 °C, the PL yield drops again at high dose. The PL spectra show noticeable influence of Tb–Tb crossrelaxation, which favors the green PL over the blue PL.

Room-temperature photoluminescence from Tb ions implanted in SiO2 on Si

Zinc-oxide (ZnO) nanoparticles (NPs) are fabricated in silica glasses (SiO2) by implantation of Zn+ ions of 60 keV up to 1.0×1017ions∕cm2 and following thermal oxidation. After the oxidation at 700 °C for 1 h, the absorption in the visible region due to Zn metallic NPs disappears and a new absorption edge due to ZnO appears at ∼3.25eV. Cross-sectional transmission electron microscopy confirms the formation of ZnO NPs of 5–10 nm in diameter within the near-surface region of ∼80nm thick and larger ZnO NPs on the surface. Under He–Cd laser excitation at λ=325nm, an exciton luminescence peak centered at 375 nm with FWHM of 113 meV was observed at room temperature.

/pdf/fabrication-of-zno-nanoparticles-in-sio2-by-ion-implantation-2a8bpo5gti.pdf

Fabrication of ZnO nanoparticles in SiO2 by ion implantation combined with thermal oxidation

Absorption spectra of silica glasses implanted by Ni negative-ions of 60 keV to 3.8 × 10 16 ions/cm 2 were studied with excluding incoherent multiple reflection (ICMR) due to substrates. Two absorption peaks of 3.3 and 6.0 eV due to Ni nanoparticles were observed. To our knowledge, this is the first direct observation of the 6.0 eV peak which was usually smeared by ICMR. A next concern is identification of the surface plasmon resonance (SPR) of Ni nanoparticles from the two peaks, 3.3 and 6.0 eV. However, two types of well-known SPR criteria predict completely different energies each other, 2.8 and 5.9 eV, respectively. Validity of these criteria is discussed. It is found that one of the criteria includes a non-realistic assumption. Moreover, both the criteria are found to give just an absorption maximum and do not specify the SPR. Using different size dependences between sp-electrons (surface plasmon) and d-electrons, an alternative criterion is proposed. The 3.3 eV band is identified as the SPR of Ni nanoparticles in SiO 2 .

Criteria for surface plasmon resonance energy of metal nanoparticles in silica glass

Magnetic nanoparticles are fabricated in silica glass (SiO2) using high-flux implantation of 60 keV negative nickel ions Ni−. Photo-absorption measurements and cross-sectional transmission electron microscopy (XTEM) observation confirm the formation of metallic Ni nanoparticles in SiO2, and exclude the possible formation of Ni silicides (Ni3Si, Ni2Si, NiSi) and oxides (NiO) as major products. The mean diameter of the nanoparticles was ∼2.9 nm, and the observed depth distribution was similar to a prediction from the TRIDYN code when sputtering was accounted for. Temperature- and field-dependences of magnetization show that the nanoparticles are in the superparamagnetic state at T>∼50 K. From the peak temperature of the zero-field cooling (ZFC) magnetization, which is ∼27 K, and the mean diameter determined from XTEM, it is suggested that the magnetic anisotropy constant K of the nanoparticles is enhanced by ∼8 times as compared to the bulk value.

Hiro Amekura

Papers

Role of thermodynamics in the shape transformation of embedded metal nanoparticles induced by swift heavy-ion irradiation.

Room-temperature photoluminescence from Tb ions implanted in SiO2 on Si

Fabrication of ZnO nanoparticles in SiO2 by ion implantation combined with thermal oxidation

Criteria for surface plasmon resonance energy of metal nanoparticles in silica glass

Nickel nanoparticles in silica glass fabricated by 60 keV negative-ion implantation