Phase-Coexistence and Thermal Hysteresis in Samples Comprising Adventitiously Doped MnAs Nanocrystals: Programming of Aggregate Properties in Magnetostructural Nanomaterials

Small changes in the synthesis of MnAs nanoparticles lead to materials with distinct behavior. Samples prepared by slow heating to 523 K (type-A) exhibit the characteristic magnetostructural transition from the ferromagnetic hexagonal (α) to the paramagnetic orthorhombic (β) phase of bulk MnAs at Tp = 312 K, whereas those prepared by rapid nucleation at 603 K (type-B) adopt the β structure at room temperature and exhibit anomalous magnetic properties. The behavior of type-B nanoparticles is due to P-incorporation (up to 3%), attributed to reaction of the solvent (trioctylphosphine oxide). P-incorporation results in a decrease in the unit cell volume (∼1%) and shifts Tp below room temperature. Temperature-dependent X-ray diffraction reveals a large region of phase-coexistence, up to 90 K, which may reflect small differences in Tp from particle-to-particle within the nearly monodisperse sample. The large coexistence range coupled to the thermal hysteresis results in process-dependent phase mixtures. As-prepared type-B samples exhibiting the β structure at room temperature convert to a mixture of α and β after the sample has been cooled to 77 K and rewarmed to room temperature. This change is reflected in the magnetic response, which shows an increased moment and a shift in the temperature hysteresis loop after cooling. The proportion of α present at room temperature can also be augmented by application of an external magnetic field. Both doped (type-B) and undoped (type-A) MnAs nanoparticles show significant thermal hysteresis narrowing relative to their bulk phases, suggesting that formation of nanoparticles may be an effective method to reduce thermal losses in magnetic refrigeration applications

Synthesis and Characterization of Discrete Fe_<i>x</i>Ni_2–<i>x</i>P Nanocrystals (0 < <i>x</i> < 2): Compositional Effects on Magnetic Properties

Ternary Fe_<i>x</i>Ni_2–<i>x</i>P (0 < <i>x</i> < 2) phases exhibit a range of useful properties that can be augmented or tuned by confinement to the nanoscale including hydrotreating catalytic activity for small <i>x</i> and near-room temperature ferromagnetism for high <i>x</i>. In this work, a solution-phase arrested-precipitation method was developed for the synthesis of Fe_<i>x</i>Ni_2–<i>x</i>P over all values of <i>x</i> (0 < <i>x</i> < 2). The synthesis involves preparation of Ni–P amorphous particles, introduction of the Fe precursor to form amorphous Fe–Ni–P particles, and high-temperature conversion of Fe–Ni–P particles into crystalline ternary phosphide nanocrystals. The ternary Fe_<i>x</i>Ni_2–<i>x</i>P nanocrystals crystallize in the hexagonal Fe₂P-type structure, and the morphology of the nanocrystals showed a distinct compositional dependence, transitioning from about 11 nm diameter spheres to rods with aspect ratios approaching 2 as the Fe fraction is increased (<i>x</i> ≥ 1.2). Lattice parameters do not follow Vegard’s law, consistent with Mössbauer data showing preferential site occupation by Fe of the tetrahedral over the square pyramidal sites at low Fe concentrations, and the opposite effect for <i>x</i> > 0.8. Magnetic measurements of Fe_<i>x</i>Ni_2–<i>x</i>P (<i>x</i> = 1.8, 1.4, and 1.2) nanorods showed a strong compositional dependence of the Curie temperature (<i>T</i>_C) that differs from observations in bulk phases, with the highest <i>T</i>_C (265 K) obtained for <i>x</i> = 1.4

Control of Composition and Size in Discrete Co_<i>x</i>Fe_2–<i>x</i>P Nanoparticles: Consequences for Magnetic Properties

In this work, a solution-phase method was developed for the synthesis of Co_<i>x</i>Fe_2–<i>x</i>P nanoparticles over all <i>x</i> (0 ≤ <i>x</i> ≤ 2). The nanoparticles vary in size, ranging from 17 to 20 nm in diameter with standard deviations ≤ 14%. The synthesis involves preparation of Co_<i>x</i>Fe_1–<i>x</i> alloy nanoparticles and high temperature conversion into crystalline ternary phosphide nanocrystals. The target composition can be controlled by the initial metal precursor ratio, and the size of Co_<i>x</i>Fe_2–<i>x</i>P (from 12 to 22 nm) can be tuned by varying the oleylamine/metal ratio. Mössbauer data show that Fe has a strong preference for the square pyramidal site over the tetrahedral site. Magnetic measurements on Co_<i>x</i>Fe_2–<i>x</i>P nanoparticles showed a strong compositional dependence of the Curie temperature (<i>T</i>_C); CoFeP and Co_0.7Fe_0.3P have <i>T</i>_C’s > 340 K and are superparamagnetic at room temperature