resumo
The MnAs compound shows a first-order transition at T-C approximate to 42 degrees C, and a second-order transition at T-t approximate to 120 degrees C. The first-order transition, with structural (hexagonal-orthorhombic), magnetic (FM-PM), and electrical conductivity changes is associated to magnetocaloric, magnetoelastic, and magnetoresistance effects. We report a study in a large temperature range from -196 degrees up to 140 degrees C, using the gamma-gamma perturbed angular correlations method with the radioactive probe Br-77 --> Se-77, produced at the On-Line Isotope Mass Separator (ISOLDE)-CERN facility. The electric field gradients and magnetic hyperfine fields are determined across the first-and second-order phase transitions encompassing the pure and mixed phase regimes in cooling and heating cycles. The temperature irreversibility of the first-order phase transition is seen locally at the nanoscopic scale sensitivity of the hyperfine field, by its hysteresis, detailing and complementing information obtained with macroscopic measurements (magnetization and X-ray powder diffraction). To interpret the results hyperfine parameters were obtained with first-principles spin-polarized density functional calculations using the generalized gradient approximation with the full potential (linear) augmented plane wave plus local orbitals method (WIEN2K code) by considering the Se probe at both Mn and As sites. A clear assignment of the probe location at the As site is made and complemented with the calculated densities of states and local magnetic moments. We model electronic and magnetic properties of the chemically similar MnSb and MnBi compounds, complementing previous calculations.
palavras-chave
MAGNETOOPTICAL PROPERTIES; ELECTRONIC-STRUCTURE; MAGNETIC-PROPERTIES; PHASE-TRANSITION; TRANSFORMATION; FIELDS; ANISOTROPY; MOSSBAUER; COMPOUND; PRESSURE
categoria
Physics
autores
Goncalves, JN; Amaral, VS; Correia, JG; Lopes, AML
nossos autores
João Nuno Santos Gonçalves
Investigador
Vitor Brás Sequeira Amaral
Professor Catedráticoagradecimentos
This work was supported by the Portuguese Foundation for Science and Technology FCT, with Projects No. CERN-FP-109357-2009, No. CERN-FP-109272-2009, the German BMBF funding resources and by the ISOLDE collaboration with approved Project No. IS487. J. N. Goncalves acknowledges FCT PhD Grant No. SFRH/BD/42194/2007. The authors gratefully acknowledge S. Gama for supplying the MnAs samples, R. Soares for XRD measurements, and H. Haas for useful discussions.