• Christensen, Mogens
• Andersen, Henrik Lyder
• Garbus, Pelle Gorm
• Ahlburg, Jakob Voldum
• Granados-Miralles, Cecilia

Abstract

Strong permanent magnets with a high energy-product are vital for a great number of electronic devices, these can be found in transformers, loudspeakers, windmills etc. Normally the preferred type of magnets are Rare Earth Metals (REM) containing magnets. REM excels since the magnetic contribution origins from the 4f orbitals which can contain up to 14 electrons. But since REM are both expensive and difficult to mine, a great demand has come for cheaper types of magnets with a similar magnetic performance. <br/>A candidate could be the transition metal oxides. Here the magnetic contribution origins from the 3d orbitals but these can only contain up to 10 electrons. This means that other measures have to be done in order to compete with the REM magnets.<br/>One prominent method is mixing a hard and a soft magnetic phase, on the nanoscale, to achieve an exchange coupling between the phases and enhancing the magnetic energy product. For the exchange coupling to happen it is crucial to have the right ratio between the hard and the soft phase but also to control the size of the particles since exchange coupling is a very small range effect.<br/>In this study, nanoparticles of the spinel CoFe2O4 (hard magnet) is reduced to a metallic alloy CoFe (soft magnet) by heating the sample and flowing it with hydrogen gas. It is studied in situ using neutron powder diffraction with a time resolution of 12 min. The transition from spinel to pure metal goes through an intermediate step of a metal oxide before being fully reduced. These metal oxides are antiferromagnetically ordered an is therefore considered a parasitic phase. However by fine-tuning the reaction temperature and hydrogen flow rate the occurrence of the phase can be minimized. <br/>In order to distinguish between Co and Fe Neutrons are chosen. Since neutrons have a spin it will also be possible to measure a magnetic signal and investigate the exchange-coupling. After the reduction the samples was furthermore investigated using powder x-ray diffraction and VSM (vibrating sample magnetometer).<br/>To understand the reaction mechanism, a series of experiments with varying temperature (fixed flow) or varying flow (fixed temperature) has been performed.<br/>To optimize the exchange-coupling several experiments with fixed temperature and flow, have been performed where the conversion from spinel to metal has been varied.<br/>

Topics

• nanoparticle
• impedance spectroscopy
• phase
• experiment
• powder X-ray diffraction
• Hydrogen
• neutron diffraction
• rare earth metal