Novel Approaches to Magnetostructural phase transitions in Metallic systems

The NAMM project, Novel Approaches to Magnetostructural phase transitions in Metallic systems, is fundamental of nature. Two post doctor fellows have worked for a period of three years. The starting point was actually 30 y old results from the University of Oslo, done at a time when highly advanced methods to investigate the detailed atomic arrangements of solids were much less developed than today where we can address such issues by means of very advanced TEM electron microscopes, and large scale synchrotron (X-rays) and neutron facilities. Also with respect to theoretical modeling the situation has developed strongly with current access to supercomputers. This combination, of unsolved interesting and challenging materials issues, excellent samples, along with access to new sharp methods and tools, define the basis for NAMM. The samples being studied are metallic, and contain group 15 elements, in our case arsenic. As an example; mixtures of the binary MnAs and NiAs are obtained by diffusion reactions in closed evacuated containers. Thereafter, magnetic and electric properties are measured. Results are evaluated in context of detailed information on the atomic arrangement. NAMM reports one extraordinary scientific finding: rather than observing that Mn and Ni atoms are mixed (solid solution) in the crystal structure of the solid, analogous to the mixing of water and alcohol in solutions, one observes the formation of sub-nm thick lamellae that repeat systematically (although mathematically in a modulated way). This has never been observed for any solid material earlier, neither metallic nor ceramic. This is absolutely new knowledge. The possible practical implications are not yet known, but advanced sensing technology is an option. Four publications are printed so far, while 4-5 more will be finalized during 2022.

Prosjektet har styrket vitenskaplig kompetanse for alle deltakere i prosjekter, prosjekttilstatte og fast vitenskapelig - mhp metodisk innsikt og bruk, samt ny innsikt/forståelse av nano/material - kjemi/fysikk. Det betyr et dokumentert høyere vitenskaplig nivå i internasjonal sammenheng. Prosjektet har medført nytt samarbeid, med sterke miljøer, gjennom bruk av avansert TEM (Caen, Praha), og XMCD (Soleil), i tillegg til konsolidering av bruk av synkrotron/neutron stor skala anlegg. Dette er viktig for vår fremtidige bruk av slike installasjoner mer generelt innen grunnleggende og anvendt materialforskning. Fornyelse og høy faglig kvalitet stimulerer til topp forskning, tett samarbeid med ledende grupper internasjonalt, samt gjør oss attraktive som partnere for norsk forskning; grunnleggende, samt innen forskningsinstitutter og næringsliv

Certain metallic systems undergo fascinating magnetostructural phase transitions. Our main focus is 3d-transition metal pnictides, primairly with manganese as the element with particular electronic and magnetic features. A few such compounds, for instance MnAs and ternary derivatives, are energetically at a subtle balance with respect to spin state (low versus high spin), electronic structure (localized versus itinerant), antiferro- (incommensurate variants included) or ferromagnetic, hexagonal or orthorhombically disorder, fully cation ordered or partly disordered. In this complex energy landscape first and second order magnetostructural transitions occurs, with multicritical points in relevant phase diagrams. Some such systems attracts large attention for applications in cooling and refrigeration in terms of magneto- or barocaloric materials. Our focus is on fundamental aspects, in particular on how structure and chemistry influence properties. In this respect electronic structure is a key for understanding. We will benefit from a strong past experience, and in particular with skills in sample synthesis. A large number of critical issues are addressed. To answer those, and to pinpoint the physical origin of the exciting stabilities of these compounds, we will use a battery of synchrotron and neutron methods to describe crystal/magnetic structures, at ambient, at variable temperature, pressure and magnetic fields. Physical characterization (PPMS) will provide magnetization, resistivity, seebeck coefficient, heat capacity for 2-350 K and 0-9 T field. High pressure and magnetic field based experiments are planned at large scale facilities. The electronic structure will be calculated by first principles methods, and explored experimentally by SR methods like XAS, XES, XPS, ARPES. DFT modeling will take a particular role, for stability, phase diagrams and electronic structure. National (IFE) and international collaboration is agreed (CUTN, India and CRISMAT, Caen (F).