Synthesis and Investigation of Ternary Intermetallics as Itinerant Magnets

• Other Authors: Tan, Xiaoyan (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Chemistry
• Online Access: http://purl.flvc.org/fsu/fd/FSU_2016SP_Tan_fsu_0071E_13050

Intermetallic magnetic materials, such as SmCo5 and Nd2Fe14B, play important role in our daily lives via their use in many high-tech devices and technologies. Understanding the interplay between the crystal structure, electronic structure, and magnetic properties can provide useful information for the improvement of existing and discovery of new magnetic materials. ThCr2Si2 structure is one of the most abundant intermetallic structure types that accommodates a variety of compounds with a general formula AT2X2 (A = alkali, alkaline metal or rare earth metals, T = transition metal, X = metalloids, or nonmetals). The magnetic behavior of itinerant systems with ThCr2Si2 structure type can be very sensitive to modifications of the electronic structure caused by chemical substitution, applied pressure, or magnetic field, even if such perturbations are small. Although extensive studies have been performed on ternary silicides and germanides and on FeAs-based superconductors of this structure type, much less attention has been paid to ternary rare-earth cobalt arsenides. The overarching goal of this work is to provide a synthetic methodology toward rare-earth cobalt arsenides and to investigate their structure-property relationships. We use physical pressure, chemical compression, or chemical substitution to alter the crystal and electronic structures and thus impact the magnetic behavior of these materials. The general synthetic and characterization methods are described in Chapter 2. Chapter 3 reports the synthesis of RCo2As2 (R = La, Ce, Pr, Nd) by reactions of constituent elements in molten Bi. All compounds exhibit high-temperature ferromagnetic ordering of Co magnetic moments. Electronic band structure calculations revealed a high peak in the density of states at the Fermi level, thus supporting the itinerant nature of magnetism in the Co sublattice. The magnetic ordering in the lanthanide sublattice takes place at lower temperatures, with the R moments aligning parallel or antiparallel to the Co moments. In Chapter 4, we demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu, Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. The mixed valence triggered by applied pressure in EuCo2As2 results in the increase of the Co 3d density of states at the Fermi level, promoting itinerant ferromagnetism. Similar to high-pressure form of EuCo2As2, ferromagnetic ordering of Co moments can also be achieved through chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The study of chemical compression was extended to a Ca1-xEuxCo2As2 series of solid solutions. As shown in Chapter 5, chemical pressure exerted in these structures on the Eu site causes it to display mixed valence, which varies from +2.17 for x ≤ 0.6 to + 2.14 for x ≥ 0.65. The solid solutions with x ≤ 0.6 exhibit ferromagnetic ordering while the ones with x ≥ 0.65 exhibit antiferromagnetic ordering. We demonstrate that the change in the magnetic behavior is associated with the changes in the electronic band structure and the increase in the bonding character of Co-Co interactions at the Fermi level for Eu-rich phases. The Bi-flux method used for the synthesis of RCo2As2 was also successfully extended to more complex structures, R2Co12As7 and RCo5As3, as described in Chapter 6. All R2Co12As7 compounds exhibit high-temperature ferromagnetic ordering of Co moments and low-temperature ordering of R moments. In contrast, all RCo5As3 materials are paramagnetic with the exception of antiferromagnetically ordered PrCo5As3. The dramatic difference in the behavior of R2Co12As7 and RCo5As3 also can be justified by changes in the electronic band structure. Based on the understanding of correlations between magnetic properties and electronic structure attained in preceding chapters, we demonstrate in Chapter 7 the prediction of ferromagnetism in another layered-structure material, AlFe2B2. The compound was prepared by two alternative synthetic routes, arc melting and synthesis from Ga flux. The predicted itinerant ferromagnetic behavior was confirmed experimentally, with the magnetic ordering taking place near room temperature. The measurement of magnetocaloric effect (MCE) as a function of applied magnetic field revealed an isothermal entropy change of 4.1 J kg–1 K–1 at 2 T and 7.7 J kg–1 K–1 at 5 T. These are the largest values observed thus far for any metal boride and for any magnetic material of the vast AT2X2 family of layered structures. Importantly, AlFe2B2 represents a rare case of a light-weight material prepared from earth-abundant, benign reactants, which exhibits a substantial MCE while not containing any rare-earth elements. === A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the Doctor of Philosophy. === Spring Semester 2016. === February 4, 2016. === Intermetallic, magnetic materials === Includes bibliographical references. === Michael Shatruk, Professor Co-Directing Dissertation; Vasile Ovidiu Garlea, Professor Co-Directing Dissertation; Pedro Schlottmann, University Representative; Susan Latturner, Committee Member; Hedi Mattoussi, Committee Member.