Magnetic properties and microstructural observations of oxide coated FeCo nanocrystals before and after compaction

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Abstract
A radio frequency (rf) plasma torch has been used to produce FeCo nanoparticles with a thin protective oxide coating from metal powder precursors. Structural characterization by conventional and synchrotron x-ray diffraction indicated a disordered bcc α-FeCo phase. High resolution transmission electron microscopy revealed spherical particles with several monolayer thick protective oxide coatings. Thermomagnetic measurements were carried out using a superconducting quantum interference device magnetometer and a vibrating sample magnetometer at temperatures between 5 and 1050 K. Antiferromagnetic (exchange bias) coupling was observed due to the presence of the oxide layer. Relatively high coercivities were observed (280 Oe at 5 K and 250 Oe at room temperature). Néel’s surface (interface) anisotropy model was employed to explain the origin of the observed coercivities. As produced powders were hot isostatically pressed at 1023 K and 22 ksi for 2 h. Dense structures were observed and compacted particles revealed coercivities as low as 25 Oe at room temperature.