Superparamagnetic Inclusions Research Articles

Temperature dependences of paramagnetic response in oxidized spark plasma sintered carbon nanotubes: The way of understanding the local electronic structure

Electron paramagnetic resonance (EPR) spectroscopy was used to study an electronic structure of carbon nanotubes (CNTs) consolidated by spark plasma sintering and additionally oxidized in vapor of boiling nitric acid. Two components were distinguished in the EPR absorption spectra: narrow and broad lines with different g-factors. This testified to the different surrounding of paramagnetic centers associated with the lines. Temperature dependence of spin numbers, associated with the narrow line, transfers from Curie-Weiss law to Curie law with increasing temperature. EPR susceptibility, associated with the broad line, can be attributed to a paramagnetic matrix with superparamagnetic inclusions.

Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized Near-IR Light Scattered by Aligned Grains

Magnetic fields play essential roles in protoplanetary disks. Magnetic fields in the disk atmosphere are of particular interest, as they are connected to the wind-launching mechanism. In this work, we study the polarization of the light scattered off of magnetically aligned grains in the disk atmosphere, focusing on the deviation of the polarization orientation from the canonical azimuthal direction, which may be detectable in near-IR polarimetry with instruments such as VLT/SPHERE. We show with a simple disk model that the polarization can even be oriented along the radial (rather than azimuthal) direction, especially in highly inclined disks with toroidally dominated magnetic fields. This polarization reversal is caused by the anisotropy in the polarizability of aligned grains and is thus a telltale sign of such grains. We show that the near-IR light is scattered mostly by μm-sized grains or smaller at the τ = 1 surface and such grains can be magnetically aligned if they contain superparamagnetic inclusions. For comparison with observations, we generate synthetic maps of the ratios of U ϕ /I and Q ϕ /I, which can be used to infer the existence of (magnetically) aligned grains through a negative Q ϕ (polarization reversal) and/or a significant level of U ϕ /I. We show that two features observed in the existing data, an asymmetric distribution of U ϕ with respect to the disk minor axis and a spatial distribution of U ϕ that is predominantly positive or negative, are incompatible with scattering by spherical grains in an axisymmetric disk. They provide indirect evidence for scattering by aligned nonspherical grains.

The transition of polarized dust thermal emission from the protostellar envelope to the disc scale

ABSTRACT Polarized dust continuum emission has been observed with Atacama Large Millimeter/submillimeter Array in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disc scale, with the polarization fraction typically dropping from ${\sim } 5{{\ \rm per\ cent}}$ to ${\sim } 1{{\ \rm per\ cent}}$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal magnetohydrodynamic disc formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disc scale, which makes it more difficult to magnetically align the grains on the disc scale. Specifically, to produce the observed ${\sim } 1{{\ \rm per\ cent}}$ polarization at ${\sim } 100\, \mathrm{au}$ scale via grains aligned with the B-field, even relatively small grains of $1\, \mathrm{\mu m}$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of ∼20) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching ∼2 × 104 for millimetre-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disc scale by scattering.

Size Limit of Superparamagnetic Inclusions in Dust Grains and Difficulty of Magnetic Grain Alignment in Protoplanetary Disks

Abstract Alignment of nonspherical grains with magnetic fields is an important problem, as it lays the foundation of probing magnetic fields with polarized dust thermal emissions. In this paper, we investigate the feasibility of magnetic alignment in protoplanetary disks (PPDs). We use an alignment condition that Larmor precession should be fast compared with the damping timescale. We first show that the Larmor precession timescale is some 3 orders of magnitude longer than the damping time for millimeter-sized grains under conditions typical of PPDs, making the magnetic alignment unlikely. The precession time can be shortened by superparamagnetic inclusions (SPIs), but the reduction factor strongly depends on the size of the SPI clusters, which we find is limited by the so-called “Néel’s relaxation process.” In particular, the size limit of SPIs is set by the so-called “anisotropic energy constant” of the SPI material, which describes the energy barrier needed to change the direction of the magnetic moment of an SPI. For the most common iron-bearing materials, we find maximum SPI sizes corresponding to a reduction factor of the Larmor precession timescale of order 103. We also find that reaching this maximum reduction factor requires fine-tuning on the SPI sizes. Lastly, we illustrate the effects of the SPI size limits on magnetic alignment of dust grains with a simple disk model, and we conclude that it is unlikely for relatively large grains of order 100 μm or more to be aligned with magnetic fields, even with SPIs.

The alignment of interstellar dust grains: thermal flipping and the Davis–Greenstein mechanism

ABSTRACT Interstellar dust grains are non-spherical and, in some environments, partially aligned along the direction of the interstellar magnetic field. Numerous alignment theories have been proposed, all of which examine the grain rotational dynamics. In 1999, Lazarian & Draine introduced the important concept of thermal flipping, in which internal relaxation processes induce the grain body to flip while its angular momentum remains fixed. Through detailed numerical simulations, we study the role of thermal flipping on the grain dynamics during periods of relatively slow rotation, known as ‘crossovers’, for the special case of a spheroidal grain with a non-uniform mass distribution. Lazarian & Draine proposed that rapid flipping during a crossover would lead to ‘thermal trapping’, in which a systematic torque, fixed relative to the grain body, would time average to zero, delaying spin-up to larger rotational speeds. We find that the time-averaged systematic torque is not zero during the crossover and that thermal trapping is not prevalent. As an application, we examine whether the classic Davis–Greenstein alignment mechanism is viable, for grains residing in the cold neutral medium and lacking superparamagnetic inclusions. We find that Davis–Greenstein alignment is not hindered by thermal trapping, but argue that it is, nevertheless, too inefficient to yield the alignment of large grains responsible for optical and infrared starlight polarization. Davis–Greenstein alignment of small grains could potentially contribute to the observed ultraviolet polarization. The theoretical and computational tools developed here can also be applied to analyses of alignment via radiative torques and rotational disruption of grains.

Radiative Torques of Irregular Grains: Describing the Alignment of a Grain Ensemble

Abstract The radiative torque (RAT) mechanism is the most promising way of explaining observed polarization arising from aligned grains. We explore the efficiency of the grain alignment by an anisotropic radiation flow for an extensive ensemble of grain shapes, grain sizes (a), and wavelength (λ). We calculate the distribution of the ratios of the amplitudes of the two major components of the RATs, which is an essential parameter that is used in the theory of RAT alignment in Lazarian & Hoang. While this distribution is different for the different classes of grain shapes that we considered, the most probable values of the parameter are centered in the range of q max ∼ 0.5–1.5. The functional form from the calculated RATs is in good agreement with the analytical model. We find that the RAT efficiency scales as (λ/a)−2.6 for λ ≳ 1.8a, which has a slightly shallower slope than previously found in LH07. This increases the power of predictions obtained with the RAT theory. We also confirm that superparamagnetic inclusions are necessary for achieving high degrees of alignment, and constrain the parameter space describing the requirements for achieving these alignment degrees.

Radiative Grain Alignment in Protoplanetary Disks: Implications for Polarimetric Observations

Abstract We apply the theory of radiative torque (RAT) alignment for studying protoplanetary disks around a T-Tauri star and perform 3D radiative transfer calculations to provide the expected maps of polarized radiation to be compared with observations, such as with ALMA. We revisit the issue of grain alignment for large grains expected in the protoplanetary disks and find that mm-sized grains at the midplane do not align with the magnetic field since the Larmor precession timescale for such large grains becomes longer than the gaseous damping timescale. Hence, for these grains the RAT theory predicts that the alignment axis is determined by the grain precession with respect to the radiative flux. As a result, we expect that the polarization will be in the azimuthal direction for a face-on disk. It is also shown that if dust grains have superparamagnetic inclusions, magnetic field alignment is possible for (sub-)micron grains at the surface layer of disks, and this can be tested by mid-infrared polarimetric observations.

Mechanosynthesis of hydroxyapatite–ferrite composite nanopowder

In the present work, an investigation of the mechanosynthesis of calcium hydroxyapatite (HA, Ca10(PO4)6(OH)2) from a mixture of calcium oxide (СаО) and ammonium hydrophosphate ((NH4)2HPO4) and mechanotreatment of HA in a planetary mill with the use of steel drums and milling body has been performed. The obtained results have shown that the mechanosynthesis of crystalline nanodisperse HA proceeds through the stage of formation of an amorphous material. The temperature treatment of HA powders at 1000°C has enabled us to establish the influence of the treatment time on the phase composition of the powders and establish the following sequence of phase transformations: Ca10(PO4)6(OH)2→β-Ca3(PO4)2 (tmilling~2h), β-Ca3(PO4)2→α-Ca3(PO4)2 (tmilling~5h), β-,α-Ca3(PO4)2→Ca10(PO4)6(OH)2 (tmilling~7h).The mechanosynthesis and mechanotreatment of hydroxyapatite in steel drums with steel balls is accompanied by the contamination of hydroxyapatite by their wear debris (iron + manganese). A large part of oxidized iron forms superparamagnetic inclusions distributed in HA powder. A small part of Fe3+ and Mn2+ ions from the steel wear debris enters into the hydroxyapatite lattice, substituting Ca2+ ions. As a result, a nanocomposite powder consisting of hydroxyapatite, alloyed by Fe3+ and Mn2+ ions and ferrite inclusions forms. The phase composition of HA powders, the degree of their alloying by Fe3+ and Mn2+ ions, and the content of ferrite inclusions can be controlled by changing the time of mechanotreatment.

Magnetic susceptibility of Al-Co-R (R = Ce, Dy) glass-forming alloys at high temperatures

The magnetic susceptibility of Al-Co-Ce and Al-Co-Dy amorphous alloys is investigated in a wide range of temperatures (t = 20−1700 °C) and magnetic fields (B = 0.3−1.3 T). A weak gC(B) dependence in the entire temperature range and an anomalous increase in susceptibility beginning above the melting temperature of Al2R compound are revealed for all investigated compositions. The experimental data are used to calculate the parameters of the electronic structure of alloys and to estimate the size of super-paramagnetic inclusions. It follows from the experimental data that cobalt atoms are present in these alloys in the nonmagnetic state. It is found that the values of effective magnetic moments on cerium and dysprosium atoms are significantly lower than those for R3+ ions and independent of the chemical composition of alloys. The results are discussed assuming the existence in melts of microregions formed by quasi-molecules of Al2R.

Electric dipole moments and disalignment of interstellar dust grains

The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grain receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.

Alignment of Dust with Magnetic Inclusions: Radiative Torques and Superparamagnetic Barnett and Nuclear Relaxation

We consider grains with superparamagnetic inclusions and report two new condensed matter effects that enhance the internal relaxation of the energy of a wobbling grain, namely, superparamagnetic Barnett relaxation, and an extension of the range where nuclear relaxation is important to higher frequencies. In addition, we report a new effect related to grain rotation: in situations where grains otherwise rotate subthermally subject to radiative torques, they get into a state of fast rotation in the presence of superparamagnetic inclusions. This effect enhances alignment, as fast rotating grains, unlike slowly rotating ones, exhibit the perfect alignment of grain axes with respect to the ambient astrophysical, e.g., interstellar, magnetic fields. Thus measurements of the polarization degree provide a possibility of testing whether grains have superparamagnetic inclusions.

Light Scattering Study of Magnetic Latex Particles and Their Interaction with Polyelectrolytes

Commercially available latex particles with superparamagnetic inclusions have been characterized by polarized and depolarized dynamic light scattering. The particles are spherical, but the larger ones (Rg∼ 0.1 μm) with many magnetic inclusions behave in dynamic light scattering experiments like symmetric tops while the smaller particles do not. The as-received materials had a broad size distribution, but more narrowly distributed particles were prepared by selective filtration for use in probe diffusion and stability studies. The interaction of these negatively charged particles with a linear polyelectrolyte, negatively charged sodium polystyrene sulfonate, was studied by static and dynamic light scattering. Depolarized dynamic light scattering is particularly sensitive to changes in the stability of the colloids or interaction with the polyelectrolyte. The polyelectrolyte can prevent or reverse salt-induced aggregation of the superparamagnetic latex particles. Fluorescence photobleaching recovery experiments to monitor the mobility of fluorescently labeled polyelectrolyte suggest that it is not bound to the latex particles; apparently, a mechanism other than steric stabilization is involved. The dependence of the diffusion of the superparamagnetic latex particles upon added salt is discussed in terms of the ion atmosphere surrounding the polyelectrolyte.

Temperature magnetic hysteresis of precipitation-hardened alloys

The temperature variation of the residual magnetization of the precipitation-hardened alloys Alnico and Ticonal is considered. The temperature hysteresis of the magnetization due to the irreversible magnetization reversal of single-domain particles is related to the temperature dependence of the anisotropy constant and the value of the demagnetizing field. The magnetic state of the matrix phase affects the irreversible process of magnetization reversal of the strongly magnetic particles. The residual magnetization may include a component due to superparamagnetic inclusions in the matrix phase, giving a contribution to the temperature hysteresis in heating-cooling cycles.