Iron is rich in natural resources and environmental benign. It has been widely used as
ferromagnetic as well as structural materials. Iron nitride,
Fe16N2, has been expected to have the largest magnetic moment among the ferromagnetic materials. It will be used in magnetic core, recording media and so on in high performance by controlling its magnetic domain size. Difficulty in its preparation has been recently solved by nano-sized powder preparation. Large magnetic coersivity of 20kOe was also recently reported on e-Fe2O3 fine powder in rod shape. Tunnel type magnetoresistance effect for magnetic sensor application can be expected on granular materials where ferromagnetic fine particles are dispersed in nonmagnetic matrix. The nano structure control in nitrides and oxides of iron will bring us to a new era of magnetic materials.
2. Research results
(1) Fe16N2 fine powder prepared in low temperature nitridation and its magnetic properties
It was prepared by nitridation of iron oxide fine power in ammonia flow after the reduction in hydrogen ammonia. Reproducibility was recently much improved by removing the residual humidity in the reaction system. Its magnetic coersivity was about 0.9kOe and the magnetization was saturated to 216emu/g in a magnetic field of 4T.
Fig. 3. Magnetic hysteresis loops for a-Fe starting powder (solid line), and products nitrided at 130oC for 50 hours (dotted line)
and 100 hours (broken line).
(2) Ferromagnetic granular thin films prepared by post annealing of sputter deposited iron based nitride
Nitrides of 3d transition metals thermally decompose releasing nitrogen at relatively low temperature because of the contribution of their anti-bonding chemical bond nature. Fe, Co and Fe0.7Co0.3 were sputter deposited with Al in nitrogen atmosphere to improve the thermal stability of their nitride thin films. Their respective mixture with AlN was obtained by the post annealing of the cosputtered nitride thin films. Nanostructure effects are now being investigated to improve their magnetoresistivity of about 1%.
3. Future view
Nanostrucure needs to be controlled intensively to obtain high performance of the
functional materials. Post annealing has been done by the sample heating in furnace.
We have started to study laser heating effect to precipitate the metal dots in ordered way.
Size and shape control of e-Fe2O3 particle will
also be interesting in its precipitation from silica matrix and also in inverse micelle.