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>

Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

Magnetite (Fe3O4) is of fundamental importance for the Verwey transition near TV = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to an intriguing variation of the Verwey transition: an initial drop of TV to a minimum at 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution between inhomogeneous to homogeneous doping regimes. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on the Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

Exchange bias and Verwey transition in Fe5C2/Fe3O4 core/shell nanoparticles

This report presents new findings of exchange bias and related structural and magnetic properties in iron carbide/magnetite (Fe5C2/Fe3O4) core/shell nanoparticles.

Revealing a masked Verwey transition in nanoparticles of coexisting Fe-oxide phases

We provide direct evidence of a Verwey transition in a nanoparticle ensemble of mixed iron-oxide phases via temperature-dependent Raman spectroscopy.

On the distribution of Verwey transition temperatures in natural magnetites

SUMMARY The Verwey transition in magnetite is a crystallographic phase transition occurring in the temperature range 80–125 K and depends on stoichiometry and cation substitution, which may in turn serve as an indicator of the conditions under which magnetite was formed or altered in nature. We have analysed the distribution of Verwey transition temperatures (TV) in a large set of samples (N = 1110) from a wide variety of rocks, sediments, and other natural and synthetic materials containing magnetite, mined from the database of the Institute for Rock Magnetism and from published studies. The analysis is restricted to measurements of remanence while warming through the transition from which TV was determined by the derivative method. Our analysis showed that the TV distribution exhibited a generally bimodal distribution of Verwey transition temperatures, both for the entire data set and for almost all of the lithological subsets. There is a sharp peak for values in the range 118–120 K, and a broad, relatively flat or polymodal distribution from about 98 to 118 K. The upper end of the distribution was sharp, with only a few values exceeding 124 K, and the tail on the lower end extended down to about 80 K. Virtually all of the sample types exhibited polymodal distributions, almost always with one peak near 120 K, and with one or more additional peaks at lower temperatures. Biogenic magnetites produced by magnetotactic bacteria had the lowest modal value of TV (100 K). Loesses (103.5 K) and igneous extrusives (102.5 K) also had low modal transition temperatures and distributions with dominant low-TV peaks. Lithological groups with the highest modal transition temperatures were modern soils (119.5 K), silicate minerals with exsolved magnetite (119 K) and sedimentary rocks (119 K). Numerical experiments confirmed that the derivative method for the determination of TV was reasonably robust and that the observed distributions cannot be explained as an artefact related to the determination of TV from individual thermomagnetic runs but rather is a general characteristic of natural magnetites. The results provide context for studies that interpret TV in particular samples in terms of natural processes or conditions during formation or alteration of magnetite.