Dissertation

2019

#### Degree Name

Doctor of Philosophy

#### Department

Physics and Astronomy

#### Major

Condensed Matter Physics

Paul C. Canfield

#### Abstract

Permanent magnets are very essential in numerous applications such as automobiles, wind turbines, computers, electrical motors and many more. The demand for the high-flux permanent magnet is increasing day by day and doubles roughly in every ten years. Surging demand for portable mechanical and digital devices need miniaturization of energy storage devices such as permanent magnets and batteries. Current rare-earth-based high flux permanent magnets such as Nd$_2$Fe$_{14}$B magnets are critical" and Sm based magnets (SmCo$_5$ and Sm$_2$Co$_{17}$) are likely to be critical", meaning that the raw materials' supply security is not guaranteed despite their great importance in clean energy technologies. Additionally, the mining and the purification of these critical materials create large ecological destruction and generate huge amount of acidic hazardous waste and tons of greenhouse and toxic gases.

Moreover, there is a gap in maximum energy product $(BH)_{max}$ between the existing transition metal-based magnets (e.g. Alnico and ferrites) and the above-mentioned rare-earth-based magnets. If new permanent magnets, performing at the higher end of this gap were to be discovered, the dependency of the magnet industries on the critical rare earth elements could be greatly reduced.

This thesis is a summary of results obtained in a search of relatively more abundant, environment friendly and cheaper transition metal based, or Ce-based, ferromagnets for permanent magnets applications. In this work, transition-metal-based ferromagnets (i.e. Fe$_5$B$_2$P, HfMnP, ZrMnP and AlFe$_2$B$_2$), antiferromagnetic AlMn$_2$B$_2$ and Ce-based ferromagnets (Ce$_{3-x}$Mg$_x$Co$_9$ and Ce(Co$_{1-x-y}$Cu$_x$Fe$_y$)$_5$) are synthesized in single crystalline forms and anisotropic magnetic properties were studied.

Fe$_5$B$_2$P is a tetragonal transition metal ferromagnetic compound with a Curie temperature of 655$\pm$2 K and a room temperature magnetocrystalline anisotropy energy density, $\sim$ 0.35 MJ/m$^3$, comparable to hexaferrites. For comparison with a high-flux system, Nd$_2$Fe$_{14}$B has a room temperature magnetocrystalline anisotropy of $\sim$ 5 MJ/m$^3$. HfMnP and ZrMnP are two additional phosphorous based compounds that were identified as orthorhombic ferromagnetic materials with Curie temperatures of 320 K and 371 K respectively. They also exhibit hard axis high anisotropy fields of 4.5 T and 10.5 T respectively at 50 K. AlFe$_2$B$_2$ is an itinerant, rare-earth free, anisotropic, magnetocaloric material with a Curie temperature of 274 K. Similarly, AlMn$_2$B$_2$ is an antiferromagnetic material with a N\'eel temperature of \textit{T}$_\rm N$ = 313$\pm$2 K with low dimensional magnetic properties inferred via smaller critical exponent $\beta = 0.21\pm 0.02$.

Although Ce is the most abundant and easiest to purify rare-earth element, Ce based permanent magnets have been overlooked in the past due to their low anisotropy and rapid decline of the magnetic properties with temperature. These properties were assumed to be associated with the ambivalent and itinerant nature of Ce in intermetallic compounds. An appropriate chemical substitution also known as rehabilitation can be an effective way to improve the ferromagnetic properties and to decrease the content of the critical element in a permanent magnet. The rehabilitation is the process of development of four major intrinsic properties namely uniaxial anisotropy, high saturation magnetization, high Curie temperature and magnetocrystalline anisotropy by the method of noncritical elements alloying such as Mg or Fe.

Mg, a non-magnetic element, alloying in CeCo$_3$ induced a quantum phase transition of Pauli paramagnetic CeCo$_3$ to isostructural, ferromagnetic, solid solution Ce$_{3 - x}$Mg$_x$Co$_9$ at as low as $0.35\leq x \leq 0.40$ along with a significant magnetic anisotropy energy density, useful for a gap magnet with $x \sim 1.40$. In this study, the Curie temperature increases linearly with the Mg content in the solid solution and reaches as high as 450 K for the highest Mg content (\textit{x}$\sim$1.4). As an additional benefit of Mg, we discovered a Mg-flux growth method of high-flux ferromagnetic compound Sm$_2$Co$_{17}$. Then the low-temperature anisotropy field and Curie temperature were determined. Similarly, the optimum amount of Fe and Cu addition to CeCo$_5$ phase adjusts anisotropy and Curie temperature of the system along with the development of 13~Mega-Gauss-Oersted (MG Oe) (103.45 kJ/$m^3$: See conversion table in the Appendix~\ref{tbl:Unitconversion}) energy-product in single-crystalline permanent magnets after appropriate heat treatment.

In summary, exploratory synthesis (including mostly single and sometime polycrystalline samples) of various magnetic systems were performed using flux-growth methods on Fe-, Mn- and Co-rich compounds to identify potential noncritical, ferromagnets for permanent magnet applications. In addition to identifying new ferromagnetic systems (HfMnP, ZrMnP and Ce$_{3-x}$Mg$_x$Co$_9$) the anisotropic magnetic properties were characterized for the materials under study. Ce$_{3-x}$Mg$_x$Co$_9$ system shows promising properties to be useful ferromagnetic material. A 13 MG Oe energy product was developed in Cu, Fe and Ta substituted single-crystalline CeCo$_5$ permanent magnets. These results can serve as proof of the principle of our research goal of discovery and characterization of non-critical, ferromagnetic materials for gap-magnets applications.

Tej Nath Lamichhane

en

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307 pages

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