Abstract
Cobalt nano-rods with the hexagonal close-packed (hcp) structure were prepared by reduction of the long-chain carboxylate Co (II) precursor in polyol. The application of an external magnetic field (µ0H = 1.25 T) during the nucleation and growth steps resulted in a noticeable modification of the mean aspect ratio (length/diameter) of the particles. The particle morphology was also modified as the nano-rods did not exhibit conical heads at their extremities anymore, which are observed for particles prepared without application of an external magnetic field. Besides, the stacking faults density along the c axis of the hcp structure in the cobalt nano-rods has been found to decrease with the increase in the applied magnetic field. The coercive field of randomly oriented nano-rods increased with the aspect ratio, showing the highest value (i.e., 5.8 kOe at 300 K) for the cobalt nano-rods obtained under the highest applied magnetic field. For partially oriented Co nano-rods in toluene solution, the magnetic properties were significantly enhanced with a coercive field of 7.2 kOe at 140 K, while the magnetization saturation reached 92% of the bulk. The MR/MS value was about 0.8, indicating a good orientation of the anisotropic particles relative to each other, making them suitable for the preparation of permanent magnets via a bottom-up approach.
Highlights
Magnetic materials present several interests in the field of high-density magnetic recording [1], spintronics, and microwave applications [2], and can be used as new building blocks for the preparation of permanent magnets [3]
For the syntheses conducted in an open reactor (OR), Figure 1 shows that the obtained phases correspond to Co whatever the intensity of the applied magnetic field
It is remarkable to notice that the NRs obtained during the polyol synthesis under the application of 1.25 T presents high magnetic performances: A high coercive field (5.83 kOe) and a Ms approaching the saturation magnetization of the bulk (92% of the cobalt bulk value) at 300 K
Summary
Magnetic materials present several interests in the field of high-density magnetic recording [1], spintronics, and microwave applications [2], and can be used as new building blocks for the preparation of permanent magnets [3]. Micromagnetic simulations, as well as experimental results, have demonstrated that the conical heads at the ends of the nano-rod particles strongly contribute to the coercivity decrease due to the high demagnetizing field at the conical endings. This is a major issue excluding them from the permanent magnet applications [9,10,11]. The resulting particles display higher mean lengths (up to 570 nm) than those obtained without external magnetic fields, while the mean diameter is still in the nanometer range (15 nm) This magnetic field-assisted polyol process leads to a decrease in the stacking faults along the c axis of the hcp structure. Our results show that the NRs obtained under the application of the magnetic field present good hard magnetic properties at room temperature
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