Structural and magnetic properties of Mn-X and Co-X (X = Ge, V) nanostructured alloys fabricated by non-equilibrium techniques
Date
2020
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Publisher
University of Delaware
Abstract
In this thesis, two non-equilibrium techniques, the cluster beam deposition and melt-spinning, were used to prepare new and novel structures with interesting magnetic properties. The aim was to explore the effects of the size, surface, chemical disorder, and composition on the structural and magnetic properties of nanoclusters and nanostructures in some Mn-Ge, Co-Ge and Co-V alloys. ☐ Mn5Ge3 nanoparticles were prepared by the cluster-beam deposition technique and the effects of size and surface were investigated was on their structural and magnetic properties. Nanoparticles with sizes between 7.2 and 12.6 nm were fabricated with a single-step deposition process without the need for any heat-treatment and with control over the phase purity and crystallinity by varying the argon pressure and power in the cluster gun. It was shown that all the nanoparticles crystallize in the hexagonal Mn5Si3-type crystal structure, which is also the structure of bulk Mn5Ge3. The nanoparticles of 7.2 and 10 nm were found to be superparamagnetic at room temperature (with blocking temperatures of 130 and 170 K, respectively) whereas the nanoparticles of 12.6 nm are ferromagnetic with a Curie temperature of 300 K. The magnetic properties of nanoparticles are inferior to those of bulk. The saturation magnetization, magnetocrsytalline anisotropy and coercivity at 50 K were found to increase drastically from 31 to 172 emu/cm3, 0.4 to 2.9 Mergs/cm3, and 0.4 to 1.3 kOe, respectively, as the size was increased. A core-shell particle morphology was suggested as a possible explanation of the observed magnetization decrease suggested with a 2.8-nm shell having zero magnetization which can be caused by different reasons including a possible Ge gradient and surface oxidation. ☐ Nanocrystalline Co-Ge intermetallic alloys with a wide range of compositions Co62+xGe38-x (x = 0, 1, 2, 3, 4, 4.7, 8 and 18) were made by melt-spinning motivated by inconsistencies in the literature data regarding phase equilibria in the Co-rich part of the Co-Ge phase diagram in order to understand the effect of disorder on their magnetic properties. We resolve these contradictions by showing that the structural and magnetic properties of Co62+xGe38-x melt-spun alloys depend on the cooling rate, the composition, and, as a result, on the lattice parameters and especially the c/a ratio of the phases present. A new orthorhombic phase with a high Curie temperature of 805 K has been discovered for the first time. The observed enhanced magnetic properties in our annealed melt-spun ribbons is explained by atomic rearrangements in the two phases present in the melt-spun alloys induced by melt-spinning and annealing leaving the orthorhombic phase with a higher c/a. ☐ Co-Ge nanoclusters with composition Co2Ge were fabricated by the cluster beam deposition method and their structural and magnetic properties were investigated with the aim to understand the mechanism behind the high Tc observed in the melt-spun Co-Ge alloys by directly observing the nanostructures with elemental distribution inside them as well as quantifying the magnetic moment per cluster. As-made particles with an average size of 5.5 nm exhibit a mixture of hexagonal and orthorhombic crystal structures. Thermomagnetic measurements showed that the as-made particles are superparamagnetic at room temperature with a blocking temperature of 20 K. When the particles are annealed at 823 K for 12 h, their size is increased to 13 nm and they develop a new orthorhombic crystal structure, with a Curie temperature of 815 K. This is drastically different behavior from bulk which is ferromagnetic at cryogenic temperatures only. X-ray diffraction (XRD) measurements suggest the formation of a new Co-rich orthorhombic phase (OP) with slightly increased c/a ratio in the annealed particles and this is believed to be the reason for the drastic change in their magnetic properties as previously mentioned in the ribbons. ☐ The effects of nanostructuring and chemical disorder on the structural and magnetic properties of Co-V NPs and melt-spun alloys were investigated. Our data show that the as-made Co0.71V0.29 and Co0.75V0.25 nanoparticles are superparamagnetic at room temperature (RT) with blocking temperatures of 90 and 137 K, respectively unlike the bulk which are paramagnetic down to cryogenic temperatures. To understand the enhanced magnetic properties observed in the NPs, melt-spun Co3+xVx alloys with 0 ≤ x ≤ 0.70 were prepared. At 50 K, alloys with 0 ≤ x ≤ 0.50 have been found to be a mixture of ferromagnetic clusters in a paramagnetic matrix whereas the samples with higher Co contents (x ≥ 0.5) show ferromagnetism. Some preliminary quantum phase transition analyses were performed on the melt-spun alloys to shed light on understanding the enhanced magnetic properties observed in the NPs because these analyses can reveal the changes in the magnetic properties for small changes in the composition. It was shown that the clusters could show long-range ferromagnetism when x is above a critical value xc (quantum critical point) which can be due to formation of inter-connected individual clusters. We believe this analysis can reveal the reason behind the enhancement in the magnetic properties of the nanoparticles.
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Keywords
Two non-equilibrium techniques, Cluster beam deposition, Melt-spinning, New and novel structures, Magnetic properties