Composition-lattice interactions in ternary transition-metal boride ferromagnetic systems

Abstract

Successful development of novel magnetic materials leverages understanding the materials intrinsic magnetic behavior and underlying structural-magnetic property correlations. Of particular interest are materials that undergo coupled magnetic phase transitions sensitive to changes in crystal structure (magnetostructural phase transitions). Magnetostructural phase transformations may be induced by applying multiple stimuli (temperature, pressure, magnetic field) and are often easily tuned by chemical substitution. Composition induced structure changes provide a method for studying the sensitivity of a materials magnetic response to specific bond alterations and changes in the electronic environment. This research aims to quantify the impact of detailed chemistry on the magnetic phase transitions in layered systems where structure and magnetism are strongly coupled. The focus of this Dissertation is to understand structure-composition-property correlations in AlFe2B2 and Fe5SiB2 based technologically relevant new magnetic material systems. Both magnetic systems have layered crystal structures that contribute to their large magnetocrystalline anisotropy, or preference for a material to magnetize along a particular crystallographic direction. The magnitude of the magnetocrystalline anisotropy energy can be used to assess the strength of magnetism-structure interactions and is an important characteristic for magnetic material performance. Additionally both ferromagnetic materials readily undergo chemical substitutions, although prior to this Dissertation connections between chemical modification and observed magnetic response in these systems are not full realized. This Dissertation research provides insight into the fundamental driving forces underlying the magnetic responses of AlFe2B2, Fe5(Si,Ge)B¬2 and related transition-metal borides. Bulk forms of elementally substituted AT2X2 (A = Al, Ga, Ge; T = Mn, Fe, Co, Ni; X = B, C) and Fe5(Si0.75Ge0.25)B2 compounds, are synthesized and studied to explore interactions between structure and magnetism. Results confirm that the ferromagnetic intermetallic compound AlFe2B2 undergoes a magnetostructural phase transformation that is sensitive to Fe-Al antisite defects and detailed chemistry that alter the c-axis and magnetic transition temperature. Within the Fe5SiB2 system, Ge substitution has been identified as a mechanism of magnetocrystalline anisotropy enhancement. These findings provide guidelines for tuning the magnetic phase transition and magnetocrystalline anisotropy in intermetallic ferromagnetic systems.