title: Phase-coexistence and thermal hysteresis in samples comprising adventitiously doped MnAs nanocrystals: programming of aggregate properties in magnetostructural nanomaterials.
abstract: 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.
authors.affiliation: Department of Chemistry, Wayne State University , Detroit, Michigan 48202, United States.
authors.forename: Yanhua
authors.initials: Y
authors.lastname: Zhang
authors.forename: Rajesh
authors.initials: R
authors.lastname: Regmi
authors.forename: Yi
authors.initials: Y
authors.lastname: Liu
authors.forename: Gavin
authors.initials: G
authors.lastname: Lawes
authors.forename: Stephanie L
authors.initials: SL
authors.lastname: Brock