Elucidation Of The Cation And The Anion Doping Mechanism Of Nanoparticulate Manganese Arsenide: Effect Of Doping On The Magnetostructural Properties

<p>The mechanism of solution-phase synthesis of P-doped type-B MnAs nanoparticles has been quantitatively assessed and the knowledge obtained is extended to independently control nanoparticle size and P incorporation in order to understand the roles of size and dopant concentration on the magnetostructural properties. During the solution-phase synthesis, the dimensions of the nanoparticles change as the monomer concentration in the bulk solution is depleted and the particles become uniform in size and shape when the temperature stabilizes. The temperature at which the nanoparticles were isolated controls the particle size and polydispersity. High temperature isolation of nanoparticles is required to achieve narrow polydispersity and ultimately leads to a highly reproducible product. Adventitious phosphorus incorporation from the solvent TOPO occurs during nucleation, and P is subsequently lost, likely due to self-purification.</p> <p>The reaction conditions for synthesis of nanoparticles of similar sizes with different P concentrations and different particle sizes with similar P concentrations were established and the effects on magnetic and structural properties were evaluated. Temperature-dependent XRD studies and magnetic measurements suggest that the transformation from β to α structure upon cooling is reversible, but occurs with varying degrees of hysteresis. As a consequence of P-doping, the phase transition temperature has shifted below room temperature and a large region of phase co-existence is observed. The transition temperature depends on the concentration of P concentrations (~2%) resulting in suppression of the phase transition temperature as much as 150 K when probed by XRD vs ca. 50 K by magnetic susceptibility. In contrast, ca 1% P doping results in suppression of the magnetostructural transition by ca. 50 K with XRD and magnetic measurements producing similar values. Differences in the apparent transition temperature and phase coexistence region are likely a consequence of heterogeneity of P. The magnetic and structural properties appear to be correlated.</p> <p>The conditions for Fe-doping on type-B MnAs nanoparticles are established and the consequence on magnetic and structural properties is probed. The core-shell morphology, %P, particle size and β-MnAs structure are retained upon incorporation of 2.6% Fe at relatively lower temperature (100 oC) with a short reaction time (15 minutes). Iron incorporation appears to compress lattice parameters by 0.6% with respect to the initial type-B MnAs nanoparticles. The β to α complete structural transition appears to stay same after treatment with iron while hysteresis is increased by ca. ~20 K by magnetic susceptibility. The apparent increase in hysresis is likely a consequence of heterogeneity of Fe.</p> <p>Overall, the changes in intrinsic properties observed with doping on the cation and anion lattice may be arising mainly due to volumetric effect in unit cell. This dissertation research reveals synthetic pathways to produce inaccessible phases from accessible phases by exploiting ion-exchange routes and direct syntheses.