Structural, Magnetic and Electronic Properties of Nanostructured Magnetic Materials

<p>This thesis was motivated by the different properties exhibited by magnetic nanoparticles when compared with the bulk. For example the coercivity and magnetocrystalline anisotropy vary with the particle size and the finite particle size can affect the spin-wave dispersion. When the nanoparticle radius becomes small enough it is possible to observe superparamagnetism with negligible hysteresis. The transport properties can also be different in nanoparticle composites when compared with the bulk. It is particularly interesting if the nanoparticles have a degree of electronic spin polarization because it is then possible to observe spin-dependent tunnelling. This thesis reports the results from a study of the structural, magnetic, and electronic properties of two partially electronically spin-polarized nanostructured compounds, iron-nickel alloy and magnetite, that were made using a new arc-discharge method, ion implantation and annealing, and a co-precipitation method. It was found that permalloy powders could be made by arc-discharge where there were a range of particle sizes from nms to 10s of microns. Magnetoresistance was observed where it is due to the ordinary magnetoresistance and spin-dependent tunnelling between the particles. It was also possible to make magnetite using the arc-discharge process and the powders contained nanoparticles, large faceted nanoparticles, and larger particles in the 10s of micron range. The temperature dependence of the saturation magnetization changes at 127 K, which can be attributed to the charge-ordering Verwey transition. A large magnetoresistance was observed and attributed to spin-dependent tunnelling between the magnetite particles. It was less than predicted due to a spin-disordered interfacial region. The electrical resistance was modelled in terms of small nanoparticles coating the larger particles and electrostatic charging during tunnelling between small nanoparticles. Magnetite powders were also synthesized via a chemical co-precipitation method where nanoparticles with diameters of ~14 nm were observed. The Verwey transition was only observed in the zero-field cooled field-cooled magnetization for the arc-discharge powders. It was observed for the magnetite powders made using both methods in the temperature dependence of the saturation moment. The saturation magnetic moment for powders made using both methods has a power law dependence on temperature with an exponent of 3/2 at low temperatures and a higher value above the Verwey transition temperature 2. There was also a large magnetoresistance due to spin-dependent tunnelling for magnetite nanoparticle made using a chemical co-precipitation method and the electrical resistance could be modelled in terms of electrostatic charging during tunnelling. NixFe₁₋x nanoparticles were made for the first time by ion beam implantation. Small superparamagnetic nanoparticles occurred after implantation. The saturation moment after implantation did not follow the Bloch’s T³/² for x=0.82, which is likely to be due to spin-waves propagating in the nanoparticle/NiyFe₁₋ySizOn matrix. A bi-modal particle size distribution of mostly spherical nanoparticles was observed for x=0.82 after annealing. An x=0.45 sample showed large asymmetric NixFe₁₋x nanoparticles with minimal smaller nanoparticles. The different nanoparticle morphologies is likely to be due to the different nucleation centres and the different initial concentration profiles. The saturation moment had an exponent of 3/2 at low temperatures and there was a contribution from surface disordered spins. A higher Ni fluence with x=0.53 lead to the formation of superparamagnetic nanoparticles that had a higher blocking temperature, indicating the formation of larger nanocrystallites. There was an enhancement in the permeability.</p>

Structural, Magnetic and Electronic Properties of Nanostructured Magnetic Materials

<p>This thesis was motivated by the different properties exhibited by magnetic nanoparticles when compared with the bulk. For example the coercivity and magnetocrystalline anisotropy vary with the particle size and the finite particle size can affect the spin-wave dispersion. When the nanoparticle radius becomes small enough it is possible to observe superparamagnetism with negligible hysteresis. The transport properties can also be different in nanoparticle composites when compared with the bulk. It is particularly interesting if the nanoparticles have a degree of electronic spin polarization because it is then possible to observe spin-dependent tunnelling. This thesis reports the results from a study of the structural, magnetic, and electronic properties of two partially electronically spin-polarized nanostructured compounds, iron-nickel alloy and magnetite, that were made using a new arc-discharge method, ion implantation and annealing, and a co-precipitation method. It was found that permalloy powders could be made by arc-discharge where there were a range of particle sizes from nms to 10s of microns. Magnetoresistance was observed where it is due to the ordinary magnetoresistance and spin-dependent tunnelling between the particles. It was also possible to make magnetite using the arc-discharge process and the powders contained nanoparticles, large faceted nanoparticles, and larger particles in the 10s of micron range. The temperature dependence of the saturation magnetization changes at 127 K, which can be attributed to the charge-ordering Verwey transition. A large magnetoresistance was observed and attributed to spin-dependent tunnelling between the magnetite particles. It was less than predicted due to a spin-disordered interfacial region. The electrical resistance was modelled in terms of small nanoparticles coating the larger particles and electrostatic charging during tunnelling between small nanoparticles. Magnetite powders were also synthesized via a chemical co-precipitation method where nanoparticles with diameters of ~14 nm were observed. The Verwey transition was only observed in the zero-field cooled field-cooled magnetization for the arc-discharge powders. It was observed for the magnetite powders made using both methods in the temperature dependence of the saturation moment. The saturation magnetic moment for powders made using both methods has a power law dependence on temperature with an exponent of 3/2 at low temperatures and a higher value above the Verwey transition temperature 2. There was also a large magnetoresistance due to spin-dependent tunnelling for magnetite nanoparticle made using a chemical co-precipitation method and the electrical resistance could be modelled in terms of electrostatic charging during tunnelling. NixFe₁₋x nanoparticles were made for the first time by ion beam implantation. Small superparamagnetic nanoparticles occurred after implantation. The saturation moment after implantation did not follow the Bloch’s T³/² for x=0.82, which is likely to be due to spin-waves propagating in the nanoparticle/NiyFe₁₋ySizOn matrix. A bi-modal particle size distribution of mostly spherical nanoparticles was observed for x=0.82 after annealing. An x=0.45 sample showed large asymmetric NixFe₁₋x nanoparticles with minimal smaller nanoparticles. The different nanoparticle morphologies is likely to be due to the different nucleation centres and the different initial concentration profiles. The saturation moment had an exponent of 3/2 at low temperatures and there was a contribution from surface disordered spins. A higher Ni fluence with x=0.53 lead to the formation of superparamagnetic nanoparticles that had a higher blocking temperature, indicating the formation of larger nanocrystallites. There was an enhancement in the permeability.</p>

Magnetic Properties of a Holocene Sediment Core from the Yeongsan Estuary, Southwest Korea: Implications for Diagenetic Effects and Availability as Paleoenvironmental Proxies

The sensitivity of magnetic properties, which characterize the mineralogy, concentration, and grain size distribution of magnetic minerals, to environmental processes may provide useful information on paleoenvironmental changes in estuarine environments. Magnetic property studies of estuaries are less common than other environments and, due to the west coast of South Korea having an abundance of estuaries, it provides a good place to study these processes. In this study, we analyzed a variety of magnetic properties based on magnetic susceptibility, hysteresis parameters, progressive acquisition of isothermal remanent magnetization and first-order reversal curve data from a Holocene muddy sediment core recovered from the Yeongsan Estuary on the west coast of South Korea. We examined diagenetic effects on magnetic properties and tested their availability as proxies of paleoenvironmental change. The presence of generally low magnetic susceptibility, ubiquitous greigite-like authigenic magnetic component, and very fine magnetic particle occurrence suggested that the analyzed sediments had undergone considerable early diagenetic alteration. Electron microscopic observations of magnetic minerals support this suggestion. Our results confirm that the use of initial bulk susceptibility as a stand-alone environmental change proxy is not recommended unless it is supported by additional magnetic analyses. We recognized the existence of ferromagnetic-based variabilities related to something besides the adverse diagenetic effects, and have examined possible relationships with sea-level and major climate changes during the Holocene. The most remarkable finding of this study is the two distinct intervals with high values in magnetic coercivity (Bc), coercivity of remanence (Bcr), and ratio of remanent saturation moment to saturation moment (Mrs/Ms) that were well coincident with the respective abrupt decelerations in the rate of sea-level rise occurred at around 8.2 and 7 thousand years ago. It is then inferred that such condition with abrupt drop in sea-level rise rate would be favorable for the abrupt modification of grain size distribution toward more single-domain-like content. We modestly propose consideration of the Bc, Bcr, and Mrs/Ms variability as a potential indicator for the initiation/occurrence of sea-level stillstand/slowstand or highstand during the Holocence, at least at estuarine environments in and around the studied area.

Predictive modeling to estimate the demand for intensive care hospital beds nationwide in the context of the COVID-19 pandemic

Introduction SARS CoV-2 pandemic is pressing hard on the responsiveness of health systems worldwide, notably concerning the massive surge in demand for intensive care hospital beds. Aim This study proposes a methodology to estimate the saturation moment of hospital intensive care beds (critical care beds) and determine the number of units required to compensate for this saturation. Methods A total of 22,016 patients with diagnostic confirmation for COVID-19 caused by SARS-CoV-2 were analyzed between March 4 and May 5, 2020, nationwide. Based on information from the Chilean Ministry of Health and ministerial announcements in the media, the overall availability of critical care beds was estimated at 1,900 to 2,000. The Gompertz function was used to estimate the expected number of COVID-19 patients and to assess their exposure to the available supply of intensive care beds in various possible scenarios, taking into account the supply of total critical care beds, the average occupational index, and the demand for COVID-19 patients who would require an intensive care bed. Results A 100% occupancy of critical care beds could be reached between May 11 and May 27. This condition could be extended for around 48 days, depending on how the expected over-demand is managed. Conclusion A simple, easily interpretable, and applicable to all levels (nationwide, regionwide, municipalities, and hospitals) model is offered as a contribution to managing the expected demand for the coming weeks and helping reduce the adverse effects of the COVID-19 pandemic.

Magnetic and Martensitic Transitions in Ni2Mn1+xSn1-x Alloys

The magnetic phase diagram of Ni2Mn1+xSn1−x based ferromagnetic (FM) shape memory alloys for varied concentrations of x have been studied. With increasing concentration of Mn, the FM Curie temperature (TC) decreases, while the martensitic transition temperature (TM) goes higher. For x = 0.44 and 0.48, TM is close to the onset of ferromagnetism, and two distinct magnetic transitions are observed corresponding to the TC’s of the FM phases of martensite and austenite respectively. The isothermal magnetization at 5 K indicates saturating behaviour at high fields and the saturation moment drops linearly with x. The samples show reasonably large negative magnetoresistance around TM, however the magnitude drops with increasing x. The magnetoresistance is found to be highly irreversible with respect to the applied magnetic field and field induced arrested state is observed for all the FM samples studied around the first order martensitic transition. The present investigation clearly indicates complex magnetic ground state of the Ni2Mn1+xSn1−x samples with competing magnetic interactions.

Co2CrIn: A Further Magnetic Heusler Compound

A further example of the class of Heusler compounds is presented. Co2CrIn is L21 ordered (face centered cubic, space group Fm3̅m) with a lattice constant of a = 6.0596(2) Å . The crystal structure was determined from powder diffraction data by means of the Rietveld method. The magnetic properties of Co2CrIn were measured by means of SQUID magnetometry. The material turns out to be a soft ferromagnet with a saturation moment of 1.2 μB at 5 K