Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

Platelets of strontium hexaferrite (SrFe12O19, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulationsThis work is supported by the Spanish Ministry of Economy and Competitiveness through Projects MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, MAT2015-66888-C3-1-R and by the European Commission through Project H2020 No. 720853 (Amphibian). These experiments were performed at the CIRCE, MISTRAL and BOREAS beamlines of the ALBA Synchrotron Light Facility. G.D.S. acknowledges the European Youth Employement Initiative and the Autonomous Community of Madrid for a one-year fellowship. Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI for the use of TE

A beyond mean-field study of the Tavis- Cummings model

AIP Conference Proceedings 2150, 040001-1–040001-6A beyond mean-field study of the Tavis-Cummings (TC) model is developed. This is the simplest model for describing the interaction of a radiation field with a system composed by an array of two-level atoms. The first correction to the mean-field ground state energy and the energy gap between the ground and the first excited states are computed. For the ground-state energy our result improves the mean-field calculation, as expected. For the gap, that cannot be calculated at mean-field level, a drop down to zero is obtained at the critical point where the system undergoes a second-order quantum phase transition.Ministerio de Economía y Competitividad, European regional development fund (FEDER) FIS2017-88410-PConsejería de Economía, Innovación, Ciencia y Empleo de la Junta de Andalucía FQM-16