Synthesis of 1-D iron oxide nano-sticks: Tuning the interfacial defects and surface of nano-sticks for fine-tuned exchange biasing

Exchange biasing in single-phase nanoparticles rely highly upon the interface of the nano-particles, which can accommodate uncompensated spins and defects on their surface. Herein, we report the synthesis of high aspect ratio akaganeite nano-sticks with 2.2 +/- 0.15 mu m in length and 116 +/- 20 nm in diameter by understanding various kinetics parameters within the system. By stabilizing the longer length 1-D symmetry, we achieved near room temperature ferromagnetism as well as exchange biasing of 30 Oe at 250 K in akaganeite nano-sticks. The existence of exchange biasing at such high temperature was manifested to natural core-shell structure generated by large uncompensated spins, due to large amount of defects as well as surface effects per unit nano stick. Subsequently the akaganeite nano-sticks were optimally heat-treated at 600 degrees C to convert them into hematite nano-sticks with morphological retention as well as controlled surface pores. It was found that near room temperature exchange bias with high exchange biasing field of 346 Oe at 250 K (approximately 10 times of as synthesized sample) was obtained in case of heat-treated sample. The illustration of higher exchange bias in this research (as compared with other single phase system) are well backed with higher length, defects and surface of the nano-sticks and can be regarded as a model system for exchange biasing of 1-D single phase nanoparticle systems. (C) 2019 Elsevier B.V. All rights reserved.